Revised Procedure for Determining Large Excess Reactivity of Reactors

Abstract

Existing experimental methods for determination of large positive reactivity, the fuel addition method and the neutron absorption substitution method were analyzed. It is found that both the experimental methods are possibly affected to the substantially large systematic error, when the value of potential excess multiplication factor comes into the range close to 20%Δk. To cope with this difficulty, a revised procedure is validly proposed. The revised procedure evaluates the value of the excess multiplication factor as the accumulation of the consecutive increases of the effective multiplication factor in a virtual core, which are converted from those in an actual core multiplied by a calculated conversion factor f. The conversion factor f is to be determined in principle by calculation. Numerical experiments were done on a slab reactor with one group diffusion model. The systematic errors observed in the existing methods are mostly swept out in the proposed revised procedure. Influence of the uncertainties in the group constants on determination of large positive reactivity has been confirmed to be small. The present paper describes only fundamental validity of the revised procedure. Detailed core calculations are required to obtain the precise values of the conversion factor f on an actual reactor, which should be performed in accordance with the steps taken in the experiments.