Assembly Based Modular Ray Tracing and CMFD Acceleration for BWR Cores with Different Fuel Lattices

The geometry module of the DeCART direct whole core calculation code has been extended in order to analyze BWR cores which might have a mixed loading of different fuel types. First, an assembly based modular ray tracing scheme was implemented for the Method of Characteristic (MOC) calculation, and a CMFD formulation applicable for unaligned mesh conditions was then developed for acceleration the MOC calculation. The new calculation feature has been validated by comparing DeCART BWR assembly calculations with the MCU Monte Carlo calculations. A good agreement identified by the maximum eigenvalue difference of 120 pcm and the maximum pin power error of about 1% has been achieved. The CMFD scheme is shown to reduce the number of MOC iterations by factors of 12-25 without loss of accuracy. (authors)