| I. Materials > The Uranium-Plutonium Chain > Fabrication of Uranium-based Fuel |
Fuel for natural uranium graphite gas reactors
Natural uranium graphite gas reactors, all shut down, used fuel made of uranium metal. This fuel was in the form of cartridges composed of hollow cylinders of natural uranium clad in magnesium-zirconium alloy. A sleeve of graphite often supported the cartridges, and in certain versions, a graphite bar filled the otherwise empty interior.
Manufacture consisted of the following operations: melting and molding metal, thermal treatment by heating and cooling, machining, cleaning, and cladding.
Sicn at its Annecy and Veurey-Voroize sites, and, until about 1970, Cerca at Romans, manufactured the UNGG fuel.
Fuel for light water and heavy water reactors
Light water and heavy water reactors use uranium oxide fuel. For light water reactors, the uranium is enriched to about 3.5 percent; for heavy water reactors, natural uranium or very low-enriched uranium is generally used. Cylindrical fuel rods in zircaloy, containing uranium oxide pastilles, are grouped into assemblies.
The core of a 925 MW pressurized water reactor (PWR) contains 157 assemblies corresponding to 72.5 t of uranium divided among 41,500 rods.
The fabrication of uranium oxide is carried out by the dry method or the wet method, the dry method being generally preferred.
The stages of the fabrication process using conversion by the wet method include:
-- the vaporization of the UF6 by heating in a furnace;
-- conversion of the UF6 to UO2 in the presence of overheated water vapor, by the hydrolysis of UF6, followed by reduction of the compound UO2F2 by hydrogen. The UO2 is then in the form of a powder;
-- fabrication of the pastilles, by homogenization of the powder, the addition of a lubricant, transfer of the powder into a matrix for compression and compacting. The pastilles are heated in a furnace, then monitored;
-- fabrication of the assemblies, by introduction of the pastilles into tubes in zircaloy (an aluminum-zirconium alloy) or in stainless steel; the fabrication of structural pieces; the creation of the skeleton by the assembly of guide tubes with grills; and the introduction of the fuel rods into the skeleton.
The wet procedure has the inconvenience of producing a greater quantity of effluents than the dry method. Because it allows one "to start from almost any form and obtain almost anything that one wants," it is often used for the recovery of fissile material from rejects and wastes [Lamy 93a].
In France, uranium oxide fuel is today produced by Franco-Belge de fabrication de combustible (FBFC), a subsidiary of Framatome and Cogéma, in its factory at Romans. It uses the dry method. A subsidiary of FBFC, FBFC Internationale, operates a factory at Dessel in Belgium, which uses a part of the uranium oxide powder created in the other FBFC factory.
Sicn at Veurey-Voroize, CICE/Cicaf at Corbeville and Sfec at Bollène (the last two plants have been dismantled), carried out stages of the production of uranium oxide fuel for PWR and heavy water reactors.
The procedure for the fabrication of Repu fuel is the same as that for the fabrication of standard fuel, except that supplementary protection is necessary. This fuel is manufactured in a special line in the FBFC factory at Romans.
Fuel for research reactors and the Célestin
Uranium-aluminum and silicide fuel
The majority of the research reactors in France are of the pool type and use fuel composed of thin, clad, metallic plaques, containing enriched uranium.
Generally the plaque contains core composed of fissile material dispersed in the form of grains in an aluminum matrix [RGN xi-xii.85]. The cladding is generally composed of an aluminum alloy. The reactor core is made up of several hundred fuel plaques grouped into fuel elements, slightly separated from one another.
Until 1978, the fissile material was generally an alloy of uranium and aluminum of the type UAl or UAlx (a mixture of the compounds UAl3 and UAl4).
In 1978 the US Department of Energy (DOE), in cooperation with other entities, established an international RERTR program (Reduced Enrichment Research and Test Reactor). Its purpose was the development of new fuels with a low level of enrichment to avoid the proliferation of highly enriched uranium. In order that a reactor can attain the same level of performance with low-enriched uranium as with high-enriched uranium, the quantity of uranium 235 in each plaque must remain identical. It is therefore necessary to dilute the uranium 235 in more uranium 238 and to realize this dilution, to increase the density of the uranium.
At present the standard RERTR fuel is a compound of uranium, a silicide, U3Si2, with a uranium density of 4.8 g/cm3 and therefore an enrichment of less than 20% uranium 235. Use of this fuel has been authorized by the US Nuclear Regulatory Commission [Travelli 94].
Researchers are now developing fuels with 8 or more grams of uranium per cubic centimeter. Until late 2003 the most promising materials appeared to be alloys of uranium and molybdenum (U-Mo). The target date for having a new U-Mo fuel qualified and ready to use was 2006. However, swelling in U-Mo test fuels under irradiation has caused the target date for qualification to be pushed back to the end of 2010 at best. Some researchers are questioning whether U-Mo will ever replace silicide fuel. The French "MTR new fuels working group" is composed of the CEA (neutronics), Cerca (manufacturing aspects), and Cogéma (reprocessability) [NucF 19.iv.99, 29.iii.04; Krull 98].
