MAAP code predictions of in-vessel thermal hydraulics in boiling water reactors

The Modular Accident Analysis Program (MAAP) has become a useful tool in Level 1 (front-end) and Level 2 (back-end) probabilistic risk assessment (PRA) studies in the nuclear industry. A limited amount of work has been published on the utilization of the MAAP code in developing success criteria for Level 1 and Level 2 PRAs; there have been a number of publications related to the use of the MAAP Code in describing severe accident progression, quantifying source term releases, and benchmarking the code results against experimental data as well as predictions of other severe accident codes. During the past seven years, the probabilistic risk assessment group at Northeast Utilities has extensively utilized the MAAP Code in analyzing design basis (DBA) and beyond Design Basis accident sequences. In this context, it is worthwhile to report some of Northeast Utilities' experiences with this Code. The effort in this paper is directed towards establishing more confidence in the reliability of the MAAP Code's predictions and to demonstrate the utilization of this code in developing Level 1 PRA success criteria