This title appears in the Scientific Report :
2015
Evaluation of waste form behaviour in a geological repository for high-level radioactive waste in Boom Clay in the Netherlands
Evaluation of waste form behaviour in a geological repository for high-level radioactive waste in Boom Clay in the Netherlands
The primary objective of the Dutch research programme into geological disposal of radioactive waste (OPERA - OnderzoeksProgramma Eindberging Radioactief Afval) is the development of initial conditional safety cases for generic repositories in the Tertiary Boom Clay and Zechstein rocksalt formations...
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Personal Name(s): | Deissmann, Guido (Corresponding Author) |
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Haneke, Kirsten / Filby, André / Wiegers, Rob / Bosbach, Dirk | |
Contributing Institute: |
Nukleare Entsorgung; IEK-6 |
Published in: | 2015 |
Imprint: |
2015
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Conference: | Clays in Natural and Engineered Barriers for Radioactive Waste Confinement - 6th International Conference, Brussels (Belgium), 2015-03-23 - 2015-03-26 |
Document Type: |
Poster |
Research Program: |
Nuclear Waste Management |
Publikationsportal JuSER |
The primary objective of the Dutch research programme into geological disposal of radioactive waste (OPERA - OnderzoeksProgramma Eindberging Radioactief Afval) is the development of initial conditional safety cases for generic repositories in the Tertiary Boom Clay and Zechstein rocksalt formations in the Netherlands. The generic disposal concept for high-level radioactive waste (HLW) in Boom Clay pursued within this context is based on the Belgian supercontainer concept [1]. In this concept, cementitious materials are extensively used in the engineered barrier system (i) as buffer within the waste container, (ii) as backfilling grout, and (iii) in the construction material for the disposal gallery linings. Thus the post closure conditions in the near-field will be governed by the interaction of Boom Clay porewaters with cementitious materials, leading to highly alkaline conditions in the vicinity of the emplaced wastes. The HLW inventory for the OPERA safety case comprises in particular vitrified wastes (HLW glass) from the reprocessing of light water reactor (LWR) fuels from commercial nuclear reactors and spent research reactor fuels, besides spent uranium targets from molybdenum isotope production, and non-heat generating wastes such as compacted hulls and ends from fuel assemblies.In this paper we present and discuss initial results of the evaluation of the corrosion behaviour of and the radionuclide release from vitrified HLW and spent fuel from research reactors under the disposal conditions encountered in a geological repository in Boom Clay in the Netherlands. The overall aim of this project is (i) to provide a coherent picture of the radionuclide release from HLW disposed in a generic repository in Boom Clay, (ii) to increase the understanding of waste form evolution (i.e. leaching and dissolution behaviour with time) and the radionuclide migration behaviour in the near-field, and finally (iii) to develop source terms for relevant radionuclides that are the main contributors to dose in the long-term in support of the post closure safety assessments. The results derived address directly the safety function "delay and attenuation of releases" relevant after the failure of the waste canisters when the waste forms come into contact with the near-field water. Due to the variability of the porewater composition encountered in the Boom Clay in the Netherlands and the generic nature of the OPERA safety case (i.e. without selection of a repository site), various scenarios regarding the composition of the near-field water resulting from the interaction of Boom Clay porewater with the cementitious materials and their evolution with time will be discussed.Vitrified HLW within the OPERA inventory comprise predominantly R7T7 glasses from spent fuel reprocessing in La Hague, France, and, to a lesser extent, glasses from the reprocessing of fuels from the Dodewaard NPP in Sellafield, UK. The dissolution behaviour of the glasses depends on their composition (e.g. Mg-bearing glasses such as MW produced in the Sellafield vitrification plant exhibit generally a lower durability than the French R7T7 glasses) as well as on the post closure conditions in the near-field. In general, HLW glass dissolution is expected to be more rapid at high pH in alkali-rich waters compared to waste emplacement in a non-cementitious, clay-based repository, leading to the formation of altered glass and secondary silicate phases, some of which may retain certain radionuclides (e.g. [2]). However, the presence of elevated calcium concentrations such as in evolved cement porewaters buffered by portlandite appears to reduce the glass dissolution rate (e.g. [3]), suggesting a favourable effect on HLW glass durability under high pH conditions. Thus the evolution of near-field chemistry and the extent of cement alteration resulting from the exchange with Boom Clay porewaters will directly impact the glass corrosion rates, the formation of glass alteration layers and secondary phases as well as radionuclide release with time.Research reactor fuels relevant within the OPERA safety case are plate-shaped dispersion type fuels consisting of uranium-aluminide (UAlx) or uranium-silicide (e.g. U3Si2) fuel particles dispersed in an aluminium matrix that is metallurgicaly bonded to aluminium cladding, employing either high-enriched uranium (HEU, up to 80 to 95 wt.% 235U) or low-enriched uranium (LEU, up to 20 wt.% 235U). Corrosion data obtained for these types of fuels in salt brines, granite waters, or clay waters indicate a – with respect to the time scales relevant for geological disposal – practically instantaneous corrosion after coming into contact with groundwater (i.e. after failure of the waste canisters) [4, 5]. A similar behaviour is suggested under disposal conditions in a cementitious repository in Boom Clay, due to high aluminium corrosion rates under alkaline conditions leading to a fast disintegration of the fuels. The amount, nature and stability of potential secondary phases formed during the corrosion process (e.g. layered double hydroxide phases (LDH)) that may retain certain radionuclides depend on the near field conditions and is as such the degree of interaction of the Boom Clay pore waters with the cementitious materials and the stage of cement degradation at the time of the failure of the waste canisters. Uranium and some actinides are assumed to be solubility limited under the expected near-field conditions.References[1] Verhoef, E., Neeft, E., Grupa, J., Poley, A. 2011. Outline of a disposal concept in clay. OPERA-PG-COV008. COVRA, Vlissingen, 1–17.[2] Lemmens, K., Cachoir, C., Ferrand, K., Mennecart, T., Gielen, B., Vercauter, R. 2012. Interaction of cementitious materials with high-level waste matrices. In: Cementitious materials in safety cases for geological repositories for radioactive waste: role, evolution and interaction, NEA/RWM/R(2012)3, 129–130.[3] Utton, C.A., Hand, R.J., Bingham, P.A., Hyatt, N.C., Swanton, S.W., Williams, S.J. 2013. Dissolution of vitrified wastes in a high-pH calcium-rich solution. Jour. Nucl. Mat. 435, 112–122.[4] Brücher, H., Curtius, H. 2007. Investigations into the behaviour of research reactor fuel elements in repository relevant aquatic phases. Transactions 11th International Topical Meeting Research Reactor Fuel Management (RRFM), Lyon, France, 11– 15 March 2007, 5 p.[5] Curtius, H., Kaiser, G., Müller, E., Bosbach, D. 2011. Radionuclide release from research reactor spent fuel. Jour. Nucl. Mat., 416, 211–215.AcknowledgementsThe research leading to these results has received funding from the Dutch research programme on geological disposal, OPERA. OPERA is financed by the Dutch Ministry of Economic Affairs and the public limited liability company Elektriciteits-Produktiemaatschappij Zuid-Nederland (EPZ), and is coordinated by COVRA. |