Chemical Effects at the Reaction Front in Corroding Spent Nuclear Fuel
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0985-NN01-03
Chemical Effects at the Reaction Front in Corroding Spent Nuclear Fuel Jeffrey A. Fortner, A. Jeremy Kropf, James L. Jerden, and James C. Cunnane Chemical Engineering, Argonne National Laboratory, CMT/205, 9700 S. Cass Avenue, Argonne, IL, 60439 ABSTRACT Performance assessment models of the U. S. repository at Yucca Mountain, Nevada suggest that neptunium from spent nuclear fuel is a potentially important dose contributor. A scientific understanding of how the UO2 matrix of spent nuclear fuel impacts the oxidative dissolution and reductive precipitation of Np is needed to predict the behavior of Np at the fuel surface during aqueous corrosion. Neptunium would most likely be transported as aqueous Np(V) species, but for this to occur it must first be oxidized from the Np(IV) state found within the parent spent nuclear fuel. In this paper we present synchrotron X-ray absorption spectroscopy and microscopy findings that illuminate the resultant local chemistry of neptunium and plutonium within uranium oxide spent nuclear fuel before and after corrosive alteration in an air-saturated aqueous environment. We find the Pu and Np in unaltered spent fuel to have a +4 oxidation state and an environment consistent with solid-solution in the UO2 matrix. During corrosion in an airsaturated aqueous environment, the uranium matrix is converted to uranyl (UO22+) mineral assemblage that is depleted in Np and Pu relative to the parent fuel. The transition from U(IV) in the fuel to a fully U(VI) character across the corrosion front is not sharp, but occurs over a transition zone of ~ 50 micrometers. We find evidence of a thin (~ 20 micrometer) layer that is enriched in Pu and Np within a predominantly U(IV) environment on the fuel side of the transition zone. These experimental observations are consistent with available data for the standard reduction potentials for NpO2+/Np4+ and UO22+/U4+ couples, which indicate that Np(IV) may not be effectively oxidized to Np(V) at the corrosion potential of uranium dioxide spent nuclear fuel in air-saturated aqueous solutions. BACKGROUND Specimens of unclad fragments of commercial spent nuclear fuel (CSNF) that had been subjected to corrosion testing for up to 10 years under hydrologically unsaturated conditions were examined to elucidate radionuclide release pathways and mechanisms. These unsaturated tests had been used to investigate oxidative dissolution of the fuel, release of radionuclides, and formation of alteration phases that can sequester radionuclides in an oxidizing environment that is hydrologically relevant to that expected at Yucca Mountain [1]. Commonly known as the Argonne “drip tests,” this series of investigations are described in detail elsewhere [1]. These tests provided substantial information on the relative release rates of various radionuclides, which were generally found to be highly incongruent. Notably, the tests had shown that neptunium was released at a normalized rate roughly comparable to that of uranium, but orders of magnitude more slowly than that
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