Uranium Fuel Processing

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

2.2.1 Uranium Fuel Processing

The process of reducing uranium ore to metal begins with the discovery and mining of uranium in

ore bodies. Most medium grade ore consists of oxides of uranium, of which carnotite

(K2(UO2)2(VO2.3H2O)) is predominant. Although some ore is mined using in situ leach techniques, most is

hard-rock mined with a small amount removed by open pit mining. Uranium ore is milled by crushing,

leaching, extracting, and precipitating, usually to ammonium diuranate ((NH4)2U2O7) commonly called

yellow cake. The radioactivity of this product is low because the decay products have been stripped away

and it is in an unenriched form. The yellow cake is purified and converted to UF4 and then further

fluorinated to uranium hexafluoride (UF6). Gaseous diffusion enrichment changes the uranium isotopic, but

not the chemical, composition of the gas. The UF6 is hydrolyzed to uranyl oxyfluoride, which is precipitated

with an ammonia solution to ammonium diuranate. This precipitate is filtered or centrifuged, dried, and

calcined. The uranium compound is reduced to UO2 powder, which is pelletized, sintered, and encapsulated

in tubes for reactor usage.

Laser enrichment can use feed forms including metal and UF6.

Steel was an early cladding material that was discontinued because of its thermal-neutron poison

characteristics. Fuel bundles used in commercial LWRs are now made of fuel pins that consist of pellets of

UO2. The pellets are stacked into free-standing cladding tubes of a zirconium or zirconium-tin alloy.

Differences in fuel design between the two common types of nuclear reactors in use in the United States,

pressurized water reactors (PWRs) and boiling water reactors (BWRs), are rod diameter and cladding

thickness.

Reactor fuel for the Canadian pressurized heavy water reactors (CANDU-PHWR) is similar but the

cladding need not be free-standing. Additionally, the fuel pins are smaller in diameter. Breeder reactors

like the Fast Flux Test Facility (FFTF) use a mixture of PuO2 and depleted UO2. In the case of the FFTF,

the pelle ts are loaded into stainless steel cladding tubes (which have a smaller effect on fast neutrons).

Uranium carbide (UC2) microspheres were developed as an alternative to UO2, primarily for the

high-temperature gas-cooled reactor. These fuel particles, developed for high thermal and radiation

stability, prevent the release of fuel and fission products over a wide range of conditions.

2.2.2 Uranium Metal

Conversion of UF6 to uranium metal involves, first, the production of UF4, commonly called green

salt. Enriched uranium green salt is reacted with granular calcium to produce metal slag. This product is

then reacted with magnesium or calcium to reduce the material to metal. Depleted uranium green salt is

more commonly reacted with magnesium to produce DU metal as a derby. In both cases, most of the

uranium decay products are concentrated in the calcium or magnesium slag, leaving the metal relatively

pure and with a reduced level of radioactivity. Buildup of decay products to near-equilibrium levels takes

about six months.

The metallic uranium is processed into desired forms using machining, melting, casting, and other

treatments. This very dense metal is usually alloyed with another metal for greater stability. Uranium is a

reactive metal that oxidizes easily. In the newly minted metal, a very thin surface layer tends to undergo

rapid oxidation. This surface layer may protect the rest of the metal from further corrosion, and prevent the

generation of removable contamination. Certain environmental conditions, particularly moist

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