THE IMPORTANCE OF PORE FLUIDS IN THE RADIOACTIVE WASTE PROGRAMS OF CANADA AND THE USA

The programs for disposal of radioactive waste in Canada and the USA began in the 1970's with the recognition by geoscientists and regulators, that disposal at depth in a stable rock formation was the best option for nuclear fuel waste generated by electrical power reactors. Prior to 1987, both countries studied igneous rock formations on the Canadian Shield, in parts of Manitoba and Ontario, and extensions into Maine and Minnesota. Granite was the favored host because it was abundant, had a high compressive strength, occurred in extensive batholiths, tended to be less fractured than other rock types, and was lacking in valuable mineral resources. After 1987, the granite program ended in the USA and attention was focused on the tuff formations of Yucca Mountain, Nevada.

A common aspect of site characterization procedures required in both programs was the need to understand pore fluids in the matrix of these rock types so that the long-term geochemical stability of a waste repository could be assessed. Pore fluids may be saline (granite) or dilute (tuff) but their geochemical and isotopic composition contains information on matrix flow, residence time, sources of dissolved salts, water-rock interactions and, most importantly, potential for corrosion of metal containers for the waste. Salt halos are expected to form around excavations due to evaporation of pore fluids and these halos will grow as the fluids migrate towards the excavation, driven by hydraulic gradients. In the case of a Shield disposal site, re-saturation following closure will create brines that may contact all surfaces of the container. In the Yucca Mountain tuff, high salinities also may be attained but in a lower fluid volume, potentially causing corrosion by seepage drips. Examples of pore-fluid composition are given together with expected brine compositions. The Canadian pore-fluid studies may be relevant to future USA programs because the search for a second repository is beginning and may include granitic rocks in a saturated environment.