To achieve an effective fission chain reaction, neutrons produced during fission must be captured by fissionable nuclei, which then split, releasing more neutrons. In most fission reactor designs, the nuclear fuel is not sufficiently refined to absorb enough fast neutrons to carry on the chain reaction, due to the lower cross section for higher-energy neutrons, so a neutron moderator must be introduced to slow the fast neutrons down to thermal velocities to permit sufficient absorption. Common neutron moderators include graphite, ordinary (light) water and heavy water. A few reactors (fast neutron reactors) and all nuclear weapons rely on fast neutrons.
Cosmogenic neutrons are produced from cosmic radiation in the Earth's atmosphere or surface, as well as in particle accelerators. They often possess higher energy levels compared to neutrons found in reactors. Many of these neutrons activate atomic nuclei before reaching the Earth's surface, while a smaller fraction interact with nuclei in the atmospheric air.[3] When these neutrons interact with nitrogen-14 atoms, they can transform them into carbon-14 (14C), which is extensively utilized in radiocarbon dating.[4]
Neutron radiation is often called indirectly ionizing radiation. It does not ionize atoms in the same way that charged particles such as protons and electrons do (exciting an electron), because neutrons have no charge. However, neutron interactions are largely ionizing, for example when neutron absorption results in gamma emission and the gamma ray (photon) subsequently removes an electron from an atom, or a nucleus recoiling from a neutron interaction is ionized and causes more traditional subsequent ionization in other atoms. Because neutrons are uncharged, they are more penetrating than alpha radiation or beta radiation. In some cases they are more penetrating than gamma radiation, which is impeded in materials of high atomic number. In materials of low atomic number such as hydrogen, a low energy gamma ray may be more penetrating than a high energy neutron.
Health hazards and protection
In health physics, neutron radiation is a type of radiation hazard. Another, more severe hazard of neutron radiation, is neutron activation, the ability of neutron radiation to induce radioactivity in most substances it encounters, including bodily tissues.[5] This occurs through the capture of neutrons by atomic nuclei, which are transformed to another nuclide, frequently a radionuclide. This process accounts for much of the radioactive material released by the detonation of a nuclear weapon. It is also a problem in nuclear fission and nuclear fusion installations as it gradually renders the equipment radioactive such that eventually it must be replaced and disposed of as low-level radioactive waste.
Neutron radiation protection relies on radiation shielding. Due to the high kinetic energy of neutrons, this radiation is considered the most severe and dangerous radiation to the whole body when it is exposed to external radiation sources. In comparison to conventional ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and slowed (absorbed) by light nuclei so hydrogen-rich material is more effective at shielding than iron nuclei. The light atoms serve to slow down the neutrons by elastic scattering so they can then be absorbed by nuclear reactions. However, gamma radiation is often produced in such reactions, so additional shielding must be provided to absorb it. Care must be taken to avoid using materials whose nuclei undergo fission or neutron capture that causes radioactive decay of nuclei, producing gamma rays.
Neutrons readily pass through most material, and hence the absorbed dose (measured in grays) from a given amount of radiation is low, but interact enough to cause biological damage. The most effective shielding materials are water, or hydrocarbons like polyethylene or paraffin wax. Water-extended polyester (WEP) is effective as a shielding wall in harsh environments due to its high hydrogen content and resistance to fire, allowing it to be used in a range of nuclear, health physics, and defense industries.[6] Hydrogen-based materials are suitable for shielding as they are proper barriers against radiation.[7]
Concrete (where a considerable number of water molecules chemically bind to the cement) and gravel provide a cheap solution due to their combined shielding of both gamma rays and neutrons. Boron is also an excellent neutron absorber (and also undergoes some neutron scattering). Boron decays into carbon or helium and produces virtually no gamma radiation with boron carbide, a shield commonly used where concrete would be cost prohibitive. Commercially, tanks of water or fuel oil, concrete, gravel, and B4C are common shields that surround areas of large amounts of neutron flux, e.g., nuclear reactors. Boron-impregnated silica glass, standard borosilicate glass, high-boron steel, paraffin, and Plexiglas have niche uses.
Because neutrons that strike the hydrogen nucleus (proton, or deuteron) impart energy to that nucleus, they in turn break from their chemical bonds and travel a short distance before stopping. Such hydrogen nuclei are high linear energy transfer particles, and are in turn stopped by ionization of the material they travel through. Consequently, in living tissue, neutrons have a relatively high relative biological effectiveness, and are roughly ten times more effective at causing biological damage compared to gamma or beta radiation of equivalent energy exposure. These neutrons can either cause cells to change in their functionality or to completely stop replicating, causing damage to the body over time.[8] Neutrons are particularly damaging to soft tissues like the cornea of the eye.
