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  • 1.
    Branger, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Enhanced verification of irradiated nuclear fuel using Cherenkov light2019Conference paper (Other academic)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is one instrument available to authority inspectors to verify spent nuclear fuel assemblies in wet storage. Verification with the DCVD relies on a comparison between the measured Cherenkov light intensity to a predicted one. This work describes the development of an improved the prediction model, to further enhance the DCVD performance. By considering more fuel parameters in the predictions, predictions that are more accurate can be provided for fuel assemblies with a greater range of burnups, cooling times and irradiation histories. Furthermore, by considering the effect of the storage situation, the accuracy of the predictions can be further enhanced. By using the improved prediction model, the DCVD can be put into regular use to reliably verify fuel assemblies with a wider range of burnups and cooling times than before. The improved prediction model will be available to authority inspectors shortly.

  • 2.
    Branger, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Enhancing the performance of the Digital Cherenkov Viewing Device: Detecting partial defects in irradiated nuclear fuel assemblies using Cherenkov light2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is an instrument used by authority safeguards inspectors to verify irradiated nuclear fuel assemblies in wet storage based on Cherenkov light emission. It is frequently used to verify that parts of an assembly have not been diverted, which is done by comparing the measured Cherenkov light intensity to a predicted one.

    This thesis presents work done to further enhance the verification capability of the DCVD, and has focused on developing a second-generation prediction model (2GM), used to predict the Cherenkov light intensity of an assembly. The 2GM was developed to take into account the irradiation history, assembly type and beta decays, while still being usable to an inspector in-field. The 2GM also introduces a method to correct for the Cherenkov light intensity emanating from neighbouring assemblies. Additionally, a method to simulate DCVD images has been seamlessly incorporated into the 2GM.

    The capabilities of the 2GM has been demonstrated on experimental data. In one verification campaign on fuel assemblies with short cooling time, the first-generation model showed a Root Mean Square error of 15.2% when comparing predictions and measurements. This was reduced by the 2GM to 7.8% and 8.1%, for predictions with and without near-neighbour corrections. A simplified version of the 2GM for single assemblies will be included in the next version of the official DCVD software, which will be available to inspectors shortly. The inclusion of the 2GM allows the DCVD to be used to verify short-cooled assemblies and assemblies with unusual irradiation history, with increased accuracy.

    Experimental measurements show that there are situations when the intensity contribution due to neighbours is significant, and should be included in the intensity predictions. The image simulation method has been demonstrated to also allow the effect of structural differences in the assemblies to be considered in the predictions, allowing assemblies of different designs to be compared with enhanced accuracy.

    List of papers
    1. On Cherenkov light production by irradiated nuclear fuel rods
    Open this publication in new window or tab >>On Cherenkov light production by irradiated nuclear fuel rods
    Show others...
    2017 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 12, article id T06001Article in journal (Refereed) Published
    Abstract [en]

    Safeguards verification of irradiated nuclear fuel assemblies in wet storage is frequently done by measuring the Cherenkov light in the surrounding water produced due to radioactive decays of fission products in the fuel. This paper accounts for the physical processes behind the Cherenkov light production caused by a single fuel rod in wet storage, and simulations are presented that investigate to what extent various properties of the rod affect the Cherenkov light production. The results show that the fuel properties has a noticeable effect on the Cherenkov light production, and thus that the prediction models for Cherenkov light production which are used in the safeguards verifications could potentially be improved by considering these properties.It is concluded that the dominating source of the Cherenkov light is gamma-ray interactions with electrons in the surrounding water. Electrons created from beta decay may also exit the fuel and produce Cherenkov light, and e.g. Y-90 was identified as a possible contributor to significant levels of the measurable Cherenkov light in long-cooled fuel. The results also show that the cylindrical, elongated fuel rod geometry results in a non-isotropic Cherenkov light production, and the light component parallel to the rod's axis exhibits a dependence on gamma-ray energy that differs from the total intensity, which is of importance since the typical safeguards measurement situation observes the vertical light component. It is also concluded that the radial distributions of the radiation sources in a fuel rod will affect the Cherenkov light production.

    Keywords
    Nuclear safeguards, Geant4, Cherenkov light, DCVD, Nuclear fuel rod
    National Category
    Subatomic Physics
    Research subject
    Physics with specialization in Applied Nuclear Physics
    Identifiers
    urn:nbn:se:uu:diva-309736 (URN)10.1088/1748-0221/12/06/T06001 (DOI)000405090900001 ()
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2016-12-07 Created: 2016-12-07 Last updated: 2018-08-17Bibliographically approved
    2. Comparison of prediction models for Cherenkov light emissions from nuclear fuel assemblies
    Open this publication in new window or tab >>Comparison of prediction models for Cherenkov light emissions from nuclear fuel assemblies
    Show others...
    2017 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 12, article id P06007Article in journal (Refereed) Published
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used by nuclear safeguards inspectors to verify irradiated nuclear fuel assemblies in wet storage based on the Cherenkov light produced by the assembly. Verification that no rods have been substituted in the fuel, so-called partial-defect verification, is made by comparing the intensity measured with a DCVD with a predicted intensity, based on operator fuel declaration. The prediction model currently used by inspectors is based on simulations of Cherenkov light production in a BWR 8x8 geometry. This work investigates prediction models based on simulated Cherenkov light production in a BWR 8x8 and a PWR 17x17 assembly, as well as a simplified model based on a single rod in water. Cherenkov light caused by both fission product gamma and beta decays were considered.The simulations reveal that there are systematic differences between the models, most noticeably with respect to the fuel assembly cooling time. Consequently, a prediction model that is based on another fuel assembly configuration than the fuel type being measured, will result in systematic over or underestimation of short-cooled fuel as opposed to long-cooled fuel. While a simplified model may be accurate enough for fuel assemblies with fairly homogeneous cooling times, the prediction models may differ by up to 18 \,\% for more heterogeneous fuel. Accordingly, these investigations indicate that the currently used model may need to be exchanged with a set of more detailed, fuel-type specific models, in order minimize the model dependant systematic deviations.

