Pergamon Ann. Nucl. Ener,g,, Vol. 24, No. 12, pp. 955 963, 1997 [(; 1997 Elsevier Science Ltd. All rights reserved PI]: S0306-4549(96)00090-4 Printed in Great Britain 0306-4549/97 $17.00 + 0.00zyxwvutsrq APPLICATION OF LOCAL BASIS PSEUDO-HARMONICS METHOD Z. D. THOME, F. C. DA SILVAt and A. C. M. ALVIM COPPE/UFRJ, Nuclear EngineeringProgramme, Federal Universityof Rio de Janeiro, PO. Box 68509,21945-970Rio de Janeiro, RJ, Brazil (Received17 September1996) Abstract—The alternative pseudo-harmonics method, in conjunction with local basis procedures, is applied to compute perturbed neutron flux shapes and eigenvalue in a two-group, seven region, 1-D representation of a reactor core. Definition of a region of interest smaller than the core domain allows a significant reduction in the number of pseudo-harmonics employed, with cor- responding reduction in computational effort, while preserving small relative error in flux shapes (maximum relative error of O. OQO/o) and no error at all in eigenvalue. (Cj 1997 Elsevier Science Ltd. 1. INTRODUCTION Perturbation methods are useful as a tool for estimating preliminary flux calculations and for sensitivity analysis. It also applies to cases where repeated calculations, varying only some parameters, are needed. Several perturbation methods use flux expansion techniques (Gandini, 1978 and 1981; Palmiotti, 1983; Gomit et al., 1985; Palmiotti, 1987; Palmiotti et al., 1987; Bruna and Sargeni, 1994) in order to represent the perturbed flux. One of them is the pseudo-har- monics method, which was originally proposed by Gomit et al. (1985) and has as one of its main advantages the fact that the basis used to represent the perturbed flux is con- structed from the eigenfunctions of a self-adjoint operator (the leakage plus removal parts of the unperturbed diffusion operator). This implies that: (i) it is not necessary to compute the unperturbed adjoint group eigenfunctions; (ii) there are neither complex eigenvalues nor eigenvector degeneracy; (iii) there is a decoupling of the group equations in the basis generation process. A modified version of the pseudo-harmonics method (da Silva et al., 1987)was developed and tested for a two-group, three region, 1-D representation of a PWR, with material tTo whom al] correspondenceshould be addressed. 955