Startup neutron source
Encyclopedia
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 rods.
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. 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. 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.
The sources can be of two types:
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.
A plutonium-238
/beryllium primary source can be utilized. As plutonium-238 would undergo neutron capture and transmutation when subjected to intense thermal neutron flux, which would dramatically shorten the source lifetime, the sources can be either affixed to some control rod
and removed from the reactor when it is powered, or clad in a cadmium
alloy, which is opaque for the thermal neutrons but transparent for the fast neutrons produced by the source.
Some neutron sources also serve as additional sources of delayed neutron
s; these serve to dampen the response rate of reactor power to control rods and power transients, allowing safer and more stable operation.
Neutron source
A Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...
used for stable and reliable initiation of nuclear chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
in nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s, when they are loaded with fresh nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
, whose neutron flux
Neutron flux
The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....
from spontaneous fission
Spontaneous fission
Spontaneous fission is a form of radioactive decay characteristic of very heavy isotopes. Because the nuclear binding energy reaches a maximum at a nuclear mass greater than about 60 atomic mass units , spontaneous breakdown into smaller nuclei and single particles becomes possible at heavier masses...
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 rods.
The sources are important for safe reactor startup. The spontaneous fission and cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
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. 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. 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.
The sources can be of two types:
- Primary sources, used for startup of a fresh reactor core; conventional neutron sourceNeutron sourceA Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...
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 captureNeutron captureNeutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
; exposition to thermal neutron flux in an operating reactor reduces their lifetime.- Californium-252 (spontaneous fissionSpontaneous fissionSpontaneous fission is a form of radioactive decay characteristic of very heavy isotopes. Because the nuclear binding energy reaches a maximum at a nuclear mass greater than about 60 atomic mass units , spontaneous breakdown into smaller nuclei and single particles becomes possible at heavier masses...
) - Plutonium-238Plutonium-238-External links:**...
-berylliumBerylliumBeryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, plutonium-239Plutonium-239Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...
-beryllium or americiumAmericiumAmericium is a synthetic element that has the symbol Am and atomic number 95. This transuranic element of the actinide series is located in the periodic table below the lanthanide element europium, and thus by analogy was named after another continent, America.Americium was first produced in 1944...
-beryllium (α,n nuclear reactionNuclear reactionIn nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
s).
- Californium-252 (spontaneous fission
- Secondary sources, originally inert, become radioactive and neutron-producing only after neutron activationNeutron activationNeutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting particles such as neutrons, protons, or alpha...
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).- SbAntimonyAntimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
-BeBerylliumBeryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
photoneutron source; antimony becomes radioactiveNeutron activationNeutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting particles such as neutrons, protons, or alpha...
in the reactor and its strong gamma emissions (1.7 MeV for 124Sb) interact with beryllium-9 by an (γ,n) reaction and provide photoneutrons. In a PWR reactor one neutron source rod contains 160 grams of antimony, and stay in the reactor for 5–7 years. The sources are often constructed as an antimony rod surrounded by beryllium layer and clad in stainless steelStainless steelIn metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
. Antimony-beryllium alloyAlloyAn alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
can be also used.
- Sb
Boron-11 can be added to the fuel; it emits neutrons by the (α,n) reaction to nitrogen-14. Deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
in heavy water emits neutrons by (γ,n) reaction to 1H.
A plutonium-238
Plutonium-238
-External links:**...
/beryllium primary source can be utilized. As plutonium-238 would undergo neutron capture and transmutation when subjected to intense thermal neutron flux, which would dramatically shorten the source lifetime, the sources can be either affixed to some control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
and removed from the reactor when it is powered, or clad in a cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
alloy, which is opaque for the thermal neutrons but transparent for the fast neutrons produced by the source.
Some neutron sources also serve as additional sources of delayed neutron
Delayed neutron
In nuclear engineering, a delayed neutron is a neutron emitted after a nuclear fission event by one of the fission products anytime from a few milliseconds to a few minutes later....
s; these serve to dampen the response rate of reactor power to control rods and power transients, allowing safer and more stable operation.