Repository design

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For cement-based repository designs, this will play no role but, for repository designs using bentonite (predominantly, but not exclusively, HLW/SF), special attention should be paid to materials such as injected cement grouts, e.g., which are virtually impossible to remove from the repository before closure (Walker and Metcalfe, 2004) and may have a significant effect on the performance of the bentonite. Generally speaking, any foreign materials brought or transported into the underground facility and used during construction, especially those that will not form part of the repository infrastructure, should be controlled and registered accurately for inclusion in post-closure safety assessments. Examples include scaffolding materials, grouting materials used to consolidate fractured rock, organic and oxidising substances.

5.4.2. Design approach and methodologies

Detailed design requirements and methods, as well as data, need to be specified to develop the EBS and disposal facility, taking into account site geological environments. General frameworks have been developed for the EBS and repository design for a wide range of geological environments (e.g., JNC, 2000; see Figure 5.4). This provides the basis for designing and constructing the repository, which is flexible enough to be tailored to specific characteristics of a potential disposal site.

Design requirements relating to specifications for each component of the EBS and the disposal facility are selected to ensure safety of the integrated repository system. Specifications that are determined to meet the design requirements based on the design analyses may have a certain range of values. Possible specifications of the EBS and the disposal facility are discussed so as to have sufficient safety margins for system performance, taking into account geological environments and the uncertainty of data. In addition, analyses of the long-term integrity of the EBS must be conducted. The results of these analyses are integrated into the safety assessment and must be reflected in the specifications of the EBS and the disposal facility.

To determine a reasonable specification range in accordance with the design requirements and the integrated design logic, design analyses are conducted with realistic design data. Data required for design include groundwater chemistry, mechanical, thermal and hydraulic properties of the rock mass and properties of EBS materials. Various tests at both laboratory and engineering scale, as well as in URLs, are conducted to support development of these analysis methods and databases. Based on the findings from these experiments and field investigations, models and codes for the design analyses can be improved. In addition, it is important to assess the long-term system safety of example specifications within integrated designs and to incorporate assessment results into improvement of subsequent designs.

The major function of the container (or overpack) is to physically contain radionuclides for a certain period of time after waste package emplacement. The design requirements, such as corrosion resistance, pressure resistance, radiation shielding, physical containment and ease of manufacture, are considered in selecting materials, in determining the shape and in specifying the thickness of the container. The container may also be designed to prevent, e.g., the HLW/SF from contacting groundwater for, e.g., at least 1000 years after emplacement in the repository, during which time relatively short-lived nuclides will have decayed to insignificant levels and radiogenic heat production will be considerably reduced.

START

Basic concept of geological disposal system

- Preventing over-conservatism

Identification of basic design requirements

- Preserving flexibility

Integrated design methodology

Selection of design requirements relating to specifications for each component

	Refined design					EBS design
						Refined design
	analysis methods	Overpack design,			Buffer design,
				Mechanical interaction		analysis methods
	with realistic	- Materials, dimensions			- Density, thickness,
						with realistic
	design data				sand mixing ratio
						design data
	- Safety margins for uncertainty		Determination of example specifications of EBS

Disposal facility design

Assumption of tunnel dimensions

Refined design

Evaluation of mechanical stability of

Evaluation of thermal effects,

Refined design

analysis methods

analysis methods

tunnels,

-Tunnel spacing and waste package,

with realistic

with realistic

-Specifications of tunnel support,

pitch

design data

design data

-Tunnel spacing and waste package pitch

Determination of tunnel spacing and waste package pitch

Refined design

Repository layout design

Realistic design data

analysis methods

with realistic

If necessary, modify

Evaluation of integrity of EBS

design data

Selection of construction, operation and closure technologies

Confirmation of repository performance by safety assessment

If necessary, modify

END

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Fig. 5.4. Integrated design methodology for the EBS and disposal facility for HLW/SF (from JNC, 2000).

Repository design

129

(a)

(b)

Fig. 5.5. Example of test manufacturing of waste containers. (a) A titanium-carbon steel composite (JNC, 2000). (b) A copper-carbon steel composite (SKB, 1998).

