(The) reconstruction of pointwise power distributions in a light water reactor core form coarse-mesh nodal calculations

Abstract

The objective of this thesis is to develop an accurate and computationally efficient method for reconstruction of pointwise power distributions from coarse-mesh nodalcalculations. Provided that homogenized parameters are properly determined in each node, the analytic nodal method code (ANM) calculates global reactor power shapes and criticality accurately. But inherent in nodal procedures, there is inevitable loss of information on local heterogeneous quantities. Hence a special technique is needed in order to recapture local properties where and when desired. In this study, an improved form function method which reflects the exponential transition of the thermal flux near the assembly surface is developed for the reconstruction of the heterogeneous fluxes. The distinctive feature of the new form function method is that the form function of the thermal flux is dependent on the form function of the fast flux which is approximated by the bi-quadratic polynomials, and the dependency is represented by the hyperbolic functions. Use of the new form function method in several PWR benchmark problems reduces the maximum errors in the reconstructed thermal flux to those in the reconstructed fast flux. In realistic PWER cores, use of this method also results in improved pointwise power reconstruction; the maximum reconstruction errors even for assemblies adjacent to the steel baffle are only half of those obtained by using the conventional bi-quadratic form function method.