Compact Heat Exchangers/Steam Generators and Decay Heat Removal in Passive Safety Systems: Comparison of Thermal Hydraulic Features

The following report aims to compare and analyse the performance of innovative heat exchangers and passive safety systems (Decay Heat Removal Systems-DHRS), in different configurations, similarly to those proposed in several new generation reactor projects. Two in-vessel heat exchangers are described and adopted, namely the microchannel type and the helical coil type. Two ex-vessel heat exchangers are used to create a closed loop, natural circulation, passive DHRS: an air cooled type and an isolation condenser type, dipped into a water pool. The comparison of the invessel heat exchangers is made in steady state conditions and in an accident scenario, with the use of RELAP5 commercial software. The new generation reactor adopted as a reference test bed for the DHRS implementation and analysis is the I2S-LWR project from Georgia-Tech (an Integral PWR of large size). In the steady state analysis, the two types of in-vessel heat exchangers designed to remove ¼ of the reactor power have been compared at full power and at reduced power considering a constant mean temperature of the primary system. The analysis allowed to highlight the features in terms of compactness, friction pressure losses and performances at different power levels. In the analysis during accident conditions, a System Black-Out (SBO) followed by the reactor scram is used. In this case, the heat exchangers compared are used as a part of the DHRS, considering two possible solutions for the ex-vessel ultimate heat sink (the isolation condenser and the air cooled heat exchanger), resulting in 4 different DHRS configurations. The simulations results are compared in two chapters grouping them by the common ultimate heat sink, in order to isolate the effect on the transient of the heat exchanger.