The integrity of MARK-I containments is severely challenged by the consequences of a postulated main steam isolation valve-anticipated transient without scram (MSIV-ATWS) at full power. To elucidate the nuclear plant’s response to this postulated beyond-design-basis accident, considerable research has been conducted, primarily in the United States. Paul Scherrer Institute is currently engaged in a sizable effort to develop the simulation models for the Transient Analysis of Reactors in Switzerland (STARS project). The current study as part of STARS aims primarily at the physical phenomena occurring during an MSIV-ATWS in a domestic boiling water reactor (B WR)-4. This plant was selected because it has the following features different from the ones widely analyzed: 1. The capacity of the torus (primary containment) is increased by roughly a factor of 2 compared with other nuclear power plants of similar design. 2. The feedwater pumps are motor driven and therefore do not automatically stop operation after a main steamline isolation event. 3. The difference between the setpoints for opening and closing the safety relief valves (SR Vs) is 12% of the opening pressure. This is considerably larger than the comparable values of the plant analyzed abroad. 4. Core power and core size are only about one-third of the corresponding values for the boiling water reactor analyzed so far. The assumed closure of the MSIVs causes a pressure increase that trips the recirculation pumps. Natural circulation is then established in the reactor pressure vessel. Later on, the SR Vs start cycling, dumping steam into the torus. Per emergency operating guidelines, the operator lowers the water level to top of active fuel (TAF) by shutting off the feedwater pumps and by inhibiting the reactor core isolation cooling injection. He then attempts to maintain the water level at this elevation (this action is not simulated in the analysis). The modeling is discussed, and the results of the simulation performed with RETRAN-03/PRE55 are described in detail. Having addressed the limitations of this analysis, it is concluded that the reactor power can be reduced to 15 to 20% of nominal power by only lowering the downcomer water level to TAF.