Study of rod ejection accident at hot zero power condition in a PWR using RELAP5

The core reactivity and power distribution within the core of a pressurized water reactor (PWR) are controlled for safe and efficient operation of the reactor. Several postulated events, however, can alter the core reactivity and power distribution and disrupt reactor operation. Such those events are the reactivity accidents. Among the different expected reactivity accidents, the control rod ejection accident (REA) is of a particular concern. This accident can occur in PWRs by mechanical failure of the control rod drive mechanism or due to inadvertent operator action. There are two cases of REA; REA at hot zero power condition, and REA at power condition. This accident results in reactivity addition, and based on the rate of reactivity addition, the reactor power may increases to levels large enough to endanger the fuel assemblies faster than the automatic protective actions. In the current work, RELAP5 code is used to study the consequences of REA at Hot Zero Power Condition in a standard four loop Westinghouse PWR reactor. A reactivity insertion of up to 75 pcm/sec is assumed in the analysis due to the rod movement. This value is consistent with the withdrawal of two control rod banks. A RELAP5 input file is prepared to simulate the reactor core and primary system. In this file, the core is simulated by two channels, a hot channel represents a one fuel assembly and an average channel represents the rest of the core. RELAP5 reactor kinetics model is used to determine the power generation with time including the core feedback effects; i.e., Doppler reactivity and moderator reactivity. The reactor is set to trip when it reaches 35% of full power, the low setting reactor trip assumed to be initiated by power range high neutron flux. The initial power level was assumed to be below the power level expected for any shutdown condition (10⁻⁹ of nominal power). Results of the study including the peak power, maximum fuel pellet temperature and fuel pellet enthalpy are presented. The effects of the rod ejection rate on REA consequences are also studied.