    Keywords
    Cherenkov and transition radiation; Cherenkov detectors; Search for radioactive and; fissile materials; Interaction of radiation with matter
    National Category
    Subatomic Physics
    Research subject
    Physics with specialization in Applied Nuclear Physics
    Identifiers
    urn:nbn:se:uu:diva-309739 (URN)10.1088/1748-0221/12/06/P06007 (DOI)000405090600007 ()
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2016-12-07 Created: 2016-12-07 Last updated: 2018-08-17Bibliographically approved
    3. Improving the prediction model for Cherenkov light generation by irradiated nuclear fuel assemblies in wet storage for enhanced partial-defect verification capability
    Open this publication in new window or tab >>Improving the prediction model for Cherenkov light generation by irradiated nuclear fuel assemblies in wet storage for enhanced partial-defect verification capability
    2015 (English)Conference paper, Published paper (Other academic)
    Place, publisher, year, edition, pages
    ESARDA, 2015
    Series
    ESARDA Bulletin, ISSN 0392-3029 ; 53
    Keywords
    DCVD; partial defect verification; Cherenkov light
    National Category
    Natural Sciences Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-253277 (URN)10.2789/099293 (DOI)
    Conference
    ESARDA Symposium May 18-21 2015, Manchester (UK).
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2015-05-25 Created: 2015-05-25 Last updated: 2018-09-07
    4. Investigating the Cherenkov light production due to cross-talk in closely stored nuclear fuel assemblies in wet storage
    Open this publication in new window or tab >>Investigating the Cherenkov light production due to cross-talk in closely stored nuclear fuel assemblies in wet storage
    Show others...
    2018 (English)In: ESARDA Bulletin, ISSN 1977-5296, no 57Article in journal (Other academic) Published
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is one of the tools available to a safeguards inspector performing verifications of irradiated nuclear fuel assemblies in wet storage. One of the main advantages of safeguards verification using Cherenkov light is that it can be performed without moving the fuel assemblies to an isolated measurement position, allowing for quick measurements. One disadvantage of this procedure is that irradiated nuclear fuel assemblies are often stored close to each other, and consequently gamma radiation from one assembly can enter a neighbouring assembly, and produce Cherenkov light in the neighbour. As a result, the measured Cherenkov light intensity of one assembly will include contributions from its neighbours, which may affect the safeguards conclusions drawn.

    In this paper, this so-called near-neighbour effect, is investigated and quantified through simulation. The simulations show that for two fuel assemblies with similar properties stored closely, the near-neighbour effect can cause a Cherenkov light intensity increase of up to 3% in a measurement. For one fuel assembly surrounded by identical neighbour assemblies, a total of up to 14% of the measured intensity may emanate from the neighbours. The relative contribution from the near-neighbour effect also depends on the fuel properties; for a long-cooled, low-burnup assembly, with low gamma and Cherenkov light emission, surrounded by short-cooled, high-burnup assemblies with high emission, the measured Cherenkov light intensity may be dominated by the contributions from its neighbours.

    When the DCVD is used for partial-defect verification, a 50% defect must be confidently detected. Previous studies have shown that a 50% defect will reduce the measured Cherenkov light intensity by 30% or more, and thus a threshold has been defined, where a ≥30% decrease in Cherenkov light indicates a partial defect. However, this work shows that the near-neighbour effect may also influence the measured intensity, calling either for a lowering of this threshold or for the intensity contributions from neighbouring assemblies to be corrected for. In this work, a method is proposed for assessing the near-neighbour effect based on declared fuel parameters, enabling the latter type of corrections.

    Keywords
    DCVD; partial defect verification; Cherenkov light; Geant4; Cross-talk
    National Category
    Subatomic Physics
    Research subject
    Physics with specialization in Applied Nuclear Physics
    Identifiers
    urn:nbn:se:uu:diva-323613 (URN)
    Conference
    Presenterad under 2017 ESARDA safeguards symposium
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2017-06-08 Created: 2017-06-08 Last updated: 2019-04-01
    5. Experimental evaluation of models for predicting Cherenkov light intensities from short-cooled nuclear fuel assemblies
    Open this publication in new window or tab >>Experimental evaluation of models for predicting Cherenkov light intensities from short-cooled nuclear fuel assemblies
    2018 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 13, article id P02022Article in journal (Refereed) Published
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used by nuclear safeguards inspectors to verify irradiated nuclear fuel assemblies in wet storage based on the recording of Cherenkov light produced by the assemblies. One type of verification involves comparing the measured light intensity from an assembly with a predicted intensity, based on assembly declarations. Crucial for such analyses is the performance of the prediction model used, and recently new modelling methods have been introduced to allow for enhanced prediction capabilities by taking the irradiation history into account, and by including the cross-talk radiation from neighbouring assemblies in the predictions.

    In this work, the performance of three models for Cherenkov-light intensity prediction is evaluated by applying them to a set of short-cooled PWR 17x17 assemblies for which experimental DCVD measurements and operator-declared irradiation data was available; (1) a two-parameter model, based on total burnup and cooling time, previously used by the safeguards inspectors, (2) a newly introduced gamma-spectrum-based model, which incorporates cycle-wise burnup histories, and (3) the latter gamma-spectrum-based model with the addition to account for contributions from neighbouring assemblies.

    The results show that the two gamma-spectrum-based models provide significantly higher precision for the measured inventory compared to the two-parameter model, lowering the standard deviation between relative measured and predicted intensities from 15.2% to 8.1% respectively 7.8%.

    The results show some systematic differences between assemblies of different designs (produced by different manufacturers) in spite of their similar PWR 17x17 geometries, and possible ways are discussed to address such differences, which may allow for even higher prediction capabilities. Still, it is concluded that the gamma-spectrum-based models enable confident verification of the fuel assembly inventory at the currently used detection limit for partial defects, being a 30% discrepancy between measured and predicted intensities, while some false detection occurs with the two-parameter model. The results also indicate that the gamma-spectrum-based prediction methods are accurate enough that the 30% discrepancy limit could potentially be lowered.

