Behavior of Inert Fission Gas Under Accelerated Irradiation Conditions for Prototypical LWR Conditions

Accelerated irradiation of fuels in reactors is expected to play a role in the Accelerated Fuel
Qualification (AFQ). Interpretation of the accelerated irradiation experiments in terms of burnup
may no longer be adequate and need to account for fission rate. We use fission gas release as an
example scenario to demonstrate the implication of accelerated irradiation on fuel behavior and
their utility in validation of underlying mechanisms. The impact of fission rate on diffusion of
fission gas atoms is recognized in mechanistic fission gas release models. Accelerated irradiation
is expected to impact other physical processes as well, such as thermal conductivity and fuel
restructuring. Our goal is to further assess the implication of elevated fission rates on fuel behavior.
Current fuel performance codes (FPC) lack a mechanistic treatment of point defect (PD)-induced
conductivity reduction. In this study, we analyze the implications of PD accumulation on the
thermal conductivity of UO2 by adopting Lucuta thermal conductivity correlation (LC). We
demonstrate that fission rate-dependent point defect concentrations have the largest impact on inpile thermal conductivity in the periphery of light-water reactor fuel. The reduction of thermal
conductivity in the low-temperature rim region acts as additional thermal resistance and leads to a
temperature notably larger than suggested by LC specifically at low burnups. During the
accelerated irradiation, further reduction in thermal conductivity is expected and hence, an increase
in fuel temperature. However, it leads to an overall reduction in the FGR and a notable change in
fuel microstructure. These microstructure evolutions are important aspects of fuel design and
development for next-generation reactors. Once additional multiphysics mechanisms tightly
coupled to temperature profile are introduced it becomes harder to deconvolve the impact of point
defects.