Technical Report NTB 12-01

The Long Term Geochemical Evolution of the Nearfield of the HLW Repository

The work presented in this report focuses on the spatial and temporal evolution of the nearfield of the high level radioactive waste repository situated in the Opalinus Clay formation.

The major components of the nearfield of such a repository are spent fuel, vitrified high-level waste, canisters (assumed for the purposes of the present report to be made of carbon steel), compacted bentonite and a concrete liner. Over the one million year time period considered in safety analysis, these components will chemically interact with one another and potentially change their retention characteristics.

As a starting reference point the "initial" (unreacted) states of the Opalinus Clay, bentonite, concrete liner (mineralogies and water chemistries) and the canister are briefly described.

The main processes considered to influence the evolution of the repository in time and space, and which often operate over different time scales, are: interactions of the concrete tunnel liner with compacted bentonite and Opalinus Clay, temperature gradients caused by the heat generating high level waste, mineralogical changes to the compacted bentonite through interactions with the corrosion products of the iron canisters, and finally, the dissolution of the spent fuel and vitrified high-level waste. The consequences of these processes (as a function of time) on the long term barrier performance of the nearfield have been estimated, particularly with respect to radionuclide solubilities and the sorption, diffusion and swelling characteristics of the bentonite.

The main conclusions drawn are as follows: The alteration depth into the bentonite due to the interaction with the concrete liner (assumed to be 15 cm thick) is likely to be much less than 13 cm over a one million year time scale, with the main reaction products being clays (illite), hydroxides, carbonates, calcium silicate hydrates, and aluminosilicates. The swelling pressure and the sorption capacity of the bentonite in this region will be reduced, but not to zero.

Experimental findings and modelling studies indicate that any alterations due to the post closure temperature transients will not change the swelling and retention properties of the outer (more than) half of the bentonite which can be relied upon to fulfil its buffer function fully.

The dissolution of spent fuel and vitrified high-level waste are not expected to have any detrimental effects on the sorption properties or the swelling capacity of the bentonite. However, the potential influence of the release of boron from the vitrified high-level waste on complexation with highly charged radionuclides needs to be addressed.

Current studies indicate that the Fe2+ released by the corrosion of the steel canisters can lead to the alteration of montmorillonite at temperatures below 100 °C to form Fe-rich smectites or non-swelling clays and chlorites. Fe-rich smectites have similar properties to the Na-montmorillonite they replace. Hence, the near-field barrier function will not be significantly influenced with respect to sorption and swelling. However, if non-swelling clay minerals or chlorites are formed, then noticeable changes are expected, reducing the bentonite swelling capacity and sorption properties.

The release of iron from canister corrosion is a slow process. Estimates based on the corrosion rate indicate that the canister corrosion and the following conversion of montmorillonite needs between 100'000 and 200'000 years. The situation described here represents a "worst case" scenario. Other iron phases like magnetite or siderite are stable in this environment and decrease the availability of Fe2+ for Montmorillonite transformation. This suggests that significant quantities of bentonite will still be available up to one million years after closure of the repository.