Optimal control rod programming of light water reactors in equilibrium fuel cycle

A generalized treatment for investigating the effects of various refueling schemes on the optimal control rod programming that maximizes the average burnup of discharged fuels in a two-region, radially one-dimensional light water moderated nuclear reactor is presented and applied to a boiling water reactor having uranium fuel of a single 235U enrichment. It is assumed that the refueling scheme has reached an equilibrium fuel cycle, and the analysis by burnup space is applied, which helps in interpreting geometrically the coupled effect of the control rod programming and the fuel burnup by the trajectory drawn in this space. Three refueling schemes are considered: parallel, series out-in, and series in-out. Scatter loading is assumed in each region and the batch number and volume fraction of each region are varied as the refueling parameters.Fuel management and poison management constitute a hierarchy relation, and the effect of the refueling schemes on burnup maximization or enrichment minimization is several times greater than that of the control rod programmings. However, the policy of the optimal control rod programming strongly depends on the refueling scheme. The power density of the inner region should be as high as possible for out-in scheme (inner high policy) and vice versa for in-out scheme (outer high policy). However, either policy can be optimal, depending on the refueling parameters for parallel schemes, and in some cases the optimal control rod programming is not unique (degenerate policy). Optimal control rod programming increases the discharge burnup or decreases the enrichment of the feed fuels by about 0 to 4% over the conventional constant power shape operation. The difference is mainly determined by the reactor design and the refueling scheme. Optimal refueling should be chosen from among parallel schemes, which have much larger freedom than series schemes.