Opalinus Clay: a solution for Switzerland's waste1 February 2003
Swiss nuclear waste management company Nagra has demonstrated the feasibility of disposal for high-level waste and spent fuel in Opalinus Clay. A corresponding comprehensive report was submitted to the Swiss federal government in December 2002.
In Switzerland, the producers of radioactive waste are responsible for its proper and safe management as demanded by the "polluter pays" principle. The approach to waste management is mainly based on the federal government ruling of 1978 on the Atomic Act which also stipulates that a project should be prepared that ensures "the permanent, safe management and final disposal of radioactive waste arising from the power plants."
A first feasibility study, Project Gewähr 1985, was submitted to the authorities by the Swiss nuclear waste management company Nagra in 1985. The study addressed the feasibility of disposal of spent nuclear fuel (SNF), vitrified high-level waste (HLW) and long-lived intermediate-level waste (ILW) in a crystalline host rock. The federal government accepted the study on 3 June 1988, although with the proviso that a siting demonstration should be delivered for SNF, HLW and ILW and that the work should be extended to include non-crystalline (ie sedimentary) host rocks.
After several years of field investigations, Nagra fulfilled this requirement by its report on the Opalinus Clay Project, submitted to the federal government in December last year. The project provides a basis for planning future waste management activities, including estimating costs and gives a framework for in-depth social discussion of waste management issues. It also represents the final step in the demonstration of disposal feasibility - in German: Entsorgungsnachweis.
The Opalinus Clay Project is intended to provide input to the deliberations of the Swiss federal government on the future strategy to be followed in connection with the management and disposal of Swiss radioactive waste. Based on the results of the project and the systematically conducted selection procedure, Nagra requested the federal government:
• To confirm that feasibility of disposal has been demonstrated as required.
• To agree to focusing future investigations on the disposal of SNF, HLW and ILW in Switzerland on the Opalinus Clay and the potential siting region in the Zürcher Weinland.
According to information from the responsible federal authorities, the detailed review of the Opalinus Clay Project documentation will take around two years. Thereafter the Waste Management Programme 2005 will be formulated, meaning that a final decision by the Federal Council can be expected in 2006.
Status of waste management
The current status of the work on the Swiss waste management programme can be summarised as follows:
• For interim storage of waste, decentralised storage facilities exist at the individual nuclear plants, as well as two centralised storage facilities: ZWILAG for the wastes from the power plants, and a facility of the federal government (BZL) for waste from medicine, industry and research. The storage capacity is sufficient to cover waste arising during the operating lifetime of the existing power plants.
• For the disposal of low- and intermediate-level waste (LILW), the Wellenberg site was selected, investigated and, finally, judged as being suitable by the safety authorities and by experts in relevant fields. An application was made for the necessary concession for an exploratory drift, but was refused by the people of the Nidwalden canton in a referendum on 22 September 2002. Consequently, Wellenberg was abandoned, and, in 2005, a formal programme for further LILW work will be delivered to the federal government for approval.
• With the Opalinus Clay Project, a demonstration of the feasibility of final disposal of SNF, HLW and ILW has been submitted to the federal authorities for their review. Nagra is proposing to the authorities that future investigations relating to deep geological disposal should focus on the Opalinus Clay and on the potential siting area Zürcher Weinland.
As part of the current parliamentary deliberations on a new Nuclear Energy Law, the waste producers are required to present the federal government with a programme that will lead to the realisation of repositories. This programme - "Programme 2005" - will be drafted by Nagra, and will draw on the high level of scientific and technical expertise that Nagra has built up over the years and which has come to be recognised internationally. Most of the technical components and options for the programme have already been defined and evaluated. Bringing all the different components together to form a comprehensive waste management strategy will, however, be possible only after the response of the authorities to the Entsorgungsnachweis project is known and the legal and political framework has been clarified. The political powers are called upon to create, with the new Nuclear Energy Law, a legal framework that will allow timely implementation of site-specific waste management facilities with the cooperation - but without the veto right - of affected cantons.
Site selection procedure
The feasibility demonstration provided by the Opalinus Clay Project basically consists of three components:
• Siting. This demonstrates that, in Switzerland, one or more sites exist with suitable (in terms of safety) geological and hydrogeological properties.
• Engineering feasibility. This demonstrates that a repository can be constructed and operated at such a site using current technology.
• Safety. This demonstrates that such a repository meets the requirements specified by the authorities regarding long-term safety.
For the siting demonstration, Nagra pursued a systematic, broadly based and transparent evaluation procedure, with sequential narrowing-down of options, leading to selection of potential sedimentary host rocks and potential siting areas. The procedure, which is documented in three interim reports, was followed with the approval of the federal safety authorities and their experts.
