Highly radioactive waste products
Direct disposal of high-level radioactive waste, which comprises spent nuclear fuel (SNF) and vitrified high-level liquid waste, in a deep geological repository is considered as an option for the management of these wastes in many countries including Germany.
In this option, the long-term safety depends on a multi-barrier system that consists of technical and geo-technical barriers such as the waste form, the canister, backfilling and sealing of the mined openings as well as on the natural barrier function of the host rock itself.
Although this multi-barrier system is designed to prevent, to some extent, the intrusion and eventual contact between the groundwater and the emplaced waste, this fact cannot be excluded. Therefore, the understanding of this interaction is critical to evaluate the safety of the disposal strategy.
Upon the contact with water, the alteration of the high-level waste and the consecutive radionuclide release depends on different coupled processes influenced by various factors as the radiation field, the groundwater composition and the environmental conditions (e.g. pH, redox potential, temperature).
In order to derive a source term, all these processes can be assigned to two steps: (1) a fast release due to waste package and cladding failure; (2) a much slower, long-term release that results from the alteration and dissolution of the UO2 matrix.
In this context, leaching experiments with SNF under various geochemical conditions are being performed to determine the influence of the pH, groundwater composition as well as the redox conditions on the dissolution behavior of the matrix and on the fast release fraction. In parallel, radionuclide of special interest for performance assessments such as 14C and 36Cl, due to their high mobility and long half-life, are analysed in SNF, high-level waste glass and irradiated Zircaloy cladding by means of various analytical techniques.
In the case of solid samples, synchrotron radiation based X-ray absorption and fluorescence spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy are used. In the case of liquid samples, a- and g-spectroscopy, liquid scintillation counting and sector field inductively coupled plasma mass spectrometry (ICP-MS) are used. In addition, gas samples are analysed by means of a multipurpose ICP-MS with customized gas inlet system.
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