Synthesis of the safety studies carried out on the GFR2400

The present paper is dedicated to the synthesis of the safety studies carried out on the 2400 MWth gas-cooled fast reactor (GFR2400) concept developed at CEA. The analysis of the reference design basis accidents investigated up to now, has shown margins up to the acceptance criteria, equal at least to 300 °C for the category 3 situations and larger than 100 °C for the category 4 situations. The dimensioning of the decay heat removal (DHR) loops and of the power conversion system (PCS) loops has been shown adequate even for bounding degraded situations including multiple failures. Furthermore, in the following part of the paper, it is shown how the main insights provided by a level 1 probabilistic safety assessment (PSA) carried out at an early stage of the design, have led to reinforce the reliability of the DHR function in high pressure conditions by using the PCS as the first mean to cool the core; in the same time, on the basis of a combination of deterministic augments and of PSA results, a design simplification process has led to add a low pressure DHR loop to replace a high pressure DHR loop. The last section is dedicated to prevention and preliminary study of severe accidents (SA). Four SA families have been identified depending on the dynamics and on the scale of the considered accident. The possibility to prevent core degradation by using an adapted accident management (nitrogen injection, use of PCS loops) has been preliminarily shown in several particularly challenging situations (loss of active means, unprotected transients, full depressurization). Finally, preliminary results regarding analytical studies carried out on phenomena involved in GFR2400 core degradation (physico-chemistry and neutron physics) are presented. Then, the application of the separate results aforementioned by considering results of analytical simplified thermalhydraulic calculations and of system calculations (carried out with the CATHARE2 code) have enabled a preliminary assessment of GFR2400 behaviour in case of core degradation. For some cases, such applications permitted to conclude on the problematic/begnin issue of a phenomenon (like air ingress in realistic scenarios) whereas in other cases, those applications have illustrated that more complex calculation tools coupling the various phenomena are necessary (like effects of water ingress for instance) as well as semi-integral experiments reproducing a fuel assembly degradation.