Uranium Dioxide Fuels

Uranium dioxide fuel is irradiated in a reactor for periods of one to two years to produce fission energy. Upon removal, the used or spent fuel contains a large inventory of fission products. These are largely contained in the oxide matrix and the sealed fuel tubing. 

However, at this time all such LMRs have been shut down and are being decommissioned. Programs are also underway to use conventional uranium dioxide fuel containing blended down highly enriched uranium... 

Schieferdecker H, Dilger H, Doerfel H, et al. 1985. Inhalation of uranium aerosols from uranium dioxide fuel element fabrication. Health Phys 48 29-48. 

C H. Knight, R. M. Cassidy, B. M. Recoskie, and L. W. Green, Dynamic ion exchange chromatography for determination of number of fissions in thorium-uranium dioxide fuels. Anal Chem., 56,474,1984. 

Massive zirconium and zircaloy are resistant to hot nitric acid. In fuel reprocessing (Chap. 10), uranium metal or uranium dioxide fuel can be dissolved by hot nitric acid while leaving the zircaloy cladding unattacked. 

Similar products can be formed by reactions of liquid sodium with solid oxides, and some of these reactions are of technical importance. The failure of fuel element canning tubes causes the contact of uranium dioxide fuel with liquid sodium at high temperatures. The stoichiometric UO2 does not react with sodium at temperatures of 400 and 600 °C due to its very high thermodynamic stability. Non-stoichiometric compounds, however, which may contain some UO3 or U3O8, are able to react. In this case, the sodium uranate-V, Na3U04, is formed, a face-centered cubic cell with a = 0.474 nm The same product is formed at temperatures above 550 °C by the reaction... 

Pajo et al. (2001a) used GD-MS to measure impurities in uranium dioxide fuel and showed that these impurities could be used to identify the original source of confiscated, vagabond nuclear materials. De las Heras et al. (2000) used GD-MS to determine neptunium in Irish Sea sediment samples. The sediment samples were compacted into a disk that was used with a tantalum secondary cathode in the glow discharge. Using a doped marine sediment standard for calibration, detection limits down to the mid pg/g level were determined. 

Two types of light water reactors, namely, the boiling water reactor (BWR) and the pressurized water reactor (PWR) are in use in the United States of America. The fuel for these reactors consists of long bundles of 2-4 wt% of enriched uranium dioxide fuel pellets stacked in zirconium-alloy cladding tubes. 

Measured Values of Release from Uranium Dioxide Fuel"... 

Calculations show that using a uranium dioxide fuel with 60% enrichment on with a zirconium hydride moderator makes it possible to obtain a core size of no more than 25 cm. The reactor is controlled by rotary steel drums with insertion from boron carbide, all drums are located in the reflector. The reactor shielding has a total thickness of 2 m, consisting of two layers light (1.6m) and heavy (0.4m) concrete. 

The radioactive products contained in the fuel are normally located in the sinterized uranium dioxide of the reactor fuel (the uranium dioxide fuel is shaped into pellets, roughly 1 cm in diameter, inserted in long zirconium alloy (zircalloy) cylinders). The matrix of these cylinders (roughly 40000), grouped in bundles to form the fuel elements, is the reactor core. 

Several hundred uranium dioxide fuel assemblies make up the core of a reactor. For a reactor with an output of 1,000 MWe, a typical core contains about 75 t of low-enrichment uranium ( 3.5% U). During the operating cycle of a nuclear reactor, several competing processes determine the final radionuclide inventory in the spent fuel. These processes are... 

The uranium dioxide fuel pellets in PWR fuel assemblies that were considered all had diameters between 0.82 and 0.97 cm. The fuel pins were all Zircaloy clad, and the metar to-water ratios in the fuel assemblies were all about the same. Factors that varied significantly were (a) amount of in the assemblies (b) overall size of the assemblies (c) thickness- of the Type 304 stainless steel in the racks and (d) thickness of water between the fuel assemblies in the stpra latUce. 

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