The main objective of the fuel management is to minimize fuel costs, thus reducing the cost of generating electricity. In this study, to achieve this objective, the optimum fuel specifications, axial blankets and burnable absorber (BA) lengths are suggested based on the optimization study of the fuel cycle costs or design parameters for each fuel cycle length.
It is confirmed that the longer cycle operation has economical advantages due to increased plant capacity factor. And the fuel cycle cost evaluation shows that the fuel specifications, enrichment and batch size, provide more benefits with increasing the average discharge burnup within the present nominal licensing limit on fuel rods of 60 GWD/MTU (equivalent discharge burnup of 48 GWD/MTU).
The results of optimization of the axial blanket length show that the length of 6 inch produces the maximum benefit over a wide range of discharge burnups and cycle lengths. However, the optimum enrichment is varied with discharge burnups and cycle lengths, and thus an equation that can generically estimate the optimum enrichment is derived in this study. The formula from optimization study can be easily used to estimate the enrichment of the axial blanket when the cycle length and region average discharge burnup are known. It can be also applied to all 12-foot cores having the similar behavior of axial power distributions. In conjunction with the axial blanket, the optimum length of BA is determined to give additional margin to the total power peaking factor, F$_Q$, limit. The use of optimum part length BA can also enhance the fuel utilization due to both the reduced residual penalty and the extended burnup resulting from the power redistribution as the cycle burnup progresses.
As a result of the longer cycle application to Kori Unit 1, it is found that most effective way to control the power peaking factor and MTC within their limits in the same time is to use HFA (Heavy Fuel Assembly) having lower H/U (Hydrogen-to-Uranium) ratio instead of OFA. Furthermore, when the relaxed limit of the power peaking factor is applied, a much lower leakage loading pattern can be established. This lower leakage pattern combining with the optimum axial blanket reduces not only the fuel cycle cost as much as 3.36%, but also the fast neutron fluence hitting the pressure vessel. Additionally, the use of HFA fuel reduces spent fuel storage requirements by as much as 4 ~ 8 assemblies in each cycle due to increased uranium loading per assemblies compared with OFA fuel
