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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
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,
** [[Californium-252]] ([[spontaneous fission]])
** [[Plutonium-238]] & [[beryllium]], (α,n) [[Nuclear reaction|reaction]]
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