The Effect of Alpha-Radiolysis on UO 2 Dissolution Determined from Batch Experiments with 238 Pu-Doped UO 2
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The Effect of Alpha-Radiolysis on UO2 Dissolution Determined from Batch Experiments with 238Pu-Doped UO2 Simcha Stroes-Gascoyne and James S. Betteridge Atomic Energy of Canada Limited, Whiteshell Laboratories, Pinawa, MB R0E 1L0, Canada, [email protected]
ABSTRACT Dissolution experiments were performed with 238Pu-doped UO2 containing doping levels of ∼1–100 Ci/kg UO2, in 0.1M perchlorate with or without 0.1M carbonate, at pH 9.5, to investigate the effects of α dose-rate on UO2 dissolution rates and on Geff, the fraction of radiolytically produced H2O2 supporting dissolution. Dissolution rates increased by only about 10 times for a ∼100-fold increase in doping level in both solutions, but increased between 10 and 100 times in 0.1M carbonate, compared to dissolution rates in 0.1M perchlorate. Geff values increased with decreasing doping level, suggesting that the dissolution reaction is limited by the reaction rate between UO2 and H2O2 at high dose rates, but becomes limited by the rate of production of H2O2 at lower dose rates. INTRODUCTION The long-term stability of used nuclear fuel in a repository will be determined by the influence of α-radiolysis, since the dose-rate from α-radiolysis will exceed that for γ/βradiolysis beyond a fuel age of a few hundred years and will persist for more than 10 000 years [1]. However, since a few hundred-year old spent fuel is not available at present, it is not possible to exclude γ/β radiolysis effects from experiments conducted with spent fuel. In this study we have measured dissolution rates for 238Pu-doped UO2 materials to assess the effects of α-radiolysis on older fuels. In addition, Geffective (Geff) values were derived, which are indicative of the fraction of radiolytically produced H2O2 that supports UO2 dissolution. It has been estimated that the most conservative value of Geff = 1 would lead to complete fuel dissolution in a repository in about 105 a [2], while a more commonly used value of Geff = 0.01 leads to complete dissolution in > 107 a [2]. Geff is, therefore, a crucial parameter for predicting the extent and rate of UO2 dissolution due to α-radiolysis. EXPERIMENTAL DETAILS The fabrication of the 238Pu-doped material has been described elsewhere [3]. Briefly, powdered UO2 and a very small quantity of 238PuO2 were mechanically mixed, pressed and sintered into pellets. It is expected that this procedure resulted in 238Pu being present as island of PuO2 in the UO2 matrix. The materials prepared contained nominally about 1, 10 and 100 Ci/kg UO2 (from 0.006, 0.06 and 0.6 wt% 238Pu). Typical used CANDU fuel contains ∼1 Ci/kg UO2 (α-activity) at a cooling time of 20 to 50 years and a burnup of 685 GJ/kg. The 238Pu-doped pellets were cut into ∼3 mm thick discs for the dissolution experiments. The dissolution behaviour of these materials (and undoped UO2) was investigated in perchlorate solution (0.1M NaClO4) and perchlorate/carbonate solution (0.1M NaClO4 + 0.1M carbonate (0.03M Na2CO3
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and 0.07M NaHCO3)) at pH 9.5, to investigate the effects of α-dos
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