The chemistry of blended cements and backfills intended for use in radioactive waste disposal

This project was initiated by Her Majesty's Inspectorate of Pollution (HMIP) at the time when UK NIREX had announced its intention to develop a repository for low and intermediate level nuclear waste in the vicinity of Sellafield. In this repository setting, two main barriers existed to the return of radio-isotopes to the biosphere: the natural, or geologic and hydrogeologic barriers, and the man-made barriers. These latter comprise relatively short-lived containers as well as an engineered backfill. The backfill was designed to condition a high pH in the repository, thereby lowering the solubility of many long-lived radionuclides yet not confine gases, which might be generated from chemical and radioactive waste within the repository vault. The Environment Agency for England and Wales had already taken independent steps to examine the suitability of alkaline backfills, based on Portland cement, limestone flour and Ca(OH)2, for the man-made barriers. Preliminary data on post-closure repository performance assessment at Sellafield suggested the importance of two additional factors which had not hitherto been considered in assessments: (i) temperature: Inclusion of heat generating waste could drive temperatures up to ∼80 deg. C in the post closure phase; (ii) salinity of deep groundwater: Much previous work has been done in initially-pure water but borehole analyses indicated high salinity at depth. Other potential deep repositories could also be saline. These impacts were likely to occur together throughout much of the post-closure phase: backfills were likely to be in prolonged contact with hot, saline groundwater. Previous studies demonstrated that cements achieve their performance by a sacrificial action. It is however essential that the cementitious materials should not dissolve too rapidly if prolonged backfill performance lifetimes are to be achieved. By dissolving cement backfills condition permeating water to a high pH and thereby lower the solubilities of many radionuclides. While this conditioning action had been demonstrated to occur for initially-pure water at ∼25 deg. C, it had not been demonstrated for hot, saline solutions. Thus the performance of cements had to be addressed in hot, saline conditions, and a range of elevated temperatures, but the question was how? Many centuries, perhaps millennia, had to be compressed into a relatively brief period of laboratory assessment. It is well known that accelerated testing of materials has many pitfalls and 'performance' may not be independent of the test criteria, particularly in accelerated tests. The programme was designed to provide accelerated data but also to take into account previous relevant experience. Programme of research: aims (i) To undertake a coordinated research programme to determine the influence of saline and environments and elevated temperatures on cementitious material and backfill. (ii) To extend the capabilities of existing thermodynamic models of backfills and blended cements to encompass the range of environmental conditions likely to be experienced at proposed repository sites