Zircaloy-UO2 and -water reactions and cladding temperature estimation for rapidly-heated fuel rods under an RIA condition

Abstract

Zircaloy cladding chemical reactions with coolant water and UO2 fuel at elevated temperatures under a reactivity initiated accident (RIA) condition were studied from a metallurgical point of view on the basis of the Nuclear Safety Research Reactor (NSRR) experiments. The cladding-fuel chemical reaction was extensively analyzed and found to be explainable from equilibrium phase diagrams. The systematic estimation methods of maximum cladding temperature were proposed and examined from metallographies. Maximum cladding temperature can be estimated from measured oxidation thicknesses in the temperature range of 1,000-1,600° C, from melting microstructures in the range of 1,600-1,950° C and also from the volume fraction of the precipitates, (U, Zr)O2 – x, in once-molten oxygen-stabilized α-Zircaloy in 1,950-2,400° C. The estimation by the method proposed in the paper is more valid than thermocouple indications at high temperatures, since thermocouples perturb the temperatures they are measuring or fail at the extremely high temperatures. The results are thought to be applicable also to understand general fuel rod behavior under hypothetical accident conditions which cause severe fuel damage. © 1982, Taylor & Francis Group, LLC.