Hydrogen generation during cladding/coolant interactions under reactivity initiated accident conditions

Hydrogen generation in chemical reaction of Zircaloy cladding with coolant water under reactivity initiated accident conditions (RIAs) has been studied with the in-pile experiments in the Nuclear Safety Research Reactor (NSRR). PWR type segmented fuel rods were subjected to pulse irradiation to simulate the power excursion of an RIA. Transient measurements of the void fraction with a densimeter developed for the in-pile experiment detected prompt generation of hydrogen with an increase in cladding surface temperature. The hydrogen generation ceases in the initial 4 seconds of the power excursion even in the experiments resulting in the melting of the fuel cladding and severe damage. The total amount of hydrogen generated during the power burst was estimated by integration of the densimeter data and also by metallographic examinations of the oxidized cladding. The total amount of hydrogen produced increased with an elevation in the maximum temperature of the cladding surface, and the hydrogen generation was intensified as a function of damage to the fuel cladding. The applicability of the PRECIP-II code to the analysis of hydrogen generation under an RIA condition was also examined. (author)