PARCS vs CORE SIM neutron noise simulations

Abstract

In a nuclear reactor, even operating at full power and steady-state conditions, fluctuations are detected in the recording of any process parameter. These fluctuations (also called noise) could be of various origins, such as, turbulence, mechanical vibrations, coolant boiling, etc. The monitoring and complete comprehension of those parameters should thus allow detecting, using existing instrumentation and without introducing any external perturbation to the system, possible anomalies before they have any inadvertent effect on plant safety and availability. In order to reproduce and study the induced neutron noise in a nuclear reactor core, it is compulsory to develop suitable tools. Existing time-domain codes were originally not developed for this type of calculations. Modifications of those codes and the development of an associated intricate methodology are necessary for enabling noise calculations. This involves, in some cases, changes in the source code and the development of new auxiliary tools to ensure accurate reproductions of the core behavior under the existence of a neutron noise source. In the proposed work, the time-domain neutron diffusion code PARCS is used to model the effect of stationary perturbations representative of given neutron noise sources. In order to validate the feasibility of the time-dependent methodology thus developed, comparisons with the results of simulations performed in the frequency domain, using the CORE SIM tool, developed at Chalmers University of Technology, are performed. The development of a few test cases based on a real reactor model are undertaken as the basis for such comparisons and a methodology aimed at assessing the time-domain simulations versus the frequency-domain simulations is established. It is demonstrated that PARCS, although not primarily developed for neutron noise calculations, can reproduce neutron noise patterns for reasonable frequencies. However, it is also observed that unphysical results are occasionally obtained.

Analysis of the neutron noise induced by fuel assembly vibrations

“…Quite surprisingly, continuous-energy Monte Carlo simulations of the linearized noise equations independently performed by CEA and Kyoto University have also pointed out that in some circumstances the spectral peak at 2ω 0 might become (much) larger than the one at ω 0 1 . Neutron noise analysis is nowadays mostly based on state-of-the-art numerical codes, capable of taking into account arbitrary cross section variations in the time or frequency domain (Demazière, 2011;Yamamoto, 2013;Rouchon et al, 2017;Rohde et al, 2018;Olmo-Juan et al, 2019;Vidal-Ferrándiz et al, 2020;Chionis et al, 2020), as witnessed by the renewed interest stimulated by the CORTEX H2020 project (Demazière et al, 2018). For the case of frequency domain, newly developed transport (as opposed to diffusion) solvers (Rouchon, 2016;Yi et al, 2019) and Monte Carlo methods (Yamamoto, 2013;Rouchon et al, 2017) allow addressing realistic applications at the scale of fuel assemblies or full reactor cores with unprecedented accuracy (Demazière, 2011;Rouchon, 2016;Yamamoto, 2018;Mylonakis et al, 2019;Rouchon et al, 2019).…”

Operational modal analysis for characterization of mechanical and thermal–hydraulic fluctuations in simulated neutron noise

“…In the last decade, there has been a noticeable increase in neutron noise levels in certain German, Swiss, and Spanish reactors, of similar design (RSK/ESK 2013;Spanish Nuclear Safety Council 2011). This has led to a rise in the scientific interest and effort to understand its phenomenology as can be seen in different papers (Bermejo, Montalvo, and Ortego 2017;Olmo-Juan et al 2019;Seidl et al 2015;Tran, Pázsit, and Nylén 2015;Viebach et al 2018Viebach et al , 2019.…”

A time and frequency domain analysis of the effect of vibrating fuel assemblies on the neutron noise

“…Also, the influence of coherent lateral deflections for all FAs on the neutron flux was considered in (Seidl et al, 2015) and (Viebach et al, 2018). In the last years, some research aimed at comparing time domain and frequency domain calculations was reported (Chionis et al, 2017;Viebach et al, 2019;Olmo-Juan et al, 2019).…”