Can sealing of rock hosting a repository for highly radioactive waste be relied on?

Multibarrier systems are commonly proposed for effective isolation of highly radioactive waste (HLW). Presently considered concepts take the host rock as a barrier claiming it to retard migration of possibly released radionuclides from HLW containers to the biosphere. This capacity is small unless water-bearing fracture zones intersecting the blasted waste-containing tunnels and excavation-disturbance zones around them can be sealed by grouting and construction of bulkheads, but this is effective only for a very limited period of time as explained in the paper. The disturbed zones thence make the entire repository serve as a continuous hydraulic conductor causing quick transport of released radionuclides up to the biosphere. The dilemma can be solved by accepting the shortcircuiting function of the disturbed zones along the tunnels on the condition that totally tight waste containers be used. Deep holes bored in the site selection phase through the forthcoming repository can be effective pathways for radionuclides unless they are properly sealed. They are small-scale equivalents of tunnels but do not have any excavation damage and can be effectively sealed by using clay and concrete of new types. Applying this principle to very deep boreholes with a diameter of a few decimeters would make it possible to safely store slim, tight HLW canisters for any period of time.

• Hazardous Waste; Repositories; Crystalline Rock; Rock Sealing Engineered Barriers

1. INTRODUCTION

1.1. Background

Radionuclides leaking out from canisters with highly radioactive waste (HLW) must not contaminate the groundwater as required by national and international (IAEA) codes. The commonly applied multibarrier principle implies that the host rock, the waste canisters, and the clay surrounding them shall combine to retard migration of radionuclides escaping from the canisters. Comprehensive research in Sweden and Finland (Swedish Nuclear Fuel and Waste Management Co and POSIVA OY, respectively) has led to the proposal of using thinwalled copper canisters with spent nuclear fuel placed inside an iron core [1,2]. Such canisters, representing a first barrier, will not remain tight if sheared by significant instantaneous or repeated seismic events, and radionuclides being released from the failed canisters would enter the second barrier consisting of very dense smectite-rich clay [1]. Theoretically, several types of radionuclides would be hindered by ion-exchange mechanisms and sorption but the hydrothermal processes generated by the heat production may significantly reduce the isolating capacity of the clay [3], leaving the rock as the only remaining obstacle to contamination of the biosphere.

1.2. Scope

The pathways of contaminated groundwater are interconnected fracture zones intersecting repository rooms, and boreholes made in the site selection and construction phases [2]. We will examine them here with special respect to the performance and longevity of grouts, and assess the possibility of sealing fracture zones for the required period of time, i.e. at least 100,000 years. The introductory part describes the systems of flow paths in repository rock on different scales including both natural fracture zones and excavation-induced changes in aperture of natural fractures and creation of fracture-rich zones (EDZ). This is followed by describing their transmissivity in terms of hydraulic conductivity as measured in situ, and ways of blocking the flow paths by constructing bulkheads and by grouting. The subsequent part is focused on the short-circuiting role of deep boreholes and on new ways of sealing them effectively. The lack of practically important excavation disturbance opens the possibility of disposing HLW canisters in very deep holes, i.e. where groundwater flow in the rock is nearly none [4].

2. TRANSPORT PATHS OF FLOWING GROUNDWATER CONTAMINATED BY RADIONUCLIDES

2.1. Water-Bearing Zones in a HLW Repository

2.1.1. Canister Deposition Holes

Migration of radionuclides within and from a deep underground repository depends on how the tunnels and shafts are constructed. Common HLW repository concepts imply that tunnels of a few hundred meter length and a cross section area of about 20 m² are constructed with a spacing of 30 - 40 meters and that holes with a diameter of about 2 m for placing waste canisters are bored from the tunnel floors to about 8 m depth [1]. The canisters are proposed to be surrounded by blocks of expansive clay (smectite) that sorbs water from the rock and expands to form a very dense embedment of the canisters, establishing also a tight contact with the rock. The source of radionuclides is leakage from failed or initially imperfect canisters containing HLW from which radionuclides like cesium, strontium and iodine, as well as actinides, can move through the surrounding “buffer clay” to the rock.

Figure 1 shows that transport of released radionuclides can take place in water-bearing fractures intersecting the deposition holes and in the 1 - 2 cm thick boring-disturbed zone around them [2]. From there they reach the tunnel floor and move on in groundwater flowing there under prevailing hydraulic gradients. The boring-disturbance plays a minor role for the flow [5], but is believed to be important for diffusive migration of radionuclides by having a higher porosity than the crystal matrix of undisturbed rock.

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