(A) new method for the incorporation of spectral effects to one group nodal simulators

Abstract

The Becker type thermal leakage correction incorporated into KINS2 (KAERI Improved Nodal System Version 2) has been found to have a tendency to overestimate the thermal flux transients which occur at the interfaces of nodes having different material compositions. This is because, in the Becker type correction, the thermal transients occuring at the each interface of a three dimensional node are estimated by one dimensional diffusion theory and the overall thermal leakage of the node is represented by the simple summation of the one dimensional results. In this thesis, a new method for the thermal leakage correction was developed to improve the accuracy of the Becker type correction. Some features of the new method could be summarized as follows. Firstly, in the new method the effect of the fast neutron leakage which is neglected in the Becker type correction, is accounted for, more accurate thermalto-fast flux ratio at the node center can be obtained. Secondly, in order to diminish the overestimation of the thermal transients, the overall thermal transients of a three dimensional node are represented by the weighted relation between the node center flux ratio and the six facial flux ratios of the node. The new method was applied to the analysis of the KNU-1 Cycle-1 BOC core. The results show that the new method predicts more accurately the nodal flux ratio distribution and the nodal power distribution than the Becker method, and that the new method can be used to improve the calculational accuracy of one group nodal simulators.