Editing Boiling water reactor (section)

=== Simplified boiling water reactor - never licensed ===
Parallel to the development of the ABWR, General Electric also developed a different concept, known as the '''simplified boiling water reactor''' (SBWR). This smaller 600 [[watt#Conventions in the electric power industry|megawatt electrical]] reactor was notable for its incorporation—for the first time ever in a light water reactor{{Citation needed|date=August 2015}}—of "[[Passive nuclear safety|passive safety]]" design principles. The concept of passive safety means that the reactor, rather than requiring the intervention of active systems, such as emergency injection pumps, to keep the reactor within safety margins, was instead designed to return to a safe state solely through operation of natural forces if a safety-related contingency developed.

For example, if the reactor got too hot, it would trigger a system that would release soluble neutron absorbers (generally a solution of borated materials, or a solution of [[borax]]), or materials that greatly hamper a chain reaction by absorbing neutrons, into the reactor core. The tank containing the soluble neutron absorbers would be located above the reactor, and the absorption solution, once the system was triggered, would flow into the core through force of gravity, and bring the reaction to a near-complete stop. Another example was the [[Isolation Condenser system]], which relied on the principle of hot water/steam rising to bring hot coolant into large heat exchangers located above the reactor in very deep tanks of water, thus accomplishing residual heat removal. Yet another example was the omission of recirculation pumps within the core; these pumps were used in other BWR designs to keep cooling water moving; they were expensive, hard to reach to repair, and could occasionally fail; so as to improve reliability, the ABWR incorporated no less than 10 of these recirculation pumps, so that even if several failed, a sufficient number would remain serviceable so that an unscheduled shutdown would not be necessary, and the pumps could be repaired during the next refueling outage. Instead, the designers of the ''simplified boiling water reactor'' used thermal analysis to design the reactor core such that natural circulation (cold water falls, hot water rises) would bring water to the center of the core to be boiled.

The ultimate result of the passive safety features of the SBWR would be a reactor that would not require human intervention in the event of a major safety contingency for at least 48 hours following the safety contingency; thence, it would only require periodic refilling of cooling water tanks located completely outside of the reactor, isolated from the cooling system, and designed to [[Nuclear reactor heat removal|remove reactor waste heat]] through evaporation. The ''simplified boiling water reactor'' was submitted{{when|date=February 2021}} to the United States [[Nuclear Regulatory Commission]], however, it was withdrawn{{when|date=February 2021}} prior to approval; still, the concept remained intriguing to General Electric's designers, and served as the basis of future developments.{{citation needed|date=February 2021}}