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{{Short description|Neutron source used to start nuclear reactors}}
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The startup sources are typically inserted in regularly spaced positions inside the [[reactor core]], in place of some of the [[fuel rod]]s.
The sources are important for safe reactor startup. The spontaneous fission and ambient radiation such as [[cosmic ray]]s serve as weak neutron sources, but these are too weak for the reactor instrumentation to detect; relying on them could lead to a "blind" start, which is a potentially unsafe condition.<!--with a minuscule chance of going supercritical and causing partial [[core meltdown]] or at least fuel element damage--> Blind startups were used in the early days of the American nuclear submarine program, before corrosion problems of the clading of startup sources were resolved. (Leaking of the first neutron sources contaminated the reactors, making maintenance dangerous.)<ref>{{cite book|url=https://books.google.com/books?id=SkrVDKMconIC&dq=neutron+startup+source&pg=PA224|page=224|title=Canada enters the nuclear age: a technical history of Atomic Energy of Canada Limited|publisher=McGill-Queen's Press - MQUP|year=1997 |isbn=0-7735-1601-8|author=Atomic Energy of Canada}}</ref> The sources are positioned so that the neutron flux they produce is always detectable by the reactor monitoring instruments. When the reactor is in a shutdown state, the neutron sources serve to provide signals for neutron detectors monitoring the reactor, to ensure the detectors are operable.<ref name="pat1">{{US patent|4208247}} Neutron source</ref> The equilibrium level of neutron flux in a subcritical reactor is dependent on the neutron source strength; a certain minimum level of source activity has to be ensured in order to maintain control over the reactor when in strongly subcritical state, namely during startups.<ref>{{cite web|url=http://ocw.mit.edu/NR/rdonlyres/Nuclear-Engineering/22-05Fall-2006/4D228A81-EC19-43CD-8C8D-B4AC34851DF9/0/lecture25.pdf |title=Microsoft Word - lecture25.doc |date= |access-date=2010-03-28 |url-status=dead |archive-url=https://web.archive.org/web/20110629124040/http://ocw.mit.edu/NR/rdonlyres/Nuclear-Engineering/22-05Fall-2006/4D228A81-EC19-43CD-8C8D-B4AC34851DF9/0/lecture25.pdf |archive-date=June 29, 2011 }}</ref>
The sources can be of two types:<ref name="nucleng">{{cite book|url=https://books.google.com/books?id=EMy2OyUrqbUC&dq=neutron+startup+source&pg=PA27 |title=Nuclear Engineering Handbook |author=Ken Kok|page=27|publisher=CRC Press|year=2009 |isbn=978-1-4200-5390-6}}</ref>
* '''Primary sources''', used for startup of a fresh reactor core; conventional [[neutron source]]s are used. The primary sources are removed from the reactor after the first fuel campaign, usually after a few months, as [[neutron capture]] resulting from the thermal neutron flux in an operating reactor changes the composition of the isotopes used, reducing their useful lifetime as neutron sources.
** [[Californium-252]] ([[spontaneous fission]])
** [[
** [[americium-241]] & beryllium, (α,n) [[Nuclear reaction|reaction]]
** [[polonium]]-210 & beryllium, (α,n) [[Nuclear reaction|reaction]]
** [[radium]]-226 & beryllium, (α,n) [[Nuclear reaction|reaction]]<ref name="tpub" />
When [[plutonium-238]]/beryllium primary sources are utilized, they can be either affixed to [[control rod]]s which are removed from the reactor when it is powered, or clad in a [[cadmium]] alloy, which is opaque to thermal neutrons (reducing transmutation of the plutonium-238 by neutron capture) but transparent to [[fast neutron]]s produced by the source.<ref name="pat1" />
* '''Secondary sources''', originally inert, become radioactive and neutron-producing only after [[neutron activation]] in the reactor. Due to this, they tend to be less expensive. Exposure to thermal neutrons also serves to maintain the source activity (the radioactive isotopes are both burned and generated in neutron flux).
** [[Antimony|Sb]]-[[Beryllium|Be]] [[photoneutron]] source; antimony [[neutron activation|becomes radioactive]] in the reactor and its strong gamma emissions (1.7 MeV for <sup>124</sup>Sb) interact with [[beryllium-9]] by an (γ,n) reaction and provide [[photoneutron]]s. In a [[Pressurized water reactor|PWR reactor]] one neutron source rod contains 160 grams of antimony, and stays in the reactor for 5–7 years.<ref>{{cite book|url=https://books.google.com/books?id=SJOE00whg44C&dq=neutron+startup+source&pg=PA147 |title=The radiochemistry of nuclear power plants with light water reactors|author=Karl-Heinz Neeb|page=147|publisher=Walter de Gruyter|year=1997 |isbn=3-11-013242-7}}</ref> The sources are often constructed as an antimony rod surrounded by beryllium layer and clad in [[stainless steel]].<ref name="tpub">{{cite web|author=Integrated Publishing |url=http://www.tpub.com/content/doe/h1019v1/css/h1019v1_108.htm |title=Neutron Sources Summary |publisher=Tpub.com |date= |accessdate=2010-03-28}}</ref><ref>{{cite web|url=http://www.lib.ncsu.edu/specialcollections/digital/text/engineering/reactor/murray/MurNBabneutron040953.html |title=Memorandum from Raymond L. Murray to Dr. Clifford K. Beck |publisher=Lib.ncsu.edu |date= |accessdate=2010-03-28}}</ref> Antimony-beryllium [[alloy]] can be also used.
The chain reaction in the first critical reactor, [[Chicago Pile-1|CP-1]], was initiated by a radium-beryllium neutron source. Similarly, in modern reactors (after startup), delayed neutron emission from fission products suffices to sustain the amplification reaction while yielding controllable growth times. In comparison, a bomb is based on immediate neutrons and grows exponentially in nanoseconds.
==References==
{{Reflist|30em}}
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