Implementation of the full-scale emplacement (FE) experiment at the Mont Terri rock laboratory

Abstract

Opalinus Clay is currently being assessed as the host rock for a deep geological repository for high-level and low-and intermediate-level radioactive wastes in Switzerland. Within this framework, the 'Full-Scale Emplacement' (FE) experiment was initiated at the Mont Terri rock laboratory close to the small town of St-Ursanne in Switzerland. The FE experiment simulates, as realistically as possible, the construction, waste emplacement, backfilling and early post-closure evolution of a spent fuel/ vitrified high-level waste disposal tunnel according to the Swiss repository concept. The main aim of this multiple heater test is the investigation of repository-induced thermo-hydro-mechanical (THM) coupled effects on the host rock at this scale and the validation of existing coupled THM models. For this, several hundred sensors were installed in the rock, the tunnel lining, the bentonite buffer, the heaters and the plug. This paper is structured according to the implementation timeline of the FE experiment. It documents relevant details about the instrumentation, the tunnel construction, the production of the bentonite blocks and the highly compacted 'granulated bentonite mixture' (GBM), the development and construction of the prototype 'backfilling machine' (BFM) and its testing for horizontal GBM emplacement. Finally, the plug construction and the start of all 3 heaters (with a thermal output of 1350 Watt each) in February 2015 are briefly described. In this paper, measurement results representative of the different experimental steps are also presented. Tunnel construction aspects are discussed on the basis of tunnel wall displacements, permeability testing and relative humidity measurements around the tunnel. GBM densities achieved with the BFM in the different off-site mock-up tests and, finally, in the FE tunnel are presented. Finally, in situ thermal conductivity and temperature measurements recorded during the first heating months are presented.

Interactions of bentonite with metal and concrete from the FEBEX experiment: mineralogical and geochemical investigations of selected sampling sites

“…They are required both to validate the safety functions and to investigate the processes taking place after closure of the repository. Worldwide, only a few heated real-scale tests have been launched: the prototype repository experiment in crystalline rock at Äspö, Sweden (Johannesson et al , 2007; Dohrmann & Kaufhold, 2014; Svemar et al , 2016), the PRACLAY experiment in Boom Clay at the HADES rock laboratory in Belgium (Bernier et al , 2007), the ALC (full-scale demonstrations experiment at Bure URL) experiment in Callovo-Oxfordian Clay at ANDRA's rock laboratory in France (Gugala, 2015), the recently installed FE experiment (full-scale emplacement experiment) in Opalinus Clay at Mont Terri in Switzerland (Müller et al , 2017) and the FEBEX test investigated in the present study.…”

Properties of Granulated Bentonite Mixtures for Radioactive Waste Disposal: A Review

“…The GBM and pellet mixtures presented in this review comprise several bentonite materials, mainly MX-80, Serrata, Kunigel (V1 and GX), FoCa, and GMZ. MX-80 GBM from Wyoming, USA, is a sodium montmorillonite bentonite and was adopted by NAGRA in the FE experiment [7]. Serrata GBM produced from bentonite deposits (the same origin as FEBEX bentonite) in the zone of Serrata (Almería, Spain) was used in the EB experiment at the Mont Terri rock laboratory [8].…”

“…It was shown that the fraction of fines could be an important factor to achieve higher compaction dry density. The best mixture was used in the FE experiment at the Mont Terri rock laboratory [7] where the target DD of 1.45 g/cm 3 was achieved.…”

“…MX80 GBM was used in a full-scale in-situ demonstration experiment as buffer around the heaters mimicking waste in an FE experiment [7]. The authors measured a gradual increase of thermal conductivities over a period of one year after the emplacement of GBM, indicating a slow wetting of GBM due to water inflow at a very low that the rate of thermal increase was higher at points located close to local water inflow spots (e.g., near rock bolts).…”

“…Many design concepts consider pre-compacted blocks of bentonite as a buffer material, e.g., the prototype repository in-situ test carried out at the Äspö Hard Rock Laboratory in Sweden [4]. Alternatively, granulated bentonite mixtures (GBM) consisting of granules (highly compressed pellets) and powders in a mix of graded proportions have also been considered as the buffer material [6] as adopted by National Cooperative for the Disposal of Radioactive Waste (NAGRA) in the prototype Full-scale Emplacement (FE) experiment [7] and Engineered Barrier (EB) experiment [8,9] at Mont Terri rock laboratory in Switzerland. GBM is purported to improve emplacement operations including ease of transportation and in-situ placement/backfilling, achieve good compaction attributes to meet a required emplacement dry density.…”