Neutron flux fluctuations are a natural phenomenon of nuclear reactors. Approximately since 2001, pressurized water reactors built by Kraftwerk Union AG have exhibited an unexplained cycle-by-cycle change of the magnitude of these fluctuations. The change has also drawn attention to long-known but also unexplained spatial correlations of the fluctuations in these reactors. The thesis at hand aims to contribute to a better understanding of both the observed change in magnitude and the immanent correlations.
Based on the findings of previous research and on the own analysis of measured raw data, a hypothesis was developed, which states that a synchronous coolant-flow driven vibration of major parts of the fuel-assembly ensemble triggers the main contribution to the observed neutron flux fluctuations. The fluctuation correlations are supposed to result from the correlations of the fuel-assembly vibration. This idea was tested using the time-domain reactor dynamics code DYN3D and complementary using the frequency-domain neutronic tool CORE SIM. For this purpose, simplified mechanical models of the synchronous fuel-assembly vibration and models coupling the vibration to the neutron kinetics were developed and implemented. Two effects are distinguished: In case of the “reflector effect”, all fuel assemblies vibrate synchronously, in a way that the main resulting perturbation acts
in the radial reflector as a fluctuation of the water layer between the outer fuel assemblies and the core shroud. In case of the “fuel-assembly pitch effect”, the fuel assemblies are unequally involved in the synchronous vibration, in a way that the main perturbations are induced within the reactor core as fluctuations of the fuel-assembly gaps.
Both the simulations with DYN3D and the simulations with CORE SIM showed that a synchronized fuel-assembly vibration is a possible main source of the concerned neutron flux fluctuations. In particular, the uniform fluctuation of the gaps between all fuel assemblies, corresponding to high-amplitude fuel-assembly vibrations in the core center and low-amplitude fuel-assembly vibrations in the outer core regions, gave the best approximation to the measured data. A C-like axial vibration-shape provides the best agreement.
The simulation results show that the developed hypothesis should be further investigated. In particular, the proposed synchronized vibration of the fuel assemblies suggest correlations of the neutron flux fluctuations with mechanical signals, which have not been taken into account so far. The simulations presented here enable further improvements of the understanding of the neutron flux fluctuations in the concerned reactors by additional measurements involving also specific modes of reactor operation.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:76318 |
| Date | 18 October 2021 |
| Creators | Viebach, Marco |
| Contributors | Hurtado Gutierrez, Antonio Miguel, Weiß, Frank-Peter, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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