Content deleted Content added
mNo edit summary Tags: Mobile edit Mobile app edit iOS app edit |
Copyedit for readability |
||
| (6 intermediate revisions by 4 users not shown) | |||
Line 1:
{{Short description|Neutron source used to start nuclear reactors}}
A '''
▲'''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 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
The sources can be of two types:<ref name="nucleng">{{cite book|url=https://books.google.com/books?id=EMy2OyUrqbUC
* '''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]]
Line 16 ⟶ 15:
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" />
* '''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 <sup>124</sup>Sb) interact with [[beryllium-9]] by an (γ,n) reaction and provide [[photoneutron]]s. In a [[Pressurized water reactor|PWR reactor]] one neutron source rod contains 160 grams of antimony, and stays in the reactor for 5–7 years.<ref>{{cite book|url=https://books.google.com/books?id=SJOE00whg44C
The chain reaction in the first critical reactor, [[Chicago Pile-1|CP-1]], was initiated by a radium-beryllium neutron source. Similarly, in modern reactors (after startup), delayed neutron emission from fission products suffices to sustain the amplification reaction while yielding controllable growth times. In comparison, a bomb is based on immediate neutrons and grows exponentially in nanoseconds.
| |||