    Keywords
    Cherenkov detectors; Search for radioactive and fissile materials; Simulation methods and programs; Radiation calculation
    National Category
    Subatomic Physics
    Research subject
    Physics
    Identifiers
    urn:nbn:se:uu:diva-346692 (URN)10.1088/1748-0221/13/02/P02022 (DOI)000425937900001 ()
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2018-03-20 Created: 2018-03-20 Last updated: 2018-08-17Bibliographically approved
    6. On the inclusion of light transport in prediction tools for Cherenkov light intensity assessment of irradiated nuclear fuel assemblies
    Open this publication in new window or tab >>On the inclusion of light transport in prediction tools for Cherenkov light intensity assessment of irradiated nuclear fuel assemblies
    2019 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 14, article id T01010Article in journal (Refereed) Published
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used to verify irradiated nuclear fuel assemblies in wet storage by imaging the Cherenkov light produced by the radiation emitted from the assemblies. It is frequently used for partial defect verification, verifying that part of an assembly has not been removed and/or replaced. In one of the verification procedures used, the detected total Cherenkov light intensities from a set of assemblies are compared to predicted intensities, which are calculated using operator declarations for the assemblies.

    This work presents a new, time-efficient method to simulate DCVD images of fuel assemblies, allowing for estimations of the Cherenkov light production, transport and detection. Qualitatively, good agreement between simulated and measured images is demonstrated. Quantitatively, it is shown that relative intensity predictions based on simulated images are within 0.5% of corresponding predictions based solely on the production of Cherenkov light, neglecting light transport and detection. Consequently, in most cases it is sufficient to use predictions based on produced Cherenkov light, neglecting transport and detection, thus substantially reducing the time needed for simulations.

    In a verification campaign, assemblies are grouped according to their type, and the relative measured and predicted intensities are compared in a group. By determining transparency factors, describing the fraction of Cherenkov light that is blocked by the top plate of an assembly, it is possible to adjust predictions based on the production of Cherenkov light to take the effect of the top plate into account. This procedure allows assemblies of the same type bit with different top plates to be compared with increased accuracy. The effect of using predictions adjusted with transparency factors were assessed experimentally on a set of Pressurized Water Reactor 17x17 assemblies having five different top plate designs. As a result of the adjustment, the agreement between measured and predicted relative intensities for the whole data set was enhanced, resulting in a reduction of an RMSE from 14.1% to 10.7%. It is expected that further enhancements may be achieved by introducing more detailed top-plate and spacer descriptions.

    Keywords
    Nuclear safeguards, Geant4, Cherenkov light, DCVD, Nuclear fuel
    National Category
    Subatomic Physics
    Research subject
    Physics with specialization in Applied Nuclear Physics
    Identifiers
    urn:nbn:se:uu:diva-357151 (URN)10.1088/1748-0221/14/01/T01010 (DOI)000457930800001 ()
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2018-08-13 Created: 2018-08-13 Last updated: 2019-03-05Bibliographically approved
    7. Experimental study of background subtraction in Digital Cherenkov Viewing Device measurements
    Open this publication in new window or tab >>Experimental study of background subtraction in Digital Cherenkov Viewing Device measurements
    2018 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 13, no 8, article id T08008Article in journal (Refereed) Published
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is an imaging tool used by authority inspectors for partial defect verification of nuclear fuel assemblies in wet storage, i.e. to verify that part of an assembly has not been diverted. One of the currently adopted verification procedures is based on quantitative measurements of the assembly's Cherenkov light emissions, and comparisons to an expected intensity, calculated based on operator declarations. A background subtraction of the intensity data in the recorded images is necessary for accurate quantitative measurements. The currently used background subtraction is aimed at removing an electronics-induced image-wide offset, but it is argued here that the currently adopted procedure may be insufficient.

    It is recommended that a standard dark-frame subtraction should be used, to remove systematic pixel-wise background due to the electronics, replacing the currently used offset procedure. Experimental analyses show that a dark-frame subtraction would further enhance the accuracy and reliability of DCVD measurements. Furthermore, should ageing of the CCD chip result in larger systematic pixel-wise deviations over time, a dark-frame subtraction can ensure reliable measurements regardless of the age of the CCD chip. It can also help in eliminating any adverse effects of malfunctioning pixels. In addition to the background from electronic noise, ways to compensate for background from neighbouring fuel assemblies and ambient light are also discussed.

    Keywords
    Nuclear safeguards, Cherenkov light, DCVD, Nuclear fuel
    National Category
    Subatomic Physics
    Research subject
    Physics with specialization in Nuclear Physics
    Identifiers
    urn:nbn:se:uu:diva-357150 (URN)10.1088/1748-0221/13/08/T08008 (DOI)000442556100001 ()
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2018-08-13 Created: 2018-08-13 Last updated: 2019-08-01Bibliographically approved
  • 3.
    Branger, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Studies of Cherenkov light production in irradiated nuclear fuel assemblies2016Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is an instrument used by authority inspectors to assess irradiated nuclear fuel assemblies in wet storage for the purpose of nuclear safeguards. Originally developed to verify the presence of fuel assemblies with long cooling times and low burnup, the DCVD accuracy is sufficient for partial defect verification, where one verifies that part of an assembly has not been diverted. Much of the recent research regarding the DCVD has been focused on improving its partial defect detection capabilities.

    The partial-defect analysis procedure currently used relies on comparisons between a predicted Cherenkov light intensity and the intensity measured with the DCVD. Enhanced prediction capabilities may thus lead to enhanced verification capabilities. Since the currently used prediction model is based on rudimentary correlations between the Cherenkov light intensity and the burnup and cooling time of the fuel assembly, there are reasons to develop alternative models taking more details into account to more accurately predict the Cherenkov light intensity.

    This work aims at increasing our understanding of the physical processes leading to the Cherenkov light production in irradiated nuclear fuel assemblies in water. This has been investigated through simulations, which in the future are planned to be complemented with measurements.

