Void reactivity decomposition for the Sodium-cooled Fast Reactor in equilibrium fuel cycle

Abstract

The Sodium-cooled Fast Reactor (SFR) is one of the most promising Generation IV systems with many advantages, but has one dominating neutronic drawback – a positive sodium void reactivity. The aim of this study is to develop and apply a methodology, which should help better understand the causes and consequences of the sodium void effect. It focuses not only on the beginning-of-life (BOL) state of the core, but also on the beginning of open and closed equilibrium (BOC and BEC, respectively) fuel cycle conditions. The deeper understanding of the principal phenomena involved may subsequently lead to appropriate optimization studies. Various voiding scenarios, corresponding to different spatial zones, e.g. node or assembly, have been analyzed, and the most conservative case – the voiding of both inner and outer fuel zones – has been selected as the reference scenario. On the basis of the neutron balance method, the corresponding SFR void reactivity has been decomposed reaction-, isotope-, and energy-group-wise. Complementary results, based on generalized perturbation theory and sensitivity analysis, are also presented. The numerical analysis for both neutron balance and perturbation theory methods has been carried out using appropriate modules of the ERANOS code system. A strong correlation between the flux worth, i.e. the product of flux and adjoint flux, and the void reactivity importance distributions has been found for the node- and assembly-wise voiding scenarios. The neutron balance based decomposition has shown that the void effect is caused mainly by the influence of the sodium removal on the neutron spectrum, and this has been confirmed by the perturbation method. The neutron balance based decomposition has also shown that the prime consequence of the sodium voiding is the reduction of the absorption rate (especially of 238U capture), which provides the strongest positive component of the void reactivity. The changes in neutron production and leakage provide smaller and negative contributions. Sensitivity analysis results for individual isotopic reactions have been found to be, in a relative sense, consistent with the neutron balance based isotope-wise decomposition. Finally, the neutron balance method has been applied to analyze the void reactivity differences between the three different SFR fuel cycle states considered, viz. BOL, BOC and BEC.