DEC-A identification and impact of post-Fukushima improvements on risk reduction

After the Fukushima accident, nuclear power plants (NPPs) were required to demonstrate their reliability by increasing their safety provisions and equipment to mitigate accidents. These requirements must cover equipment, procedure improvements, and regulatory development. One of the most important actions has been the introduction of the analysis of the Design Extension Conditions (Multiple Failure Conditions outside the design basis envelope, DEC) into the design requirements of NPPs to further improve safety. On this subject, in the framework of the SPAR-CSN project to create a Standardized Plant Analysis Risk (SPAR) model for Spanish NPPs, a novel methodology has been developed to identify sequences that are outside the design basis but not reaching fuel damage, DEC-A, making use of the probabilistic safety assessment level 1 model for a generic plant, 3-loop pressurized water reactor Westinghouse design. Using this method, we have been able to identify the DEC-A sequences with high relative risk by comparing their frequencies with their corresponding DEC-B sequences (with fuel damage). As shown in this study, the addition of post-Fukushima improvements highly reduces the relative risk of the DEC sequences.