    The simulations performed reveal that the Cherenkov light production depends on fuel rod dimensions, source distribution in the rod and initial decay energy in a complex way, and that all these factors should be modelled to accurately predict the light intensity. The simulations also reveal that for long-cooled fuel, Y-90 beta-decays may contribute noticeably to the Cherenkov light intensity, a contribution which has not been considered before.

    A prediction model has been developed in this work taking fuel irradiation history, fuel geometry and Y-90 beta-decay into account. These predictions are more detailed than the predictions based on the currently used prediction model. The predictions with the new model can be done quickly enough that the method can be used in the field. The new model has been used during one verification campaign, and showed superior performance to the currently used prediction model. Using the currently used model for this verification, the difference between measured and predicted intensity had a standard deviation of 15.4% of the measured value, and using the new model this was reduced to 8.4%.

    List of papers
    1. Improving the prediction model for Cherenkov light generation by irradiated nuclear fuel assemblies in wet storage for enhanced partial-defect verification capability
    Open this publication in new window or tab >>Improving the prediction model for Cherenkov light generation by irradiated nuclear fuel assemblies in wet storage for enhanced partial-defect verification capability
    2015 (English)Conference paper, Published paper (Other academic)
    Place, publisher, year, edition, pages
    ESARDA, 2015
    Series
    ESARDA Bulletin, ISSN 0392-3029 ; 53
    Keywords
    DCVD; partial defect verification; Cherenkov light
    National Category
    Natural Sciences Physical Sciences
    Identifiers
    urn:nbn:se:uu:diva-253277 (URN)10.2789/099293 (DOI)
    Conference
    ESARDA Symposium May 18-21 2015, Manchester (UK).
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2015-05-25 Created: 2015-05-25 Last updated: 2018-09-07
    2. Comparison of prediction models for Cherenkov light emissions from nuclear fuel assemblies
    Open this publication in new window or tab >>Comparison of prediction models for Cherenkov light emissions from nuclear fuel assemblies
    Show others...
    2017 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 12, article id P06007Article in journal (Refereed) Published
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used by nuclear safeguards inspectors to verify irradiated nuclear fuel assemblies in wet storage based on the Cherenkov light produced by the assembly. Verification that no rods have been substituted in the fuel, so-called partial-defect verification, is made by comparing the intensity measured with a DCVD with a predicted intensity, based on operator fuel declaration. The prediction model currently used by inspectors is based on simulations of Cherenkov light production in a BWR 8x8 geometry. This work investigates prediction models based on simulated Cherenkov light production in a BWR 8x8 and a PWR 17x17 assembly, as well as a simplified model based on a single rod in water. Cherenkov light caused by both fission product gamma and beta decays were considered.The simulations reveal that there are systematic differences between the models, most noticeably with respect to the fuel assembly cooling time. Consequently, a prediction model that is based on another fuel assembly configuration than the fuel type being measured, will result in systematic over or underestimation of short-cooled fuel as opposed to long-cooled fuel. While a simplified model may be accurate enough for fuel assemblies with fairly homogeneous cooling times, the prediction models may differ by up to 18 \,\% for more heterogeneous fuel. Accordingly, these investigations indicate that the currently used model may need to be exchanged with a set of more detailed, fuel-type specific models, in order minimize the model dependant systematic deviations.

    Keywords
    Cherenkov and transition radiation; Cherenkov detectors; Search for radioactive and; fissile materials; Interaction of radiation with matter
    National Category
    Subatomic Physics
    Research subject
    Physics with specialization in Applied Nuclear Physics
    Identifiers
    urn:nbn:se:uu:diva-309739 (URN)10.1088/1748-0221/12/06/P06007 (DOI)000405090600007 ()
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2016-12-07 Created: 2016-12-07 Last updated: 2018-08-17Bibliographically approved
    3. On Cherenkov light production by irradiated nuclear fuel rods
    Open this publication in new window or tab >>On Cherenkov light production by irradiated nuclear fuel rods
    Show others...
    2017 (English)In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 12, article id T06001Article in journal (Refereed) Published
    Abstract [en]

    Safeguards verification of irradiated nuclear fuel assemblies in wet storage is frequently done by measuring the Cherenkov light in the surrounding water produced due to radioactive decays of fission products in the fuel. This paper accounts for the physical processes behind the Cherenkov light production caused by a single fuel rod in wet storage, and simulations are presented that investigate to what extent various properties of the rod affect the Cherenkov light production. The results show that the fuel properties has a noticeable effect on the Cherenkov light production, and thus that the prediction models for Cherenkov light production which are used in the safeguards verifications could potentially be improved by considering these properties.It is concluded that the dominating source of the Cherenkov light is gamma-ray interactions with electrons in the surrounding water. Electrons created from beta decay may also exit the fuel and produce Cherenkov light, and e.g. Y-90 was identified as a possible contributor to significant levels of the measurable Cherenkov light in long-cooled fuel. The results also show that the cylindrical, elongated fuel rod geometry results in a non-isotropic Cherenkov light production, and the light component parallel to the rod's axis exhibits a dependence on gamma-ray energy that differs from the total intensity, which is of importance since the typical safeguards measurement situation observes the vertical light component. It is also concluded that the radial distributions of the radiation sources in a fuel rod will affect the Cherenkov light production.

