Prediction of the Oxidative Dissolution Rates of Used Nuclear Fuel in a Geological Disposal Vault Due to the Alpha Radio
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PREDICTION OF THE OXIDATIVE DISSOLUTION RATES OF USED NUCLEAR FUEL IN A GEOLOGICAL DISPOSAL VAULT DUE TO THE ALPHA RADIOLYSIS OF WATER S. Sunder, D.W. Shoesmith and N.H. Miller AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada ROE ILO ABSTRACT Effects of alpha radiolysis of water on the corrosion of nuclear fuel (U0 2) have been investigated in solutions at pH = 9.5, i.e., a value close to that expected in groundwaters at the depth of the disposal vault proposed in the Canadian nuclear fuel waste management program, CNFWMP. The corrosion potentials of U0 2 electrodes exposed to the products of alpha radiolysis of water were monitored as a function of alpha flux and exposure time in a specially designed thin-layer cell. The oxidative dissolution rates of U0 2 are calculated from the steady-state values of the corrosion potential using an electrochemical model. A procedure to predict the dissolution rate of used nuclear fuel in groundwater as a function of fuel cooling time is described, and illustrated by calculating the dissolution rates for the reference used fuel in the CNFWMP (Bruce CANDU reactor fuel, burnup 685 GJ/kg U). It is shown that the oxidative dissolution of used fuel in the CNFWMP will be important only for time periods < 600 a at this burnup and assuming no decrease in pH. INTRODUCTION The concept of direct disposal of used nuclear fuel in a geological disposal vault is being considered in several countries, including Canada, U.S.A. and Sweden [1-6]. Transport by groundwater is the only credible mechanism for the migration to the biosphere of radionuclides contained in the used fuel bundles in the disposal vault. Used fuel is largely UO2 with only a small fraction of other actinides and fission products. A great majority (>90%) of these radionuclides are present as solid solutions within the UO 2 matrix [3]. Hence, dissolution of U0 2 is a major potential pathway for the release of radionuclides contained within used fuel, and an evaluation of a disposal concept requires a knowledge of the dissolution rate of the used fuel matrix as a function of time. The dissolution rate of U0 2 in water depends upon its surface oxidation, which is governed by the solution redox conditions [3,4,7]. Although, the groundwaters at the planned depth of the disposal vault in the Canadian Nuclear Fuel Waste Management Program (CNFWMP), 500-1000m, are generally reducing [8], their redox condition may be altered on contact with the used fuel surface due to the radiolysis of water by ionizing radiation [3-6,9-12]. This contact is not expected to be established until the disposal containers fail after a minimum of five hundred years. As their radiation fields change (decrease) with time [5,6,13], it is not possible to determine the dissolution rates of used fuel for times > container failure from the leaching behaviour of presently available used fuels which are generally < 30 a old. Here we describe an experimental strategy to predict used fuel dissolution rates as a function of cooling time, including time p
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