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Modelling Oxidative Dissolution of Spent Fuel

Published online by Cambridge University Press:  03 September 2012

Ivars Neretnieks
Ivars Neretnieks*
Affiliation:
Department of Chemical Engineering and Technology, Royal Institute of Technology S-100 44 Stockholm, SWEDEN
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Abstract

Spent nuclear fuel will, by the radiation, split nearby water into oxidizingand reducing compounds. The reducing compounds are mostly hydrogen that willdiffuse away. The remaining oxidizing compounds can oxidize the uraniumoxide of the fuel and make it more soluble. The oxidised uranium willdissolve and diffuse away. The nuclides previously incorporated in the spentfuel matrix can then be released and also migrate away from the fuel.

A model is proposed where the produced oxidizing species compete forreaction with the fuel and for escaping out of the system. The chemicalreaction rate of oxygen and fuel is taken from literature values based onexperiments. The escape rate of oxidants to a receding redox front in thebackfill is modelled assuming a redox reaction of oxidizing component andreducing component in the surrounding. The rate of movement of the redoxfront is determined from the rate of production of oxidants. This isestimated using a previously devised model that has been calibrated to insitu observed radiolysis.

Three cases are modelled. In the first case it is assumed that the reducingcompound is insoluble and that the reaction between oxygen and reducingmineral is very fast. In the second case it is assumed that the reducingcomponent has a known solubility and that it can migrate to meet the oxygenand quickly react. In a third case a finite reaction rate is modelledbetween the oxygen and the reducing species.

The sample calculations show that if the reducing mineral has to be suppliedfrom the backfill a large fraction of the spent fuel could be oxidised. Ifthe corrosion products of a degraded steel canister can supply the reducingspecies and the redox reaction is fast, very small amounts of the fuel couldbe oxidised. Literature data indicate that the redox reaction rate may notbe so fast that it can be considered instantaneous and then a considerablefraction of the fuel could be oxidised. The model gives a means of exploringwhich mechanisms and data may be of most importance for radiolytic fueldissolution, but the realism of the data and the model must be testedfurther. There is a lack of understanding and data on reaction rates,heterogeneous as well as homogeneous. This is crucial to the results.

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Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 465: Symposium II – Scienfific Basis for Nuclear Waste Management XX , 1996 , 573
Copyright
Copyright © Materials Research Society 1997

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References

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