An efficient grid depression reconstruction model on axial assembly power distribution was developed for MHI nuclear design code system GalaxyCosmo-S. The objective of this paper is to present the background, methodology and its application of the new model in GalaxyCosmo-S.

In order to consider the grid depression effect to the homogeneous axial power distribution obtained from 3D nodal core calculation, the new model employs the concept of the pin-power reconstruction model widely used in modern core design codes. In the new model, axial heterogeneous assembly power distribution is calculated by synthesizing the grid form function to the axial homogeneous power distribution by nodal calculation. The form function is pre-produced by fitting the local grid depression data processed from the measured axial thimble reaction rate in the grid position. By incorporating the measured data, the form function can reflect the precise grid depression information. According to the present study, it was shown that the form function has a burnup dependency for its depth, and it is prepared for each fuel type and axial grid position.

In order to confirm the applicability of the present method to the existing PWRs, the predicted axial power distribution by GalaxyCosmo-S was compared with the measured data by the movable detector (M/D). As a result, the good agreements were confirmed without any specific trends for burnup condition.

In addition, the difference of axial power distribution between predicted and measured data was statistically analyzed for multiple plants, cycles and burnup conditions. From the results, it is confirmed that the systematic over- or under-estimations of the power distribution observed in the grid homogenized model are reduced by the grid depression model. So this model is suitable for the 3D power distribution analysis and FQ uncertainty evaluation.

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