Bimodal Analysis and Spatial Control

Abstract

This paper deals with a bimodal xenon kinetics model which is useful in assessing the effects of spatial xenon on CANDU-PHW reactor spatial control systems. The bimodal model has been implemented in a computer program called BIMODEX and applied to the CANDU PHW-850 MWe core. BIMODEX trades detailed accuracy for simplicity and low computational cost, However it retains the basic nonlinear features of more detailed models and provides sufficient quantitative information to be useful as a reactor control system design tool.

The bimodal model, as implemented in BIMODEX is described in this paper. Briefly BIMODEX is a variant of the SMOKIN code [l] and like SMOKIN, it uses a modal expansion technique based upon the Lambda modes [2] generated by the MONIC code [3]. However, BIMODEX restricts modelling to two modes at a time, i.e. fundamental mode plus one spatial harmonic, Reactor spatial control is idealized, With spatial control "on" the flux mode amplitude trajectories in time must be pre-specified and the model calculates the resulting xenon and iodine trajectories and the spatial control effort needed to maintain reactivity balance. With spatial control "off", spatial control reactivity terms are frozen and the flux shape behaviour is governed by the feedback effects of spatial xenon. Computational results for the CANDU-PHW 850 MWe core illustrate the type of information that BIMODEX can provide concerning reactor spatial control.