Nuclear reactors fuel elements

Results of uranium weight determination in nuclear reactor fuel elements. 

Carbide-based cermets have particles of carbides of tungsten, chromium, and titanium. Tungsten carbide in a cobalt matrix is used in machine parts requiring very high hardness such as wire-drawing dies, valves, etc. Chromium carbide in a cobalt matrix has high corrosion and abrasion resistance it also has a coefficient of thermal expansion close to that of steel, so is well-suited for use in valves. Titanium carbide in either a nickel or a cobalt matrix is often used in high-temperature applications such as turbine parts. Cermets are also used as nuclear reactor fuel elements and control rods. Fuel elements can be uranium oxide particles in stainless steel ceramic, whereas boron carbide in stainless steel is used for control rods. 

The maximum heat flux achievable with nucleate boiling is known as the critical heat flux. In a system where the surface temperature is not self-limiting, such as a nuclear reactor fuel element, operation above the critical flux will result in a rapid increase in the surface temperature, and in the extreme situation the surface will melt. This phenomenon is known as burn-out . The heating media used for process plant are normally self-limiting for example, with steam the surface temperature can never exceed the saturation temperature. Care must be taken in the design of electrically heated vaporisers to ensure that the critical flux can never be exceeded. 

D. R. Odander, Fundamental Aspects of Nuclear Reactor Fuel Elements, National Technical Information Service, Springfield, Va., 1976, Chapt. 17. 

The absorption of radiation leads to an increase in die tenqierature of the absorber. An exanqile of this is the absorption of the kinetic energy of fission products in nuclear reactor fuel elements which is a main source of the heat production in reactors. The absorption of decay energy of radioactive nuclides in appropriate absorbing material can be used in a similar - albeit more modest - way as an energy source. 

A. R. Kaufmann, Ed., Nuclear Reactor Fuel Elements Metallurgy and Fabrication, John Wiley and Sons, New York and London (1962). 

USDC 85] Fundamental aspects of nuclear reactor fuel elements, TID 26711-Pl, National Technical Information Service, US Department of Commerce, Springfield, Va 22161 USA, 1985. 

Except for large scale accidental releases (e.g. nuclear explosions or catastrophic accidents at nuclear plants), water will be the main transport medium of plutonium to man. Therefore the size and location of plutonium sources, its pathways to man and its behaviour in natural waters are essential knowledge required for the evaluation of its ecological impact. That information, combined with radiological health standards, allows an assessment of the overall risk to the public from plutonium e.g. from a waste repository for spent unreprocessed reactor fuel elements in deep granite bedrock (8, 9). ... 

Development efforts in the nuclear industry are focusing on the fuel cycle (Figure 6.12). The front end of the cycle includes mining, milling, and conversion of ore to uranium hexafluoride enrichment of the uranium-235 isotope conversion of the enriched product to uranium oxides and fabrication into reactor fuel elements. Because there is at present a moratorium on reprocessing spent fuel, the back end of the cycle consists only of management and disposal of spent fuel. 

Hcit, W., Huschka, H, Rind, W., and Kaiser, G.G., Status of qualification of high-temperature reactor fuel element spheres, Nuclear Technology, 1985, 69, 44 54. 

In 1942, the Mallinckrodt Chemical Company adapted a diethylether extraction process to purify tons of uranium for the U.S. Manhattan Project [2] later, after an explosion, the process was switched to less volatile extractants. For simultaneous large-scale recovery of the plutonium in the spent fuel elements from the production reactors at Hanford, United States, methyl isobutyl ketone (MIBK) was originally chosen as extractant/solvent in the so-called Redox solvent extraction process. In the British Windscale plant, now Sellafield, another extractant/solvent, dibutylcarbitol (DBC or Butex), was preferred for reprocessing spent nuclear reactor fuels. These early extractants have now been replaced by tributylphosphate [TBP], diluted in an aliphatic hydrocarbon or mixture of such hydrocarbons, following the discovery of Warf [9] in 1945 that TBP separates tetravalent cerium from... 

