Fuel Element Manufacture

Fuel elements consist of nuclear fuel in cladding tubes 

Fuel elements consist of a bundle of fuel rods 

The nuclear fuel pellets are generally filled in thin-walled cladding tubes to hinder leaching by coolant in the reactor core and to prevent release of fission products into the coolant circuit. In light-water reactors, for example, zirconium alloy (zirkaloy) cladding is used. 

The fuel rods for boiling and pressurized water reactors are constructed similarly. They are filled with helium to improve the heat transfer from the pellets to the cladding tube and to withstand better the pressure in the reactor and contain no fuel at the top end of the fuel rods to improve fission gas retention. The latter can be ensured by holding the fuel in place with the aid of a spiral spring. Both ends of the cladding tube are welded gas tight. The fuel rods for pressurized water reactors are manufactured with a helium pressure of ca. 23 bar and ca. 5 bar for fuel rods for boiling water reactors. 

The actual fuel elements in pressurized water reactors consist of individual fuel rods and control rod tubes mounted in a self-supporting construction of spacers fitted with a top and feet. Fuel elements for boiling water reactors, by comparison, have no control rod tubes, the fuel element zirkaloy claddings being used to guide the control rods and the coolant. 

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The UF4 may be treated, as above, with magnesium or calcium to yield enriched uranium metal or reacted with water and a hydroxide salt to give UO3, from which enriched UO2 suitable for fuel element manufacture may be obtained by reaction with hydrogen. 

Several types of nuclear reactors have already been mentioned in the previous section with respect to the use of nuclear fuel and the manufacture of fuel elements. The various types of nuclear reactors are distinguished on the basis of the following aspects ... 

Magnox Reactor waste streams include a wide range of materials such as ion exchange (IX) resins, sludge, Magnox fuel element debris (FED), reactor graphite and carbon and stainless steels. Some wastes will exhibit heterogeneity and for many construction materials such as steels there will be radionuclides present which are neutron activation products of trace impurities and were un-quantified at the time of manufacture. 

These particles are embedded in a graphite matrix, the fuel elements either being manufactured as balls (6 cm in the diameter) or hexagonal blocks (key width 360 mm). 

Plutonium is usually precipitated in reprocessing plants as its oxalate, which is converted into plutonium(lV) oxide from which mixed oxide fuel elements for light-water reactors or fast breeder reactors can be manufactured. Considerable knowledge over the manufacture of mixed oxide fuel elements has been built up over the years in the USA, France, Great Britain, Japan, Belgium and the Federal Republic of Germany. Thus up to the end of 1993, just in the Federal Republic of Germany, 4.5t of fissile... 

Mehner, A. W. et al. Spherical Fuel Elements for Advanced HTR Manufacture and Qualification by Irradiation Testing. J. Nucl. Mater. 171, 9-18 (1990),... 

Complex FCC oxides of the fluorite type represent oxygen-conduction solid electrolytes (SOE s). They comprise a typical class of materials for the manufacture of sensors of oxygen activity in complex gas mixtures, oxygen pumps, electrolyzers and high-temperature fuel elements. These materials are based on doped oxides of cerium and thorium, zirconium and hafnium, and bismuth oxide. Materials based on zirconium oxide, for example, yttrium stabilized zirconia (YSZ) are the most known and studied among them. This fact is explained both by their processibility and a wide spectrum of practical applications and by the possibility to conduct studies on single crystals, which have the commercial name "fianites" and are used in jewelry. 

Heavy-metal manufacturing contamination of fuel element coolant hole surfaces. 

The direct Sr-90 release is negligibly small, as shown in Table 4.2-20. The dominant contribution to the strontium release is from the release and subsequent decay of its Kr-90 precursor, which accounts for 88 percent of total release the remaining contribution is from heavy-metal manufacturing contamination of the fuel element coolant hole surfaces (a minimum fractional release of 5 x 10 is assumed for all fission products to account for possible heavy metal contamination on the fuel element coolant hole surfaces). The predicted 40-year Sr-90 plateout inventory of 0.20 Ci is below the "Maximum Expected" criterion of 0.34 Ci. 

Currently, design and manufacture work is under way on independent loop for in-pile tests of the fuel elements intended for BREST-300 lead cooled reactor. 

Several experimental nuclear reactors have been put into service in Argentina since the early 1960s. All of them use aluminium clad fuel, most of the MTR type. RA3, located at the Ezeiza Atomic Centre (near Buenos Aires) is the most powerful of these experimental reactors. It started up burning 90% enriched uranium, and the fuel plates were made of pure (99.7%) aluminium It was converted to use 20% enriched uranium at the end of the 1980s, and at that time the fuel plates started to be manufactured with 6061 alloy. Some of the earliest irradiated fuel elements were inserted into RA6 in Bariloche (some 1700 km south-west of Buenos Aires), a zero power reactor, where they have been in service for almost 20 years. 

Alloys 1060, 6061 and 6262 are presently being used in IPEN for the manufacture of FAs for the lEA-Rl reactor. The compositions of the alloys are given in Table 6.4. Coupons of the three alloys, 1060 in the processed and scratched condition (to simulate the effect of scratches formed during handling of fuel elements in the reactor coolant), and various combinations of bimetallic couples were mounted in the rack (Fig. 6.2). Four coupons under each set of conditions were exposed. Coupon preparation and pretreatment were as mentioned earlier. This rack was also introduced into the lEA-Rl reactor, close... 

In-pile loop tests of this type of element were successfully carried out for 5700 hr at a maximum fuel surface temperature of 1500° to 1700°F and for appropriate burnup conditions (45). A full core loading of these fuel elements was manufactured for the EBOR reactor. 

The fuel cycle starts with the mining of the uranium. It continues with its chemical isolation, possibly an isotopic enrichment of (see O Chap. 51 in Vol. 5 on Isotope Separation ), the manufacture of the fuel elements, and their use in the reactor. If a final storage of spent fuel elements as such, as practiced in some countries, is not preferred, the cycle continues with a dissolution of the fuel elements. The remaining uranium and the newly formed plutonium are separated fi om the fission products. Plutonium can be reintroduced into the reactor in the form of Mixed OXide (MOX) fuel elements. Uranium has to pass through an enrichment plant in order to increase the content of from about 0.8% in the spent fuel to about 3%. The enriched fraction will be returned to new fuel elements. 

Finally, plutonium in the form of PUO2 together with UO2 will be used for the manufacture of new MOX-fuel elements. Uranium with a remaining content of about 0.8% in will return to the enrichment plant. The fission products will go to final storage as described in the next section. 

Fuel element development A comprehensive program, aim at resolving every detailed design and manufacturing aspect of the fuel element is going on. 

The small power size of one HTR-Module unit (200 MWth) and the low power density leads principally to high specific investment costs compared to bigger reactors. However, this effect is compensated to a large degree by technical simplifications, (e.g. by omission of a full pressure containment and active safety systems). Additionally the high proof and control effort for nuclear components can be reduced drastically due to the fact that most components of the HTR-Module (e.g. water steam cycle, He-blower) are not safety relevant. The proof effort could be restricted mainly to the fuel elements. A further cost reduction can be reached by manufacturing in larger quantities of modular units. 

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