A J Hooper and J Mathieson
United Kingdom Nirex Limited
Curie Avenue
Harwell, Didcot
Oxfordshire OX11 ORH
United Kingdom

Nirex is the organisation responsible in the United Kingdom for the development of a deep repository for the safe disposal of solid intermediate-level and certain low-level radioactive wastes. In 1991 following preliminary geological investigations at two sites, Dounreay in Caithness, NE Scotland, and Sellafield in Cumbria, NW England, investigations were focused on the site at Sellafield. In 1994 Nirex proposed that, as the next stage in the detailed characterisation of the site, an underground Rock Characterisation Facility (RCF) was required. The relevant planning authority refused permission to develop the RCF and that refusal was upheld by the Secretary of State for the Environment in March 1997 following a public inquiry held between September 1995 and February 1996. This paper describes the necessary role that Nirex identifies for underground research in the development of a robust case for the safety of deep disposal by reference to its work on a potential repository site at Sellafield.

Nirex is the organisation responsible in the United Kingdom for the development of a deep repository for the safe disposal of solid intermediate-level (ILW) and some long-lived low-level (LLW) radioactive wastes. The reprocessing of spent nuclear fuel from the first generation of power reactors in the UK (at the British Nuclear Fuels plc Works at Sellafield in Cumbria) has resulted in a relatively larger component of long-lived ILW (characterised as TRU wastes in many programmes) than in those countries where there is no reprocessing. These long-lived wastes require a deep, rather than near-surface, repository to ensure long-term safety.

Since 1987 it has been Government policy that a deep repository should be developed for all categories of ILW. Government confirmed that policy in its Radioactive Waste Management White Paper (Ref. 1) published in July 1995.

It is the responsibility of Nirex to identify a suitable site and to propose the development of a repository there. Three principal permissions will be required before a repository can be constructed and put into operation:

• planning permission under the Town and Country Planning Act 1990 : while the planning application is made to the local planning authority, the Government has indicated that the planning application for the repository will be "called in" for determination by the Secretary of State after a full public inquiry;
• a nuclear site licence under the Nuclear Installations Act (Ref. 2) : issued by the Nuclear Installations Inspectorate of the Health and Safety Executive; and
• authorisation to dispose of waste under the Radioactive Substances Act 1993 (Ref. 3) : granted by the Environment Agency in England and Wales.

The first two of these permissions must be gained before repository construction commences, the third prior to commencement of disposal of waste in the repository.

The Nirex deep waste repository programme is funded by the principal producers of radioactive wastes (British Nuclear Fuels plc, Magnox Electric, United Kingdom Atomic Energy Authority, British Energy and the Ministry of Defence) on the basis of the polluter-pays principle.

In 1994, as a key step in its research and development programme, Nirex applied for permission to construct an underground research laboratory or Rock Characterisation Facility (RCF) at a site near Sellafield in west Cumbria. The RCF was to be part of an extensive programme of scientific investigations to determine whether or not the site would be suitable to host a deep repository. If, from the RCF and other studies undertaken during the site confirmation stage, Nirex had judged the prospective post-closure safety performance of a repository in the Sellafield area to be acceptable and repository construction was confirmed to be safe at acceptable cost, then it would have been ready to propose development of a repository at Sellafield. On 17 March 1997 the Secretary of State for the Environment dismissed Nirex's appeal against the refusal by Cumbria County Council to allow construction of the RCF, at the Longlands Farm site near Sellafield. Following this decision, the investigations that had been in progress at Sellafield since 1989, were wound down.

The role of the RCF in the national disposal strategy was to build confidence in the assessments of post-closure performance of a repository potentially to be located in the same geological setting as the underground laboratory (although not necessarily in exactly the same location). In order to fulfil this role, two main scientific objectives were established for the RCF :

• to obtain information that is only available from a phase of underground investigations; and
• to test models of the geology, hydrogeology and geotechnics of the rock mass.

The information that is only available from an RCF is related to:

• the nature of rock mass fractures and their influence on groundwater flow;
• the response of the groundwater system to the stressing induced by construction of the RCF;
• collection of hydrochemical information not able to be obtained from deep boreholes drilled from the surface, especially measurements related to pH, Eh and alkalinity;
• measurements of the geomechanical response of the rock mass to excavation of underground openings; and
• data on aspects such as gas migration, sealing and tracer migration. These measurements were planned for latter stages of the RCF because such data were not judged to be required to inform the decision on site suitability, but were required to inform the safety case to support a subsequent application for authorisation to dispose of waste should site suitability have been established sufficiently to apply for and be granted permission to construct a repository.

The process that had been adopted for testing models within the science programme for the first phase of work in the RCF is summarised in the flow chart shown in Figure 1. Although the flow chart is shown largely as a series of sequential operations there would inevitably have been iterations during the process as views developed regarding, for example, the measurements needed to test the models and the design of the programme of measurements.

