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Encyclopedia > Very high temperature reactor
Very high temperature reactor scheme.

The Very High Temperature Reactor is a Generation IV reactor concept that uses a graphite-moderated nuclear reactor with a once-through uranium fuel cycle. This reactor design envisions an outlet temperature of 1,000°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 thermo-chemical sulfur-iodine cycle. Image File history File links Vhtr. ... Image File history File links Vhtr. ... Generation IV reactors (Gen IV) are a set of theoretical nuclear reactor designs currently being researched. ... Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ... This does not cite any references or sources. ... Core of a small nuclear reactor used for research. ... General Name, Symbol, Number uranium, U, 92 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Standard atomic weight 238. ... In a gaseous fission reactor the reaction takes place in a core which is bounded and created by magnetic field. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... The sulfur-iodine cycle is a series of thermochemical processes used to produce hydrogen. ...

Contents

Nuclear reactor design

Neutron moderator

Some United States and Russian designs refer to a prismatic block core configuration, where hexagonal graphite blocks are stacked to fit in a circular pressure vessel. Pebble bed designs are also being studied and have been used at lower temperatures than those envisioned for the VHTR. Pebble bed designs usually have a core where the pebbles are in an annulus, and there is a graphite center spire. Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Steel Pressure Vessel A pressure vessel is a closed, rigid container designed to hold gases or liquids at a pressure different from the ambient pressure. ... An annulus In mathematics, an annulus (the Latin word for little ring, with plural annuli) is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. ... A modern spire on the Lancaster University Chaplaincy Centre A spire is a tapering conical or pyramidal structure on the top of a building, particularly a church tower. ...


Nuclear fuel

The fuel is usually referenced to be uranium dioxide in a TRISO configuration, however, uranium carbide or uranium oxycarbide are also possibilities. 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. UO2 A black, radioactive, crystalline powder, once used in the late 1800s to mid-1900s in ceramic glazes. ... Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. ... Uranium carbide, a carbide of uranium, is a hard refractive ceramic material. ...


Coolant

Helium

This helium cooled reactor type is the dominant one being studied; its primary design uses a 600-MW thermal core with a helium outlet temperature of 1,000°C. Helium has been used in most high temperature gas reactors (HTGR) to date. Helium is an inert gas, so it will not react with any materials except through its stored heat. General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Standard atomic weight 4. ... An inert gas is any gas that is not reactive under normal circumstances. ...


Molten salt

The molten salt cooled variant, the LS-VHTR, previously called the advanced high temperature reactor (AHTR), uses a molten salt for cooling in a prismatic core. It is essentially a standard VHTR design that uses molten salt as a coolant instead of helium. The molten salt would pass through holes drilled in the graphite blocks. The LS-VHTR has many attractive features, including: the ability to work at very high temperatures (the boiling point of most molten salts being considered are >1,400°C), low pressure cooling that can be used to more easily match hydrogen production facility conditions (most thermo-chemical cycles require temperatures in excess of 750°C), better electric conversion efficiency than a helium-cooled VHTR operating at similar conditions, passive safety systems, and better retention of fission products in case an accident occurred. Because it is relatively untested, this proposed version requires somewhat more research. For other uses, see Salt (disambiguation). ... A coolant, or heat transfer fluid, is a fluid which flows through a device in order to prevent its overheating, transferring the heat produced by the device to other devices that utilize or dissipate it. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Passive nuclear safety describes a safety feature of a nuclear reactor that does not require operator action or electronic feedback in order to shut down safely in the event of a particular type of emergency (usually overheating resulting from a loss of coolant or loss of coolant flow). ... Fission products are the residues of fission processes. ... This article covers notable accidents involving nuclear devices and radioactive materials. ...


Control

In the prismatic designs, control rods 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. A control rod is a rod made of a chemical element capable of absorbing many neutrons without decaying themselves. ... The Pebble Bed Reactor is an advanced nuclear reactor design. ... Nuclear weapon designs are often divided into two classes, based on the dominant source of the nuclear weapons energy. ... Isotopes of certain elements absorb free neutrons creating higher isotopes of the same element. ...


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 inertia 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. Volumetric heat capacity (VHC) describes the ability of a given volume of a substance to store heat while undergoing a given temperature change, but without undergoing a phase change. ...


References

See also

Generation IV reactors (Gen IV) are a set of theoretical nuclear reactor designs currently being researched. ... Graphite Pebble for Reactor The pebble bed reactor (PBR) or pebble bed modular reactor (PBMR) is an advanced nuclear reactor design. ...

External links

  • IAEA HTGR Knowledge Base
  • ORNL NGNP page
  • INL Thermal-Hydraulic Analyses of the LS-VHTR

  Results from FactBites:
 
GEN-4 : Technology : Systems (335 words)
The VHTR is designed to be a high-efficiency system, which can supply electricity and process heat to a broad spectrum of high-temperature and energy-intensive processes.
The reference reactor is a 600 MWth core connected to an intermediate heat exchanger to deliver process heat.
Fuel particles are coated with successive material layers, high temperature resistant, then formed either into fuel compacts embedded in graphite block for the prismatic block-type core reactor, or formed into graphite coated pebbles.
Generation IV reactor - Wikipedia, the free encyclopedia (1144 words)
The reactor is helium-cooled, with an outlet temperature of 850°C and using a direct Brayton cycle gas turbine for high thermal efficiency.
Several fuel forms are being considered for their potential to operate at very high temperatures and to ensure an excellent retention of fission products: composite ceramic fuel, advanced fuel particles, or ceramic clad elements of actinide compounds.
This reactor concept is cooled by liquid sodium and fueled by a metallic alloy of uranium and plutonium.
  More results at FactBites »

 
 

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