In the United States, there are two main types of commercial power reactors: PWR (Pressurized Water Reactor) and BWR (Boiling Water Reactor). When Gibson is explaining the basic workings of the plant to Kimberly Wells, the diagram on the board shows a PWR. This is indicated by the two-loop system in which the water is pumped through the reactor under high pressure to prevent boiling, then through a steam generator to create steam for the turbine using clean secondary water. Later, the dialog of the characters in the control room suggests they are dealing with a BWR, where water is allowed to boil in the reactor vessel, and steam is directly piped to the turbine, with no steam generator. Godell is concerned that the high water level in the reactor might reach the steam lines, of which there are none on a PWR vessel. Once Goddell and the operators realize the water level is low, the dialogue refers to Auxilary Feedwater, which is a PWR system. Also, in the action hearing later, the investigator talks about how the operators began cutting off feedwater and releasing steam in order to lower the reactor water level; this would happen only on a BWR.
