A multi cycle optimization approach and program system is developed for a low-leakage in-corefuel management. In this system, the multi cycle optimization process is achieved in three stages:At first, use a linear programming method to find an optimum power sharing distribution and optimum cycle length distribution for the successive several cycles. which yields maximum multi cycle total cycle length. Secondly, detenmine the optimum fuel arrangment on the basis of the Former stage results by employing a linear programming method or directly search method for successive every one of the multi cycle. In the third stage, optimize the burnable poison assignment by Flexible Tolerance Method (FTM).

The paper introduces a three-dimensional nodal and subchannel analysis code package for the core transient in PWPs-SNENM/RTSP, including the model of neutronics and thermohydraulics used in the package. The preliminary results of applying this code package to the analysis of Steam-Line Break accident for Qinshan NPP are given and also evaluated briefly.