The process of fabricating UAlx-Al and U3Si2-Al fuel consists of two main stages:
--creation of the core by the metallurgy of powders: fusion of the uranium and aluminum, breaking up and grinding, addition of an aluminum powder of the A5 type to form a matrix, and compaction;
--colaminage of the core in a box of aluminum alloy, first by fabrication of a "sandwich," then of a plaque by lamination [RGN ix-x.89; RGN xi-xii.85].
Cerca is the only French manufacture of the UAlx-Al and U3Si2-Al fuel for research reactors. It also manufactured the UAl-Al fuel for the Célestin reactors at Marcoule. We do not know whether fuel for the Célestin is still manufactured nor a fortiori where.
--updated 25.iv.04
Caramel Fuel
In the seventies, the CEA, in collaboration with its subsidiaries Sfec and Technicatome, developed a fuel with a low level of enrichment, Caramel. The CEA used complete cores of Caramel in the Isis and Osiris reactors. It recently abandoned use of Caramel in Osiris to the benefit of silicide fuel. Caramel fuel was and, it seems, may still be, used in submarines (see below).
Caramel fuel is composed of flat pastilles of uranium oxide enriched to about 7.5%. Each pastille has almost the shape and size of a true caramel, hence the name.
A layer of pastilles is constructed inside a thin plaque, and each plaque clad individually in a zircaloy alloy. A pile of these plaques is itself clad. The whole is subjected to a thermal treatment under high pressure, which has the effect of creating a tight metallic cell around each pastille.
The CEA equipped a workshop of Sfec at Bollène and workshops at Cadarache and at Saclay to fabricate Caramel fuel from UF6. The Bollène workshop no longer exists. A portion at least of its activities relating to Caramel have been transferred to Veurey-Voroize. The Saclay workshop, if it is the Atelier de réalisation d’assemblages combustibles (Arac), and Veurey-Voroize no longer fabricate Caramel [Con viii.95]. We do not know whether the fuel is still fabricated at Cadarache.
Fuel for Naval Propulsion
We have very little information about fuel for military ships, whose propulsion reactors are all of the PWR type; but it seems that the navy used various fuels, with low enriched and high enriched uranium.
The first three missile-launching submarines (SNLE), Le Redoutable (1969-1991), Le Terrible (since 1971), and Le Foudroyant (since 1973), were furnished with fuel in the form of zircaloy tubes containing uranium oxide pastilles. It seems probable that the uranium was low enriched, perhaps between 3.5 and 6 percent [see LeMo 4.vi.74; TechIn B 3140]. On the other hand, Le Tonnant (since 1978), the fifth SNLE, and probably L’Indomptable (since 1976), the fourth, used a metallic fuel "with highly enriched uranium and a very long life" [Arme xii.77; LeMo 19.ix.79].
The nuclear attack submarines (SNA), the first of which Le Rubis, went critical in 1981, used Caramel fuel, at least for a time [LeMo 10.vii.79]. We do not know if the SNA continue to use this fuel.
In 1996 Technicatome was developing a fuel for submarines that contains uranium enriched to the same level as the uranium fuel used in power reactors, or about 3.25% uranium 235. A brochure of Technicatome that was published that year states, "In 1995 we perfected new manufacturing processes using uranium with commercial enrichment and made prototype fuel elements that will be loaded into the RNG reactor in 1996." The year 1996 was that of the closing of the P:ierrelatte plant, which until then produced highly enriched uranium for the military.
In 1999, according to Technicatome, the CEA was trying to develop submarine fuel that would have the same enrichment as the PWR fuel and that would be used by submarines in the future Barracuda class of SNA and their future parallel on land, the Réacteur d’essais à terre (RES). The CEA would like to design fuel with the same enrichment as commercial PWR fuel [RGN iii-iv.99]. In a press release of December 22, 2006, the defense ministry announced that the Barracuda will, in fact, use nuclear fuel "with a civilian enrichment, i.e., identical to that of EDF's power plants."
However, it is difficult to know with certainty whether the fuel with commercial enrichment is already used in the present class of nuclear attack submarines, in the SNLE (nuclear, missile-launching submarines) of the new generation (Le Triomphant and Le Téméraire, which entered into service in 1997 and in 1999 respectively, or in the airplane carrier Charles-de-Gaulle, also of the new generation. "An official document notes that for LeTriomphant "the fuel furnished by the CEA in the form of low-enriched uranium powder, was transformed into bars with squared sections in the Technicatome workshops" [InfoDG x.93]. The words "lightly enriched" could, however, refer to the 7.5% enrichment of Caramel fuel rather than to the 3.25% to 4% of commercial fuel. A description of fuel posted in 2004 on the CEA's web site adjacent to an account of SNAs, new generation SNLE, and the Charles-de-Gaulle appears to refer to Caramel fuel but does not state the enrichment of the uranium.
The CEA itself is responsible for the development and the fabrication of the fuel for naval propulsion. Prior to 1974, the industrial contractor for nuclear propulsion was its own department of nuclear propulsion. Since, this role has been filled by its subsidiary Technicatome [ColB 15.xii.90]. In 2002 the workshops of Technicatome at Cadarache fabricate the naval fuel. The Sfec factory at Bollène and a workshop at Saclay, probably l’Arac (above) have helped to manufacture military Caramel fuel. It is possible that the Sicn factory at Veurey-Voroize likewise had a military role.
--revised January 6, 2007
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