Various materials, e.g., carbon steel, Cu-Fe or Ti-Fe composites (Figure 5.5) are under consideration, depending on the repository concept. To date, carbon steel appears to satisfy design requirements and it has been successfully used as a structural material. The container thickness can be estimated based on corrosion resistance, pressure resistance and radiation shielding function. Feasibility of manufacturing has been demonstrated for carbon steel, pure copper and composite containers.

To ensure the long-term safety of geological disposal, the buffer is required to have favourable physical and chemical characteristics, such as self-sealing and mechanical buffering, good thermal conductivity, a low hydraulic conductivity, a colloid filtration function and a chemical buffering capability to restrict radionuclide migration.

For HLW/SF, extensive international studies have identified pure bentonite that can perform the required functions. Bentonite-quartz sand mixtures have also been examined for the buffer material to increase cost effectiveness (e.g., JNC, 2000; Nagra, 2002). Considering the changes in properties of the buffer caused by mixing in the sand, the quartz sand mixing ratio and dry density that would meet the functional requirements of the buffer have been studied (e.g., Biggin et al., 2003). For example, it is possible to set a mixture of 70 wt% bentonite and 30 wt% quartz sand with a dry density of 1.6 Mg m 3 as a buffer material specification for both the in-situ compaction and the block fabrication method.

The requirements which influence the thickness of the buffer include radionuclide retardation and thermal conductivity, as well as stress buffering capability, self-sealing ability and workability. As for the stress buffering function, the relationship between the

130

H. Umeki

pressure resistance thickness of the container and buffer thickness should be considered, taking into account the mechanical interaction between the container and buffer, which is caused by the consolidation reaction force of the buffer, assuming an increase in container volume as a result of corrosion. The consolidation reaction force of the buffer is determined from the void ratio of the buffer material after rock mass creep deformation and corrosion expansion, based on the relationship between the void ratio and the effective stress obtained from consolidation tests. The consolidation reaction force depends largely on the thickness of the buffer.

Although, for HLW/SF, the focus is on a bentonite-based material, there is a wide envelope of variants of bentonite/sand ratio, buffer thickness and dry density within which acceptable performance may be found (e.g., Figure 5.6).

For non-cementitious L/ILW (e.g., hulls and ends) disposal tunnels, a bentonite-based buffer may also be used to ensure diffusion-dominant transport processes and high radionuclide retardation. The thickness of the buffer may be decided based on its marginal effects on radionuclide release. For most cementitious L/ILW repository designs, the use of bentonite is avoided due to current uncertainties over the degree of alteration of the bentonite by hyperalkaline leachates from the cement (see also comments in Chapters 8 and 10). Nevertheless, the SFR cementitious repository in Sweden utilises a bentonite buffer, but this was built before any awareness of the potential problem. The L/ILW cementitious repository which is currently under construction at Rokkasho in north-east Japan is also likely to include bentonite in the design, but this is not yet definite. Interestingly, in the recent SA on TRU waste (FEPC & JAEA, in preparation), selected model representations are presented which show that alteration of bentonite should not be significant, something which flies in the face of most laboratory (Adler et al., 1999), URL (Ma¨der et al., 2004), modelling (Soler and Ma¨der, 2002a,b) and natural analogue (Smellie et al., 2001) data currently available. Nevertheless, as noted in Chapter 8, more R&D is required in this area before a conclusive verdict can be reached.

	1
	0.9
	0.8
[m]	0.7
thickness
	0.6
Buffer
	0.5
	0.4
	0.3
	0.2
	0.8

b.	a.			c.

Region with selfsealing function

Region with colloid filtration function

Region of thickness and density of buffer compatible with the thickness of the overpack

Dry density and thickness of buffer specified

Buffer thickness

Buffer thickness

40 cm

40 cm

Hard rock

Soft rock

1

1.2

1.4

1.6

1.8

2

2.2

Dry density of buffer [Mg m–3 ]

Fig. 5.6. Relationship between buffer material thickness and density to satisfy design requirements (JNC, 2000).