Effects on materials
High-energy neutrons damage and degrade materials over time; bombardment of materials with neutrons creates collision cascades that can produce point defects and dislocations in the material, the creation of which is the primary driver behind microstructural changes occurring over time in materials exposed to radiation. At high neutron fluences this can lead to embrittlement of metals and other materials, and to neutron-induced swelling in some of them. This poses a problem for nuclear reactor vessels and significantly limits their lifetime (which can be somewhat prolonged by controlled annealing of the vessel, reducing the number of the built-up dislocations). Graphite neutron moderator blocks are especially susceptible to this effect, known as Wigner effect, and must be annealed periodically. The Windscale fire was caused by a mishap during such an annealing operation.
Radiation damage to materials occurs as a result of the interaction of an energetic incident particle (a neutron, or otherwise) with a lattice atom in the material. The collision causes a massive transfer of kinetic energy to the lattice atom, which is displaced from its lattice site, becoming what is known as the primary knock-on atom (PKA). Because the PKA is surrounded by other lattice atoms, its displacement and passage through the lattice results in many subsequent collisions and the creations of additional knock-on atoms, producing what is known as the collision cascade or displacement cascade. The knock-on atoms lose energy with each collision, and terminate as interstitials, effectively creating a series of Frenkel defects in the lattice. Heat is also created as a result of the collisions (from electronic energy loss), as are possibly transmuted atoms. The magnitude of the damage is such that a single 1 MeV neutron creating a PKA in an iron lattice produces approximately 1,100 Frenkel pairs.[9] The entire cascade event occurs over a timescale of 1 × 10−13 seconds, and therefore, can only be "observed" in computer simulations of the event.[10]
The knock-on atoms terminate in non-equilibrium interstitial lattice positions, many of which annihilate themselves by diffusing back into neighboring vacant lattice sites and restore the ordered lattice. Those that do not or cannot leave vacancies, which causes a local rise in the vacancy concentration far above that of the equilibrium concentration. These vacancies tend to migrate as a result of thermal diffusion towards vacancy sinks (i.e., grain boundaries, dislocations) but exist for significant amounts of time, during which additional high-energy particles bombard the lattice, creating collision cascades and additional vacancies, which migrate towards sinks. The main effect of irradiation in a lattice is the significant and persistent flux of defects to sinks in what is known as the defect wind. Vacancies can also annihilate by combining with one another to form dislocation loops and later, lattice voids.[9]
The collision cascade creates many more vacancies and interstitials in the material than equilibrium for a given temperature, and diffusivity in the material is dramatically increased as a result. This leads to an effect called radiation-enhanced diffusion, which leads to microstructural evolution of the material over time. The mechanisms leading to the evolution of the microstructure are many, may vary with temperature, flux, and fluence, and are a subject of extensive study.[11]
Radiation-induced segregation results from the aforementioned flux of vacancies to sinks, implying a flux of lattice atoms away from sinks; but not necessarily in the same proportion to alloy composition in the case of an alloyed material. These fluxes may therefore lead to depletion of alloying elements in the vicinity of sinks. For the flux of interstitials introduced by the cascade, the effect is reversed: the interstitials diffuse toward sinks resulting in alloy enrichment near the sink.[9]
Dislocation loops are formed if vacancies form clusters on a lattice plane. If these vacancy concentration expand in three dimensions, a void forms. By definition, voids are under vacuum, but may became gas-filled in the case of alpha-particle radiation (helium) or if the gas is produced as a result of transmutation reactions. The void is then called a bubble, and leads to dimensional instability (neutron-induced swelling) of parts subject to radiation. Swelling presents a major long-term design problem, especially in reactor components made out of stainless steel.[12] Alloys with crystallographic isotropy, such as Zircaloys are subject to the creation of dislocation loops, but do not exhibit void formation. Instead, the loops form on particular lattice planes, and can lead to irradiation-induced growth, a phenomenon distinct from swelling, but that can also produce significant dimensional changes in an alloy.[13]
Irradiation of materials can also induce phase transformations in the material: in the case of a solid solution, the solute enrichment or depletion at sinks radiation-induced segregation can lead to the precipitation of new phases in the material.[14]
The mechanical effects of these mechanisms include irradiation hardening, embrittlement, creep, and environmentally-assisted cracking. The defect clusters, dislocation loops, voids, bubbles, and precipitates produced as a result of radiation in a material all contribute to the strengthening and embrittlement (loss of ductility) in the material.[15] Embrittlement is of particular concern for the material comprising the reactor pressure vessel, where as a result the energy required to fracture the vessel decreases significantly. It is possible to restore ductility by annealing the defects out, and much of the life-extension of nuclear reactors depends on the ability to safely do so. Creep is also greatly accelerated in irradiated materials, though not as a result of the enhanced diffusivities, but rather as a result of the interaction between lattice stress and the developing microstructure. Environmentally-assisted cracking or, more specifically, irradiation-assisted stress corrosion cracking (IASCC) is observed especially in alloys subject to neutron radiation and in contact with water, caused by hydrogen absorption at crack tips resulting from radiolysis of the water, leading to a reduction in the required energy to propagate the crack.[9]
The neutron is a subatomic particle, symbol n or n0 , which has no electric charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, they are both referred to as nucleons. Nucleons have a mass of approximately one atomic mass unit, or dalton. Their properties and interactions are described by nuclear physics. Protons and neutrons are not elementary particles; each is composed of three quarks.
