Editing Boiling water reactor (section)

== Evolution ==
=== Early concepts ===
The BWR concept was developed slightly later than the PWR concept. Development of the BWR started in the early 1950s, and was a collaboration between [[General Electric]] (GE) and several US national laboratories.

Research into nuclear power in the US was led by the three military services. The Navy, seeing the possibility of turning submarines into full-time underwater vehicles, and ships that could steam around the world without refueling, appointed [[Captain (naval)|Captain]] [[Hyman Rickover]] to run their nuclear power program. Rickover decided on the PWR route for the Navy, as the early researchers in the field of nuclear power feared that the direct production of steam within a reactor would cause instability, while they knew that the use of pressurized water would definitively work as a means of heat transfer. This concern led to the US's first research effort in nuclear power being devoted to the PWR, which was highly suited for naval vessels (submarines, especially), as space was at a premium, and PWRs could be made compact and high-power enough to fit into such vessels.

But other researchers wanted to investigate whether the supposed instability caused by boiling water in a reactor core would really cause instability. During early reactor development, a small group of engineers accidentally increased the reactor power level on an experimental reactor to such an extent that the water quickly boiled. This shut down the reactor, indicating the useful self-moderating property in emergency circumstances. In particular, [[Samuel Untermyer II]], a researcher at [[Argonne National Laboratory]], proposed and oversaw a series of experiments: the [[BORAX experiments]]—to see if a ''boiling water reactor'' would be feasible for use in energy production. He found that it was, after subjecting his reactors to quite strenuous tests, proving the safety principles of the BWR.<ref name=anl>{{Citation |title=Boiling Water Reactor Simulator with Passive Safety Systems - IAEA |page=14 |format=PDF (11 MB) |publisher=[[International Atomic Energy Agency|IAEA]] |date=October 2009 |url=http://www.iaea.org/NuclearPower/Downloads/Simulators/Advanced.BWR.Manual.2009-10.pdf |access-date=8 June 2012}}</ref>

Following this series of tests, GE got involved and collaborated with [[Argonne National Laboratory]]<ref>{{Cite journal|url=https://www.osti.gov/servlets/purl/4115425|doi=10.2172/4115425|title=Nuclear Reactors Build, Being Built, or Planned in the United States as of June 30, 1970|year=1970|publisher=Office of Scientific and Technical Information (OSTI) |doi-access=free}}</ref> to bring this technology to market. Larger-scale tests were conducted through the late 1950s/early/mid-1960s that only partially used directly generated (primary) nuclear boiler system steam to feed the turbine and incorporated heat exchangers for the generation of secondary steam to drive separate parts of the turbines. The literature does not indicate why this was the case, but it was eliminated on production models of the BWR.

=== First series of production ===
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 | image1 = Reaktor.svg
 | caption1 = Cross-section sketch of a typical BWR Mark I containment

 | image2 = Browns Ferry Unit 1 under construction.jpg
 | caption2 = [[Browns Ferry Nuclear Power Plant|Browns Ferry]] Unit 1 drywell and wetwell under construction, a BWR/4 using the Mark I containment. In the foreground is the lid of the drywell or primary containment vessel (PCV).
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{{See also|GE BWR}}

The first generation of production boiling water reactors saw the incremental development of the unique and distinctive features of the BWR: the torus (used to quench steam in the event of a transient requiring the quenching of steam), as well as the drywell, the elimination of the heat exchanger, the steam dryer, the distinctive general layout of the reactor building, and the standardization of reactor control and safety systems. The first, General Electric ([[General Electric|GE]]), series of production BWRs evolved through 6 iterative design phases, each termed BWR/1 through BWR/6. (BWR/4s, BWR/5s, and BWR/6s are the most common types in service today.) The vast majority of BWRs in service throughout the world belong to one of these design phases.

* 1st generation BWR: BWR/1 with [[Containment building#Boiling water reactors|Mark I]] containment.
* 2nd generation BWRs: BWR/2, BWR/3 and some BWR/4 with Mark I containment. Other BWR/4, and BWR/5 with Mark-II containment.
* 3rd generation BWRs: BWR/6 with Mark-III containment.

