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 [[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 ana potentially unsafe condition.<!--with chance of going supercritical and causing partial [[core meltdown]] or at least fuel element damage--><ref>{{cite book|url=https://books.google.com/books?id=SkrVDKMconIC&pg=PA224&dq=neutron+startup+source&lr=&as_drrb_is=q&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=&num=50&as_brr=3&cd=1#v=onepage&q=neutron%20startup%20source&f=false|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 the neutron flux they produce is always detectable by the reactor monitoring instruments. When the reactor is in shutdown state, the neutron sources serve to provide signals for neutron detectors monitoring the reactor, to ensure they 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 |format=PDF |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&pg=PA27&dq=neutron+startup+source&lr=&as_drrb_is=q&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=&num=50&as_brr=3&cd=4#v=onepage&q=neutron%20startup%20source&f=false |title=Nuclear Engineering Handbook |author=Ken Kok|page=27|publisher=CRC Press|year=2009 |isbn=1-4200-5390-6}}</ref>
