Zircaloy cladding

To complete the broad picture of what is meant by burn-out, it is useful to consider further the particular burn-out condition which produces tolerable wall temperatures. Such a condition must occur, for example, in any practical, once-through boiler system, where a change from, for example, liquid water at inlet to superheated steam at outlet takes place in a single heated channel. Normal operation of nuclear reactors beyond burn-out has also been contemplated, and Collier et al. (C5) have described successful experiments with irradiated Zircaloy-clad rods operating continuously beyond burn-out... 

Zircaloy Cladding by the Zirflex Process, Report A/ Conf. 15/2429, June 1958, "Second United Nations International Conference on Peaceful Uses of Atomic Energy,"... 

I0.6.8.I Cladding failure in oxide fuel pins of nuclear reactors. The long-term operational performance of nuclear fuel pins is critically governed by the reactions that occur in the gap between the fuel and its cladding. Ball et al. (1989) examined this for the cases of (1) Zircaloy-clad pellets of U02+, in a pressurised water reactor (PWR) and (2) stainless-steel-clad pellets of (U, P)02+, in a liquid-metal-cooled fast-breeder reactor (LMFBR). In particular they were interested in the influence of O potential on Cs, I, Te and Mo and the effects of irradiation on the gaseous species within the fuel-clad gaps. 

Levine, H.S., "An Assessment of Processes for Conversion of Zircaloy Cladding Waste to Zirconate Ion Exchange Material," SAND75-O643, July (1976). 

As shown in Fig. 12, the active core is made up of 241 fuel assemblies, all of which are mechanically identical. As indicated by Fig. 13. each fuel assembly contains 236 Zircaloy-clad, UO2 fuel rods retained in a structure consisting of Zirculoy spacer grids welded at about 15-inch (38.1-centimeter) intervals to five Zirealoy control element assembly guide tubes which, in turn, are mechanically fastened at each end to stainless... 

Shearing the Zircaloy-clad fuel rods into approximately one-inch lengths so the U02 can be dissolved... 

Zirconium is the principal FP to arise in oxidation state (IV). Where Zircaloy clad fuel is involved, nonradioactive zirconium isotopes may also be present from fuel can residues. As with ruthenium, there may be a variety of nitrato complexes present in the solution including the aquated complexes Zr(N03)s where x = 1-6, and hydroxy nitrato complexes. However, species containing ZrO " " are not expected to be present since this ion is unstable in aqueous media and is rapidly hydrated to Zr(OH)2. The extraction chemistry is further complicated by the formation of inextractable polymeric species when the Zr" concentration exceeds ca. 10 M. An example of such oligomerization is afforded by the [Zr(0H)2(H20)4]4 ion which contains four Zr ions in a square arrangement linked by two /u-OH ligands on each square edge. Four water molecules complete the Zr coordination sphere in an approximately D2d dodecahedral geometry. 

ZIrcaloy cladding containing UOg pallets (see detail) (204) Zircaloy guide for control rods, water filled (20)... 

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. 

Methods that have been proposed for chemical removal of zircaloy cladding in nonaqueous processing of spent fuel include conversion to gaseous ZrCL by reaction with HCl above 350°C or solution in molten zinc, which dissolves 10 a/o (atom percent) zirconium at 900°C. 

Another example of chemical decladding is afforded by the Zirflex process, which was proposed for zircaloy-clad UOj fuel before mechanical decladding was fully developed. In the Zirflex process [S17], zirconium or zircaloy cladding is dissolved as ammonium fluozirconate in a boiling solution of ammonium fluoride containing ammonium nitrate, the latter added to reduce hydrogen evolution. Overall reaction is approximately... 

In voloxidation, stainless steel-clad UO2 is oxidized with flowing air or oxygen at 575 to 650°C. With zircaloy-clad fuel, these gases may be unsafe because of the danger of a zirconium fire. Less reactive N2O4 has been proposed as an oxidant for such fuel. 

Fuel element UOj pellets, 1.43 cm diameter 49 rods per assembly 444 assemblies in core enrichment 2.6% Gd203 burnable poison zircaloy cladding 0.8 mm. 

During operation, a slow corrosion of the can is unavoidable. As long as the corrosion products stick to the surface, corrosion rates drop with time. For zircaloy clad fuel in water cooled reactors the corrosion rate follows a parabolic equation (in the normal operating temperature range)... 

Zircaloy clad oxide fuel elements can be stored for decades in storage pools with very little risk of leakage. Metal fuels, especially those canned in magnesium or aluminum alloys, are less resistant and should not be stored as such in this manner for a prolonged time. The corrosion resistance of aluminum or magnesium clad fuel can be improved by electrolytic treatment yielding a protective oxide layer. 

Hart, R. R., Rennie, J., Unlii, K. and Rios-Martinez, C. (1997) Gallium Interactions with Zircaloy Cladding, Procs. Plutonium Futures - The Science Topical Conf. on Plutonium and Actinides, pp.105-106, Santa Fe, NM, August 25-27, Los Alamos National Laboratory. 

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