The design of the RCF could be considered as comprising two parts; the engineering construction of the shafts and galleries to permit access to the underground rock mass, and the scientific programme to be carried out within the constructed excavations. These two design parts required careful integration both to ensure that the scientific objectives would not be compromised by the construction activities and that the construction activities would not be unreasonably impacted by the requirements to undertake the programme of scientific work. Such impacts would have had the potential for significantly increasing the costs of the total RCF project.

The main elements of the planned RCF were to be a series of horizontal galleries at a depth between 650 and 900 m below Ordnance Datum (bOD), accessed by two vertical shafts located at the centre of the facility (the North and South Shafts). The planning of the RCF was based on an assumption that the galleries were to be excavated at 650 m bOD. An illustrative layout is shown in Figure 2, indicating the three phases of excavation that were envisaged. The actual layout of the galleries was to be reviewed as information from the early stages of the RCF programme became available. The galleries were required for two purposes: to provide an opportunity for directly exploring a greater proportion of the Potential Repository Zone than would have been possible from Phase 1 excavation (see Figure 2) and to provide adequate space to allow experiments to be carried out without one experiment interfering with another and invalidating the results.

The location of the South Shaft was coincident with that of a Nirex deep borehole (Borehole RCF3), which is located about 100 m south of Longlands Farm, near Gosforth in west Cumbria.

The shafts would have been sunk through approximately 520 m of sedimentary rocks (principally Permo-Triassic sandstones of the Sherwood Sandstone Group) into underlying basement rocks of the Borrowdale Volcanic Group. The Borrowdale Volcanic Group of rocks were the potential host rock for the deep waste repository and also for the main galleries of the proposed RCF.

Construction of the RCF was programmed in three phases. The first phase was to comprise shaft sinking, excavation of connecting roadways and testing to evaluate models of the formations penetrated. This was likely to have taken about 3.5 years. The second phase was to involve excavation of a system of galleries within 100 - 200 m of the shafts; these were to be designed for specific experiments. The third phase was to extend galleries up to a maximum total axial length of 975 m to allow isolated experiments to be conducted, and to enable investigations to the periphery of the Potential Repository Zone to be conducted using drilled boreholes. Sections of both shafts through the sedimentary rocks would have been lined with concrete to restrict groundwater inflow. A backwall grouting programme was to further restrict groundwater inflows by sealing up small cavities and fissures between the lining and the rock mass as shaft sinking progressed, thus limiting the ‘open’ period of rock sections. The total duration of the RCF programme was to have been about 10 years.

It was clearly recognised that construction of the RCF would perturb the groundwater pressures and hydrochemistry at the site. The undisturbed conditions incorporate the effects of natural features and processes which account for spatial and/or temporal variability in the system. Prior to construction of the RCF, baseline conditions for groundwater pressures and hydrochemical conditions needed to be established. Sufficient information needed to have been acquired so that:

• the disturbance created by the RCF could be measured with confidence and used to test and develop models of the site; and
• there was a sufficient database on undisturbed conditions that the properties of the Potential Repository Zone could be interpreted within the context of regional groundwater flow and hydrochemical models.

The information on baseline conditions was assembled (Ref. 4) and subjected to independent peer review by an international review group, the results of which were published (Ref. 5). The terms of reference for the peer review were to examine critically the baseline information presented by Nirex and to express opinions on the following:

1. the appropriateness of the definition of baseline as adopted by Nirex in relation to the purpose for which Nirex would use the information; and
2. the sufficiency of the information obtained in relation to establishing baseline, in accordance with the established definition, prior to the commencement of RCF shaft construction.

The International Review Group considered that the two criteria adopted by Nirex were sound and objective within the requirements of the Sellafield studies and the Group unequivocally supported the Nirex conclusions that baseline conditions had been established.

Following the announcement of the refusal of planning permission for the RCF the investigations at Sellafield have been wound down. The only data acquisition activities that are continuing relate to the operation of the hydrogeological monitoring system. This is a unique system providing integrated monitoring of meteorological conditions, surface water hydrology, shallow and deep groundwater pressures. The data acquired from the investigations carried out at the site have been interpreted and reported.

Much of the detailed information that is now available on the planning of the science programme to be undertaken in the RCF was produced to a programme having as its key milestone the commencement of the RCF construction. This information was thus not available for input into the RCF planning inquiry that was called following initial refusal of planning permission for the RCF.

The failure of Nirex to obtain planning permission for the underground rock laboratory at Sellafield has, in the words of the United Kingdom Parliamentary Office of Science and Technology (Ref. 6), "stopped dead in its tracks the search for a long-term disposal route for intermediate level radioactive waste". Parliamentary interest in this is rising and the House of Lords Select Committee on Science and Technology has announced an enquiry starting in early 1998 into ‘The Management of Nuclear Waste (civil and military) in the United Kingdom’.

The information on the scientific work carried out by Nirex provides a detailed record of what has been achieved and on approaches that can be adopted for addressing scientific aspects of radioactive waste disposal and the role that underground investigations can play in the process. In establishing the way forward to implement government policy, socio-political issues, particularly related to achieving public confidence in the process, and questions related to the institutional and regulatory frameworks under which such work should proceed are likely to be of equal or greater importance than matters related specifically to the scientific aspects of the evaluation of sites for radioactive waste disposal.

