Durability of cement paste as engineered barrier in borehole waste repository

Abstract

The Radioactive Waste Management Laboratory of the Nuclear and Energy Research Institute, in São Paulo, Brazil, is developing the concept of a repository for disposal of sealed radioactive sources. The concept is a deep borehole drilled a few hundred meters below surface in a granite batholith. Portland cement paste is the material intended to backfill the annular space between the steel casing and the geological formation around the borehole. The hardened cement paste is intended to function as barrier against water flow between the different strata of the geological setting crossed by the borehole and also as an additional barrier against inflow of water and migration of the radionuclides present in the sealed sources. A service life of thousands of years is a necessary characteristic of the engineered barriers in this repository because many sealed sources are long-lived. The durability of cementitious materials is known only for short periods and must be evaluated for long periods. This research aims at evaluating the durability of Portland cement paste under the repository conditions foreseen in that disposal facility, by accelerated tests in laboratory. In this paper we present results of mechanical strength, mass, and volume variations of cement samples under irradiation, high temperature and immersion in saline solutions, as a function of time.