Numerical Study of Burnup Characteristics of 300 MWe LMFBR Based on Three-Dimensional Calculation

摘要

The burnup characteristics of a 300 MWe LMFBR are studied numerically based on three-dimensional calculations.The analysis covers the effects brought by differences in the assumed positions of the control rods and by changing the number of energy groups adopted for calculating the burnup reactivity loss and the detailed space distribution of major Pu isotopes after prescribed periods of irradiation. Results are also compared between three- and two-dimensional models used for the calculations. The results reveal significant influence of control rod position on the depletion and buildup of Pu isotopes not only in the core region but also in the blanket region.Burnup calculations based on two energy groups prove to provide a fairly good approximation of the results obtained with three groups ; this is not the case with one-group calculations, so long as the conventional method is adopted for reducing the energy groups. The errors introduced by the approximative methods in estimating the changes brought to the amounts of Pu isotopes resulting from irradiation increases with the atomic weight, in the order 2391311, 240Pu and 241Pu. Hence for accurate estimation of breeding characteristics, it would be necessary to perform three-dimensional burnup calculations with account taken of the individual positions of control rods distributed in discrete channels over the cross sectional plane. The use of an adequate number of energy groups is also essential unless a suitable improved one-group method is adopted.