Technischer Bericht NTB 93-02

Endlager für kurzlebige schwach- und mittelaktive Abfälle (Endlager SMA)Vergleichende Beurteilung der Standorte Bois de la Glaive, Oberbauenstock, Piz Pian Grand und Wellenberg

Following a period of siting investigations and evaluation of results lasting some ten years, in Summer 1993 Nagra (the National Cooperative for the Disposal of Radioactive Waste) was able to recommend a site for a repository for shortlived wastes to the Federal Council. Provided the suitability of this site is confirmed, investigations will be continued and, ultimately, the repository constructed. The Federal Council will consider Nagra's recommendation, and reach a decision thereon, as part of the procedure for granting a general repository license. This procedure will be initiated once the supporting documentation for the license application has been prepared. The decision of the Federal Council requires to be ratified by the Federal Assembly.

Wellenberg recommended as a repository site

Similar investigation programmes were carried out at all four sites under evaluation. Based on the results of these investigation programmes, Nagra selected Wellenberg in the community of Wolfenschiessen (Canton Nidwalden) for recommendation to the Government. In a direct comparison with the other potential sites, Wellenberg offers distinct advantages, both in terms of demonstrating long-term safety (influenced by geology) and from the point of view of environmental impact. Investigations will be discontinued at the three sites which were not selected – Bois de la Glaive in the community of Ollon (VD), Oberbauenstock in the community of Bauen (UR) and Piz Pian Grand in the communities of Rossa and Mesocco (GR). In terms of safety, there is no reason for ruling out any of these sites; they will therefore be held in reserve up to the time when the repository has actually been constructed.

A comprehensive evaluation procedure

Nagra's site evaluation procedure began as long ago as 1978. The initial 100 siting options were gradually narrowed down to give a total of four potential sites in three different host rocks. The "EVALUATION" table (page XXV) provides an overview of the procedure.

Geological and hydrogeological data are required for site evaluation. Such data can be obtained in part from studies which have already been performed, but special targeted investigation programmes are also necessary for acquiring new data. Important factors to be taken into account when considering a site for evaluation are therefore the extent of information which already exists from construction of underground facilities (tunnels, drifts) and the technical potential for performing site characterisation (explorability).

Different levels of existing information – different levels of site investigation

In the case of Bois de la Glaive, Oberbauenstock and Piz Pian Grand, the amount of information already available (partly from existing underground constructions) ranged from good to very good, but explorability from the earth's surface is often very restricted. When investigations were initiated at Wellenberg, there was no information available from previous exploration of the area and the assumption of a large volume of host rock was initially largely hypothetical in nature. The price for making up this lack of previous knowledge was a considerably more intensive exploration programme than at the other three sites. Thanks to the favourable topography at Wellenberg, this was able to proceed in a satisfactory manner.

At Oberbauenstock and Piz Pian Grand, the planned investigations were completed in 1988. By 1993, the level of information available on the other sites was such that a comparative assessment of site suitability could be carried out, although a greater investment of work effort (Wellenberg) and time (licensing procedure, Bois de la Glaive) did prove necessary. The relative lack of data on the Wellenberg site was made up by drilling several deep boreholes; at Bois de la Glaive it proved necessary to resort to time-consuming expropriation procedures before the site investigations could proceed.

Results of the investigations

Several criteria carrying different weight are considered when assessing site suitability. Top priority is assigned to criteria relevant to radiological safety (safety during repository operations and long-term safety following sealing of the facility). In this context, failure to fulfil radiological safety requirements results in a site being ruled out for consideration. Long-term safety is influenced by the geological situation – if, for example, the disposal caverns cannot be located away from unsuitable rock zones then the site must be abandoned.

Site assessment also has to take account of engineering feasibility (rock mechanical parameters), environmental impact and land use planning regulations (observation of particular environmental protection laws, transportation issues, zoning regulations, agriculture, etc.). The results of the evaluation are presented in the "ASSESSMENT" table (page XXVI).

