In the late 1950s, two distinct light-water-cooled nuclear reactor steam supply systems became commercially available: the pressurized water reactor (PWR) system and the boiling water reactor (BWR) system. The commercial PWR systems largely grew out of the technology developed for naval nuclear submarine reactors, while early BWR technology was developed mainly at Argonne National Laboratory (ANL) and the Nuclear Energy Division (NED) of General Electric Company (GE) . The BWR steam supply system is attractive due to its basic simplicity and potential for greater thermal efficiency, better reliability, and lower capital cost than other competing light water reactor (LWR) systems. Two reactor containment concepts were also developed during the 1950s . The first and earliest type was dry containment, which surrounded the reactor with a large, passive, leakproof shell, capable of withstanding the pressures caused from a pipe break and complete discharge of the reactor coolant. The pressure suppression containment concept soon followed, in which the reactor coolant energy, released from a postulated pipe rupture, is contained by a surrounding drywell shell that discharges through vents to a large water pool. The pressure suppression containment concept is the one that has been adopted for modern BWRs.