Effect of Cladding Surface Pre-oxidation on Rod Coolability under Reactivity Initiated Accident Conditions

Abstract

The effect of cladding surface pre-oxidation on the rod coolability under reactivity initiated accidents (RIAs) was investigated. NSRR experiments on irradiated fuel rods have shown higher rod coolability than that of fresh rods, which arose from suppressed DNB and early quench at the surface. To identify the dominating factor, possible factors such as pellet cracking, porosity increase and so on, were assessed. The most probable factor, i.e., the effect of cladding surface pre-oxidation, was examined by pulse irradiation experiments on fresh rods with three cladding surface conditions, no oxide layer, 1 μm and 10(o.m-thick oxide layers. Temperature measurements showed the DNB thresholds in terms of cladding temperature and fuel enthalpy increase at the pre-oxidized surface. The cladding temperature at quench also rises at the pre-oxidized cladding, leading to a reduced film boiling duration. These shifts of the critical and minimum heat flux points could be caused by the surface wettability increase. The test results indicate the dominating factor is the wettability change probably due to the surface chemical potential change by pre-oxidation rather than the thermal conductivity change in the oxide layer, because the results do not depend on the oxide layer thickness, but on the presence of the surface oxide.