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During long cycles, burnable poisons are used to flatten out the variations in the reactivity, their actual composition and distribution in the fuel assembly can have a significant impact on the power distribution. In this paper, Erbium is used as a burnable poison in a PWR fuel assembly that is burned up to a very high burnup level reaching 70 GWd/t. The optimum composition and distribution of the Erbium isotopes in the assembly are determined to provide reactivity control and obtain flat power distribution. Four models are simulated using MCNPX 2.7 code; the first model used natural Erbium distributed uniformly in all the fuel pins of the assembly, while the other three models used enriched Erbium with different Erbia (Er2O3) concentrations and distributions. The results obtained for k-infinity and pin power peaking factors showed that the suggested loadings of the Erbium bearing pins can sustain very long burnup cycles up to 70 GWd/t. The suggested loading of the Erbia (Er2O3) bearing rods, either in natural or enriched form distributed uniformly in the fuel assembly, can keep the power distribution in the assembly more flattened. Moreover, the peak pin power values are lower compared to the enriched models with partial Erbium distribution.

Optimum Erbium Isotopes Composition and Distribution for Power Flattening in Advanced PWR Fuel Assembly