Startup neutron source: Difference between revisions

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{{Short description|Neutron source used to start nuclear reactors}}

A '''Startupstartup neutron source''' is a [[neutron source]] used for stable and reliable initiation of [[nuclear chain reaction]] in [[nuclear reactor]]s, when they are loaded with fresh [[nuclear fuel]], whose [[neutron flux]] from [[spontaneous fission]] is insufficient for a reliable startup, or after prolonged shutdown periods. Neutron sources ensure a constant minimal population of neutrons in the reactor core, sufficient for a smooth startup. Without them, the reactor could suffer fast power excursions during startup from state with too few self-generated neutrons (new core or after extended shutdown).

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 therefore 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 theythe 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 therefore 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, and thus reducesreducing their useful lifetime as neutron sources.

** [[Californium-252]] ([[spontaneous fission]])

** [[Plutonium-238]] & [[beryllium]], (α,n) [[Nuclear reaction|reaction]]