Modeling of Near Field Actinide Concentrations in Radioactive Waste Repositories in Salt Formations: Effect of Buffer Ma
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Modeling of Near Field Actinide Concentrations in Radioactive Waste Repositories in Salt Formations: Effect of Buffer Materials W. Schuessler, B. Kienzler, S. Wilhelm*, V. Neck and J.I. Kim Institut für Nukleare Entsorgung, Forschungszentrum Karlsruhe Postfach 3640, D-76021 Karlsruhe, Germany Email: [email protected] *Present address: Colenco AG, Mellingerstraße 207, CH-5405 Baden, Switzerland ABSTRACT Engineered barrier systems are designed to reduce the near field actinide concentrations in case of water penetration into a repository. In this paper, the influence of buffer materials, such as MgO/CaO and clays, on the solubilities of Am, Np, Pu, and U is studied. The analysis is performed for low level cemented waste forms in a rock salt formation in contact with MgCl2 saturated salt brine (Q-brine). The evolution of the geochemical milieu by cement corrosion is calculated using reaction path modeling supported by the code EQ3/6. The influence of different buffer materials is analyzed with respect to their impact on the solution chemistry and corresponding actinide concentrations. INTRODUCTION Safe disposal of radioactive waste relies on multi-barrier systems [1]. Part of these systems are engineered barriers which provide for a plugging of the openings as well as for the chemical buffering of solutions and sorption capacity. Geochemical evaluation of buffer and backfill materials under repository relevant conditions focuses on two main topics i) the long-term geochemical stability and the chemical properties in case of solution access and ii) the geochemical buffering of the solution with respect to Eh, pH, and pCO2 [2,3]. The geochemical stability of buffer and backfill materials in contact with salt solutions and the evolution of the solution composition depends on the waste forms (cement and organic components), the remaining void volume, and different boundary conditions, such as pH and temperature etc. In this paper the actinide solubility is investigated. In the case of radioactive waste disposal in German salt formations, brines having a high ionic strength have to be considered in this analysis. Different buffer materials, i.e., MgO, CaO and clays, were selected and their behavior in contact with cemented waste forms and with a MgCl2 saturated salt brine (Q-brine (Table 1)) was investigated.
Table 1. Composition of Q-brine [mol⋅(1000 mol H2O)-1] [4] NaCl
KCl
MgCl2
MgSO4
pH
6.8
17.4
77.3
3.2
ρ
T
mol mol mol mol 3 3 3 3 10 mol H 2 O 10 mol H 2 O 10 mol H 2 O 10 mol H 2 O
[ C]
g 3 cm
55
1.30
o
5.5
Methods The geochemical milieu, actinide concentrations, and possible precipitation of solid phases are computed by geochemical model calculations using the EQ3/6 Rel.7.2a package [5]. A thermodynamic equilibrium between the solution and solid phases is assumed. The initial solution composition is computed, including all species involved in further reactions. The cemented waste forms consist of different solid mineral phases. After c
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