As a first step, Nagra published a broad review of the potential suitability of sedimentary formations occurring in Switzerland based on existing information. As a result, clay-rich rock strata like Opalinus Clay and the Lower Freshwater Molasse (USM) began to emerge as leading options. Potential siting regions were identified.
This was followed by a regional investigation phase (1990-1993) with specific field activities, with the emphasis on obtaining further geological information to allow the selection of a priority sedimentary host rock option and potential siting areas. A regional study of the USM led to this formation being classified as a reserve option with large spatial potential, but with some reservations regarding explorability. For the Opalinus Clay option, studies led in 1994 to the identification of a priority area for local investigations in the Zürich canton (the northern part of the Zürcher Weinland region). This selection was approved by the regulatory authorities and their experts.
After 1994, the next step was a detailed characterisation of the Opalinus Clay and the Zürcher Weinland region, with the following key components:
• A 3D seismic campaign covering an area of approximately 50km2.
• An exploratory borehole (Benken).
• Experiments in the Opalinus Clay as part of the international research programme in the Mont Terri Rock Laboratory (Jura canton).
• Regionally based comparative studies of the Opalinus Clay, as well as comparisons with clay formations being investigated in other countries with a view to geological disposal.
The facility is designed to meet specific requirements specified in detail by the federal safety authorities, and is also consistent with the recommendations of the Expert Group on Disposal Concepts for Radioactive Waste (EKRA), which was established in 1999 by the Swiss Department of the Environment, Transport, Energy and Communication (DETEC) following broad public discussion on issues relating to the permanent safe management and final disposal of radioactive waste.
EKRA came to the conclusion that geological disposal is the only option that fulfils requirements relating to the long-term safety of man and the environment. It also recommended that, to take into account the societal requirement for a cautious approach to decisions that might be difficult to reverse, final closure of the repository should take place only after an extended monitoring phase. EKRA formulated a proposal for the layout of a geological repository which allows stepwise progress from a monitored facility to a closed repository, with the option of continuing certain monitoring activities even after (partial) closure.
The concept of a stepwise procedure, in which a deep geological storage facility is closed after an extended monitoring phase and thus becomes a final repository, was also incorporated into the draft of the new Swiss Nuclear Energy Law. Although parliamentary debate had not been concluded at the time of preparing the Opalinus Clay Project, the geological disposal concept was undisputed by both chambers.
The repository considered in the Opalinus Clay Project is consistent with the corresponding provisions of the new Nuclear Energy Law and also contains the key features of EKRA's concept (test, pilot and main facilities).
The wastes to be emplaced in the planned repository are spent fuel elements and radioactive waste from reprocessing. For the purpose of designing the facilities and for assessing long-term safety, it is assumed that waste will be produced by existing nuclear plants and that these will generate 192GWe-year within their lifetimes. This scenario gives the following waste volumes:
• SNF: 2065 disposal canisters.
• Vitrified HLW: 730 canisters.
• Solidified long-lived ILW: around 4360m3 (so-called cemented waste option).
To cover all eventualities, an extreme variant also is considered which corresponds to an energy production of 300GWe-year.
The demonstration of engineering feasibility for the underground facilities is based on measured rock mechanical parameters. It required the drawing up a general procedure for handling and emplacement of radioactive waste, construction of the engineered safety barriers, design of the facility in accordance with safety regulations, reviewing operational safety and ventilation during the operational phase and rough design of transport and handling equipment. It also involved special studies on retrievability and closure of the facility.
The overall conclusion is that a deep geological repository for SNF, HLW and ILW in the Opalinus Clay of the Zürcher Weinland can be constructed, operated, monitored and, if so demanded, closed within a few years using current technology and in accordance with legally prescribed requirements. Societal requirements in terms of monitoring and control are fulfilled and retrievability of the waste is also ensured.
The primary objective of a geological repository is the long-term isolation of radioactive materials from the human environment. This is achieved through a system of multiple safety barriers. In the case of a repository in Opalinus Clay:
• The repository would be constructed in a stable, deep underground environment, isolated from human activities, with minimal risk of disruptive geological events and an absence of unfavourable geological processes.
• The selected host rock has an extremely low hydraulic permeability and a homogeneous pore structure, with self-healing of any fractures. This ensures that radioactive substances will not be transported to any significant degree by flowing groundwater and protects the engineered safety barriers from environmental influences.
• The chemical conditions in the selected host rock, which are stable in the long-term, provide a series of geochemical retention mechanisms and ensure the long-term stability of the engineered safety barriers.
• The bentonite (a natural clay) which is used as a backfill material between the waste and the rock has similar properties to the surrounding rock and functions as an efficient transport barrier.
• Embedding the waste in an inert matrix ensures long-term stability of the waste product.