    Keywords
    Nuclear safeguards, Geant4, Cherenkov light, DCVD, Nuclear fuel rod
    National Category
    Subatomic Physics
    Research subject
    Physics with specialization in Applied Nuclear Physics
    Identifiers
    urn:nbn:se:uu:diva-309736 (URN)10.1088/1748-0221/12/06/T06001 (DOI)000405090900001 ()
    Funder
    Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
    Available from: 2016-12-07 Created: 2016-12-07 Last updated: 2018-08-17Bibliographically approved
  • 4.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Andersson Sundén, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Comparison of prediction models for Cherenkov light emissions from nuclear fuel assemblies2017In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 12, article id P06007Article in journal (Refereed)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used by nuclear safeguards inspectors to verify irradiated nuclear fuel assemblies in wet storage based on the Cherenkov light produced by the assembly. Verification that no rods have been substituted in the fuel, so-called partial-defect verification, is made by comparing the intensity measured with a DCVD with a predicted intensity, based on operator fuel declaration. The prediction model currently used by inspectors is based on simulations of Cherenkov light production in a BWR 8x8 geometry. This work investigates prediction models based on simulated Cherenkov light production in a BWR 8x8 and a PWR 17x17 assembly, as well as a simplified model based on a single rod in water. Cherenkov light caused by both fission product gamma and beta decays were considered.The simulations reveal that there are systematic differences between the models, most noticeably with respect to the fuel assembly cooling time. Consequently, a prediction model that is based on another fuel assembly configuration than the fuel type being measured, will result in systematic over or underestimation of short-cooled fuel as opposed to long-cooled fuel. While a simplified model may be accurate enough for fuel assemblies with fairly homogeneous cooling times, the prediction models may differ by up to 18 \,\% for more heterogeneous fuel. Accordingly, these investigations indicate that the currently used model may need to be exchanged with a set of more detailed, fuel-type specific models, in order minimize the model dependant systematic deviations.

  • 5.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Andersson Sundén, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    On Cherenkov light production by irradiated nuclear fuel rods2017In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 12, article id T06001Article in journal (Refereed)
    Abstract [en]

    Safeguards verification of irradiated nuclear fuel assemblies in wet storage is frequently done by measuring the Cherenkov light in the surrounding water produced due to radioactive decays of fission products in the fuel. This paper accounts for the physical processes behind the Cherenkov light production caused by a single fuel rod in wet storage, and simulations are presented that investigate to what extent various properties of the rod affect the Cherenkov light production. The results show that the fuel properties has a noticeable effect on the Cherenkov light production, and thus that the prediction models for Cherenkov light production which are used in the safeguards verifications could potentially be improved by considering these properties.It is concluded that the dominating source of the Cherenkov light is gamma-ray interactions with electrons in the surrounding water. Electrons created from beta decay may also exit the fuel and produce Cherenkov light, and e.g. Y-90 was identified as a possible contributor to significant levels of the measurable Cherenkov light in long-cooled fuel. The results also show that the cylindrical, elongated fuel rod geometry results in a non-isotropic Cherenkov light production, and the light component parallel to the rod's axis exhibits a dependence on gamma-ray energy that differs from the total intensity, which is of importance since the typical safeguards measurement situation observes the vertical light component. It is also concluded that the radial distributions of the radiation sources in a fuel rod will affect the Cherenkov light production.

  • 6.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson Svärd, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Wernersson, Erik L. G.
    Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computerized Image Analysis and Human-Computer Interaction.
    Improved DCVD assessments of irradiated nuclear fuel using image analysis techniques2014Conference paper (Other academic)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool for measuring the Cherenkov light intensity emitted from irradiated nuclear fuel in wet storage. It is currently used in nuclear facilities where authority inspectors perform attended gross defect verification to ensure the presence of irradiated fuel material, as well as partial defect verification to ensure that a fraction of the fuel material has not been diverted. In 2013, Uppsala University (UU), supported by the Swedish Radiation Safety Authority, initiated a PhD project aimed at gaining a better understanding of the underlying physics process of the Cherenkov light emission and its detection, in order to improve and enhance the capabilities of the DCVD. The scope of this research is broad and includes modelling, simulations and experiments. As a first step, expertise on image analysis was brought into the project with the purpose to identify image analysis related opportunities and challenges relevant to the DCVD. The investigations performed so far cover general aspects of image analysis as well as aspects specific for verification of PWR fuels, where the fuel geometry may be extra challenging. Resulting from the investigation are suggestions on how to improve the measurement procedure and consequently the image quality obtained with the DCVD. This presentation describes these results and expected outcomes of their implementation.

  • 7.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Verifying PWR assemblies with rod cluster control assembly inserts using a DCVDManuscript (preprint) (Other academic)
    Abstract [en]

    One of the instruments available to authority inspectors to measure and characterize the Cherenkov light emissions from irradiated nuclear fuel assemblies in wet storage is the Digital Cherenkov Viewing Device (DCVD). Based on the presence, characteristics and intensity of the Cherenkov light, the inspectors can verify that an assembly under study is not a dummy object, as well as perform partial defect verification of the assembly.

     

    PWR assemblies are sometimes stored with a rod cluster control assembly (RCCA) inserted, which affects the Cherenkov light production and transport in the assembly. Such an insert will also block light from exiting the top of the fuel assembly, which will affect the light distribution and intensity of the Cherenkov light emissions. Whether or not this constitutes a problem when verifying the assemblies for gross or partial defects with a DCVD has not previously been investigated thoroughly.

     

    In this work, the Cherenkov light intensity of a PWR 17x17 assembly with two different RCCA inserts were simulated and analysed, and compared to the Cherenkov light intensity from an assembly without an insert. For the studied assembly and insert types, the DCVD was found to be able to detect partial defects on the level of 50% in all three cases with similar sensitivity, though with a higher measurement uncertainty due to the reduced intensity when an RCCA insert is present. Consequently, for the studied assembly and insert types, assemblies with inserts can be verified with the same methodology and using the same requirements as for assemblies without inserts, with similar partial defect detection performance.

     

    The approach used also made it possible to investigate the minimum sensitivity of the DCVD required to detect partial defects of other levels than 50% in the PWR 17x17 fuel assembly without and with RCCA inserts. These studies gave results that were in agreement with previous results, despite differences in substitution patterns, substitution materials, modeling software and analysis approach.

  • 8.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Verifying PWR assemblies with rod cluster control assembly inserts using a DCVD2019In: ESARDA Bulletin, ISSN 1977-5296, no 58, p. 35-40Article in journal (Refereed)
    Abstract [en]

    One of the instruments available to authority inspectors to measure and characterize the Cherenkov light emissions from irradiated nuclear fuel assemblies in wet storage is the Digital Cherenkov Viewing Device (DCVD). Based on the presence, characteristics and intensity of the Cherenkov light, the inspectors can verify that an assembly under study is not a dummy object, as well as perform partial defect verification of the assembly.

