Nonlinear iterative nodal method applied to neutron flux modal analysis

The CCCMA (Correction Coupling Coefficient Modal Analysis) method has been proposed to improve the spatial resolution of the neutron flux modal analys is. The improvement has been achieved by including the higher order intra-n odal information through the NLM (Nonlinear Iterative Method)-correction co upling coefficient. The new code system; ENTREE/ACCORD-N, has been develope d in which both the polynomial and semi-analytical Legendre flux, expansion functions hale been implemented. The application to the two-dimensional ho mogeneous and heterogeneous cores that were created from the stability test point in Ringhals Unit 1 was demonstrated. The neutral line angle, eigenva lue and power shape obtained by the CCCMA were compared with those by the F DMA (Finite Difference Modal Analysis) of various mesh sizes. The CCCMA was shown to improve the solution accuracy significantly in the heterogeneous core. The ratios of the NLM-correction to FDM (Finite Difference Method) co upling coefficients in the fuel region gave a quantitative explanation of t hese trends. For the Ist azimuthal mode which is important in the regional instability analysis, the CCCMA produced almost the same level of accuracy as that of the (4 x 4) FDMA solution without any additional computational c ost. The methodology has been also extended to include the discontinuity fa ctor.