Gas releases and rod internal pressure of BN-added UO2 fuel

Abstract

Gadolinium (Gd), in the form of gadolinia (Gd2O3), has been used as integral burnable absorber to UO2 fuel by different companies. Recently, alternative burnable absorber materials are under investigation. In this paper, the results of the study on the use of boron mixed in UO2 as an integral burnable absorber are presented. Boron nitride was selected as the form of B to be added to UO2 fuel based on the stability of B during sintering. Fuel performance analysis code FRAPCON-UNI, which is based on FRAPCON code and has the modifications for boron and helium generation and gasses release models, was used for the calculation of fuel performance. Helium generation was calculated using a model and the result was verified against ORIGEN calculation results. Gas release calculation was based on Forsberg Massih model and different characteristics of the gasses were considered. Power histories of maximum burnup rods (B rod, no B rod, and Gd rod) were selected from reactor physics data. The results indicated significant increases in rod internal pressure by 500 ppm boron addition at the end of life with minimum influence of a grain boundary boron fraction due to high diffusivity of helium and nitrogen.