Power Coefficient of Reactivity Determination for HTPBR and its Application for Reactivity Initiated Transients

Safety evaluation is one of the most important aspects of the nuclear reactors. Here some safety studies have been carried out on High Temperature Gas cooled Reactors (HTGR) with fuel in the form of pebbles. This is one of the six types of GEN IV reactors that are being developed internationally with emphasis on features like inherent safety, nuclear proliferation resistance and high thermal to electric conversion efficiency. This work pertains to the dynamic simulation of the reactor with, as well as without reactivity feedback. It involves the solution of point kinetics equations with reactivity feedback arising from the power rise. First, the reactivity feedback arising from the power rise is calculated in the form of power coefficient of reactivity. The steady state temperatures are calculated at two different steady state power levels of the reactor using the heat balance between the fission power produced in the reactor and the heat removed by the coolant. A simplified model of the reactor and lumped model of heat transfer is developed and used. The temperature rise in going from one power level to other power level is calculated. This is multiplied by typical value of the temperature coefficient of reactivity available in the literature. The reactivity changes divided by the power rise provides the power coefficient of reactivity. For performing dynamic analysis of the reactor, the kinetics equations along with feedback are solved. The methodology is verified against the analytical expressions available. Simulations are carried out for the case of raising the power of the reactor from low power to high power and transients at full power and shutting down of the reactor by reactor SCRAM. It is observed that for raising the power of the reactor, the reactivity addition beyond 20pcm/second may not be acceptable as the higher reactivity addition rates result in lower instantaneous reactor period resulting in SCRAM of the reactor. This information is useful in designing the short term and long term decay heat power removal system. Moreover, it provides insight on the determination of the maximum permissible reactivity addition rates in the reactor and optimization of the reactivity feedback due to rise in power.