Irradiation effects on fuels for space reactors

A review of irradiation-induced swelling and gas release experience is presented here for the three principal fuels UO/sub 2/, UC, and UN. The primary advantage of UC and UN over UO/sub 2/ is higher thermal conductivity and attendant lower fuel temperature at equivalent pellet diameter and power density, while UO/sub 2/ offers the distinct benefit of well-known irradiation performance. Irradiation test results indicate that at equivalent burnup, temperature, and porosity conditions, UC experiences higher swelling than UO/sub 2/ or UN. Fission gas swelling becomes important at fuel temperatures above 1320 K for UC, and at somewhat higher temperatures for UO/sub 2/ and UN. Evidence exists that at equivalent fuel temperatures and burnups, high density UO/sub 2/ and UN experience comparable swelling behavior; however, differences in thermal conductivity influence overall irradiation performance. The low conductivity of UO/sub 2/ results in higher thermal gradients which contribute to fuel microcracking and gas release. As a result UO/sub 2/ exhibits higher fractional gas release than UN, at least or burnups up to about 3%.