Models for transient analyses in advanced test reactors

Several strategies are developed worldwide to respond to the world's increasing demand for electricity. Modern nuclear facilities are under construction or in the planning phase. In parallel, advanced nuclear reactor concepts are being developed to achieve sustainability, minimize waste, and ensure uranium resources. To optimize the performance of components (fuels and structures) of these systems, significant efforts are under way to design new Material Test Reactors facilities in Europe which employ water as a coolant. Safety provisions and the analyses of severe accidents are key points in the determination of sound designs. In this frame, the SIMMER multiphysics code systems is a very attractive tool as it can simulate transients and phenomena within and beyond the design basis in a tightly coupled way. This thesis is primarily focused upon the extension of the SIMMER multigroup cross-sections processing scheme (based on the Bondarenko method) for a proper heterogeneity treatment in the analyses of water-cooled thermal neutron systems. Since the SIMMER code was originally developed for liquid metal-cooled fast reactors analyses, the effect of heterogeneity had been neglected. As a result, the application of the code to water-cooled systems leads to a significant overestimation of the reactivity feedbacks and in turn to non-conservative results. To treat the heterogeneity, the multigroup cross-sections should be computed by properly taking account of the resonance self-shielding effects and the fine intra-cell flux distribution in space group-wise. In this thesis, significant improvements of the SIMMER cross-section processing scheme are described. A new formulation of the background cross-section, based on the Bell and Wigner correlations, is introduced and pre-calculated reduction factors (Effective Mean Chord Lengths) are used to take proper account of the resonance self-shielding effects of non-fuel isotopes. Moreover, pre-calculated parameters are applied to the non-fuel multigroup neutron cross-sections to take account of the different neutron spectra in the fuel and non-fuel regions. These techniques have been validated in the present work for a wide range of water-cooled thermal systems near steady-state conditions by benchmarking the extended SIMMER version against the reference neutronics codes and experimental results, for the criticality, the kinetic parameters, and the main reactivity effects. In this work, it is proven that the deployment of the new approach leads to more accurate SIMMER results for a large variety of situations during a transient. It is also shown that these parameters can be evaluated for few representative reactor states and that they can be interpolated more easily than the microscopic cross-sections as is usually done in the safety codes for LWRs. Thus, the employment of the Bondarenko method and of the pre-calculated parameters provides a very efficient SIMMER cross-section processing scheme during transient simulations.