Most nuclear reactors use a
chain reactionA chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events...
to induce a controlled rate of
nuclear fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
in fissile material, releasing both
energyNuclear power is power produced from controlled nuclear reactions. Commercial plants in use to date use nuclear fission reactions....
and free
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s. A reactor consists of an assembly of nuclear fuel (a
reactor coreA nuclear reactor core is that portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.- Description :...
), usually surrounded by a
neutron moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
such as
regular waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
,
heavy waterHeavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O...
,
graphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
, or zirconium hydride, and fitted with mechanisms such as control rods that control the rate of the reaction.
Nuclear reactor physics is the branch of science that deals with the study and application of chain reaction to induce controlled rate of fission for energy in reactors.
The physics of
nuclear fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
has several quirks that affect the design and behavior of nuclear reactors. This article presents a general overview of the physics of nuclear reactors and their behavior.
Criticality
In a nuclear reactor, the
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
population at any instant is a function of the rate of neutron production (due to fission processes) and the rate of neutron losses (via non-fission absorption mechanisms and leakage from the system). When a reactor’s neutron population remains steady from one generation to the next (creating as many new neutrons as are lost), the fission chain reaction is self-sustaining and the reactor's condition is referred to as "critical". When the reactor’s neutron production exceeds losses, characterized by increasing power level, it is considered "supercritical", and; when losses dominate, it is considered "subcritical" and exhibits decreasing power.
The "six-factor formula" is the neutron life-cycle balance equation, which includes six separate factors, the product of which is equal to the ratio of the number of neutrons in any generation to that of the previous one; this parameter is called the effective multiplication factor (k), a.k.a. K
eff. k =
Lfρ
LthfηЄ, where
Lf = "fast non-leakage factor"; ρ = "resonance escape probability";
Lth = "thermal non-leakage factor";
f = "thermal fuel utilization factor"; η = "reproduction factor"; Є = "fast-fission factor".
k = (Neutrons produced in one generation)/(Neutrons produced in the previous generation)
When the reactor is critical, k = 1. When the reactor is subcritical, k < 1. When the reactor is supercritical, k > 1.
"
ReactivityReactivity refers to the rate at which a chemical substance tends to undergo a chemical reaction in time. In pure compounds, reactivity is regulated by the physical properties of the sample. For instance, grinding a sample to a higher specific surface area increases its reactivity. In impure...
" is an expression of the departure from criticality. δk = (k - 1)/k
When the reactor is critical, δk = 0. When the reactor is subcritical, δk < 0. When the reactor is supercritical, δk > 0. Reactivity is also represented by the lowercase Greek letter rho (
ρ). Reactivity is commonly expressed in decimals or percentages or pcm (per cent mille) of Δk/k. When reactivity ρ is expressed in units of delayed neutron fraction β, the unit is called the
dollar.
If we write 'N' for the number of free neutrons in a reactor core and
for the average lifetime of each neutron (before it either escapes from the core or is absorbed by a nucleus), then the reactor will follow differential equationA differential equation is a mathematical equation for an unknown function of one or several variables that relates the values of the function itself and its derivatives of various orders...
(the evolution equation)
where is a constant of proportionality, and is the rate of change of the neutron count in the core. This type of differential equation describes
exponential growthExponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
or
exponential decayA quantity is said to be subject to exponential decay if it decreases at a rate proportional to its value. Symbolically, this can be expressed as the following differential equation, where N is the quantity and λ is a positive number called the decay constant.The solution to this equation is:Here...
, depending on the sign of the constant , which is just the expected number of neutrons after one average neutron lifetime has elapsed:
Here, is the probability that a particular neutron will strike a fuel nucleus, is the probability that the neutron, having struck the fuel, will cause that nucleus to undergo fission, is the probability that it will be absorbed by something other than fuel, and is the probability that it will "escape" by leaving the core altogether. is the number of neutrons produced, on average, by a fission event -- it is between 2 and 3 for both
235U and
239Pu.
If is positive, then the core is supercritical
and the rate of neutron production will grow exponentially until some other effect stops the growth. If is negative, then the core is "subcritical" and the number of free neutrons in the core will shrink exponentially until it reaches an equilibrium at zero (or the background level from spontaneous fission). If is exactly zero, then the reactor is critical and its output does not vary in time .
Nuclear reactors are engineered to reduce and . Small, compact structures reduce the probability of direct escape by minimizing the
surface areaSurface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved then the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...
of the core, and some materials (such as
graphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
) can
reflectReflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
some neutrons back into the core, further reducing .
