The thermal conductivity of columnar grains in irradiated UO2 fuel elements (irradiation tests X-225 and X-227)

An irradiation experiment was performed to determine whether the columnar (essentially single-crystal) grains grown in UO2 fuel elements under irradiation at high heat ratings showed an enhanced thermal conductivity. No indication was found of any alteration in thermal conductivity within the limits of error of the experiment. Although this does not exclude the possibility that enhanced conductivity can occur in uranium dioxide, it indicates that it is not necessarily a property of single crystal material. Further information was obtained on the dependence of diametral, expansions of Zircaloy sheathed UO2 fuel elements on heat rating, which confirms that the metallographic identification of the extent of central melting is substantially correct. However there are indications that metallographic examination may underestimate the extent of central melting in a fuel element that has undergone prolonged low power operation subsequent to the time at which melting was a maximum. (author)