Nuclear Fuel, Uranium Enrichment, Fuel Fabrication, MOX

Seminar on Nuclear Science and Technology

for Diplomats

P. Adelfang

(+)Division of Nuclear Fuel Cycle and Waste Technology (NEFW)Department of Nuclear Energy (NE)

IAEA, Vienna, February 6-8, 2007

Ends of the Nuclear Fuel Cycle

The Reactor: Core of the Nuclear Fuel Cycle

The Nuclear Fuel Cycle

Uranium Mining and Milling

Yellow Cake

Final Product of Milling Step – 70 to 80 % uranium

Conversion, Enrichment and Fuel Fabrication

Natural Uranium

235U only fissile nuclide – only 1 atom of 235U in 140 atoms of 238U

Enrichment

• A number of enrichment processes have been demonstrated in the laboratory

• Only two, the gaseous diffusion process and the centrifuge process, are operating on a commercial scale

• In both of these, UF6 gas is used as the feed material• Molecules of UF6 with U-235 atoms are about one

percent lighter than the rest, and this difference in mass is the basis of both processes

• Large commercial enrichment plants are in operation in France, Germany, Netherlands, UK, USA, and Russia, with smaller plants elsewhere

Enrichment: UF6 Feed Container

Enrichment: Centrifuge Process • vacuum tubes, each containing a

rotor one to two metres long and 15-20 cm diameter.

• rotors are spun rapidly, at 50,000 to 70,000 rpm

• heavier molecules with U-238 increase in concentration towards the cylinder's outer wall

• there is a corresponding increase in concentration of U-235 molecules near the centre.

• enriched gas forms part of the feed for the next stages, depleted UF6 gas goes back to the previous stage (cascade)

• very high speeds:, outer wall spinning cylinder 400 and 500 metres per second= 1 million times the acceleration of gravity

Enrichment: Centrifuge Process

Enrichment: Gaseous Diffusion Process

• involves forcing UF6 under pressure through a porous membranes

• as 235U molecules are lighter than the 238U molecules they move faster and have a slightly better chance of passing through the pores in the membrane

• the UF6 which diffuses through the membrane is thus slightly enriched, while the gas which did not pass through is depleted in 235U

• this process is repeated many times in a series of diffusion stages called a cascade

• enriched UF6 is withdrawn from one end of the cascade and depleted UF6 is removed at the other end

• the gas must be processed through some 1400 stages to obtain a product with a concentration of 3% to 4% 235U

Enrichment: Gaseous Diffusion Process

The large Tricastin enrichment plant in France (beyond cooling towers). The nuclear reactors in the foreground provide power for it.

Conversion, Enrichment and Fuel Fabrication

Conversion and Nuclear Fuel Fabrication

• UF6, in solid form in containers, is heated to gaseous form, and the UF6 gas is chemically processed to form LEU uranium dioxide (UO2) powder

• this powder is then pressed into pellets, sintered into ceramic form (fuel pellets)

• pellets are then loaded into Zircaloy tubes that are afterwards hermetically closed (fuel rods)

• rods are constructed into fuel assemblies• fuel assemblies are made with different dimensions

and number of fuel rods, depending on the type reactor

UO2, Pellets and Fuel Assembly

Fuel Rods

Fuel Assembly

MOX Fuel

MOX Fuel

MOX Fuel• Plutonium, made in power reactors and from dismantled nuclear

weapons, is a valuable energy source when integrated in the nuclear fuel cycle

• Over one third of the energy produced in most nuclear power plants comes from plutonium. It is created there as a by-product.

• 'MOX' is derived from 'mixed oxides', and refers to reactor fuel made from a mixture of plutonium and uranium oxide

• For use in a light water reactor, the proportion of plutonium is about 5%. This is a similar fissile content as low enriched uranium fuel

• MOX is formed into ceramic fuel pellets, extremely stable and durable, and which are sealed in metal (usually zirconium) tubes, which in turn are assembled into fuel elements

• In most cases a part of the reactor core can be loaded with MOX fuel elements without engineering or operational modifications to the reactor

• Plutonium is radiologically hazardous, particularly if inhaled, so must be handled with appropriate precautions

MOX Fuel: Glove Boxes

MOX Fuel: Glove Boxes

IAEA

…atoms for peace.

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