The RELAP5 computer code is used to simulate four small-scale loss-of-coolant accident (LOCA) experiments conducted at Idaho National Engineering Laboratory (INEL). The purpose of the study is to help assess RELAP5 under conditions similar to those expected during a large-break LOCA at INEL’s Advanced Test Reactor (ATR). During an ATR large-break LOCA, it is expected that the primary system pressure will rapidly decrease from the initial operating pressure (∼2.55 MPa) to subatmospheric conditions governed by the primary coolant temperature. Flashing will occur in the high points of the system and air ingress from the break is possible. The RELAP5 code had not previously been assessed under these conditions. The results show that RELAP5 accurately predicted pressures, water levels, and air ingress behavior, thus providing confidence in the ability of the code to simulate an ATR large-break LOCA.