(The) flux reconstruction method for nodal calculations

Abstract

Since the nodal method can calculate the eigenvalue and the node-averaged power distribution in a reactor core accurately with much smaller computation time than the finite difference method, it becomes a powerful tool for nuclear fuel management study. However, the nodal method cannot give any ifformation of the flux distribution in a node. Therefore, dehomogenization is necessary for obtaining such information after a global reactor core analysis using a nodal method. The flux distribution, equivalently the power distribution, in a node is required to deplete the fuel in the node and thus to calculate node-homogenized group constants for the node which are essential for the global calculation using a nodal method. Various schemes had been studied for reconstructing local flux distribution from the node-averaged quantities obtained by the global reactor core calculation using a nodal method. Most common scheme is to approximate the local flux distribution in a node as the flux distribution obtained by the node calculation for the node-homogenized group constants with multiplying by so called a form function. First, the mode of the form function is assumed such as quadratic, sinuous or exponential mode. Then, the unknown coefficients of the form function are determined using the node-averaged quantities obtained of two by the global core calculation using the nodal method. The success of this method depends highly on the choice of appropriate mode for the form function since the most resulting error in the reconstructed local flux depends on the mode. To eliminate the difficulty in choosing an appropriate mode of form function for reconstructing local flux distribution in a node and to improve the accuracy of the reconstruction, a new method has been developed through this study. The new method is to interpolate the flux distribution on the surfaces of nodes in the reactor core and then to solve the diffusion equation for a node of interest using the flux...