Containment variants were constructed using either concrete or steel for the Primary Containment, Drywell and Wetwell in various combinations.<ref name=SAND2006-2274P>{{Citation |url=https://www.nrc.gov/reading-rm/doc-collections/nuregs/contract/cr6906/cr6906.pdf |title=Containment Integrity Research at Sandia National Laboratories – An Overview |date=July 2006 |author=Sandia National Laboratories |publisher=U.S. Nuclear Regulatory Commission |id=NUREG/CR-6906, SAND2006-2274P|access-date=13 March 2011}}</ref>

Apart from the GE designs there were others by ABB (Asea-Atom), MITSU, Toshiba and KWU (Kraftwerk Union). See [[List of boiling water reactors]].

=== Advanced boiling water reactor===
[[File:UK ABWR cross section.png|thumb|upright=1.5|Cross section of UK ABWR design Reinforced Concrete Containment Vessel]]
{{main|Advanced boiling water reactor}}
A newer design of BWR is known as the [[advanced boiling water reactor]] (ABWR). The ABWR was developed in the late 1980s and early 1990s, and has been further improved to the present day. The ABWR incorporates advanced technologies in the design, including computer control, plant automation, control rod removal, motion, and insertion, in-core pumping, and nuclear safety to deliver improvements over the original series of production BWRs, with a high power output (1350&nbsp;MWe per reactor), and a significantly lowered probability of core damage. Most significantly, the ABWR was a completely standardized design, that could be made for series production.<ref name="ABWR fact sheet">{{cite web|author1=GE Hitachi Nuclear Energy|author-link1=GE Hitachi Nuclear Energy|title=Advanced Boiling Water Reactor (ABWR) fact sheet|url=https://nuclear.gepower.com/content/dam/gepower-nuclear/global/en_US/documents/product-fact-sheets/ABWR%20Fact%20Sheet.pdf|access-date=20 June 2020|archive-url=https://web.archive.org/web/20151002085218/https://nuclear.gepower.com/content/dam/gepower-nuclear/global/en_US/documents/product-fact-sheets/ABWR%20Fact%20Sheet.pdf|archive-date=October 2, 2015|date=2010|url-status=live}}</ref>

The ABWR was approved by the United States Nuclear Regulatory Commission for production as a standardized design in the early 1990s. Subsequently, numerous ABWRs were built in Japan. One development spurred by the success of the ABWR in Japan is that General Electric's nuclear energy division merged with Hitachi Corporation's nuclear energy division, forming [[GE Hitachi Nuclear Energy]], which is now the major worldwide developer of the BWR design.

=== 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}}

=== Economic simplified boiling water reactor ===
{{main|Economic Simplified Boiling Water Reactor}}
During a period beginning in the late 1990s, GE engineers proposed to combine the features of the advanced boiling water reactor design with the distinctive safety features of the simplified boiling water reactor design, along with scaling up the resulting design to a larger size of 1,600&nbsp;[[MWt|MWe]] (4,500&nbsp;MWth). This [[Economic Simplified Boiling Water Reactor]] (ESBWR) design was submitted to the US Nuclear Regulatory Commission for approval in April 2005, and design certification was granted by the NRC in September 2014.<ref>{{Cite web|url=https://www.nrc.gov/reactors/new-reactors/design-cert/esbwr.html|title = Issued Design Certification - Economic Simplified Boiling-Water Reactor (ESBWR)}}</ref>

Reportedly, this design has been advertised as having a [[core damage frequency|core damage probability]] of only 3×10<sup>−8</sup> core damage events per reactor-year.{{Citation needed|date=March 2011}} That is, there would need to be 3 million ESBWRs operating before one would expect a single core-damaging event during their 100-year lifetimes. Earlier designs of the BWR, the BWR/4, had core damage probabilities as high as 1×10<sup>−5</sup> core-damage events per reactor-year.<ref name="NuclearNews-ESBWR">{{cite journal|last=Hinds|first=David|author2=Maslak, Chris|date=January 2006|title=Next-generation nuclear energy: The ESBWR|journal=Nuclear News|publisher=American Nuclear Society|location=La Grange Park, Illinois, United States of America|volume=49|issue=1|pages=35–40|issn=0029-5574|url=http://www.ans.org/pubs/magazines/nn/docs/2006-1-3.pdf|access-date=2009-04-04|archive-url=https://web.archive.org/web/20100704020922/http://www.ans.org/pubs/magazines/nn/docs/2006-1-3.pdf|archive-date=2010-07-04|url-status=dead}}</ref> This extraordinarily low CDP for the ESBWR far exceeds the other large LWRs on the market.