    PWR assemblies are sometimes stored with a rod cluster control assembly (RCCA) inserted, which affects the Cherenkov light production and transport in the assembly. Such an insert will also block light from exiting the top of the fuel assembly, which will affect the light distribution and intensity of the Cherenkov light emissions. Whether or not this constitutes a problem when verifying the assemblies for gross or partial defects with a DCVD has not previously been investigated thoroughly.

    In this work, the Cherenkov light intensity of a PWR 17x17 assembly with two different RCCA inserts were simulated and analysed, and compared to the Cherenkov light intensity from an assembly without an insert. For the studied assembly and insert types, the DCVD was found to be able to detect partial defects on the level of 50% in all studied cases with similar performance, though with a higher measurement uncertainty due to the reduced intensity when an RCCA insert is present. Consequently, for the studied assembly and insert types, assemblies with inserts can be verified with the same methodology as used for assemblies without inserts, with similar partial defect detection performance.

    The simulation approach used also made it possible to investigate the minimum Cherenkov light intensity reduction resulting from partial defects of other levels than 50%, in the PWR 17x17 fuel assembly with and without RCCA inserts. The results for the simulations without an insert were in agreement with previous results, despite differences in substitution patterns, substitution materials, modeling software and analysis approach.

  • 9.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Andersson Sundén, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson Svärd, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Investigating the Cherenkov light production due to cross-talk in closely stored nuclear fuel assemblies in wet storage2018In: ESARDA Bulletin, ISSN 1977-5296, no 57Article in journal (Other academic)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is one of the tools available to a safeguards inspector performing verifications of irradiated nuclear fuel assemblies in wet storage. One of the main advantages of safeguards verification using Cherenkov light is that it can be performed without moving the fuel assemblies to an isolated measurement position, allowing for quick measurements. One disadvantage of this procedure is that irradiated nuclear fuel assemblies are often stored close to each other, and consequently gamma radiation from one assembly can enter a neighbouring assembly, and produce Cherenkov light in the neighbour. As a result, the measured Cherenkov light intensity of one assembly will include contributions from its neighbours, which may affect the safeguards conclusions drawn.

    In this paper, this so-called near-neighbour effect, is investigated and quantified through simulation. The simulations show that for two fuel assemblies with similar properties stored closely, the near-neighbour effect can cause a Cherenkov light intensity increase of up to 3% in a measurement. For one fuel assembly surrounded by identical neighbour assemblies, a total of up to 14% of the measured intensity may emanate from the neighbours. The relative contribution from the near-neighbour effect also depends on the fuel properties; for a long-cooled, low-burnup assembly, with low gamma and Cherenkov light emission, surrounded by short-cooled, high-burnup assemblies with high emission, the measured Cherenkov light intensity may be dominated by the contributions from its neighbours.

    When the DCVD is used for partial-defect verification, a 50% defect must be confidently detected. Previous studies have shown that a 50% defect will reduce the measured Cherenkov light intensity by 30% or more, and thus a threshold has been defined, where a ≥30% decrease in Cherenkov light indicates a partial defect. However, this work shows that the near-neighbour effect may also influence the measured intensity, calling either for a lowering of this threshold or for the intensity contributions from neighbouring assemblies to be corrected for. In this work, a method is proposed for assessing the near-neighbour effect based on declared fuel parameters, enabling the latter type of corrections.

  • 10.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Improved Cherenkov Light Prediction Model for Enhanced DCVD Performance2018Conference paper (Other academic)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is an instrument used to verify irradiated nuclear fuel assemblies in wet storage based on the fuel’s Cherenkov light emissions. The DCVD is frequently used for partial defect verification, verifying that 50% or more of an assembly has not been diverted. The verification methodology is based on comparison of the measured Cherenkov light intensity to a predicted intensity, based on operator declarations.

    For the last five years, a dedicated PhD project at Uppsala University has been aiming at enhancing and improving the verification capabilities when using the DCVD. The project is now approaching its end, and this paper summarizes the comprehensive work performed regarding improving the prediction capabilities.

    A new prediction model has been developed, considering more fuel assembly details to ensure more accurate predictions. With the new model, the irradiation history of an assembly, the assembly design and the contributions from gamma and beta decays are taken into account. The model has also been extended to account for the radiation from neighbouring fuel assemblies, which can enter the assembly being measured and contribute to the measured Cherenkov light. The performance of the prediction model and the neighbour intensity prediction model has been validated against fuel measurements by the IAEA at a PWR facility with short-cooled fuel. The results show that the new model offers an improved prediction capability, allowing the fuel inventory to be verified with no fuel assemblies being identified as outliers requiring additional investigation. A simplified version of the prediction model will be implemented in the next DCVD software version, making it available to IAEA inspectors.

    This development of the DCVD capabilities are in line with the fourth theme of the IAEA safeguards symposium, “Shaping the future of safeguards implementation”, by resolving challenges related to the DCVD and by extending the capabilities of the instrument.

  • 11.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson Svärd, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Experimental evaluation of models for predicting Cherenkov light intensities from short-cooled nuclear fuel assemblies2018In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 13, article id P02022Article in journal (Refereed)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used by nuclear safeguards inspectors to verify irradiated nuclear fuel assemblies in wet storage based on the recording of Cherenkov light produced by the assemblies. One type of verification involves comparing the measured light intensity from an assembly with a predicted intensity, based on assembly declarations. Crucial for such analyses is the performance of the prediction model used, and recently new modelling methods have been introduced to allow for enhanced prediction capabilities by taking the irradiation history into account, and by including the cross-talk radiation from neighbouring assemblies in the predictions.

