UO2 + 5 ​vol% ZrB2 nano composite nuclear fuels with full boron retention and enhanced oxidation resistance

Abstract

The boron isotope (10B) can be used as a neutron absorber in UO2 to control the reactivity of nuclear fuel pellets, however, the boron source can react with oxygen source in UO2 to form B2O3 that vaporize readily at temperatures above 1200 °C. Unfortunately, the sintering of UO2 fuel requires hours holding at high temperature (>1700 °C), resulting in an inevitable B loss during sintering and unpredictable B concentration in final product. It is challenging to incorporate boron through a conventional sintering method. In this work, we demonstrated that spark plasma sintering (SPS), a field assisted sintering technology, can effectively densify UO2 ​+ ​5 ​vol% ZrB2 composite fuel pellets by rapid consolidation at 1600 °C for a short duration of 5 min under an applied pressure of 40 MPa. Thermogravimetric analysis (TGA) measurements confirm that ZrB2 is fully retained inside the composite fuel pellets. Inside the composite fuel pellets, nano sized ZrB2 particles are uniformly distributed along the grain boundaries of the UO2 matrix. The ZrB2 particle transforms to a glassy B2O3 phase covering the sample surface and grain boundaries of UO2 matrix after a simple post-sintering annealing at 1000 °C in flowing Argon gas for 4 h. The formed glassy B2O3 slows down the diffusion of oxygen ions and postpones the onset temperature for oxidation of UO2 from 400 °C to 550 °C. This study demonstrates the capability of SPS, an advanced fuel manufacturing technique, to achieve a full retention of ZrB2 in UO2 oxide fuel and increase oxidation resistance through a simple post-sintering annealing. The reported work holds great engineering potential for development of advanced oxide fuel for nuclear application.