Analysis of Reactivity Coefficients of Chernobyl Reactor by Cell Calculation

Abstract

A series of cell calculations for the Chernobyl reactor was performed using the SRAC code system to provide its fundamental neutronic characteristics for the accident analysis at JAERI. The calculations are based on a two-step cell modelling. The primary cell is supposed on a unit square graphite block of 25 em x 25 em which contains a fuel assembly or a control channel. The secondary cell is supposed on a unit of 16 channels where 14 fuel and 2 control channels are located so as to simulate the whole core.Detailed investigation on fractional change of reaction rates for each nuclide along with increase of void fraction was carried out. The analysis clarified the mechanism to induce the positive void coefficient, together with its burn-up dependence and the increase due to withdrawal of control rod.A comparison of the effect of void fraction on the reaction rates in the primary cell between a Monte Carlo code VIM and SRAC shows consistent results within the statistical error.The calculated results for the composition of discharged fuel, void and other reactivity coefficients, kinetic parameters and their burn-up dependence show satisfactorily good agreement with those reported by the Soviet Union and some institutions.