Startup neutron source: Difference between revisions

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[[File:RBMK Reaktor ChNPP-4.PNG|thumb|300px|[[RBMK]] reactor control rod positions at the moment of the [[Chernobyl disaster]]; blue='''startup neutron sources''' (12), yellow=shortened control rods from the reactor bottom (32), grey=pressure tubes (1661), green=[[control rodsrod]]s (167), red=automatic control rods (12)]]

'''Startup 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 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= |accessdate=2010-03-28}}</ref>

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 would lead to a "blind" start, with chance of going supercritical and causing partial [[core meltdown]] or at least fuel element damage.<ref>{{cite book|url=http://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|date=1997 |isbn=0773516018|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= |accessdate=2010-03-28}}</ref>

The sources can be of two types:<ref name="nucleng">{{cite book|url=http://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|date=2009 |isbn=1420053906}}</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 few months. Primary sources are subject to [[neutron capture]]; exposition to thermal neutron flux in an operating reactor reduces their lifetime.

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

** [[Plutonium-238]]-[[beryllium]], [[Plutoniumplutonium-239]]-beryllium or [[americium]]-beryllium (α,n [[nuclear reaction]]s).

* '''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. The exposition 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 ¹²⁴Sb) interact with [[beryllium-9]] by an (γ,n) reaction and provide [[photoneutron]]s. In a PWR reactor one neutron source rod contains 160 grams of antimony, and stay in the reactor for 5–7 years.<ref>{{cite book|url=http://books.google.com/books?id=SJOE00whg44C&pg=PA147&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=22#v=onepage&q=neutron%20startup%20source&f=false |title=The radiochemistry of nuclear power plants with light water reactors|author=Karl-Heinz Neeb|page=147|publisher=Walter de Gruyter|date=1997 |isbn=3110132427}}</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.