Dissolution of fuel

The damaged fuel from the N2 PWR is assumed to comprise UO pellets, 5% enriched, 4.5 mm diameter, 10 g-cm density, carried in Zr-Nb alloy cladding. As soon as the fuel is in contact with sea water, an immediate release of 20% of the fission product and actinide inventory is assumed from the fuel grain boundaries. The remaining fission products and the actinides are released through dissolution of fuel grains at a pessimistic rate of 30 x 10 g cm d [22]. For fuel pins with a 4.5 mm diameter and 10 g-cm density, this equates to a constant dissolution rate of 0.0011 mm-a. Once exposed to seawater, the fuel will take some 2250 years to corrode away. 

The, dissolution of fuel elements involves conditions in which UO2-PUO2 solids are Immersed in fissile solutions. Critical masses, volumes, and dimensions are required for credible. combinations of fuel, moderator, and reflec-... 

This paper presents results of two series of recent experiments. The purpose of the first series k to provide data for determining criticality safety conditions of the dissolution of fuel rods in nitric acid. The r lts of experiments are relative to r lar lattices of fuel rods with square pitches,. immersed in water plus NaNO. ... 

Contrary to low Cr steels, 14—18 Cr austenitic stainless steels exhibit high chemical compatibility with a lot of process environments, including for the fast breeder applications, the sodium coolant (potential exterior surface corrosion), the MOX oxide (inner fuel clad chemical interaction) and the nitric acid (potential dissolution of fuel pin sections in the course of the reprocessing process of used fuels). 

N. Hodge, FED-ADAP Experimental Programme Phase 2, Pilot Scale Rig -Optimisation of Acid and Temperature Parameters for Dissolution of Fuel Element Debris , UK National Nuclear Laboratory, NNL Reference NNL(13) 12493,2013. 

The rate (kinetics) and the completeness (fraction dissolved) of oxide fuel dissolution is an inverse function of fuel bum-up (16—18). This phenomenon becomes a significant concern in the dissolution of high bum-up MO fuels (19). The insoluble soHds are removed from the dissolver solution by either filtration or centrifugation prior to solvent extraction. Both financial considerations and the need for safeguards make accounting for the fissile content of the insoluble soHds an important challenge for the commercial reprocessor. If hydrofluoric acid is required to assist in the dissolution, the excess fluoride ion must be complexed with aluminum nitrate to minimize corrosion to the stainless steel used throughout the facility. Also, uranium fluoride complexes are inextractable and formation of them needs to be prevented. 

W. S. Groenier, Equipmentfor the Dissolution of Core Materialfrom S beared Power Reactor Fuels, ORNL/TM-3194, Oak Ridge National Laboratory, Oak... 

However, in the case of multimetallic catalysts, the problem of the stability of the surface layer is cmcial. Preferential dissolution of one metal is possible, leading to a modification of the nature and therefore the properties of the electrocatalyst. Changes in the size and crystal structure of nanoparticles are also possible, and should be checked. All these problems of ageing are crucial for applications in fuel cells. 

The rate of dissolution of these reaction products is slow (1.5-2 fig/day per 50-70 mg of solids). Lai and Goya 147) showed that at least 90% of these plutonium aggregates had a diameter <0.01 jum. However, it must be stressed that these forms do not necessarily represent those forms which would be produced as a result of an accidental release from a nuclear power plant or as a result of controlled release from nuclear fuel reprocessing facilities such as those which occur at Windscale in England. 

In order to assess the integrity of the system, we should know what kind of reactions would take place when the groundwater invades and the overpack is corroded. Consequently, the solidified waste or spent fuel itself will be in contact with groundwater. Since the waste would still be seriously activated, radiolysis of groundwater will take place and change the chemical condition, which might affect the dissolution of the solidified waste or UO2 of the spent fuel. 

In direct geological depository of the spent fuel, dissolution of UO2 matrix into groundwater plays a key role for the release of the radionuclides to the biosphere. Therefore mechanism of... 

In a 20-year corrosion test by Stroes-Gascoyne et al. (1997), oxidative dissolution of the fuel... 

Indications from both microscopic analyses of metallic particles from corrosion tests and evidence from the Oklo natural reactors indicate that performance assessment calculations should not assume 99Tc is easily mobilized. It is entirely inappropriate to use "Tc release as a marker for fuel corrosion because Tc is not located in the fuel matrix. The TEM examinations of corroded e-particles have shown that Mo is preferentially leached from these phases, a behaviour that is similar to the one observed at Oklo. It is interesting to note that laboratory dissolution of billion-year old 4d-metallic particles for a chemical analysis required a corrosive mix of peroxide and acid (Hidaka Holliger 1998) similar to the experience at SNF reprocessing plants. It is doubtful that the oxidation potential at the surface of an aged fuel will be sufficient to move Tc(0) from the e-metal particles. 

Amme, M. 2002. Contrary effects of the water radiolysis product H2O2 upon the dissolution of nuclear fuel in natural ground water and deionized water. Radiochimica Acta, 90, 399-406. 

KessinGer, G. F. Thompson, M. C. 2002. Dissolution of Dresden reactor fuel. Westinghouse Savannah River Company Report, WSRC-TR-2002-00448. 

Sunder, S., Shoesmith, D. W. Miller, N. H. 1997. Oxidation and dissolution of nuclear fuel (UO2) by the products of the alpha radiolysis of water. Journal of Nuclear Materials, 244, 66-74. 

Dissolution of the oxidized portion of the surface of the spent fuel, that is, the release of uranium(VI) to the solution. This is described by the following expression ... 

Ahn, T. M. 1996. Long-Term Kinetic Effects and Colloid Formations in Dissolution of LWR Spent Fuels. US Nuclear Regulatory Commission, NUREG-1564. 

The dissolution time for the unreprocessed fuel would be at least 1 million years due to the limited water supply, even if a rapid oxidation of uranium to the hexavalent state and a subse-guent formation of water soluble carbonate complexes are assumed (15). Since the conditions are reducing in the groundwater (see beTow) the dissolution time would probably be several orders of magnitude larger. The unsignificant dissolution of uranium and fission products observed in the Oklo-deposit (16) is an example of a similar extremely slow leaching process in the natural environment. 

Unfortunately, environmental problems do not solely involve dilute binary systems. To mention just one example, the dissolution of a patch of spilled diesel fuel into the groundwater (see below Illustrative Example 19.4) involves diffusion in multicomponent systems for which adequate data are extremely rare. In many cases diffusivities of diluted compounds in air or water must serve as best estimates for more complex systems. Yet, the above remarks should remind us that things may be more complicated. 

Catalyst deterioration due to gas poisoning is only avoided by careful gas cleaning. Anodic oxidation followed by dissolution of Pt and transfer to the cathode is a serious cause for Pt loss. It is potential dependent and accelerates as the cathode potential increases, for instance under partial load or in off-time, when the cathode potential drifts toward the oxygen equilibrium potential. Therefore it is of utmost importance that whenever the fuel cell is switched off, the oxygen in the cathode lumen is rapidly exchanged by inert nitrogen and that the cell voltage under operation does not surmount 0.8 V. 

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