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Development of a Methodology for Detecting Coolant Void in Lead-cooled Fast Reactors by Means of Neutron Measurements
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics. (Nuclear Fuel and Nuclear Data)
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In a lead-cooled fast reactor (LFR), small bubbles (in the order of one mm or less) may enter the coolant from a leaking steam generator. If such a leakage is undetected the small bubbles may eventually coalesce into a larger bubble in local stagnation zones under the active core. If such a bubble or void releases and passes through the core, it could drive the reactor into prompt criticality. It is therefore desirable to be able to detect the initial stages of such void formation.

In this thesis, a methodology to detect such leaks is presented together with a study on void-induced reactivity effects in various LFR's. The methodology developed is based on information from two fission chambers positioned radially outside the core. The fissile content of the fission chambers consist either of 235U or 242Pu making them sensitive to different parts of the neutron spectrum. It is shown that the information from the fission chambers can be used to obtain an early indication of the presence of a small leak within typically a month. Furthermore, it is shown that for all but the smallest LFR’s, prompt criticality due to voids passing the core cannot be excluded.

One conclusion is that the methodology may form an attractive complement to the general monitoring system of future LFR’s but, as is noted, it has potential for further developments.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 54 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1180
Keyword [en]
LFR, fission chamber, nuclear, reactor, leak, criticality
National Category
Energy Systems
Identifiers
URN: urn:nbn:se:uu:diva-232252ISBN: 978-91-554-9037-9 (print)OAI: oai:DiVA.org:uu-232252DiVA: diva2:747213
Public defence
2014-10-31, room 2001, Lägerhyddsvägen 1, Ångströmslaboratoriet, Uppsala, 08:15 (English)
Opponent
Supervisors
Available from: 2014-10-08 Created: 2014-09-16 Last updated: 2015-01-23Bibliographically approved
List of papers
1. Feasibility study of detection of coolant void in liquid metal cooled fast reactors using changes in the neutron spectrum
Open this publication in new window or tab >>Feasibility study of detection of coolant void in liquid metal cooled fast reactors using changes in the neutron spectrum
2013 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 265, 1255-1265 p.Article in journal (Refereed) Published
Abstract [en]

Formation of coolant void can lead to an increase in reactivity in metal-cooled fast reactors. Accordingly, the ability to detect formation of void and similar phenomena is highly relevant in order to counteract transient behaviour of such a reactor. As this work shows, the energy distribution of the neutron flux in a fast reactor is sensitive to formation of void. For monitoring purposes, this fact suggests the use of fission chambers with different isotopic content and thus different fission threshold energies. In such a way the monitoring system may be tailored in order to fit the purpose to obtain spectral information of the neutron flux.

In this work, simulations have been performed using the Monte-Carlo-based code SERPENT on the ELECTRA reactor design, a 0.5 MWth lead-cooled fast reactor (LFR) planned for in Sweden. The simulations show significant changes in the neutron spectrum due to the formation of void located in specific in-core regions as well as due to a homogeneous core-wide distribution of small bubbles. In an attempt to quantify and to put a number on the spectroscopic changes, the number of neutrons in the high energy region (2–5 MeV) are compared to the number of neutrons in the low-energy region (50–500 keV) and the changes caused by the introduction of void are analyzed. The implications of the findings are discussed.

National Category
Subatomic Physics
Research subject
Applied Nuclear Physics; Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-210980 (URN)10.1016/j.nucengdes.2013.10.015 (DOI)000330085500127 ()
Available from: 2013-11-18 Created: 2013-11-18 Last updated: 2017-12-06
2. Detecting neutron spectrum perturbations due to coolant density changes in a small lead-cooled fast nuclear reactor
Open this publication in new window or tab >>Detecting neutron spectrum perturbations due to coolant density changes in a small lead-cooled fast nuclear reactor
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2013 (English)In: Annals of Nuclear Energy, ISSN 0306-4549, E-ISSN 1873-2100, Vol. 58, 102-109 p.Article in journal (Refereed) Published
Abstract [en]

The lead-cooled fast reactor (LFR) is one of the nuclear reactor technologies proposed by the Generation IV International Forum (GIF). The lead coolant allows for inherent safety properties attractive from a nuclear safety point of view, but issues related to corrosion of structural materials and the possible positive coolant reactivity coefficient must be addressed before LFRs can be commercially viable. As an example, a small crack in e.g. a heat exchanger can generate a more or less homogeneous distribution of bubbles in the coolant (void) which if unnoticed, has the potential to cause criticality issues. This fact motivated an investigation of a methodology to detect such voids.

