Reconstrução da distribuição da densidade de potência heterogênea pino a pino usando o método dos pseudoharmônicos

Abstract

The method of reconstruction of heterogeneous pin power density distribution by means of a pseudo-harmonic expansion (PHRM) aims at obtaining the homogeneous neutron flux in each cell of the fuel assembly (FA). The method starts from the two-dimensional two- energy-group neutron difusion equation, considering the homogeneous nodes with the dimensions equivalent to those of the FA. The neutron difusion equation is discretized by finite differences, with mesh sizes of the order of fuel pin cell areas, to obtain a symmetric matrix associated with the leakege + removal operator. The pseudo-harmonics are the eigenvectors associated with this matrix and, therefore, constitute a basis for the space of interest. The homogeneous flux of neutrons is obtained using the pseudo-harmonic expansion and a distribution of fluxes on the FA faces, calculated from the mean values calculated by the nodal expansion method (NEM). The power density distribution is obtained from the homogeneous flux, and then the distribution of the heterogeneous pin-to-pin power density is calculated by the product of the homogeneous power by the power form function for each cell. The heterogeneous power is normalized in relation to the average power and next one determines the distribution of the pin-to-pin reconstructed heterogeneous power density. The results indicate that the method has good accuracy when compared to the reference solution values for the EPRI-9 benchmark. The major deviations are in the cells near the baffle+reflector region and these are associated with the homogenization process.