Box 8.1. Examples of URL experiments on two different EBS concepts.

The European Commission-sponsored Prototype Repository Project (PRP) is based on a repository concept similar to SKB’s KBS-3 concept for SF disposal and involved full-scale experiments in the

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Aspo URL in Sweden, with drilling of a deposition hole, installation of a bentonite buffer, deposition of a waste canister and backfilling of tunnels. Electric heaters were used within the waste canister to simulate the heating effect of spent fuel. The project highlighted practical difficulties associated with repository operations and it was noted that these may act as constraints on disposal system design. It was also noted that the effects of these constraints are often not represented in SA models and, thus, the design process must be supported by information from engineering and other studies, as well as SA. Examples of the practical difficulties encountered include:

Accurate drilling of the deposition hole avoiding surface roughness, variations in diameter and curvature.

Having sufficiently powerful pumps to extract water flowing into the 8.5 m deep deposition holes.

Preventing water inflow from affecting the bentonite prior to canister deposition.

Handling and placement of many layers of heavy ( 2 tonne) bentonite blocks to millimetre accuracy.

Accurate positioning of the canister.

Achieving sufficient compaction of the backfill.

The PRP has shown that, despite the difficulties, accurate deposition hole drilling is possible, that the use of larger bentonite blocks would simplify the buffer emplacement process, that engineering solutions exist for limiting the inflow of water to the deposition hole (use of plastic liners) and that sufficient compaction of the backfill can be achieved for mixtures containing up to 30 per cent bentonite, but that the transport of the backfill mixture to its site of emplacement is an inefficient process.

Another example is a large-scale test for gas migration in the EBS and the adjacent geosphere (GMT) for a L/ILW silo-type repository which is being carried out at Nagra’s Grimsel Test Site (Shimura et al., 2006). As mentioned in the previous section, the gas permeability is an important issue in L/ILW repository system design and the GMT experiment considers the events related to gas generation and migration, which could influence the repository performance. Excess gas pressure could cause high stresses on the concrete silo and other EBS components leading to mechanical failure and pushing contaminated water out from the waste packages and the disposal cavern into the geosphere. Escaping gas could transport volatile radionuclides through the EBS and the surrounding host rock.

It is aimed through the experiment to assess the function of the whole system (the EBS and adjacent geosphere) in terms of gas migration, evaluate models of gas migration through the engineered barriers and into the geosphere, provide data for further improvement of the EBS design and emplacement procedures and demonstrate the construction and EBS emplacement for the repository. The experimental procedure consists of excavating a cavern, emplacing the engineered barrier systems (concrete silo, sand/bentonite buffer) and injecting gas in the centre of the concrete silo (Figs 8.5 and 8.6). Laboratory studies, detailed site characterisation activities (hydrogeological, rock mechanical) and modelling studies complement the main experiment. The GMT has demonstrated the feasibility of constructing and instrumenting a silo-type disposal system. The project is still ongoing, but initial results indicate that the neither the concrete nor the buffer suffered catastrophic loss of their containment ability following gas transport across them to the host rock (see Senger et al., 2006, for details).

have investigated the likely effects of gas produced (by metal corrosion and degradation of organics) in a concrete silo for L/ILW (see Figs 8.5 and 8.6 and Box 8.1).

8.3.3. Biosphere

National regulations for the disposal of radioactive waste usually require the proponent to assess the radiological impact of a proposed repository on the exposed population.

Invariably, the radiological impact is defined in terms of risk or dose or, rarely, both (e.g., Nagra, 2002a). Such calculations require information about the habits and lifestyles of the exposed humans; what they eat for example. But, because radionuclides from a repository are unlikely to reach the biosphere for many thousands of years, information about human habits and lifestyle at that time can only be imagined and is accepted by most implementers and regulators as being very difficult to predict. In addition, climate and landscape changes could profoundly affect the environment in which these humans will live and this, too, will have an impact on the dose calculation.

Biosphere R&D therefore aims to provide information to support the construction of assessment biospheres. These are models used to calculate dose and risk for different combinations of biosphere (e.g., climate and landscape) conditions. To achieve this, information is required in three basic areas, discussed in the next three sub-sections.

8.3.3.1. Radionuclide concentration and dispersion in the biosphere

When radionuclides appear in the biosphere, they can become concentrated in environmental media, such as soil and plants, which are subsequently used by humans. This gives rise to a range of exposure pathways, of which ingestion is usually the most important. Calculation of dose starts with the construction of exposure scenarios (e.g., IAEA, 2003); these require information on parameters such as the sorption of radionuclides on soil, uptake of radionuclides by plants, and radionuclide transfer factors for, say, cattle fodder to milk. For radionuclides released in groundwater, such data have been measured and are available for key radionuclides and exposure pathways.

8.3.3.2. Climate change

Interest in climate change forced by anthropogenic carbon dioxide has generated a large body of literature on the subject. Radwaste disposal programmes are mostly interested in climate change over longer time periods than are usually found in the wider literature but, nonetheless, disposal programmes have been fortunate in that they have been able to draw on this information and expertise to build an understanding of how the future climate might develop. Recent R&D has constructed a catalogue of possible future climate states – and corresponding environments – for use in post-closure safety assessment (e.g., Pro¨hl and Texier, 2004).

8.3.3.3. Landscape change

While recognising that landscape and climate are coupled, it is still useful to outline the ways in which landscape can affect dose calculations so that the main areas of R&D can be highlighted. One of the most significant landscape changes – as far as radwaste disposal is concerned – occurs when a repository radionuclide discharge area that is currently covered by water becomes uncovered. Examples are uplift caused by glacial rebound (SKB, 1999) or tectonics (Nagra, 2002a), sea-level fall under ice age conditions (Nirex, 1997a; JNC, 2000) and sedimentation of lakes. Other important factors are the effects of cold climate periods, specifically, glacial erosion (Nagra, 1994; Nirex, 1995) and deep injection of water beneath ice-sheets (Boulton et al., 2001).

The role and treatment of the biosphere in SAs has been discussed extensively in international fora. The consensus from these discussions is that it is reasonable to assess biosphere performance in a schematic manner, compared to the assessment of the engineered barriers and geosphere, e.g., as proposed by the NEA Biosphere Working