The probability of fission, , depends on the nuclear physics of the fuel, and is often expressed as a
cross sectionIn nuclear and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles.When particles are thrown against a foil made of a certain substance, the cross section is a hypothetical area measure around the target particles that represents a...
.
Reactors are usually controlled by adjusting . Control rods made of a strongly neutron-absorbent material such as
cadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. The soft, bluish-white transition metal is chemically similar to the two other 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...
or
boronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite....
can be inserted into the core: any neutron that happens to impact the control rod is lost from the chain reaction, reducing . is also controlled by the recent history of the reactor core itself (see below).
Starter sources
The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low it may take a long time (in
235U reactors, as long as many minutes) before a chance neutron encounter starts a chain reaction even if the reactor is supercritical. Most nuclear reactors include a "starter"
neutron sourceNeutron source is a general term referring to a variety of devices that emit neutrons, irrespective of the mechanism used to produce the neutrons...
that ensures there are always a few free neutrons in the reactor core, so that a chain reaction will begin immediately when the core is made critical. A common type of neutron source is a mixture of an
alpha particleAlpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as or . They have a net spin of zero, and normally a total energy of about 5 MeV...
emitter such as
241Am (
AmericiumAmericium is a synthetic element that has the symbol Am and atomic number 95. A radioactive metallic element, americium is an actinide that was obtained in 1944 by Glenn T. Seaborg who was bombarding plutonium with neutrons and was the fourth transuranic element to be discovered. It was named for...
-241) with a lightweight isotope such as
9Be (
BerylliumBeryllium is the chemical element with the symbol Be and atomic number 4.A bivalent element, beryllium is found naturally only combined with other elements in minerals. Notable gemstones which contain beryllium include Beryl and Chrysoberyl...
-9). Once the chain reaction is begun, the starter source is removed from the core to prevent damage from the high neutron flux in the operating reactor core.
Subcritical multiplication
Even in a subcritical assembly such as a shut-down reactor core, any stray neutron that happens to be present in the core (for example from spontaneous fission of the fuel, from radioactive decay of fission products, or from a
neutron sourceNeutron source is a general term referring to a variety of devices that emit neutrons, irrespective of the mechanism used to produce the neutrons...
) will trigger an exponentially decaying chain reaction. Although the chain reaction is not self-sustaining, it acts as a multiplier that increases the
equilibriumIn nuclear physics, secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate is equal to its decay rate.-Secular equilibrium in radioactive decay:...
number of neutrons in the core. This subcritical multiplication effect can be used in two ways: as a probe of how close a core is to criticality, and as a way to generate fission power without the risks associated with a critical mass.
As a measurement technique, subcritical multiplication was used during the
Manhattan ProjectThe Manhattan Project was the codename for a project conducted during World War II to develop the first atomic bomb. The project was led by the United States, and included scientists from Denmark, The United Kingdom and Canada...
in early experiments to determine the minimum critical masses of
235U and of
239Pu. It is still used today to calibrate the controls for nuclear reactors during startup, as many effects (discussed in the following sections) can change the required control settings to achieve criticality in a reactor. As a power-generating technique, subcritical multiplication allows generation of nuclear power for fission where a critical assembly is undesirable for safety or other reasons. A subcritical assembly together with a neutron source can serve as a steady source of heat to generate power from fission.
Including the effect of an external neutron source ("external" to the fission process, not physically external to the core), one can write a modified evolution equation:
where is the rate at which the external source injects neutrons into the core. In
equilibriumIn nuclear physics, secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate is equal to its decay rate.-Secular equilibrium in radioactive decay:...
, the core is not changing and dN/dt is zero, so the equilibrium number of neutrons is given by:
If the core is subcritical, then is negative so there is an equilibrium with a positive number of neutrons. If the core is close to criticality, then is very small and thus the final number of neutrons can be made arbitrarily large.
Neutron moderators
To improve and enable a chain reaction, uranium-fueled reactors must include a
neutron moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
that interacts with newly produced fast neutrons from fission events to reduce their kinetic energy from several
MeVMeV and meV are multiples and submultiples of the electron volt unit referring to 1,000,000 eV and 0.001 eV, respectively.Mev or MEV may refer to:In entertainment:* Musica Elettronica Viva, an Italian musical group...
to several
eVeV may mean:* Electronvolt, a unit of energye. V. may mean:* Eingetragener Verein, a registered club in Germany#eV may mean:* Most Valuable player ShaQEV may mean:* Estonia's official name is Eesti Vabariik...
