Hydrogen distribution in a BWŔs RPV and Mark II drywell during the progression of a severe accident with and without emergency coolant injection

Abstract

This study is directed to determine potential risks associated to hydrogen concentrations in an originally fully inertized drywell of a BWR Mark II primary containment, as a consequence of the progression of a severe accident. An unmitigated case and a case with partial recovery of emergency coolant injection are studied. The base case accident scenario to determine hydrogen in-core generation and release rates to drywell is the Large Break Loss-of-Coolant Accident of a BWR, occurring simultaneously with a total loss of power. Two alternative scenarios are simulated by allowing the recovery of the High Pressure Coolant System during the progression of the base case scenario. The code RELAP/SCDAPSIM is used to simulate the different stages in the accident scenarios, and the code GASFLOW was used to compute the hydrogen distribution in a fully pre-inertized drywell. Results of hydrogen and fission product generation in core, as well as state of core degradation are presented. Then, a comparison of results of hydrogen concentration in drywell between the base case and the cases with coolant injection are presented. Results show that, for the base case, by the time of reaching primary containment venting pressure setpoint, the drywell upper zone reaches about 9 % of hydrogen concentration. For the other two cases, the resulting peak hydrogen concentrations were 12 % and 15 %.