IMPLEMENTATION OF STRONG IMPLICIT PROCEDURE FOR THE ENERGY EQUATIONS IN SUBCHANNEL CODE ATHAS

摘要

The poor computational efficiency is one of the main defects of the early developed subchannel codes. With the increased requirement of LWR subchannel pin-by-pin analysis and coupled local neutronics evaluations, it is necessary to improve the computational efficiency of subchannel codes. ATHAS is a subchannel code using the drift flux five equations model and the full implicit algorithm. An exhaustive analysis of the CPU times by code for different stages in the solution process revealed that more than 80 percent of the total CPU time is used for solving energy equations in ATHAS. The energy equations in ATHAS is solved by Gauss-elimination method and it is found that solving the algebraic equations of energy equations accounts for large proportion of the CPU times. In order to improve the computational efficiency, a SIP (strong implicit procedure) method is implemented in the ATHAS code. As a result, the energy matrix will be decomposed into the product of upper triangular matrix and lower triangular matrix as well as an additional nonzero small matrix. Based on the upper triangular matrix and lower triangular matrix, energy equations will be solved iteratively. A whole fuel assembly problem, which has 324 channels, has been calculated in this paper. The calculation results showed that for a 324 channels problem, 77 percent of the total CPU time can be saved by implementing the SIP method into ATHAS code.