Thermal reactor
Encyclopedia
A thermal reactor is a nuclear reactor
that uses slow or thermal neutrons. Most power reactors are of this type. These type of reactors use a neutron moderator
to slow neutron
s until they approach the average kinetic energy
of the surrounding particles, that is, to reduce the speed of the neutron
s to low velocity thermal neutrons. The nuclear cross section
of uranium-235
for slow thermal neutrons is about 1000 barns
. For fast neutrons it is in the order of 1 barn.
Thermal neutrons will be more likely to cause uranium-235
to fission
when it is struck by the neutrons, in this case thermal neutrons, and fewer neutrons will be captured by uranium-238
. If at least one neutron from the U-235 fission strikes another nucleus and causes it to fission, then the chain reaction
will continue. If the reaction will sustain itself, it is said to be critical, and the mass of U-235 required to produce the critical condition is said to be a critical mass.
Thermal reactors consists of the following:
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...
that uses slow or thermal neutrons. Most power reactors are of this type. These type of reactors use a neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
to slow neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s until they approach the average kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the surrounding particles, that is, to reduce the speed of the neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s to low velocity thermal neutrons. The nuclear cross section
Nuclear cross section
The nuclear cross section of a nucleus is used to characterize the probability that a nuclear reaction will occur. The concept of a nuclear cross section can be quantified physically in terms of "characteristic area" where a larger area means a larger probability of interaction...
of uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
for slow thermal neutrons is about 1000 barns
Barn (unit)
A barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process, and is best understood as a measure of the...
. For fast neutrons it is in the order of 1 barn.
Thermal neutrons will be more likely to cause uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
to fission
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 photons , and releasing a tremendous amount of energy...
when it is struck by the neutrons, in this case thermal neutrons, and fewer neutrons will be captured by uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
. If at least one neutron from the U-235 fission strikes another nucleus and causes it to fission, then the chain reaction
Chain reaction
A 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....
will continue. If the reaction will sustain itself, it is said to be critical, and the mass of U-235 required to produce the critical condition is said to be a critical mass.
Thermal reactors consists of the following:
- Neutron moderatorNeutron moderatorIn nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
to slow down the neutronNeutronThe neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s. - Nuclear fuelNuclear fuelNuclear 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...
, which is a fissile material, usually uraniumUraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
. - Containment buildingContainment buildingA containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi...
s which is designed , in any emergency, to contain the escape of radiationRadiationIn physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
. - Reactor vesselReactor vesselIn a nuclear power plant, the reactor vessel is a pressure vessel containing the Nuclear reactor coolant and reactor core.Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than by the configuration of the reactor vessel used to...
that is a pressure vesselPressure vesselA pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...
containing the coolantCoolantA coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
and reactor core. - Radiation shielding to protect people and the environment from the harmful effects of ionizing radiationIonizing radiationIonizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
. - Instrumentation to monitor and control the reactor's systems.
See also
- Enriched uraniumEnriched uraniumEnriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
- Fast neutron
- Fast neutron reactorFast neutron reactorA fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons...
- Neutron moderatorNeutron moderatorIn nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
- Nuclear fissionNuclear 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 photons , and releasing a tremendous amount of energy...
- Uranium-235Uranium-235- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
- Uranium-238Uranium-238Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...