Risk-informed analysis as a support to the preliminary design of the CEA GFR2400

The integration of safety issues in the early phase of the design of a 4. generation reactor of the concepts is expected. For this purpose, probabilistic insights are increasingly employed in the safety demonstration in combination with the deterministic approach in the frame of a so-called risk informed approach. The present paper deals with the safety assessment of the preliminary design of the GFR2400 developed by CEA and how it has been improved in order to fulfil deterministic criteria as well as to reach a risk level comparable to the generation III reactors. GFR2400 is a 2400 MWth, 3-loops, helium-cooled fast reactor developed at a pre-conceptual design stage whose secondary circuit is filled with a mixture of helium and nitrogen, the ternary circuit being filled with water vaporized in 3 steam generators according to a classical Rankine cycle. The resulting cycle efficiency is very close to 45 %. Considering the results obtained with a preliminary level 1 PSA (L1PSA) model, it emerged that an increased reliability of the DHR (Decay Heat Removal) function in high pressure conditions (not corresponding to a LOCA) was suitable to reduce the overall core damage frequency. On the other hand, some small break LOCA situations were not adequately mitigated according to the line of protection deterministic method. Both issues have been solved by design improvements. In addition, this final L1PSA model, characterized by success criteria based on transient calculations performed with the CATHARE2 code and performed in a perimeter extended to all representative internal initiating events at full operating power, permitted to propose design evolutions that did not increase significantly the CDF. In the same time, those evolutions enabled the DHR system to increase its redundancy level as required in the deterministic approach. Finally, a modified design has been reached implying a more extended covering of various accidental situations by means of a progressive DHR operating strategy