[en] The subject of this study is a postulate 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 (INEL) for the U.S. Nuclear Regulatory Commission (NRC). It has long been recognized that the dominant ATWS transient in BWRs is the main steamline isolation valve (MSIV) closure pressurization type of event. 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