The third tier has been addressed within the PRA/PSA (Probabilistic Risk Assessment/Probabilistic Safety Assessment) framework. In fact, PRA was initiated early in the IRIS design, and was used iteratively to guide and improve the design safety and reliability (thus adding ‘reliability by design’). The PRA has suggested modifications to the reactor system designs, resulting in reduction of the predicted CDF. After these modifications, the preliminary PRA level 1 analysis estimated the CDF due to internal events (including anticipated transients without scram, ATWS) to be about 2*10’8, more than one order of magnitude lower than in advanced LWRs. A subsequent evaluation of the LERF (large early release frequency) also produced a very low value, of the order of 6*10’10, which is more than one order of magnitude lower than in advanced loop LWRs, and several orders of magnitude lower than in present LWRs.

XVI-4. IRIS safety features

To complement its safety-by-design™, IRIS features limited and simplified passive systems as shown in Figure XVI-2. They include: