Very high temperature reactor
Encyclopedia
The Very High Temperature Reactor (VHTR), or High Temperature Gas-cooled Reactor (HTGR), is a Generation IV reactor
concept that uses a graphite
-moderated
nuclear reactor
with a once-through uranium
fuel cycle. The VHTR is a type of High Temperature Reactor (HTR) that can conceptually have an outlet temperature of 1000°C. The reactor core
can be either a “prismatic block” or a “pebble-bed
” core. The high temperatures enable applications such as process heat or hydrogen
production via the thermochemical sulfur-iodine cycle
.
There are two main types of HTGRs: pebble bed reactors (PBR) and prismatic block reactors (PMR). The prismatic block reactor refers to a prismatic block core configuration, in which hexagonal graphite blocks are stacked to fit in a cylindrical pressure vessel
. The pebble bed reactor
(PBR) design consists of fuel in the form of pebbles, stacked together in a cylindrical pressure vessel, like a gum-ball machine. Both reactors may have the fuel stacked in an annulus
region with a graphite center spire
, depending on the design and desired reactor power.
The russian VTGR is also a HTGR.
) in 1947. Professor Dr. Rudolf Schulten
in Germany also played a role in development during the 1950s. The Peach Bottom
reactor in the United States was the first HTGR to produce electricity, and did so very successfully, with operation from 1966 through 1974 as a technology demonstrator. Fort St. Vrain Generating Station
was one example of this design that operated as an HTGR from 1979 to 1989; though the reactor was beset by some problems which led to its decommissioning due to economic factors, it served as proof of the HTGR concept in the United States (though no new commercial HTGRs have been developed there since). HTGRs have also existed in the United Kingdom (the Dragon reactor
) and Germany (AVR and THTR-300), and currently exist in Japan (the HTTR
using prismatic fuel with 30 MWth of capacity) and China (the HTR-10
, a pebble-bed design with 10 MWe of generation). Two full-scale pebble-bed HTGRs, each with 100 - 195 MWe of electrical production capacity are under construction in China at the present as of November 2009, and are promoted in several countries by reactor designers, and ANTARES design by company AREVA France.
, however, uranium carbide
or uranium oxycarbide are also possibilities. Uranium oxycarbide combines uranium carbide with the uranium dioxide to reduce the oxygen stoichiometry. Less oxygen may lower the internal pressure in the TRISO particles caused by the formation of carbon monoxide, due to the oxidization of the porous carbon layer in the particle. The TRISO particles are either dispersed in a pebble for the pebble bed design or molded into compacts/rods that are then inserted into the hexagonal graphite blocks. The QUADRISO fuel concept conceived at Argonne National Laboratory
has been used to better manage the excess of reactivity.
, so it will generally not chemically react with any material. Additionally, exposing helium to neutron radiation does not make it radioactive, unlike most other possible coolants.
instead of helium. The pebble fuel floats in the salt, and thus pebbles are injected into the coolant flow to be carried to the bottom of the pebble bed, and are removed from the top of the bed for recirculation. The LS-VHTR has many attractive features, including: the ability to work at high temperatures (the boiling point of most molten salts being considered are >1,400°C), low pressure operation, high power density, better electric conversion efficiency than a helium-cooled VHTR operating at similar conditions, passive safety
systems, and better retention of fission product
s in case an accident
occurred.
s would be inserted in holes cut in the graphite blocks that make up the core. The VHTR will be controlled like current PBMR designs if it utilizes a pebble bed core, the control rods will be inserted in the surrounding graphite reflector
. Control can also be attained by adding pebbles containing neutron absorbers.
dose, and, if using a molten salt coolant, the corrosive
environment, of the VHTR require materials that exceed the limitations of current nuclear reactors. In a study of Generation IV reactors in general (of which there are numerous designs, including the VHTR), Murty and Charit suggest that materials that have high dimensional stability, either with or without stress, maintain their tensile strength
, ductility
, creep
resistance, etc. after aging, and are corrosion resistant are primary candidates for use in VHTRs. Some materials suggested include nickel-base superalloys, silicon carbide
, specific grades of graphite, high chromium
steels, and refractory alloys
. Further research is being conducted at US national laboratories as to which specific issues must be addressed in the Generation IV VHTR prior to construction.
and the helium coolant is single phase, inert, and has no reactivity effects. The core is composed of graphite, has a high heat capacity and structural stability even at high temperatures. The fuel is coated uranium-oxycarbide which permits high burn-up (approaching 200 GWd/t) and retains fission products. The high average core-exit temperature of the VHTR (1,000°C) permits emissions-free production of process heat.
Generation IV reactor
Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...
concept that uses a graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
-moderated
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....
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...
with a once-through uranium
Uranium
Uranium 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...
fuel cycle. The VHTR is a type of High Temperature Reactor (HTR) that can conceptually have an outlet temperature of 1000°C. The reactor core
Nuclear reactor core
A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.- Description :...
can be either a “prismatic block” or a “pebble-bed
Pebble bed reactor
The pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor , one of the six classes of nuclear reactors in the Generation IV initiative...
” core. The high temperatures enable applications such as process heat or hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
production via the thermochemical sulfur-iodine cycle
Sulfur-iodine cycle
The sulfur–iodine cycle is a three-step thermochemical cycle used to produce hydrogen.The S–I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled...
.
Overview
The VHTR is a type of High Temperature Reactor that conceptually can reach higher outlet temperatures (up to 1000°C); however, in practice the term "VHTR" is usually thought of as a gas-cooled reactor, and commonly used interchangeably with "HTGR" (high temperature gas-cooled reactor).There are two main types of HTGRs: pebble bed reactors (PBR) and prismatic block reactors (PMR). The prismatic block reactor refers to a prismatic block core configuration, in which hexagonal graphite blocks are stacked to fit in a cylindrical pressure vessel
Pressure vessel
A 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,...
. The pebble bed reactor
Pebble bed reactor
The pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor , one of the six classes of nuclear reactors in the Generation IV initiative...
(PBR) design consists of fuel in the form of pebbles, stacked together in a cylindrical pressure vessel, like a gum-ball machine. Both reactors may have the fuel stacked in an annulus
Annulus (mathematics)
In mathematics, an annulus is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. Or, it is the area between two concentric circles...
region with a graphite center spire
Spire
A spire is a tapering conical or pyramidal structure on the top of a building, particularly a church tower. Etymologically, the word is derived from the Old English word spir, meaning a sprout, shoot, or stalk of grass....
, depending on the design and desired reactor power.
The russian VTGR is also a HTGR.
History
The HTGR design was first proposed by the Staff of the Power Pile Division of the Clinton Laboratories (known now as Oak Ridge National LaboratoryOak Ridge National Laboratory
Oak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
) in 1947. Professor Dr. Rudolf Schulten
Rudolf Schulten
In the 1950s, Dr. Rudolf Schulten - professor at RWTH Aachen University - was the originator of the pebble bed reactor design, which compacts silicon carbide-coated uranium granules into hard, billiard-ball-like spheres to be used as fuel for a new high temperature, helium-cooled type of nuclear...
in Germany also played a role in development during the 1950s. The Peach Bottom
Peach Bottom Nuclear Generating Station
Peach Bottom Atomic Power Station, a nuclear power plant, is located southeast of Harrisburg in Peach Bottom Township, York County, Pennsylvania, on the Susquehanna River on the Maryland border....
reactor in the United States was the first HTGR to produce electricity, and did so very successfully, with operation from 1966 through 1974 as a technology demonstrator. Fort St. Vrain Generating Station
Fort St. Vrain Generating Station
Fort Saint Vrain Generating Station is a natural gas powered electricity generating facility located near the town of Platteville in northern Colorado in the United States. It currently has a capacity of just under 1000MW and is owned and operated by Xcel Energy, the successor to the plant's...
was one example of this design that operated as an HTGR from 1979 to 1989; though the reactor was beset by some problems which led to its decommissioning due to economic factors, it served as proof of the HTGR concept in the United States (though no new commercial HTGRs have been developed there since). HTGRs have also existed in the United Kingdom (the Dragon reactor
Dragon reactor
Dragon was a high temperature gas cooled reactor at Winfrith in England operated by UKAEA. Its purpose was to test fuel and materials for the European high temperature reactor programme, and was built and managed as an OECD/NEA international project...
) and Germany (AVR and THTR-300), and currently exist in Japan (the HTTR
HTTR
The high temperature test reactor is a graphite-moderated gas-cooled research reactor in Oarai, Ibaraki, Japan operated by the Japan Atomic Energy Agency. It uses long hexagonal fuel assemblies, unlike the competing pebble bed reactor designs....
using prismatic fuel with 30 MWth of capacity) and China (the HTR-10
HTR-10
HTR-10 is a 10 MWt prototype pebble bed reactor at Tsinghua University in China. Construction began in 2000 and it achieved first criticality in January 2003.In 2005, China announced its intention to scale up HTR-10 for commercial power generation...
, a pebble-bed design with 10 MWe of generation). Two full-scale pebble-bed HTGRs, each with 100 - 195 MWe of electrical production capacity are under construction in China at the present as of November 2009, and are promoted in several countries by reactor designers, and ANTARES design by company AREVA France.
Neutron moderator
The neutron moderator is graphite, although whether the reactor core is configured in graphite prismatic blocks or in graphite pebbles depends on the HTGR design.Nuclear fuel
The fuel used in HTGRs is coated fuel particles, such as TRISO fuel particles. Coated fuel particles have fuel kernels, usually made of uranium dioxideUranium dioxide
Uranium dioxide or uranium oxide , also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used...
, however, uranium carbide
Uranium carbide
Uranium carbide, a carbide of uranium, is a hard refractory ceramic material. It comes in several stoichiometries , such as uranium monocarbide , uranium sesquicarbide ,...
or uranium oxycarbide are also possibilities. Uranium oxycarbide combines uranium carbide with the uranium dioxide to reduce the oxygen stoichiometry. Less oxygen may lower the internal pressure in the TRISO particles caused by the formation of carbon monoxide, due to the oxidization of the porous carbon layer in the particle. The TRISO particles are either dispersed in a pebble for the pebble bed design or molded into compacts/rods that are then inserted into the hexagonal graphite blocks. The QUADRISO fuel concept conceived at Argonne National Laboratory
Argonne National Laboratory
Argonne National Laboratory is the first science and engineering research national laboratory in the United States, receiving this designation on July 1, 1946. It is the largest national laboratory by size and scope in the Midwest...
has been used to better manage the excess of reactivity.
Helium
Helium has been the coolant used in most HTGRs to date, and the peak temperature and power depend on the reactor design. Helium is an inert gasInert gas
An inert gas is a non-reactive gas used during chemical synthesis, chemical analysis, or preservation of reactive materials. Inert gases are selected for specific settings for which they are functionally inert since the cost of the gas and the cost of purifying the gas are usually a consideration...
, so it will generally not chemically react with any material. Additionally, exposing helium to neutron radiation does not make it radioactive, unlike most other possible coolants.
Molten salt
The molten salt cooled variant, the LS-VHTR, similar to the advanced high temperature reactor (AHTR) design, uses a liquid fluoride salt for cooling in a pebble core. It shares many features with a standard VHTR design, but uses molten salt as a coolantCoolant
A 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...
instead of helium. The pebble fuel floats in the salt, and thus pebbles are injected into the coolant flow to be carried to the bottom of the pebble bed, and are removed from the top of the bed for recirculation. The LS-VHTR has many attractive features, including: the ability to work at high temperatures (the boiling point of most molten salts being considered are >1,400°C), low pressure operation, high power density, better electric conversion efficiency than a helium-cooled VHTR operating at similar conditions, passive safety
Passive nuclear safety
Passive nuclear safety is a safety feature of a nuclear reactor that does not require operator actions or electronic feedback in order to shut down safely in the event of a particular type of emergency...
systems, and better retention of fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...
s in case an accident
Nuclear and radiation accidents
A nuclear and radiation accident is defined by the International Atomic Energy Agency as "an event that has led to significant consequences to people, the environment or the facility...
occurred.
Control
In the prismatic designs, control rodControl 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...
s would be inserted in holes cut in the graphite blocks that make up the core. The VHTR will be controlled like current PBMR designs if it utilizes a pebble bed core, the control rods will be inserted in the surrounding graphite reflector
Neutron reflector
A 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, steel, and tungsten carbide, or other materials...
. Control can also be attained by adding pebbles containing neutron absorbers.
Materials challenges
The high temperature, high neutronNeutron
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...
dose, and, if using a molten salt coolant, the corrosive
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
environment, of the VHTR require materials that exceed the limitations of current nuclear reactors. In a study of Generation IV reactors in general (of which there are numerous designs, including the VHTR), Murty and Charit suggest that materials that have high dimensional stability, either with or without stress, maintain their tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
, ductility
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
, creep
Creep (deformation)
In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
resistance, etc. after aging, and are corrosion resistant are primary candidates for use in VHTRs. Some materials suggested include nickel-base superalloys, silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
, specific grades of graphite, high chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
steels, and refractory alloys
Refractory metals
Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group differs...
. Further research is being conducted at US national laboratories as to which specific issues must be addressed in the Generation IV VHTR prior to construction.
Safety features and other benefits
The design takes advantage of the inherent safety characteristics of a helium-cooled, graphite-moderated core with specific design optimizations. The graphite has large thermal inertiaVolumetric heat capacity
Volumetric heat capacity , also termed volume-specific heat capacity, describes the ability of a given volume of a substance to store internal energy while undergoing a given temperature change, but without undergoing a phase change...
and the helium coolant is single phase, inert, and has no reactivity effects. The core is composed of graphite, has a high heat capacity and structural stability even at high temperatures. The fuel is coated uranium-oxycarbide which permits high burn-up (approaching 200 GWd/t) and retains fission products. The high average core-exit temperature of the VHTR (1,000°C) permits emissions-free production of process heat.
See also
- CAREMCAREMCAREM is an Argentine nuclear reactor designed by CNEA .- History :The reactor was first developed as a request of the Argentine Navy to power the TR-1700 submarine of German design. In 1984 it was presented publicly for the first time during an IAEA conference in Peru...
- TINTETime-dependent Neutronics and TemperaturesTIme-dependent Neutronics and TEmperatures is a two-group diffusion code for the study of nuclear and thermal behavior of high temperature reactors. It was developed by Forschungszentrum Jülich in Germany, formally known as KFA , to investigate HTGRs in 2D geometry....
- Generation IV reactorGeneration IV reactorGeneration IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...
- Pebble bed reactorPebble bed reactorThe pebble bed reactor is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of very high temperature reactor , one of the six classes of nuclear reactors in the Generation IV initiative...
- HTTRHTTRThe high temperature test reactor is a graphite-moderated gas-cooled research reactor in Oarai, Ibaraki, Japan operated by the Japan Atomic Energy Agency. It uses long hexagonal fuel assemblies, unlike the competing pebble bed reactor designs....
- List of nuclear reactors
- Next Generation Nuclear PlantNext Generation Nuclear PlantA Next Generation Nuclear Plant is a generation IV version of the Very High Temperature Reactor that could be coupled to a neighboring hydrogen production facility. It could also produce electricity and supply process heat...
- Nuclear physicsNuclear physicsNuclear 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 generation and nuclear weapons technology, but the research has provided application in many fields, including those...
- Nuclear reactor physicsNuclear reactor physicsNuclear 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....
- UHTREXUHTREXThe Ultra High Temperature Reactor Experiment was an experimental gas cooled nuclear reactor experiment starting in 1959 and lasting about 12 years UHTREX was located at Los Alamos Scientific Laboratory. The reactor first achieved full power in 1969...