The dissolution of simulant vitrified intermediate level nuclear waste in young cement water

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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.40

The dissolution of simulant vitrified intermediate level nuclear waste in young cement water Colleen Mann,1 Jeremy R. Eskelsen2, Donovan N. Leonard2 Eric Pierce2 and Claire L Corkhill1 1 Immobilisation Science Laboratory, Department of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK

2

Oak Ridge National Laboratory, Oak Ridge, TN, USA

ABSTRACT

It is pertinent to the safety case for geological disposal in the UK that the behaviour of vitrified wastes in proximity to cementitious materials is understood. In this study, vitrified simulant intermediate level nuclear waste (ILW) was subject to dissolution in a synthetic cement water solution to simulate disposal conditions. Results show that the presence of alkali / alkaline earth elements in the cementitious solution can be favourable, at least in the shortterm, leading to lower dissolution rates associated with incorporation of these elements into the altered layer of the glass. Corresponding author. Email address: [email protected] (C L Corkhill)

INTRODUCTION In recent years, the effects of disposing of vitrified nuclear waste in high pH, cement rich environments have been investigated; in particular, the UK and Belgium are considering geological disposal scenarios where glass will be in close proximity to cementitious materials. In the UK, the possibility for co-disposal of vitrified intermediate level waste (ILW) with cementitious ILW is being explored; if this is in a hard-rock geology (e.g. granite), a Portland cement / limestone blend material, known as Nirex Reference Vault Backfill (NRVB) [1], may be employed as a backfill. It is therefore necessary to undertake performance assessments to determine the compatibility of vitrified ILW and NRVB [1].

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Portland cement is composed of Ca-silicates, Ca-aluminate, Ca-Al-ferrite and other minor components including Ca-sulphates, calcite, Mg-hydroxide and Na/K sulphates[2]. Within a geological disposal facility (GDF), when Portland cement comes into contact with groundwater, K and Na will be the first elements to leach from the cement in the form of hydroxides, giving rise to solutions with high pH (pH > 13). With continual replenishment of groundwater, the cement hydrate phase, portlandite Ca(OH) 2, will be leached, resulting in a pH of 12. Once the Ca is depleted from portlandite, calcium silicate hydrate (C-S-H) phases will be dissolved, reducing the pH of the groundwater to between 10 and 11 [2][3]. Under such hyperalkaline conditions, the solubility of Si, Al and Zr from the glass matrix increases [2][5][6] and, when alkali and alkaline earth elements released from the cement interact with the silica gel, it can inf

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