Startup neutron source: Difference between revisions

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[[File:RBMK Reaktor ChNPP-4.svg|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 rod]]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 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 an unsafe condition.<!--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|dateyear=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= |accessdate=2010-03-28}} {{Dead link|date=October 2010|bot=H3llBot}}</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|dateyear=2009 |isbn=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 few months, as [[neutron capture]] resulting from the thermal neutron flux in an operating reactor changes the composition of the isotopes used, and thus reduces their useful lifetime as neutron sources.

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

** [[Plutonium-238]]-[[beryllium]], [[plutonium-239]]-beryllium or [[americium]]-beryllium (α,n) [[nuclear reactions]].

* '''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 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|dateyear=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.

[[Boron-11]] can be added to the fuel; it emits neutrons by the (α,n) reaction to [[nitrogen-14]]. [[Deuterium]] in heavy water emits neutrons by (γ,n) reaction to ¹H.<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"/>

The chain reaction in the first critical reactor, CP-1, was initiated by neutron sources generated during its prior subcritical test runs. The very first of those could have been initiated by ambient cosmic-ray neutrons. Similarly, in modern reactors (after startup), delayed neutron emission from fission products suffices to sustain the amplification reaction while yielding controllable growth times. (In comparisioncomparison, a bomb is based on immediate neutrons and grows exponentially in nanoseconds.)

The chain reaction in the first critical reactor, CP-1, was initiated by neutron sources generated during its prior subcritical test runs. The very first of those could have been initiated by ambient cosmic-ray neutrons. Similarly, in modern reactors (after startup), delayed neutron emission from fission products suffices to sustain the amplification reaction while yielding controllable growth times. (In comparision, a bomb is based on immediate neutrons and grows exponentially in nanoseconds.)

==References==