Safety analysis of a compact integral small light water reactor

Abstract

Small modular reactors (SMRs) hold great promise in meeting a diverse market while reducing the risk of delays during nuclear construction compared to large gigawatt-sized reactors. However, due to lack of economy of scale, their capital cost needs to be reduced. Increasing the compactness or power density of the nuclear island is one way to reduce capital cost. This work first assesses the transient analysis of a compact integral small light water reactor to examine its safety performance. Subsequently, a parametric optimization study with the goal of increasing its power density (i.e. improve its market competitiveness) while maintaining safety is performed. A model of the reactor is established using RELAP5/3.3gl, with reference to the features of Nuward SMR. Nuward is a compact 170 MWe Pressurized Water Reactor, whose key features include the use of Compact Steam Generators and a large water tank in which the containment submerges for passive heat removal.

A transient analysis of the reference reactor after Loss of Flow Accident, Station Blackout, and Loss of Coolant Accident is carried out. Following all three accidents, the integrity of the fuel and the reactor is maintained. The passive cooling system is estimated to provide 12 - 13 days of grace period. The parametric optimization study indicates that the size of the tank can be reduced to half and still maintain sufficient margin to both short-term and long-term safety goals after all three transients with an estimated grace period of 7 - 8 days. In addition, the configuration of the passive safety system can be rearranged to reduce the size of the containment to 76% of the reference design without affecting its safety performance. By increasing the coolant enthalpy change, which also results in a higher thermal efficiency, the electrical output of the reference design can be enhanced by 44% without major design changes.

If the number of pumps in the vessel are increased by 2, the electrical output can be enhanced by 102% while satisfying all safety criteria. The uprated power that satisfies a 72-hour grace period requires a tank size that is 32.5% of the reference design. Such compact and simplified nuclear steam supply system can partially address the lack of economy of scale for the reference SMR and improve its market competitiveness.