R. Farmakes, ed.. Fast Reactor Fuel Element Technology, Proc. Int. Conf, April 13-15, 1971, New Orleans American Nuclear Society, La Grange, IL, 1971. 

The americium and curium isotopes formed during irradiation of nuclear reactor fuels are diverted into the high-level waste (HLW) stream during fuel reprocessing. The HLW is thus the biggest potential source for these elements, and R+D activities to develop a process for the recovery of Am and Cm from HLW were started in 1967. A major condition was that the process to be developed must not essentially increase the waste amount to be processed further, must not use strongly corrosive reagents, and must be compatible with the final waste solidification procedure. 

Once the radioactive fission products are isolated by one of the separation processes, the major problem in the nuclear chemical industry must be faced since radioactivity cannot be immediately destroyed (see Fig. 10-7c for curie level of fission-product isotopes versus elapsed time after removal from the neutron source). This source of radiation energy can be employed in the food-processing industries for sterilization and in the chemical industries for such processes as hydrogenation, chlorination, isomerization, and polymerization. Design of radiation facilities to economically employ spent reactor fuel elements, composite or individually isolated fission products such as cesium 137, is one of the problems facing the design engineer in the nuclear field. 

Radioactive wastes come directly from nuclear-reactor-fuel reprocessing plants and from industries employing radioactivity for processing work. The dominating elements from nuclear reactor fuels are cesium 137 and strontium 90, with the latter th,e controlling isotope owing to low permissible concentration values (Table 10-2). Rodger cites an example to illustrate the severity of the problem. In the year a.d. 2000 the installed reactor capacity on a world-wide basis is predicted to be 2.2 X 10 Mw. If this system is operated for 50 years, the Sr steady-state level (rate of production = rate of decay) would be 8.6 X 10 curies, which would require 5 per cent of the entire world ocean volume to dilute to the maxi-... 

The basic nuclear reactor fuel materials used today are the elements uranium and thorium. Uranium has played the major role for reasons of both availability and usability. It can be used in the form of pure metal, as a constituent of an alloy, or as an oxide, carbide, or other suitable compound. Although metallic uranium was used as a fuel in early reactors, its poor mechanical properties and great susceptibility to radiation damage excludes its use for commercial power reactors today. The source material for uranium is uranium ore, which after mining is concentrated in a "mill" and shipped as an impure form of the oxide UjO (yellow cake). The material is then shipped to a materials plant where it is converted to uranium dioxide (UO2), a ceramic, which is the most common fuel material used in commercial power reactors. The UO2 is formed into pellets and clad with zircaloy (water-cooled reactors) or stainless steel (fast sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack by the coolant, prevents the escape of fission products, and provides geometrical integrity. 

Lanthanides (La, Ce, Pr, Nd, Sm, and Eu) constitute about one-fourth of the total fission products produced in nuclear fission of uranium or plutonium. Thus, accurate estimation of Nd or La in the dissolver solution of spent fuel can be used to determine the number of fissions that have occurred in nuclear reactor fuel. The bum-up or atom percent fission (number of fissions per 100 initial heavy element atoms such as U and Pu) is an important parameter and the... 

Nuclear Criticality Safety of Reactor Fuel Elements—ANS-8.17,01115 Toffer(UNC Nuclear Industries). G. E. Whitesides (Union Carbide, NuclDiv)... 

A final check on e nuclear design predictions was made by using the actual reactor fuel elements in-a core mock-up experiment in DIMPLE. These experiments also provided an opportunity to make calibrations of flux wires, evaluate approach to critical procedures and make isotopic reaction rate elements with special demountable fuel pins. This resulted in the saving of in-line time during the commissioning of the reactor. 

High-level waste from the isolation of plutonium-239 contains massive amounts of plutonium-238 formed by various nuclear reactions in reactor fuel elements. A rough estimate indicates that as of 1985 there may be as much as 2 tons of plutonium-238 mixed with heavier plutonium isotopes in stored spent fuel elements and process residues accumulated in the USA and by the European Economic Community [5]. 

---