INEL BWR severe accident ATWS study

The subject of this study is a postulated anticipated transient without scram (ATWS) at unit one of the Browns Ferry nuclear plant, a boiling water reactor (BWR). The development work is being conducted at the Idaho National Engineering Laboratory for the US Nuclear Regulatory Commission. It has long been recognized that the dominant ATWS transient in BWRs is the main steamline isolation valve closure pressurization type of event. This is because the steam supply system is isolalted from its condenser or thermal sink, and because the resultant pressurization causes a large power excursion, or because other ATWS transients ultimately degrade into this type. Resolution of this ongoing safety concern can only be effected through plant transient simulation. The analytic tool used in this study is RELAP5/MOD1.6. This version of RELAP5 has the capability to simulate BWR plants in that several special process models, such as a jet pump momentum mixer model, have been installed. The code has the capability to transport dissolved boron throughout the 1-D hydrodynamic solution using a strict donoring formulation, and to calculate the effect of this poison on the reactor power. Extensive checkout and verification of the Browns Ferry BWR model was performed. Some results are presented of the comparison of the model to plant transient data during a two pump trip test. Data comparison is quite favorable