Important differences

None of the four potential sites was dropped from the evaluation procedure on the grounds of the geological situation. There are nevertheless significant differences between the sites in terms of the risk of geological surprises of a negative nature being revealed by future investigations.

Thanks to its good exploration potential, Wellenberg comes out particularly well in this respect. At the other marl site (Oberbauenstock), the available volume of rock has been assessed as only just sufficient, making the potential for avoiding unsuitable areas of rock somewhat restricted.

At Bois de la Glaive, predictions regarding the available volume of rock have been confirmed inter alia by gravimetric survey. What does present problems at this site is demonstrating the suitability of the site from a safety viewpoint. In view of the fact that better alternatives exist, there is no real incentive to run the risk of concrete and metals corroding in the aggressive anhydrite formation. Similarly, there is no reason to have to overcome the transportation problems and the poor explorability of the Piz Pian Grand site.

Wellenberg: advantages compared with the other sites

The main advantage of the Wellenberg site lies in its large volume of compact, low-permeability marl host rock, which promises a large degree of flexibility when designing the optimum layout of the disposal caverns. This "reserve" element means that any less suitable or damaged rock zones can be avoided when the repository is being constructed. There are also good road and rail links to the site. In view of these distinct advantages, the less favourable engineering environment recedes somewhat into the background – the increased costs and effort of constructing a repository at this site can be justified on the grounds of the positive safety aspects.

The requirement for a comparative assessment of potential sites often obscures the fact that, comparison aside, the primary aim of the evaluation procedure is to identify a site which is suitable from a safety point of view. At the selected site, great significance is therefore attached to demonstrating long-term safety to the degree required for the general license application. Thanks to the good explorability of the Wellenberg site, it proved possible to attain a high level of information and the safety demonstration is accordingly more reliable. The foregoing requirement can therefore be taken as fulfilled.

Government-imposed provisos and political requirements fulfilled

Site selection proceeded against the background of the Federal Council's requirement for comparable (in so far as possible) geological information on all the sites, as well as political pressure to treat all the sites equally. In all four cases, the level of information obtained from the investigation programmes was sufficient to allow a comparative assessment of geological and other relevant criteria. Differences in the extent of existing information available at the commencement of the site investigations were taken into account.

The fact that political acceptance varies from site to site meant that additional time had to be invested in legal procedures. However, no site was abandoned for political reasons. The requirement of the Federal Council to carry out "concrete investigations" at Bois de la Glaive cost Nagra more than five years in terms of legal procedures and dealings with the community of Ollon before the necessary work could be carried out.

Restriction to short-lived radioactive wastes

The repository is intended for short-lived low- and intermediate-level wastes. This will include wastes from the operation and subsequent decommissioning of Swiss nuclear power plants, as well as waste arising from medicine, industry and research. The activity of the waste will be dominated by radionuclides with half-lives less than 30 years; the component of longer-lived nuclides will be negligible from a safety viewpoint. An integrated quality assurance system will ensure that no unacceptable wastes will be emplaced in the repository.

Control measures foreseen for the repository

The legal requirement is that the underground repository should provide the required level of safety without any surveillance measures. Nagra is planning additional possibilities for control of the facility: the backfilled caverns can be monitored as long as the access tunnel remains open and the repository environment can be supervised with no time restrictions.

These measures are not in any way intended to make up for a safety concept which falls short of the mark. They have no active safety function and simply provide the last tangible evidence for future generations that disposal of the waste has been carried out correctly.

Retrieval of the waste is possible in principle but, in the interests of long-term safety, no provisions will be made for technically and economically feasible retrieval.

Where do we go from here

In 1994, the supporting documentation for a general license application will be prepared and the application submitted. Certain legal steps have to be taken prior to this, for example setting up a company responsible for constructing and operating the repository. This company will be domiciled in the siting community. The question of compensation for a community which hosts the repository should also be clarified and set down in contractual form. The government licensing procedure is expected to take around four years. If everything proceeds according to plan, construction of the repository will begin before the turn of the century.