Assessment of the Nagra Concept

Figure 1 illustrates the Nagra concept for disposal. The repository is located in Opalinus Clay of the Zürcher Weinland (typical thickness about 100 m) at a depth of about 650 m below the surface. Passive safety is achieved through the use of multiple barriers designed to isolate the waste and to ensure that future radiological exposures from any released radionuclides are below regulatory limits. The regulatory guidelines (HSK/KSA, 1993) specify a maximum annual dose rate to individuals of 0.1 mSv per year.

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Figure 1: Plan view of the repository concept proposed by Nagra [from Nagra (2002a), Fig. 4.5-1]

The reference disposal concept corresponds to waste generated from production of 192 GWa (e) of nuclear power, equivalent to operation of the current nuclear power reactors for 60 years. This results in the generation of the following wastes (Nagra, 2002a):

• SF: 2 065 canisters containing 3 217 t initial heavy metal (UO2 and MOX).

• Vitrified HLW: 730 canisters resulting from the reprocessing of 1 195 t SF.

• Long-lived ILW: A total number of 1 680-1 880 waste drums of different types coming from COGEMA and BNFL, representing a total volume of about 1 400 m³ (or ~ 500 m³ for the ILW high force compacted wastes option).

In the Nagra concept, SF and vitrified HLW would be placed in thick-walled steel canisters that should provide absolute containment of the wastes for a period of 10 000 years or more. The canisters would be placed horizontally in 2.5 m diameter, 800 m long emplacement tunnels that would be sealed with bentonite following waste emplacement. The tunnels would be spaced 40 m apart.

Emplacement tunnels for HLW and SF would be excavated as needed and backfilled and sealed concurrently with waste emplacement, so that a given tunnel would remain open for a maximum of two years. Once all waste had been emplaced in the main facility, the access ramp would remain open during an extended period of monitoring but the main facility would be sealed to ensure long-term passive safety, even in the event that closure of the access ramp did not eventually take place as foreseen. The IRT finds that this approach to waste emplacement (including the use of multiple seals to compartmentalise and isolate waste packages) is feasible and prudent.

In common with the approach followed by a number of other countries (for example France, Sweden and Finland), Nagra proposes that long-lived ILW be disposed of in tunnels excavated in the same facility. The ILW drums would be loaded inside containers and placed in larger emplacement tunnels that would be sealed with mortar. Emplacement tunnels for ILW would be separated from tunnels used for the emplacement of SF and HLW wastes to ensure that the geochemical environment in the SF and HLW tunnels would be favourable.

Separate tunnels would be used for ILW: one for ILW waste containing organic and potentially complexing compounds and the others for ILW wastes comprising inorganic components. The IRT considers such separation of wastes to be good safety practice.

As proposed by the Swiss government advisory group EKRA, the facility would include a test facility and a pilot facility. Implementation would follow a cautious, stepwise approach and would involve an extended period of monitoring, following waste emplacement, during which retrieval of the waste would be relatively easy. Periodic reviews would be carried out that would provide for possible reversal of decisions, including even the retrieval of emplaced wastes. This approach is consistent with that proposed in a number of other countries and is consistent with recent international reflections (NEA, 1995; NEA, 1999; IAEA, 2004). Others (Simmons and Baumgartner 1994, SKB, 2000) have proposed constructing in situ test facilities similar in concept to those advocated by EKRA, and have proposed keeping the disposal facility open for an extended period of monitoring following waste emplacement [see, for example, AECL (1994) and US-DOE (2000)]. Nagra has made an important contribution with its analysis of the safety consequences of abandoning such a facility in the monitoring stage (see further discussion in Section 4.7).

The IRT finds that the disposal concept proposed by Nagra has all the desirable elements of a monitored retrievable geological repository. The multiple barriers for SF and HLW perform a number of functions appropriate to the chemical and geological environment in the proposed repository. The barriers include:

• durable waste forms (SF and HLW), surrounded by

• long-lived canisters that, except possibly for a small number of premature failures, have the potential to provide absolute containment of the wastes for a period of 10 000 years or longer, during which time the radioactivity of the wastes will decrease substantially,

• a bentonite buffer that fulfils several functions, including providing the conditions to ensure sealing of the excavation disturbed zone (EDZ) and isolation of the waste packages from the host rock and from each other, and

• the host rock, namely the Opalinus Clay of the Zürcher Weinland, which provides a geologically stable environment and which ensures that the movement of contaminants from the repository to the surface environment is constrained by the hydrological and retentive properties of the Opalinus Clay.

In the Nagra concept, the most important of these barriers, because of its properties, is the host rock. Nagra argues, convincingly in the view of the IRT, that the geologically stable environment of the Opalinus Clay of the Zürcher

Weinland is structurally simple, the clay is self-sealing, there is negligible advective water flow, the clay is chemically stable with good retention capabilities, and it has acceptable engineering properties for construction.

Further, the absence of resource potential in the area reduces the likelihood of inadvertent human intrusion. The properties of the host rock are discussed in detail in Chapter 4.

The IRT notes, however, that Nagra has drilled only one deep borehole into the Opalinus Clay of the Zürcher Weinland – the Benken borehole – and that consequently additional work would be expected before going underground to confirm the characteristics of the reference disposal site and to gather information for detailed engineering design.