, making them more likely to induce fission. This is because
235U is much more likely to undergo fission when struck by one of these thermal neutrons than by a freshly-produced neutron from fission.
Neutron moderators are materials that interact weakly with the neutrons but absorb kinetic energy from them. Most moderators rely on either weakly bound
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
or a loose crystal structure of another light element such as
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
to transfer kinetic energy from the fast-moving neutrons.
Hydrogen moderators include
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
(H
2O),
heavy waterHeavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O...
(
DDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
2O), and zirconium hydride (ZrH
2), all of which work because a hydrogen nucleus has nearly the same mass as a free neutron: neutron-H
2O or neutron-ZrH
2 impacts excite rotational modes of the molecules (spinning them around).
DeuteriumDeuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in of hydrogen...
nuclei (in heavy water) absorb kinetic energy less well than do light hydrogen nuclei, but they are much less likely to absorb the impacting neutron. Water or heavy water have the advantage of being transparent
liquidLiquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surface where the liquid is not constrained by a container....
s, so that, in addition to shielding and moderating a reactor core, they permit direct viewing of the core in operation and can also serve as a working fluid for heat transfer.
Crystal structure moderators rely on a floppy crystal matrix to absorb phonons from neutron-crystal impacts.
GraphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
is the most common example of such a moderator. It was used in
Chicago Pile-1Chicago Pile-1 was the world's first artificial nuclear reactor. CP-1 was built on a rackets court, under the abandoned west stands of the original Alonzo Stagg Field stadium, at the University of Chicago. The first artificial, self-sustaining, nuclear chain reaction was initiated within CP-1, on...
, the world's first man-made critical assembly, and was commonplace in early reactor designs including the Soviet
RBMKRBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy which means "High Power Channel-type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union for use in nuclear power plants to produce nuclear power from nuclear fuel....
nuclear power plants, of which the Chernobyl plant was one.
Moderators and reactor design
The amount and nature of neutron moderation affects reactor controllability and hence safety. Because moderators both slow and absorb neutrons, there is an optimum amount of moderator to include in a given geometry of reactor core. Less moderation reduces the effectiveness by reducing the term in the evolution equation, and more moderation reduces the effectiveness by increasing the term.
Most moderators become less effective with increasing temperature, so under-moderated reactors are stable against changes in temperature in the reactor core: if the core overheats, then the quality of the moderator is reduced and the reaction tends to slow down (there is a "negative temperature coefficient" in the reactivity of the core). Water is an extreme case: in extreme heat, it can boil, producing
effective voidIn nuclear engineering, the void coefficient is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids form in the reactor moderator or coolant...
s in the reactor core without destroying the physical structure of the core; this tends to shut down the reaction and reduce the possibility of a fuel meltdown. Over-moderated reactors are unstable against changes in temperature (there is a "positive temperature coefficient" in the reactivity of the core), and so are less inherently safe than under-moderated cores.
Some reactors use a combination of
moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
materials. For example,
TRIGATRIGA is a class of small nuclear reactor designed and manufactured by General Atomics of the USA. TRIGA is an acronym of "Training, Research, Isotopes, General Atomics"...
type research reactors use ZrH
2 moderator mixed with the
235U fuel, an H
2O-filled core, and C (graphite) moderator and
reflectorA neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, lead, steel, tungsten carbide, or other materials...
blocks around the periphery of the core.
Delayed neutrons and controllability
Fission reactions and subsequent neutron escape happen very quickly; this is important for nuclear weapons, where the object is to make a nuclear core release as much energy as possible before it physically
explodesAn explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
. Most neutrons emitted by fission events are
promptIn nuclear engineering, a prompt neutron is a neutron immediately emitted by a nuclear fission event, as opposed to a delayed neutron which is emitted by one of the fission products anytime from a few milliseconds to a few minutes later...
: they are emitted essentially instantaneously. Once emitted, the average neutron lifetime in a typical core is on the order of a millisecond, so if the exponential factor is as small as 0.01, then in one second the reactor power will vary by a factor of (1+0.01)
1000, or more than ten thousand. Nuclear weapons are engineered to maximize the power growth rate, with lifetimes well under a millisecond and exponential factors close to 2; but such rapid variation would render it practically impossible to control the reaction rates in a nuclear reactor.
Fortunately, the effective neutron lifetime is much longer than the average lifetime of a single neutron in the core. About 0.65% of the neutrons produced by
235U fission, and about 0.75% of the neutrons produced by
239Pu fission, are not produced immediately, but rather are emitted by
radioactive decayRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
of fission products, with an average lifetime of about 15 seconds. These delayed neutrons increase the effective average lifetime of neutrons in the core, to nearly 0.1 seconds, so that a core with of 0.01 would increase in one second by only a factor of (1+0.01)
10, or about 1.1 -- a 10% increase. This is a controllable rate of change.
