Geochemical optimisation of a disposal system for high-level radioactive waste

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Abstract

Geochemical exploration commonly utilises observations from nature to help find, and explain the genesis of, anomalies such as ore bodies. Radioactive waste disposal involves the inverse process of using such expertise to design a geochemical anomaly (the repository) which will persist for geological timescales. The particular challenge is developing a disposal concept which is “robust” — based on materials and processes which can be relied upon to function in a predictable manner.

With regard to long-term performance, experience has indicated that geochemical barriers generally tend to be more robust in this sense than mechanical or hydrological barriers. They are also often easier to support with natural analogue arguments. With the aim of developing a practical design of engineered barriers which will provide convincing long-term safety in a wide range of geological environments, optimisation of the materials under consideration is examined from a geochemical perspective.

Introduction

The disposal of high-level radioactive waste (HLW) in a deep geological repository aims to ensure safety by a combination of natural and engineered barriers. A stable geological setting is essential but, over hundreds of thousands of years until key radionuclides decay to insignificance, it is inevitable that a number of processes could potentially lead to a release of radioactivity.

The engineered barrier system (EBS), which includes the waste itself along with associated overpacks and backfill/buffer, is designed to ensure complete containment for the period when the waste is most hazardous (∼103 to 104 years). Thereafter, it acts to limit the release rate of the remaining long-lived radionuclides so that, when combined with retardation/radioactive decay and dilution in the geosphere, no radiological hazard is likely — even in the distant future.

This paper focuses on the vitrified high-level waste arising from the reprocessing of power reactor fuel. It is not possible to summarise the general science and technology supporting such disposal projects in a short paper — the interested reader is referred to the recent projects “H12” and Entsorgungsnachweis which provide comprehensive reviews and are the source of numerical data presented below (available in full on the Internet at www.jnc.go.jp; www.nagra.ch). Although not discussed further, the principles presented in this paper are applicable to other types of high-activity wastes — e.g. spent fuel and plutonium, if declared as waste.

The EBS concept considered here comprises a mild steel canister (or overpack) and compacted bentonite backfill (Fig. 1) — the key safety barriers found in many disposal designs for vitrified HLW or SF (e.g. Nagra, 1985, Nagra, 2002a, JNC, 2000a).

Only relatively recently has effort been put into consideration of the practicality of emplacing such an EBS in the repository. Generally, it has been assumed that it is possible to build such an EBS in situ — in a disposal borehole or tunnel. However, from consideration of the practicality of emplacement alone there are clear advantages in HLW disposal concepts which involve pre-fabricated EBS units (Apted, 1998, Toyota and McKinley, 1998). This is especially so if quality assurance is important, for high emplacement rates and for “wet” host rocks when the EBS includes a swelling clay like compacted bentonite.

Two extreme variants of the pre-fabricated EBS concept have been previously described — one in which the engineered barriers are reduced to a minimum (a cost-effective option for high performance host rocks (Apted et al., 2001)) and one in which the flexibility provided by pre-fabrication is utilised to produce a multi-component EBS (e.g. McKinley et al., 2001). The latter is particularly appropriate for fractured hard rocks (sediment or crystalline) where, despite the very good average rock properties, a strong EBS is required for the localised cases in which groundwater flow in fractures occurs. An intermediate option involves pre-fabrication of a previously examined standard option — as for example the “KBS-3H” concept, based on the Swedish KBS-3 concept (KBS, 1983), which is presently being evaluated in the Swedish and Finnish national radwaste disposal programmes.

When the EBS is pre-fabricated within a steel handling shell, however, there are options to add more components to the EBS while still ensuring that high quality levels are maintained. These variants can be tailored to the geological environment, operational boundary conditions, safety goals, cost constraints, etc. as discussed below.

Section snippets

The geochemical barrier

A wide range of repository designs for deep disposal of high-level radioactive waste have been developed for different geological environments. Many countries are considering siting repositories for HLW in host rocks which are saturated and hence a major concern is dissolution of the vitrified HLW and rapid transport of radionuclides to the human environment.

For saturated host rocks, the basic safety concept for HLW focuses predominantly on the geochemical barriers to the release of

The “standard” EBS

Borosilicate glass surrounded by a steel canister and a layer of highly compacted bentonite is here considered as the “standard” EBS for HLW disposal.

The bentonite buffer fulfils several roles:

  • It acts as a plastic mechanical buffer to protect the integrity of the canister against small rock movements on fractures. Plasticity allows the buffer to remain intact despite rock displacements

  • It forms an extremely low hydraulic conductivity hydrological barrier, so that solute transport is

Robustness of the bentonite buffer

Evaluation of the geochemistry of the near field (i.e. the repository plus the adjacent host rock which is perturbed by the construction/presence of the repository) indicates that the key parameters on which reliable, long-term safety depends are the glass corrosion rate and the low solubilities of many important, safety-relevant radionuclides — requiring that the bentonite acts as an effective colloid filter (McKinley, 1985, McKinley, 1989, JNC, 2000a). Bentonite is, indeed, a critical barrier

Design constraints

The fundamental dichotomy in designing an optimal EBS is to balance the positive attributes of additional barriers (performance, robustness, public acceptance) against the negative aspects of increased system complexity (especially in terms of modelling long-term performance), cost and difficulty of fabrication and transportation of a large/heavy unit.

To minimise the negative aspects – especially in terms of geochemical complexity – an aim was to use, whenever possible, materials which are

Development of design variants

The starting point for the optimisation of a prefabricated EBS is the “Multiple Component Module (MCM)” design (McKinley et al., 2001) which included (Fig. 2):

  • 0)

    Vitrified HLW (in a stainless steel fabrication flask) encapsulated in a steel canister

  • 1)

    An inner sand layer

  • 2)

    A pure bentonite layer

  • 3)

    An outer sand/bentonite layer

  • 4)

    A geotextile layer

  • 5)

    A thin steel handling shell.

The key justifications for inclusion of these components were:

  • Inner sand layer: acts as a reservoir for gas produced by anaerobic

Conclusions

The EBS optimisation study indicates how an understanding of the main geochemical process influencing the performance of the engineering barriers in a repository can be combined with appraisal of engineering practicality (and cost) to devise robust, cost-effective designs. Indeed, the focus on well-understood materials and geochemical constraints (corrosion rates, solute transport rates, gas production rates, solubility limits) results in an EBS whose performance can be predicted over long

Acknowledgements

The work reported in this paper has benefited from discussions with many of our colleagues, but particular thanks to Dr. Mick Apted for constructive criticism and Lawrence Johnson for input to, and review of, a draft of this manuscript. The two anonymous reviewers are also thanked for comments, particularly with respect to making the manuscript more approachable for a wider audience.

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