Clad Uranium Metal Fuel Elements

Magnox-Clad Uranium Metal Fuel Elements 

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A. Magnox-Clad Uranium Metal Fuel Elements... 

Irradiation testing of refractory metal clad uranium nitride fuel elements. 

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. 

In the case of using metal alloyed (10% of Zr) uranium fuel with 75% effective density of theoretical one, the reactor can utilize waste uranium as the make-up. Thus the highest EUU is ensured (about 20%). In this case the bum-up depth achieves about 20% of h.a. (that is justified at experimental assemblies of EBR-2 reactor), fast neutron damaging dose on the fuel element cladding material accounts for approximately 430 dpa (it is twice the value that has been achieved by tests for ferritic and martensitic steels), the total operation period of FEA is about 30 years (that is three times over that gained for RI operation at the NS). 

The investigations performed made it possible to recommend unalloyed metallic fuel to be used in absorbers and the hiel elements of radial blankets, and axial blankets (smeared density - 80%) and core fuel elements (smeared density - 70%) with gas bonded pins. As a certain protection against interaction between fiiel and cladding a special oxide film is recommended to be applied to the fuel column. For core U-Pu fuel pins an additional protective coating on the cladding is recommended. Metallic uranium alloyed with zirconium and niobium was irradiated in BN-350 and BOR-60 as fuel material for absorbers and axial blankets. At present one subassembly with U-Pu-10%Zr is under irradiation in BOR-60, and one subassembly with U-10%Zr has been discharged from BOR-60 core with a maximum bum-up of about 10%. 

The fuel elements are concentric tubes of metallic uranium enriched to 0.947 w/o u. The fuel cladding Is Zircaloy-2. Additional lattice dimensions are given in Table C-1 and a cross-eectlonal view of the lattice is shown in Figure C-1. 

Example 6.2. A heavy-water-moderated reactor has fuel in the form of natural uranium metal rods of diameter 1 in., with cladding of 0.04-in. aluminum. The heat generation rate per unit volume of a fuel element at the center plane of the reactor is 1.85 x 10 Btu/h ft. The maximum fuel temperature is 690°F and the heat transfer coefficient to the coolant is 5500 Btu/h ft °F. If the thermal conductivities of uranium and aluminum are 18.5 Btu/h ft °F and 131 Btu/h ft F, respectively, calculate the bulk coolant temperature at the center plane. 

Fuel type Cylindrical fuel elements with fuel pellets made of uranium-plutonium oxide, nitride or metallic fuel in steel claddings ... 

Two fuel forms have the potential to satisfy the GFR requirements (1) a ceramic plate-type fuel element and (2) a ceramic pin-type fuel element. The reference material for the structure is reinforced ceramic comprised of a sUicon carbide composite matrix ceramic. The fuel compound is made of pellets of mixed uranium-plutonium-minor actinide carbide. A leak-tight barrier made of a refractory metal or of Si-based multilayer ceramics is added to prevent fission products from diffusing through the clad [1]. 

One system would use pellets of uranium dioxide in stainless steel cans, whereas the other system would use uranium metal and zirconium as the cladding material. Zirconium was the preferred choice as stainless steel absorbed too many neutrons. Testing fuel elements was a lengthy process They had to be designed, fabricated, set up within a channel of a reactor, irradiated for a considerable period of time and then removed. More time had to be given for the fission products to decay, after which the fuel rods could be examined. Depending on availability of reactor space and other factors, this whole sequence could take up to 15 months. Eurthermore, the fuel elements for a submarine reactor would have to be extremely reliable — far more reliable than for a land-based reactor since it would be impossible to remove defective fuel elements whilst at sea. 

The atomic density of hydrogen in many metal hydrides is greater than that in liquid H2 or in H20. Metal hydrides are efficient moderators (Fig. 1) and neutron shielding materials, and help to minimize the core shield volume. Metal-clad yttrium hydride moderators capable of operation at 1000°C in air, uranium-zirconium hydride rods as a combination fuel-moderator element are examples, and metal-clad zirconium hydride units as moderator elements for operation up to 600°C° °. The hydrogen atom density in hydrides, Ah, the number of hydrogen atoms per cubic centimeter of hydride X 10 , is calculated from the hydrogen-to-metal atom ratio, H/M, the density of the hydride p, and the molecular weight W by ... 

The H Reactor lattice is rectangular the vertical spacing is nine Inches and the horizontal spacinsf is eig ht inches The process tubes are xiominal gnarter-inch vall Zlrcalloy-2 2 7-inch I.D tubes. The bt up fueX elements are concentric tubes of metallic uranium enriched to 0 9l(7 /o fuel clad is... 

The irradiated N Reactor fuel consists of slightly enriched metallic uranium bonded to a layer of zirconium alloy (Zircaloy-2), hereinafter referred to as the cladding. The cladding provides the primary barrier against the escape of fission products and fissile materials from the fuel assembly. The fuel assembly (see Color Illustration 3-3) includes two components an inner and an outer tube-shaped element, assembled into a tube-in-tube arrangement. 

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