A number of research projects that were complementary to the proposed programme of underground research in the RCF have been progressed and, where appropriate, will continue.

Nirex has valued its participation in the work at the Äspö Hard Rock Laboratory operated by SKB in Sweden. Two research projects have been of special interest, namely the Zone of Excavation Disturbance Experiment (ZEDEX) and the Tracer Retention Understanding Experiment (TRUE) - Block Scale (Ref. 7).

The ZEDEX Project involves collaboration between SKB, ANDRA (France) and Nirex. The objectives for Nirex were as follows :

• to understand the mechanical disturbance of a hard, fractured rock mass as a function of its method of excavation;
• to demonstrate the capability to measure significant changes in the hydrogeological and geotechnical properties of the rock mass as a result of excavation; and
• to improve understanding of the hydraulic significance of the excavation disturbed zone with respect to long-term repository performance.

The final report on the ZEDEX Project is due for publication in 1998. The results of the Project place the participants in a good position to carry out meaningful analyses of the nature and extent of geomechanical and hydraulic disturbances caused by excavations at prospective repository sites.

The TRUE-Block Scale Project involves collaboration between SKB, ANDRA (France), ENRESA (Spain), POSIVA (Finland) and Nirex. The objectives for Nirex are as follows :

• to understand solute transport in a fractured rock mass for which the hydraulic properties have been adequately characterised; and
• to test models of solute transport over lengthscales of relevance to repository post-closure performance assessments.

As in the case of the ZEDEX Project, the participation in TRUE-Block Scale was additionally intended to inform the design of experiments to be conducted at an appropriate stage of the RCF Project.

Significant quantities of gas would be generated from the inventory of ILW and LLW designated for deep disposal in the United Kingdom, as a result of either the aqueous corrosion of metallic wastes and waste containers to produce hydrogen, or the microbial degradation of organic materials to produce carbon dioxide and methane. Nirex has conducted research into gas migration through water-saturated rocks over a number of years (Refs. 8 and 9). The computer codes and models that are currently available have been applied in the most recently published assessment of the post-closure performance of a repository at Sellafield (Ref. 10). Useful research can be carried out to develop more sophisticated models of two-phase gas/water flow through geological media which can then be applied to test the sensitivity of repository performance to a range of flow conditions. However, to build confidence in the application of such models to a specific geological setting, underground research is almost certainly necessary to obtain the required input information, for example on the relative permeabilities for gas and water (Ref. 8).

The Nirex repository concept for the safe disposal of ILW includes an important contribution from the chemical containment that would be afforded through the alkaline conditioning of groundwater within the repository by cement mineral phases in a specially-formulated backfill material. Research is continuing to build confidence in this important component in the multi-barrier containment system. In relation to underground research the key objective identified for the RCF was to understand the nature and properties of new mineral phases that would be created as alkaline water migrated away from a repository. A good understanding of generic aspects of the relevant processes has been derived from participation in the Maqarin (Jordan) natural analogue project (Ref. 11).

Particularly given the richly varied deep geological conditions which occur across the United Kingdom, the strategy for research in relation to underground conditions, in the absence of investigations at a specific site, will necessarily focus on the development of scientific capabilities and improvement of understanding of generic processes. Clearly such a strategy will continue to benefit from collaboration with other national programmes where there are shared interests.

1. "Review of Radioactive Waste Management Policy - Final Conclusions", Cm 2919, Her Majesty's Stationery Office, London (1995).
2. "Nuclear Site Licences under the Nuclear Installations Act 1965 (as amended) - Notes for Applicants", HSE Books, Health and Safety Executive (1994).
3. Radioactive Substances Act 1993, Her Majesty's Stationery Office, London (1993).
4. "Rock Characterisation Facility, Longlands Farm, West Cumbria : Report on Baseline Groundwater Pressures and Hydrochemistry", Nirex Science Report SA/96/003 (1996).
5. "Rock Characterisation Facility, Longlands Farm, West Cumbria : Report on Baseline Groundwater Pressures and Hydrochemistry - Summary Report", Nirex Science Report S/96/004 (1996).
6. "Radioactive Waste -Where Next?", Report by the Parliamentary Office of Science and Technology, London (November 1997).
7. "Äspö Hard Rock Laboratory : 10 Years of Research", SKB (Sweden) Report, Eds. M. Hammarström and O. Olsson (June 1996).
8. "Post-closure Performance Assessment : Gas Generation and Migration", Nirex Science Report S/94/003 (1994).
9. "Nirex Gas Generation and Migration Research : Report on Current Status in 1994", Nirex Science Report S/96/002 (1996).
10. "An Assessment of the Post-closure Performance of a Deep Waste Repository at Sellafield, Volume 4 : The Gas Pathway", Nirex Science Report S/97/012 (1997).
11. "A Natural Analogue Study of the Maqarin Hyperalkaline Groundwaters . 1. Source Term Description and Thermodynamic Database Testing, Nagra (Switzerland) Technical Report Series, NTB 91-10, Ed. W. R. Alexander (1992).

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