Irradiation Behavior of Large Grain UO2 Fuel Rod by Active Powder.

Abstract

The internal pressures in the large and normal grain UO2 pellet rods irradiated at a linear heat rate of <29 kW/m up to 21 GWd/t U were studied to clarify the effect of in-pile densification and fission gas release. The large grain (grain size: 23 μm) pellets were manufactured from the active powder produced by the new ADU (ammonium diuranate) process. The pore structure of the large grain pellets mainly consisted of coarse pores that were formed by adding a poreformer. The pore structure of the normal grain (8 μm) pellets made from the current powder consisted of fine pores. In-pile and out-of-pile densification of the large grain pellets were smaller than those of the normal pellets and occurred owing to the disappearance of fine pores as fabricated. The increase in free volume of the large grain fuel rod did not occur significantly owing to its smaller densification during the initial irradiation period. The fuel stack length change during irradiation was predicted with the in-pile densification and swelling model. The increase in grain size decreased fission gas release by diffusion. As a result, it was found that the internal pressure of the large grain fuel rod was reduced after 10 GWd/t U.