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How Cherenkov light intensities from irradiated nuclear fuel rods in wet storage depend on rod properties
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.ORCID iD: 0000-0001-8207-3462
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics.ORCID iD: 0000-0002-3136-5665
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(English)Manuscript (preprint) (Other academic)
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 work is intended to form a solid foundation for further development of the prediction models for Cherenkov light production which are used in the safeguards verifications.

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 that light component.

The results also show a relatively complex dependence between the Cherenkov light intensity and fuel properties such as fuel rod dimensions and fission product decay energy. It is also concluded that the radial distributions of the radiation sources in a fuel rod will affect the Cherenkov light production.

Keyword [en]
Nuclear safeguards, Geant4, Cherenkov light, DCVD, Nuclear fuel rod
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
URN: urn:nbn:se:uu:diva-309736OAI: oai:DiVA.org:uu-309736DiVA: diva2:1052807
Funder
Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
Available from: 2016-12-07 Created: 2016-12-07 Last updated: 2016-12-09
In thesis
1. Studies of Cherenkov light production in irradiated nuclear fuel assemblies
Open this publication in new window or tab >>Studies of Cherenkov light production in irradiated nuclear fuel assemblies
2016 (English)Licentiate 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%.

Place, publisher, year, edition, pages
Uppsala universitet, 2016. 36 p.
Keyword
DCVD, Cherenkov light, Geant4, Nuclear fuel, Nuclear safeguards
National Category
Subatomic Physics
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-307952 (URN)
Presentation
2016-12-15, 2002, Ångströmlaboratoriet, Uppsala, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Radiation Safety Authority, SSM2012-2750Swedish National Infrastructure for Computing (SNIC), p2007011
Available from: 2016-12-08 Created: 2016-11-23 Last updated: 2016-12-22Bibliographically approved

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