• Encapsulating the HLW and SNF in corrosion-resistant canisters with high mechanical stability finally ensures complete containment of the waste for a long time period (at least 10,000 years).
A large proportion of the radioactive material emplaced in the repository will decay during the time when the canisters are still intact and thus provide complete containment. Even after the canisters lose their integrity due to corrosion and mechanical stress, the low hydraulic permeability and the retention properties of the engineered barriers ensure that the radioactive substances remain predominantly within the disposal system, where they decay. This qualitative description was confirmed by a quantitative safety analysis carried out for a series of release scenarios. In addition to the most likely future evolution of the repository system, all deviations that can be realistically envisaged are taken into account and alternative release scenarios are evaluated. In order to test the "robustness" of the system, a series of more extreme "what if?" cases are also evaluated. These consider phenomena that are outside the range of expected effects and processes, but cannot be completely ruled out and could have a negative influence on the functioning of the safety barriers.
For the purpose of evaluating safety, the federal safety authorities have specified a strict upper limit to the individual radiation dose to which a repository should give rise. The results of the safety assessment were measured against this limit. Potential concentrations of waste nuclides in the rock were also compared with natural radioactivity levels and information on natural systems that are observed to have contained relevant radionuclides over geological timescales (so-called natural analogues) were evaluated. The safety analysis reached the following conclusions:
• The safe, long-term disposal of SNF, vitrified HLW and long-lived ILW is feasible in the Opalinus Clay of the Zürcher Weinland.
• Quantitative analysis shows that the area investigated has properties capable of ensuring the required level of safety. Safety is assured for a wide range of scenarios that is sufficiently comprehensive to cover all realistically conceivable future evolutions of the disposal system. In all cases, the resulting radiation dose is below the limits specified by the safety authorities, in most cases by several orders of magnitude.
• The system has been shown to be robust - none of the uncertainties remaining would call safety into question.
• The mechanical properties of the rock and the selected engineering project allow the repository to be constructed, operated, backfilled and closed in such a way that long-term safety is ensured.
• The information base for the selected siting region is sufficiently extensive, and the geological situation sufficiently well understood, to support the findings on safety.
• The future geological evolution of the siting region can be predicted fairly accurately based on the results of extensive regional geological investigations and on the fact that the general geological situation of the region is relatively simple.
• The information on the wastes and their properties is sufficient for the purpose of safety assessment, being based on more than 20 years of scientific studies in Switzerland and broad international experience. The same is true for the system of engineered barriers.
|Properties of Opalinus Clay|
|The selected host rock – Opalinus Clay – is an argillaceous sedimentary rock that takes its name from the frequent occurrence of the fossil ammonite Leioceras Opalinum (pictured). In terms of lithology and mineralogy, Opalinus Clay is a homogeneous clay formation which was deposited uniformly over large areas of northern Switzerland. The result of this is that parameters determined at other locations (for example in the Mont Terri Rock Laboratory) are transferable to the investigation area in the Zürcher Weinland. The geometric boundaries of the tectonically quiet Opalinus Clay layer in this area were determined accurately by a 3D seismic campaign. In the investigated area, the rock fulfils the basic requirements for the geological environment of a repository: Long-term geological stability The area is located at the outermost boundary of the region influenced by the Alps, and is, in terms of tectonics, not significantly deformed. Seismically it is largely inactive, with a small uplift rate of around 0.1mm/year and a correspondingly low erosion rate. Favourable host rock properties In the whole area, the Opalinus Clay occurs with sufficient thickness to host the repository and is lithologically homogeneous. It has a very low hydraulic permeability, stable geochemical conditions and rock mechanical properties that are suitable for the construction of a repository. The formations above and below the host rock are also predominantly low permeability units and thus provide additional isolation. Robustness Based on the present situation, significant alteration of favourable rock properties due to geological events like earthquakes can be ruled out. The absence of economically viable natural resources makes a conflict of use, and hence unintentional human intrusion, unlikely. Explorability The simple topography and geological structure (homogeneity of the host rock, slight variability of facies with laterally more or less constant properties, slightly disturbed sub-horizontal bedding) ensure good explorability of the geometric conditions. It was thus possible to carry out a high-resolution 3D seismic campaign and to extrapolate the results from the Benken borehole over the entire investigation area. Predictability The past geological evolution of the investigation area is well known. Together with the simple geological structure, this leads to a good predictability of the evolution of the host rock and the potential siting area over the time period relevant for evaluating long-term safety. Flexibility An area of around 2km2 will be required for the underground repository facilities. Investigations to date indicate that, in principle, a total area of 35km2 is available for locating a repository. Hence, there is a considerable degree of flexibility in terms of placing the facilities in the potential siting area. The slight tilt of the layer also makes it possible to select the depth of the repository.|