Study On The Improvement Of Low-order Harmonics Expansion Method

Large quantity of Loading Patterns (LPs) needs to be evaluated and selected during the refueling of the reactors. The number of the LPs is so large that the traditional method of solving few groups diffusion equation cannot be applied to this problem easily. Low-order Harmonics Expansion Method (HEM) is a fast-running loading pattern evaluation method. Important core physical parameters such as effective multiply factor, power distribution and so on can be gotten without fission source iteration calculation for every LPs. So it runs much faster.The paper introduces the physical logic and the calculation flow chart of the Low-order Harmonics Expansion Method (HEM). HEM combines low-order harmonics of the reference LP with perturbation calculation of local positions. These fundamental functions can be applied to form flux prediction of new LPs. A matrix eigen-value problem can be derived by adopting the weighted residual technique. It is easy to solve this eigen-value problem to get the expansion coefficients.Based on HEM, the paper do some study on the improvement of it. For perturbation functions at different positions, they can be added all to form a total perturbation function with different coefficients each and then be combined with the low-order harmonics of the reference LP to form the flux distribution prediction of the new LP. The advantage is to reduce the number of the unknown coefficients of the flux distribution prediction and to cut down the total time of solving the coefficient matrix problem. The numerical results indicate that the modified method has the same acceptable accuracy as former HEM, and the time for solving the coefficients matrix eigen-value problem is reduced.