    In this work, the performance of three models for Cherenkov-light intensity prediction is evaluated by applying them to a set of short-cooled PWR 17x17 assemblies for which experimental DCVD measurements and operator-declared irradiation data was available; (1) a two-parameter model, based on total burnup and cooling time, previously used by the safeguards inspectors, (2) a newly introduced gamma-spectrum-based model, which incorporates cycle-wise burnup histories, and (3) the latter gamma-spectrum-based model with the addition to account for contributions from neighbouring assemblies.

    The results show that the two gamma-spectrum-based models provide significantly higher precision for the measured inventory compared to the two-parameter model, lowering the standard deviation between relative measured and predicted intensities from 15.2% to 8.1% respectively 7.8%.

    The results show some systematic differences between assemblies of different designs (produced by different manufacturers) in spite of their similar PWR 17x17 geometries, and possible ways are discussed to address such differences, which may allow for even higher prediction capabilities. Still, it is concluded that the gamma-spectrum-based models enable confident verification of the fuel assembly inventory at the currently used detection limit for partial defects, being a 30% discrepancy between measured and predicted intensities, while some false detection occurs with the two-parameter model. The results also indicate that the gamma-spectrum-based prediction methods are accurate enough that the 30% discrepancy limit could potentially be lowered.

  • 12.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson Svärd, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Experimental study of background subtraction in Digital Cherenkov Viewing Device measurements2018In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 13, no 8, article id T08008Article in journal (Refereed)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is an imaging tool used by authority inspectors for partial defect verification of nuclear fuel assemblies in wet storage, i.e. to verify that part of an assembly has not been diverted. One of the currently adopted verification procedures is based on quantitative measurements of the assembly's Cherenkov light emissions, and comparisons to an expected intensity, calculated based on operator declarations. A background subtraction of the intensity data in the recorded images is necessary for accurate quantitative measurements. The currently used background subtraction is aimed at removing an electronics-induced image-wide offset, but it is argued here that the currently adopted procedure may be insufficient.

    It is recommended that a standard dark-frame subtraction should be used, to remove systematic pixel-wise background due to the electronics, replacing the currently used offset procedure. Experimental analyses show that a dark-frame subtraction would further enhance the accuracy and reliability of DCVD measurements. Furthermore, should ageing of the CCD chip result in larger systematic pixel-wise deviations over time, a dark-frame subtraction can ensure reliable measurements regardless of the age of the CCD chip. It can also help in eliminating any adverse effects of malfunctioning pixels. In addition to the background from electronic noise, ways to compensate for background from neighbouring fuel assemblies and ambient light are also discussed.

  • 13.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson Svärd, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Improving the prediction model for Cherenkov light generation by irradiated nuclear fuel assemblies in wet storage for enhanced partial-defect verification capability2015Conference paper (Other academic)
  • 14.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson Svärd, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    On the inclusion of light transport in prediction tools for Cherenkov light intensity assessment of irradiated nuclear fuel assemblies2019In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 14, article id T01010Article in journal (Refereed)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used to verify irradiated nuclear fuel assemblies in wet storage by imaging the Cherenkov light produced by the radiation emitted from the assemblies. It is frequently used for partial defect verification, verifying that part of an assembly has not been removed and/or replaced. In one of the verification procedures used, the detected total Cherenkov light intensities from a set of assemblies are compared to predicted intensities, which are calculated using operator declarations for the assemblies.

    This work presents a new, time-efficient method to simulate DCVD images of fuel assemblies, allowing for estimations of the Cherenkov light production, transport and detection. Qualitatively, good agreement between simulated and measured images is demonstrated. Quantitatively, it is shown that relative intensity predictions based on simulated images are within 0.5% of corresponding predictions based solely on the production of Cherenkov light, neglecting light transport and detection. Consequently, in most cases it is sufficient to use predictions based on produced Cherenkov light, neglecting transport and detection, thus substantially reducing the time needed for simulations.

    In a verification campaign, assemblies are grouped according to their type, and the relative measured and predicted intensities are compared in a group. By determining transparency factors, describing the fraction of Cherenkov light that is blocked by the top plate of an assembly, it is possible to adjust predictions based on the production of Cherenkov light to take the effect of the top plate into account. This procedure allows assemblies of the same type bit with different top plates to be compared with increased accuracy. The effect of using predictions adjusted with transparency factors were assessed experimentally on a set of Pressurized Water Reactor 17x17 assemblies having five different top plate designs. As a result of the adjustment, the agreement between measured and predicted relative intensities for the whole data set was enhanced, resulting in a reduction of an RMSE from 14.1% to 10.7%. It is expected that further enhancements may be achieved by introducing more detailed top-plate and spacer descriptions.

    The full text will be freely available from 2020-02-01 00:00
  • 15.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson Svärd, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Wernersson, Erik L. G.
    Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computerized Image Analysis and Human-Computer Interaction.
    Towards unattended partial-defect verification of irradiated nuclear fuel assemblies using the DCVD2014Conference paper (Other academic)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used by authority inspectors to verify irradiated nuclear fuel assemblies in wet storage by measuring the Cherenkov light emitted. The DCVD is approved by the IAEA for gross defect verification, and is one of the few inspection tools approved for partial defect verification.

    There is interest in adapting the DCVD to work in unattended mode, so that it can be used to verify large quantities of irradiated fuel assemblies prior to moving them to difficult-to-access storage locations. This work presents methods based on image analysis that can be used to reduce the effects of different types of distortions encountered when performing measurements with the DCVD. Implementing these methods will ensure that data of high quality is obtained. Verification prior to moving fuels to difficult-to-access storage may also require a dedicated measurement station to be built, and it is argued that by constructing these stations with the DCVD in mind, many distortions can be reduced or eliminated. Thus, by implementing safeguards-by-design, it is possible to ensure that the DCVD is used in near optimal conditions.

