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Burnup Optimization of Continuous Scattered Refueling

Hiroshi Motoda

Nuclear Science and Engineering

Volume 41 / Number 1 / July 1970 / Pages 1-13


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A variational treatment of the burnup optimization of continuous scattered refueling is presented and numerical solutions are given for a slab reactor. It is made quantitatively clear how the reactor dimension, the xenon and the Doppler feedback reactivity, the burnup dependence of fission cross section and the reflector performance affect the power distribution that maximizes the average discharge exposure. Power flattening and burnup maximization are contradictory in general, but are consistent if, and only if, the condition of the perfect reflection at the core boundary is satisfied. The optimal power distribution is peaked in the central—and depleted in the outer region; and becomes flatter as the reflector performance is increased. The maximum average burnup depends on the burnup dependence of fission cross section and the strength of the Doppler and the xenon feedback reactivity, even if the average burnup calculated by the point-reactor model is the same. The former effect on the optimal power distribution is very small but the latter effects greatly contribute to power flattening. Both effects reduce the maximum burnup and the effects of the latter two are of comparable order. As the reactor becomes smaller, the maximum burnup decreases almost linearly to the neutron leakage. Optimal refueling has an advantage of more than 10% in the average burnup over the conventional flat-refueling rate method. However the difference from the flat-burnup method is very small, considering that the optimal refueling is handicapped by its very bad power distribution.

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