Neutron activation analysis (NAA) is a nuclear process used for determining the concentrations of elements in many materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on atomic nuclei. The method is based on neutron activation and thus requires a neutron source. The sample is bombarded with neutrons, causing its constituent elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element have long been studied and determined. Using this information, it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the various elements within it. A particular advantage of this technique is that it does not destroy the sample, and thus has been used for the analysis of works of art and historical artifacts. NAA can also be used to determine the activity of a radioactive sample.
Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.
Particle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam.
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:
A beta particle, also called beta ray or beta radiation, is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus, known as beta decay. There are two forms of beta decay, β− decay and β+ decay, which produce electrons and positrons, respectively.
Nuclear technology is technology that involves the nuclear reactions of atomic nuclei. Among the notable nuclear technologies are nuclear reactors, nuclear medicine and nuclear weapons. It is also used, among other things, in smoke detectors and gun sights.
A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear power. Neutron source variables include the energy of the neutrons emitted by the source, the rate of neutrons emitted by the source, the size of the source, the cost of owning and maintaining the source, and government regulations related to the source.
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely more susceptible than fast neutrons to propagate a nuclear chain reaction of uranium-235 or other fissile isotope by colliding with their atomic nucleus.
Ionizing radiation, including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel up to 99% of the speed of light, and the electromagnetic waves are on the high-energy portion of the electromagnetic spectrum.
Neutron 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 decays immediately by emitting gamma rays, or particles such as beta particles, alpha particles, fission products, and neutrons. Thus, the process of neutron capture, even after any intermediate decay, often results in the formation of an unstable activation product. Such radioactive nuclei can exhibit half-lives ranging from small fractions of a second to many years.
Nuclear fuel refers to any substance, typically fissile material, which is used by nuclear power stations or other nuclear devices to generate energy.
A reactor pressure vessel (RPV) in a nuclear power plant is the pressure vessel containing the nuclear reactor coolant, core shroud, and the reactor core.
Neutron detection is the effective detection of neutrons entering a well-positioned detector. There are two key aspects to effective neutron detection: hardware and software. Detection hardware refers to the kind of neutron detector used and to the electronics used in the detection setup. Further, the hardware setup also defines key experimental parameters, such as source-detector distance, solid angle and detector shielding. Detection software consists of analysis tools that perform tasks such as graphical analysis to measure the number and energies of neutrons striking the detector.
Radiation damage is the effect of ionizing radiation on physical objects including non-living structural materials. It can be either detrimental or beneficial for materials.
In condensed-matter physics, a collision cascade is a set of nearby adjacent energetic collisions of atoms induced by an energetic particle in a solid or liquid.
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+ or 4 2He2+ indicating a helium ion with a +2 charge (missing its two electrons). Once the ion gains electrons from its environment, the alpha particle becomes a normal (electrically neutral) helium atom 4 2He.
Radiation materials science is a subfield of materials science which studies the interaction of radiation with matter: a broad subject covering many forms of irradiation and of matter.
In condensed-matter physics, a primary knock-on atom (PKA) is an atom that is displaced from its lattice site by irradiation; it is, by definition, the first atom that an incident particle encounters in the target. After it is displaced from its initial lattice site, the PKA can induce the subsequent lattice site displacements of other atoms if it possesses sufficient energy, or come to rest in the lattice at an interstitial site if it does not.
Neutron embrittlement, sometimes more broadly radiation embrittlement, is the embrittlement of various materials due to the action of neutrons. This is primarily seen in nuclear reactors, where the release of high-energy neutrons causes the long-term degradation of the reactor materials. The embrittlement is caused by the microscopic movement of atoms that are hit by the neutrons; this same action also gives rise to neutron-induced swelling causing materials to grow in size, and the Wigner effect causing energy buildup in certain materials that can lead to sudden releases of energy.
↑ Specialist, WPI, Environmental Information Services -- Shawn Denny, Information Architect; Mike Pizzuti, Graphic Designer; Chelene Neal, Web Information Specialist; Kate Bessiere, Web Information. "Advisory Committee On Human Radiation Experiments Final Report". ehss.energy.gov. Retrieved 2017-12-21.{{cite web}}: CS1 maint: multiple names: authors list (link)
1 2 3 4 Dunand, David. "Materials in Nuclear Power Generation." Materials Science & Engineering 381: Materials for Energy Efficient Technology. Northwestern University, Evanston. 3 Feb. 2015. Lecture
↑ A. Struchbery, E. Bezakova "Thermal-Spike Lifetime from Picosecond-Duration Preequilibrium Effects in Hyperfine Magnetic Fields Following Ion Implantation". 3 May. 1999.
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