The suggested methodology is based on measurements of the “slow” and “fast” parts of the neutron spectrum because these parts respond in different ways to voiding. For detection, it is tentatively assumed that fission chambers loaded with U-235 and Pu-239, respectively, are deployed. To investigate the methodology according to sensitivity and precision, a number of scenarios have been simulated and analysed using the core simulator Serpent.

The results show that the methodology yields a sensitivity of 3% for each per cent unit of void. Assuming typical detection limits of a few per cent this implies the possibility to detect voids down to the order of 1%. From these studies it was also concluded that the positioning of the detectors relative the reactor core is crucial, which may be useful input during the design phase of a reactor in order to achieve an efficient monitoring system.

Keyword
LFR, Monitoring, Fission chamber, Void, Heat exchanger, Neutron spectrum
National Category
Subatomic Physics
Research subject
Applied Nuclear Physics; Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-198748 (URN)10.1016/j.anucene.2013.03.029 (DOI)000320481600016 ()
Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2017-12-06
3. Reactivity changes in lead-cooled fast reactors due to bubbles in the coolant
Open this publication in new window or tab >>Reactivity changes in lead-cooled fast reactors due to bubbles in the coolant
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The formation of bubbles in the coolant of a Lead-Cooled Fast Reactor (LFR) may originate from a leaking heat-exchanger and is a potential safety hazard. Small bubbles can travel with the coolant without escaping to the cover gas, causing an increasing effective voiding of the coolant in a homogeneous manner. If the small bubbles coalesce into a larger bubble located at a stagnation zone, the reactor core may eventually be exposed to a transient bubble travelling axially through the core with a resulting change in the reactivity of the system. This study is focused on the reactivity changes caused by bubbles of various sizes and for different vertical positions as the bubble rises through the core. Three different sizes of LFR’s; 50 MWth, 300 MWth and 1200 MWth,respectively were user for the study. The 300 MWth reactor design is based on the Advanced LFR European Demonstrator (ALFRED) and the two other reactors are scaled up and scaled down versions of it and these were simulated in order study the sensitivity to void as a function of reactor size. We show that LFR’s may have a positive reactivity response to transient bubbles and that the sensitivity to changes in reactivity is larger the smaller the reactor. For sufficiently large bubbles all reactors may reach prompt criticality.

National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-232236 (URN)
Available from: 2014-09-15 Created: 2014-09-15 Last updated: 2014-10-22
4. Transient Simulation of Gas Bubble in a Medium Sized Lead Cooled Fast Reactor
Open this publication in new window or tab >>Transient Simulation of Gas Bubble in a Medium Sized Lead Cooled Fast Reactor
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(English)Article in journal (Other academic) Accepted
Abstract [en]

A common problem for many liquid metal cooled fast reactor designs is the positive void worth of the coolant. In this context, an advantage of lead cooled fast reactors is the high temperature of coolant boiling. In contrast to sodium cooled fast reactors this, in practice, precludes coolant boiling. However, partial voiding of the core could result from e.g. gas bubbles entering the core from below. This would introduce a positive reactivity, if the bubble is large enough.

 

In this paper we model this type of event using a point kinetics code coupled to a heat transport code. The reactivity parameters are obtained from a Monte Carlo code. The 300 MWth reactor design Alfred is used as a test case. We show that in general the reactor design studied is robust in such events, and we conclude that small bubbles a measureable Power oscillation would occur. For very large bubbles there exist a possibility of core damage. The cladding is the most sensitive part.

National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-232238 (URN)
Available from: 2014-09-15 Created: 2014-09-15 Last updated: 2014-12-03
5. Detection of coolant bubbles in lead-cooled fast reactors
Open this publication in new window or tab >>Detection of coolant bubbles in lead-cooled fast reactors
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Previous work [1] has shown that using fission chambers coated with 242Pu and 235U, respectively, can provide the means of detecting changes in the neutron flux that are connected to coolant density changes in a small lead cooled fast reactor. Such density changes may be due to leakages of gas into the coolant, which, over time, may coalesce to large bubbles implying a high risk of causing severe damages of the core. By using the ratio of the information provided by the two types of detectors a quantity is obtained that is sensitive to these density changes and, to the first order approximation, independent of the power level of the reactor.

 

In this work we continue the investigation of this proposed methodology by applying it to the Advanced LFR European Demonstrator (ALFRED) and using realistic modelling of the neutron detectors. The results show that the methodology may be used to detect density changes indicating the initial stages of a coalescence process that may result in a large bubble. Also, it is shown that under certain circumstances, large bubbles passing through the core could be detected with this methodology.

National Category
Energy Systems
Identifiers
urn:nbn:se:uu:diva-232237 (URN)
Available from: 2014-09-15 Created: 2014-09-15 Last updated: 2014-10-22

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