Simulation of Protective Action by Shutdown-System-1 Against Hypothetical Loss of Coolant From Various Initial Flux Configurations

Abstract

The effectiveness of reactor Shutdown System 1 (SDS-1) action against hypothetical large-loss-of-coolant accidents (LOCA) has been assessed by simulating a number of power transients in a CANDU 600 core. The purpose was to confirm, using current methodologies and updated models, that shutdown system 1 (the cadmium shutoff rods) is sufficiently effective under various LOCA conditions in which there is a rapid increase in reactivity. The simulations were performed by means of a coupled set of thermohydraulics and neutron kinetics codes. ha the present study, the most limiting large-loss-of-coolant accident (100% pump suction break) was considered. Radially-crept pressure tubes were assumed; this increases the void reactivity. The LOCA was assumed to have been initiated from normal and abnormal reactor initial conditions. The abnormal conditions include side-to-side, top- to-bottom, diagonal, and end-to-end flux tilts, as well as radial flux peaking. Although some of these abnormal conditions may not be credible, they were considered in the present study in order to help define an envelope of operating conditions in which the shutdown systems have been shown to be effective. Before the initiation of the postdated accident, the reactor was assumed to have started up from a long shutdown, with moderator poison compensating for the decayed saturated fission products. In this configuration, the increased void reactivity places more stringent requirements on shutdown-system effectiveness. The evaluation of the effectiveness of SDS-1 action was made against the criterion of preventing fuel breakup. This was done by comparing the maximum energy content of the fuel in the transient to a conservative lower limit of the amount required for fuel breakup. Results of the simulations show that under a wide variety of initial flux shapes, and even with very conservative assumptions, the no fuel-breakup criterion is satisfied.