A Probabilistic Approach for the Evaluation of Reactivity Insertion Accident Effects

Reported fuel failures at low peak enthalpies for highly exposed fuel during fast reactivity transients promoted the evaluation of reactivity insertion accidents (RIAs) in light water reactors with an approach different from the ultraconservative point-kinetics licensing evaluations performed in the past. On the basis of realistic estimates for the rod worth and plant conditions, an evaluation of the consequences of the RIAs has been performed. For the pressurized water reactor and boiling water reactor (BWR) cases, rod worth became the most important parameter affecting the severity of the accident. In BWRs high subcooling can adversely affect the consequences of the event. The RIA analyses have been performed using an estimation of the distribution for these parameters in an actual plant during startup. The results show that when assumptions consistent with operating procedures are used, the probability of a significant enthalpy increase due to an RIA is greatly reduced.