Dynamic simulation of a pebble bed high temperature gas cooled reactor

A safety analysis of the PNP 500 High Temperature Gas Cooled Reactor (HTGR) was performed. The fuel is embedded in a matrix of graphite spheres containing UO2 particles - Pebble Bed Reactor, and the output power of the plant is 500 MW (t). The safety analysis is aimed at describing the system behavior following perturbations in its state variables. Anticipated transients were studied using full-scale simulation of the PNP 500, and the resulting temperatures, pressures and mass flows were compared with their permitted values. The analysis was performed in a modular form in which each module stands for a physical component. The computer code was written using DSNP - Dynamic Simulator for Nuclear Power Plants - and the modules developed for HTGR were introduced into the DSNP libraries. The results obtained by simulation of the modules were compared with those derived from analytical calculations. The Gear integration method was included in the DSNP and a new integration method was developed for stiff equations (using the second order Runge - Kutta method with exponential fitting). These integration methods were compared with older ones, and Gear's was found to give the best performance. The following transients were studied: (a) a loss of coolent accident, (b) a step perturbation in reactivity, (c) a step reduction in turbine power, (d) coastdown of the main cooling system. The PNP was found to have a slow reaction time due to its high graphite heat capacity. The consequences of the studied accidents were shown to be limited due to the inherent safety features of the HTGR. The results were compared and found to be in good agreement with those obtained from the COROX and SHOVAV-JUL codes. (author)