| I. Materials > The Uranium-Plutonium Chain > Management of Irradiated Fuel |
I. THE PRESENT SITUATION
I.A. Storage/disposal of irradiated fuel
The present storage sites are: pools for unloaded fuel that equip almost every power plant, pools at La Hague, pools at research centers (Saclay and Cadarache), pools in the military ports (Cherbourg, L’Ile Longue, and Toulon), two concrete masses at Saclay, and Cascad (a system of dry storage, in shafts, at Cadarache).
The total capacity of the storage sites at EDF’s reactors is 4000 t. The free capacity is maintained at about 1000 tons. “In the event of a sudden halt to the dispatch of irradiated fuel to La Hague, for example as a result of a blocking of shipments, storage capacities would be saturated in a year" [Bataille 98].
The main wastes created by the storage of irradiated fuel are resins and cartridges, used to purify water in the pools. A loss of cooling water or a cutoff of electricity, before the fuel in the pool is well cooled, could cause a catastrophic escape of fission products and transuranics.
I.B. Reprocessing of irradiated fuel
I.B.1 THE WET PROCEDURE
French plants like other industrial reprocessing installations use the Purex process (Plutonium and Uranium Recovery by Extraction). The main steps in reprocessing are the following: the cladding is removed by chemical or mechanical procedures or by cutting up and dissolving. For this last procedure, used at the present time at La Hague, the fuel assemblies are chopped up and soaked in a bath of nitric acid. The pieces of metal from the cladding that are not dissolved are removed. The result is an acid solution that contains a mixture of plutonium, uranium, transuranics, and fission products. The plutonium and the uranium are extracted together from that solution by using an organic solvent, usually tributyl-phosphate (TBP) mixed with a diluent, dodecane. The result is an organic solution containing the majority of the plutonium and uranium. Fission products and other transuranics remain in the aqueous solution.
The plutonium and uranium are separated from one another by reduction of the plutonium
The plutonium in the form of plutonium nitrate is purified by extraction and re-extraction liquid-liquid and concentrated by evaporation. It is then precipitated in a solution in the form of oxalate. The oxalate can be changed into metal or into plutonium oxide. For the metal, the plutonium oxalate is calcined, and fluorination is carried out by means of gaseous fluorohydric acid. Then, the fluoride is reduced by using calcium (calciothermy). For the oxide, the oxalate is filtered, spun-dry, dried out, and then calcined.
The uranium in the form of uranyl nitrate is purified by liquid-liquid extraction in several stages to eliminate the fission products, and the solution is concentrated after each stage.
The first reprocessing operation in France was carried out at Le Bouchet. Fontenay-aux-Roses then became the center of research in this area. Today, the CEA has transferred research to the new installation Atalante at Marcoule. Pilots have operated at Fontenay, at Marcoule, and at La Hague. Industrial installations are in service at La Hague and operated until late 1997 at Marcoule.
The wastes from reprocessing are described in the chapter on La Hague.
I.B.2. THE DRY PROCESS
Between 1957 and 1981, the CEA and Péchiney Ugine Kuhlmann (PUK) carried out studies on an alternative procedure, by the dry method, in particular on highly enriched uranium alloys (UAl, UZr) and on fuel for fast neutron reactors. The process included the transformation of uranium into uranium hexafluoride.
Experimentation took place at Fontenay-aux-Roses, in particular in the cell Attila, and in the Kuhlmann installations at Brignoud (Isère). The Castaing commission noted that, as compared with the Purex process, the dry process “presents certain advantages,” such as a smaller volume of waste and the absence of liquid wastes. However, France abandoned this process before even finishing R&D studies. Today, the studies on the dry method have been relaunched as part of the separation-transmutation initiative (See waste disposal).
|