A category of uranium that the industry minister does not mention is reprocessed uranium (URT), the uranium that is a product of the reprocessing of irradiated fuel.

The uranium leaves the reprocessing plant in the form of uranyl nitrate. The uranyl nitrate is ordinarily converted into other compounds for storage or for reuse. It can be transformed into UF6 and reenriched.

In general, Repu can be used like natural uranium. It serves as fuel for natural uranium gas graphite reactors and light water reactors. Nevertheless, the composition of Repu poses problems that are quite limiting, in particular in regard to the environment and health.

During its irradiation in a reactor, uranium is profoundly modified. The uranium that leaves the reprocessing plant contains all the isotopes of uranium between uranium 232 and uranium 238 except uranium 237, which is rapidly transformed into neptunium 237. If the Repu comes from a PWR, the proportion of uranium 235 has decreased to about 1%; the proportion of uranium 234 has increased to about 0.01% [Roux 88]. Repu coming from UNGG military production reactors is slightly depleted in uranium 235.

The most important isotopes from the point of view of contamination are:

--Uranium 232: an alpha emitter (half life 72 years), which rapidly produces descendents that emit alpha, beta, and gamma, notably bismuth 212 and thallium 208, emitters of hard and intense gamma radiation.

--Uranium 234: a gamma and alpha emitter with specific activity greater than uranium 235 and uranium 238. The uranium 234 absorbs neutrons and therefore changes the reactivity of uranium;

--Uranium 236: likewise an absorber of neutrons;

--Descendents of uranium 232: bismuth 212, thallium 208. If the descendents are not eliminated, they reach an equilibrium and their maximum after the Repu has been stored about ten years;

--Transuranians: neptunium 237, americium 241 and the various isotopes of plutonium;

--Fission products: cesium 134 and 137, niobium 95, ruthenium 103 and 106, technetium 99, and zirconium 95.

The proportion of uranium 232 and of other impurities in the uranium that leaves the reprocessing plant varies with the original composition of the fuel, the level of irradiation, and the length of cooling before reprocessing.

The impurities make the use of Repu more difficult than the use of natural uranium all along the uranium chain. Some stages of the chain bring about a partial purification of the Repu, but radionuclides never disappear, except by natural decay. If an impurity leaves a uranium compound, it can be found in the wastes. Moreover, elimination of decay products is only temporary if the uranium 232 remains. Because of continuing decay, bismuth and thallium always reappear.

--Uranyl nitrate, the product of Purex reprocessing plants, has a free acidity below 0.5 N and a concentration of between 200 and 400 grams of uranium per liter. It contains all the impurities listed above.

The uranyl nitrate that leaves a reprocessing plant is difficult to store because of its power of corrosion; and its reuse after storage necessitates delicate operations of redissolving with steam [Roux 88]. Therefore, it is ordinarily transformed rapidly. The compound chosen for storage today in France is U308. For fuel, uranyl nitrate is transformed into UO2 or, in order to enrich it, into uranium tetrafluoride (UF4) and thence into uranium hexafluoride (UF6);

--The oxides made from the uranyl nitrate, including U308. They are identical to uranyl nitrate in radiological and chemical contaminants, except for ruthenium 106, the level of which has decreased [Durand 91]. Thus, though U308 is more stable than uranyl nitrate, it is highly irradiating.

--Uranium tetrafluoride. UF4 retains 98% of the products of fission or of decay that are found in uranyl nitrate. Moreover, it has a hygroscopic character, which makes its storage for the purpose of later reuse a delicate matter [Roux 88]