Abstract
In this work we present the results of two spent nuclear fuel leaching experiments in simulated granitic groundwater, saturated with hydrogen under various pressures. The results show a large impact of the dissolved hydrogen already at 1 bar H2 and room temperature on the release of both the uranium and of the fission products contained in the fuel matrix. Based on the results of this study and on published data with fuel from the same rod, the importance of the oxidative dissolution of spent fuel under repository conditions as compared to its non-oxidative dissolution is discussed. The XPS-spectra of the fuel surface before the tests and after long-term leaching under hydrogen are reported and compared to reduced UO2 and SIMFUEL surfaces. The overall conclusion is that in spite of the unavoidable air contamination, hydrogen pressures of 1 bar or higher counteract successfully the oxidative dissolution of the spent nuclear fuel. The stability of the 4d-element metallic particles during fuel leaching under such conditions is also discussed, based on data for their dissolution. The metallic particles are also stable under such conditions and are not expected to release their component metals during long-term fuel leaching.
Highlights
The direct disposal of spent nuclear fuel as a waste form is currently under consideration in several countries
In most disposal concepts for high-level waste, spent nuclear fuel will be encapsulated in massive canisters made of or containing large amounts of metallic iron
The release of nonredox sensitive radionuclides such as Cs and Sr is discussed through their FIAP (Fraction of Inventory in the Aqueous Phase) and Fractional Release Rates (FRR) (Fractional Release Rate)
Summary
The direct disposal of spent nuclear fuel as a waste form is currently under consideration in several countries. In most disposal concepts for high-level waste, spent nuclear fuel will be encapsulated in massive canisters made of or containing large amounts of metallic iron. Each canister will be surrounded by compacted bentonite clay. This arrangement constitutes a multiple barrier system, including the engineered or technical barrier (waste form and backfill materials) and the geologic barrier (the host rock formation itself and its overburden). An engineered barrier may affect the geochemical environment to provide favourable conditions with respect to low solubilities of radionuclides and low dissolution rates of waste forms
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