  • 16.
    Branger, Erik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Wernersson, Erik L. G.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson, Staffan Svärd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Image analysis as a tool for improved use of the Digital Cherenkov Viewing Device for inspection of irradiated PWR fuel assemblies.2014Report (Other academic)
    Abstract [en]

    The Digital Cherenkov Viewing Device (DCVD) is a tool used to measure the Cherenkov light emitted from irradiated nuclear fuel assemblies stored in water pools. It has been approved by the IAEA for attended gross defect verification, as well as for partial defect verification, where a fraction of the fuel material has been diverted. In this report, we have investigated the current procedures for recording images with the DCVD, and have looked into ways to improve these procedures. Using three different image sets of PWR fuel assemblies, we have analysed what information and results can be obtained using image analysis techniques. We have investigated several error sources that distort the images, and have shown how these errors affect the images. We have also described some of the errors mathematically, and have discussed how these error sources may be compensated for, if the character and magnitude of the errors are known. Resulting from our investigations are a few suggestions on how to improve the procedures and consequently the quality of the images recorded with the DCVD as well as suggestions on how to improve the analysis of collected images. Specifically, a few improvements that should be looked into in the short term are:

    • Images should be recorded with the fuel assembly perfectly centered in the image, and preferably without any tilt of the DCVD relative to the fuel in order to obtain accurate measurements of the light intensity. Image analysis procedures that may aid the alignment are presented.

    • To compensate for the distorting effect of the water surface and possible turbulence in the water, several images with short exposure time should be captured rather than one image with long exposure time. Using image analysis procedures, it is possible to sum the images resulting in a final image with less distortions and improved quality.

    • A reference image should be used to estimate device-related distortions, so that these distortions are compensated for. Ideally, this procedure can also be used to calibrate individual pixels.

    • The background should be carefully taken into account in order to separate the background level from diffuse signal components, allowing for the background to be subtracted. Accordingly, each measurement campaign should be accompanied by at least one background measurement, recorded from a section in the storage pool where no fuel assemblies are present. Furthermore, the background level should be determined from a larger region in the image and not from one individual pixel, as is currently done.

    • A database of measurements should be set up, containing DCVD images, information about the applied DCVD settings and the conditions that the DCVD was used in. Any partial defect verification procedure at any time could then be tested against as much data as possible. Accordingly, a database can aid in evaluating and improving partial defect verification methods using DCVD image analysis.

    Based on the findings and discussions in this report, some long-term improvements are also suggested.

  • 17.
    Grape, Sophie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Branger, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Elter, Zsolt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Mishra, Vaibhav
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Machine learning in nuclear safeguards2019Conference paper (Other academic)
    Abstract [en]

    •Before placing spent nuclear fuel in in a geological repository, they will be characterized and their declared properties will be verified.

    •We have created large library of modelled spent nuclear fuel (SNF) assemblies and estimated their activity of gamma-ray emitting fission products, the early die-away time τ and the Cherenkov light intensity.

    •We have used Random Forest regression to evaluate the capability to determine the fuel parameters initial enrichment (IE), burnup (BU) and cooling time (CT) using data from non-destructive assay (NDA) techniques

  • 18.
    Grape, Sophie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jacobsson Svärd, Staffan
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Hellesen, Carl
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Branger, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Forskning inom teknisk kärnämneskontroll vid Uppsala universitet under 2014–20152016Report (Other academic)
    Abstract [sv]

    Uppsala universitet har inom ramen för olika avtal med SSM under 2014-2015 bedrivit ett omfattande forskningsprogram inom kärnämneskontroll. Forskningsprogrammet har under denna tid innefattat 3 doktorander med dedikerade forskningsprojekt och ett flertal seniora forskare som helt eller delvis har varit engagerade inom kärnämneskontroll.

    Denna rapport uppmärksammar särskilt fyra forskningsområden av hög relevans för den globala kärnämneskontrollen, vilka benämns; DCVD, Next Generation Safeguards Initiative, verifiering av atypiska bränsleobjekt och Generation IV kärnkraftsystem. Även andra forskningsaktiviteter har genomförts inom ramen för forskningsprogrammet, vilka dock ligger utanför redovisningen i denna rapport.

    Under perioden 2014-2015 producerades inom forskningsprogrammet 9 artiklar som skickats till vetenskapliga tidskrifter med peer-review-granskning. Därutöver gjordes medvetna satsningar på att lyfta fram forskningen på de arenor som är av störst betydelse för det internationella kärnämneskontrollarbetet, d.v.s. på de symposier och möten som arrangeras av FN:s internationella atomenergiorgan (IAEA), det europeiska samarbetsorganet ESARDA och den amerikanska organisationen INMM. Vid dessa internationella konferenser publicerades ytterligare 15 vetenskapliga artiklar med unikt innehåll under perioden. En publikationslista med samtliga forskningsarbeten som producerats under perioden redovisas i denna rapport.

  • 19.
    Zsolt, Elter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Mishra, Vaibhav
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Grape, Sophie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Branger, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Jansson, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Caldeira Balkeståhl, Li
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
    Investigating the gamma and neutron radiation around quivers for verification purposes2019Conference paper (Other academic)
    Abstract [en]

    Before encapsulation of spent nuclear fuel in a geological repository, the fuels need to be verified fors afeguards purposes. This requirement applies to all spent fuel assemblies, including those with properties or designs that are especially challenging to verify. One such example are quivers, a new type of containers used to hold damaged spent fuel rods. After placing damaged rods inside the quivers, they are sealed with a thick lid and the water is removed. The lid is thick enough to significantly reduce the amount of the gamma radiation penetrating through it, which can make safeguards verification from the top using gamma techniques difficult.

    In this paper we make a first feasibility study related to safeguards verification of quivers, aimed at investigating the gamma and neutron radiation field around a quiver using a simplified quiver geometry. The nuclide inventory of the rods placed in the quiver is calculated with Serpent and Origen-Arp, and the radiation transport is modeled with Serpent. The objective is to assess the capability of existing non-destructive assay instruments, measuring the gamma and/or neutron radiation from the object, to verify the content for nuclear safeguards purposes. The results show that the thick quiver lid attenuates the gamma radiation, thereby making gamma-radiation based verification from above the quiver difficult. Verification using neutron instruments above the quiver, or gamma and/or neutron instruments on the side may be possible. These results are in agreement with measurements of a BWR quiver using a DCVD, performed by the authors.

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