Risk reduction due to modification of normal residual heat removal system of AP1000® reactor to meet European Utility Requirements

• Verification of AP1000 DVILB event tree with TRACE code.
• Analysis of AP1000 DVILB event tree by considering European RNS design.
• AP1000 CDF reduction due to European RNS design.

As part of AP1000® design adaptation to European Utility Requirements, Westinghouse has carried out several modifications of the standard AP1000 design. One of these modifications has been the physical separation of the normal residual heat removal system (RNS) into two independent trains which improves the plant defense in depth. The AP1000 Probabilistic Risk Assessment, assumes in the standard design that in the event of direct vessel injection line break (DVILB) the RNS cannot inject efficiently, and therefore it is necessary to depressurize the Reactor Coolant System until atmospheric conditions in order to allow injection through the IRWST and to achieve safe conditions. This work presents the analysis performed for DVILB event tree taking into account the RNS modification and the effectiveness of RNS injection. With this purpose, different possibilities of DVILB event tree have been simulated with TRACE V5.0 patch 2 thermal–hydraulic code. The results show that some sequences that were classified as damage with the standard RNS design became success with the consideration of RNS improvement and therefore the contribution of DVILB to the total core damage frequency can be minimized.