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(Reuters) - The Generation IV International Forum (GIF) was created in 2000 to perform research on new types of nuclear reactors to replace the water-cooled models that make up the majority of today s global nuclear fleet.

The group, which consists mainly of public nuclear research agencies, selected the following six systems on which to focus its research efforts.

SODIUM-COOLED FAST REACTOR (SFR)

* Using superheated liquid sodium as a coolant, this is the most mature technology of the six.

* The metal s low melting point and high boiling point allow the reactor to operate at near-atmospheric pressure.

* Sodium explodes in water and catches fire in air, so research is focused on making the reactors leak-proof.

* Prototypes have been tested in France, Russia and Japan, with China, South Korea and India also proceeding with SFR projects.

LEAD-COOLED FAST REACTOR (LFR)

* Can operate at atmospheric pressure and high temperatures because of lead s high boiling point of 1,750 degrees Celsius.

* The lead must be kept hot at all times to prevent it from solidifying.

* Lead-bismuth reactors have operated successfully in Russian submarines, but conversion for commercial use requires more research.

* A European consortium is planning to build the Alfred LFR demonstration reactor in Romania.

GAS-COOLED FAST REACTOR (GFR)

* This will be cooled with helium, which is chemically inert and non-corrosive, but less efficient than liquids at evacuating heat from the core.

* France has been active in GFR development but dropped it in favor of the SFR in 2010.

* Japan and Switzerland have cut back GFR research, but a consortium of the Czech Republic, Hungary, Poland and Slovakia plans to build a small experimental GFR reactor.

SUPERCRITICAL-WATER-COOLED REACTOR (SCWR)

* Uses very hot water, above 374 degrees Celsius, as a coolant.

* As with existing boiling-water reactors, steam feeds directly into a steam turbine.

* Unlike some other fourth-generation systems, the SCWR can be developed step by step from current water-cooled reactors.

* Canada leads the way among countries working on this model.

VERY-HIGH-TEMPERATURE REACTOR (VHTR)

* Another evolutionary development of existing models, but with several potential designs, including the pebble-bed reactor.

* The pebble-bed reactor runs gas over spherical fuel elements, has relatively good safety features and the potential for deployment in oil refineries and other industries.

* Among the design challenges is the search for materials that can withstand the extreme temperatures.

MOLTEN-SALT REACTOR (MSR)

* Offers two variants: one in which nuclear fuel is dissolved in salt, the other with salt as a coolant.

* The United States conducted MSR research from 1950 to 1976.

* GIF rekindled interest in the concept, but relatively little research has taken place over the past decade.

(Reporting by Geert De Clercq; Editing by David Goodman)


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