Simplified criteria for a comparison of the accidental behaviour of Gen IV nuclear reactors and of PWRS

Highlights: • Simplified criteria devoted accident behavoir comparison of reactor concepts are presented. • Four Generation IV concepts and a PWR are assessed with the criteria. • The score obtained by each concept provides an idea of needed safety systems. • Better scores for prevention are obtained for VHTR and SFR. • Better scores for mitigation are obtained for MSFR and GFR. • The best score by combining prevention and mitigation is obtained for VHTR. A cross-comparison of four Generation IV reactor concepts and a PWR of 2nd generation is presented in this paper. 4 Gen IV reactor concepts are considered and are briefly presented in the first part of the paper: SFR of 1500 MWth, GFR of 2400 MWth, MSR of 3000 MWth and VHTR of 600 MWth. In order to perform this comparison, some simple common criteria related to accidental behavior of the reactors have been developed. The first kind of criteria aims at assessing the main physical thresholds to exceed in order to have a core degradation: phase changes of coolant and of core materials (including the effect of chemical reactions) for the various reactor concepts considered. The second set of criteria deals with kinetics aspects of the accident. On the basis of core power (after scram and without scram), on the coolant inventory and on the reactor capability to be passively cooled, the heating rate of the coolant and of the core materials are assessed thanks to simplified energy balances presented in the paper. As a result, for each reactor concept, the time to reach the physical thresholds defined above is evaluated. A third set of criteria deals with core features and aims at assessing the possible reactivity insertion that withstands each concept up to core melting (or boiling for the MSR) and the associated expected power peaks in case of coolant voiding/depressurization and in case of fissile material compaction. Finally, a last criterion set deals with the assessment of the possibility to jeopardize physical barriers confining fission products. These criteria deal with the risk of barrier loadings due to coolant and core material vaporization depending on the features of the coolant/fuel and on the operating point of each reactor concept. In the last part of the paper, a synthesis is made in order to underline the weak and strong points of each of the reactor concepts investigated in terms of severe accident prevention and mitigation.