Most nuclear reactors are hence operated in a prompt subcritical
, delayed critical
condition: the prompt neutrons alone are not sufficient to sustain a chain reaction, but the delayed neutrons make up the small difference required to keep the reaction going. This has effects on how reactors are controlled: when a small amount of control rod is slid into or out of the reactor core, the power level changes at first very rapidly due to prompt subcritical multiplication
and then more gradually, following the exponential growth or decay curve of the delayed critical reaction. Further, increases
in reactor power can be performed at any desired rate simply by pulling out a sufficient length of control rod -- but decreases are limited in speed, because even if the reactor is taken deeply subcritical, the delayed neutrons are produced by ordinary
radioactive decayRadioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter...
of fission products and that decay cannot be hastened.
Reactor poisons
Any element that strongly absorbs neutrons is called a
reactor poisonA nuclear poison, also called a neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors, when absorbing neutrons is an undesirable effect...
, because it tends to shut down (poison) an ongoing fission chain reaction. Some reactor poisons are deliberately inserted into fission reactor cores to control the reaction; boron or cadmium control rods are the best example. Many reactor poisons are produced by the fission process itself, and buildup of neutron-absorbing fission products affects both the fuel economics and the controllability of nuclear reactors.
Long-lived poisons and fuel reprocessing
In practice, buildup of reactor poisons in nuclear fuel is what determines the lifetime of nuclear fuel in a reactor: long before all possible fissions have taken place, buildup of long-lived neutron absorbing fission products damps out the chain reaction. This is the reason that
nuclear reprocessingNuclear reprocessing separates components of spent nuclear fuel.Reprocessing serves multiple purposes, whose relative importance has changed over time:*Producing plutonium for nuclear weapons...
is a useful activity: spent nuclear fuel contains about 99% of the original fissionable material present in newly manufactured nuclear fuel. Chemical separation of the fission products restores the nuclear fuel so that it can be used again.
Nuclear reprocessing is useful economically because chemical separation is much simpler to accomplish than the difficult
isotope separationIsotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...
required to prepare nuclear fuel from natural uranium ore, so that in principle chemical separation yields more generated energy for less effort than mining, purifying, and isotopically separating new uranium ore. In practice, both the difficulty of handling the highly radioactive fission products and other political concerns make fuel reprocessing a contentious subject. One such concern is the fact that spent uranium nuclear fuel contains significant quantities of
239Pu, a prime ingredient in nuclear weapons (see
breeder reactorA breeder reactor is a nuclear reactor that generates new fissile or fissionable material at a greater rate than it consumes such material. These reactors were initially considered appealing due to their superior fuel economy; a normal reactor is able to consume less than 1% of the natural...
).
Short-lived poisons and controllability
Short-lived reactor poisons in fission products strongly affect how nuclear reactors can operate. Unstable fission product nuclei transmute into many different elements (secondary fission products) as they undergo a
decay chainIn nuclear science, the decay chain refers to the radioactive decay of different discrete radioactive decay products as a chained series of transformations...
to a stable isotope. The most important such element is
xenonXenon is a chemical element represented by the symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
, because the isotope
135Xe, a secondary fission product with a half-life of about 9 hours, is an extremely strong neutron absorber. In an operating reactor, each nucleus of
135Xe is destroyed by
neutron 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 charged particles which are repelled by electrostatic...
almost as soon as it is created, so that there is no buildup in the core. However, when a reactor shuts down, the level of
135Xe builds up in the core for about 9 hours before beginning to decay. The result is that, about 6-8 hours after a reactor is shut down, it can become physically impossible to restart the chain reaction until the
135Xe has had a chance to decay over the next several hours; this is one reason why nuclear power reactors are best operated at an even power level around the clock.
135Xe buildup in a reactor core makes it extremely dangerous to operate the reactor a few hours after it has been shut down. Because the
135Xe absorbs neutrons strongly, starting a reactor in a high-Xe condition requires pulling the control rods out of the core much farther than normal. But if the reactor does achieve criticality, then the neutron flux in the core will become quite high and the
135Xe will be destroyed rapidly -- this has the same effect as very rapidly removing a great length of control rod from the core, and can cause the reaction to grow too rapidly or even become
prompt criticalIn nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. This causes a rapidly exponential increase in the number of fission events...
