Improvement of fitting method of multiband parameters for cell calculations

To accurately perform cell calculations of nuclear reactors, a new fitting procedure has been developed for calculating multiband parameters, which are necessary for effective cross section calculations. By using the new fitting procedure, the error of multiband parameters becomes always zero. Reactor cell calculations have been performed to compare the effective cross sections and the infinite multiplication factors etc. calculated using the multiband parameters obtained by the new and the conventional fitting procedures by using the cross section set based on the JENDL-3.1 (1) library with 107 energy groups. It is found that there is a small difference of the calculational results between the two fitting procedures and it is found from burnup calculations that the difference of the infinite multiplication factors is not dependent on the burnup period up to about 30 GWd/t. The onion skin effect can be exactly treated by dividing a fuel pellet to multiple regions and by using the multiband method. Thus the difference of burnup properties between two fitting procedures are investigated for the divided and the undivided fueled cells. The total inventory of Pu, Am etc. at the divided case is almost the same to the undivided case at the end of the burnup period. However it is found that the radial distribution of atomic density is slightly different between the two fitting procedures.