.
135Xe played a large part in the Chernobyl accident: about eight hours after a scheduled maintenance shutdown, workers tried to bring the reactor to a
zero power criticalZero power critical is a condition of nuclear fission reactors, that is useful for characterizing the reactor core. A reactor is in the zero power critical state if it is sustaining a stable fission chain reaction with no significant growth or decay in the reaction rate, and at a low enough level...
condition to test a control circuit. Since the core was loaded with
135Xe from the previous day's power generation, it was necessary to withdraw more control rods to achieve this. As a result, the reaction grew rapidly and uncontrollably, leading to steam explosion in the core, fire, and violent destruction of the facility.
Uranium enrichment
While many fissionable isotopes exist in nature, the only usefully
fissileIn nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate...
isotope found in any quantity is
235UUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
about 0.7% of the uranium in most ores is the 235 isotope, and about 99.3% is the inert 238 isotope. For most uses as a nuclear fuel, uranium must be enriched - purified so that it contains a higher percentage of
235U. Because
238U absorbs fast neutrons, the
critical massA critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
needed to sustain a chain reaction increases as the
238U content increases, reaching infinity at 94%
238U (6%
235U).
Concentrations lower than 6%
235U cannot go fast critical, though they are usable in a nuclear reactor with a
neutron moderatorIn nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
.
A nuclear weapon primary stage using uranium uses HEU enriched to ~90%
235U, though the secondary stage often uses lower enrichments. Nuclear reactors with water moderator can operate with only moderate enrichment of ~5%
235U. Nuclear reactors with
heavy waterHeavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ²H2O, or as deuterium protium oxide, HDO or ¹H²HO. Its physical and chemical properties are somewhat similar to those of water, H2O...
moderation can operate with natural uranium, eliminating altogether the need for enrichment and preventing the fuel from being useful for nuclear weapons; the
CANDUThe CANDU reactor is a Canadian-invented, pressurized heavy water reactor developed initially in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited , the Hydro-Electric Power Commission of Ontario , Canadian General Electric , as well as...
power reactors used in
CanadianCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
power plants are an example of this type.
Uranium enrichment is difficult because the chemical properties of
235U and
238U are identical, so physical processes such as
gaseous diffusionGaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride, UF6, through semi-permeable membranes. This produces a slight separation between the molecules containing uranium-235 and uranium-238. By use of a large cascade of many stages,...
,
gas centrifugeA gas centrifuge is a separating machine specifically developed to separate Uranium-235 from Uranium-238. The gas centrifuge relies on the principles of centripetal force accelerating molecules based upon mass. When performed in a column, this yields separations of each component. The gas...
or mass spectrometry must be used for isotopic separation based on small differences in mass. Because enrichment is the main technical hurdle to production of nuclear fuel and simple nuclear weapons, enrichment technology is politically sensitive.
Oklo: a natural nuclear reactor
Modern deposits of uranium contain only up to ~0.7%
235U (and ~99.3%
238U), which is not enough to sustain a chain reaction moderated by ordinary water. But
235U has a much shorter
half-lifeHalf-life is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....
(700 million years) than
238U (4.5 billion years), so in the distant past the percentage of
235U was much higher. About two billion years ago, a water-saturated uranium deposit (in what is now the
OkloOklo is a region near the town of Franceville, in the Haut-Ogooué province of the Central African state of Gabon.The discovery in September 1972 of several natural nuclear fission reactors in the uranium mines situated there has fired the imagination and aroused the curiosity of...
mine in
GabonGabon is a country in west central Africa sharing borders with the Gulf of Guinea to the west, Equatorial Guinea to the northwest, and Cameroon to the north, with the Republic of the Congo curving around the east and south. Its size is almost 270,000 km² with an estimated population...
,
West AfricaWest Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries distributed over an area of approximately 5 million square km:*Benin...
) underwent a naturally occurring chain reaction that was moderated by
groundwaterGroundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in...
and, presumably, controlled by the negative void coefficient as the water boiled from the heat of the reaction. Uranium from the Oklo mine is about 50% depleted compared to other locations: it is only about 0.3% to 0.7%
235U; and the ore contains traces of stable daughters of long-decayed fission products.
See also
- List of nuclear reactors
- Nuclear physics
Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei.The most commonly known applications of nuclear physics are nuclear power and nuclear weapons, but the research has provided wider applications, including those in medicine , materials...
- Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
- Nuclear fusion
In nuclear physics and nuclear chemistry, nuclear fusion is the process by which multiple like-charged atomic nuclei join together to form a heavier nucleus...