Use of Fly-Ash for Sealing a Radioactive Waste Repository
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USE OF FLY-ASH FOR SEALING A RADIOACTIVE WASTE REPOSITORY
Marc OLLAGNIER*, Christian TAUZIEDE** and Jodl OLIVIER*** * ANDRA, BP 38 - 92266 FONTENAY-AUX-ROSES Cedex - FRANCE ** INERIS, BP 2 - 60550 VERNEUIL-EN-HALATTE - FRANCE ***EDF, BP 605 - 13093 AIX-EN-PROVENCE Cedex 2 - FRANCE
ABSTRACT
The mining industry currently uses fly-ash from coal-fired power-plants to close access shafts in abandoned mines, in sedimentary formations. The technique consists of installing a plug of fly-ash, about fifty meters high, at the base of the shaft-lining. In this study, fly-ash is considered as a possible sealant for radioactive repositories in sedimentary formations, as well as in hard rock. Specific studies have been conducted in order to assess the feasibility of this technique, and to improve the long-term performance of the fly-ash seals. The hydraulic and mechanical characteristics of fly-ash, taken from the plug of an abandoned shaft, as well as from dumps of various ages, were measured. Laboratory tests showed that it is possible to reduce the hydraulic conductivity of fly-ash by a factor of thousand, with the addition of ten percent bentonite. Moreover, it seems possible to block fine fissures in the surrounding damaged rock by injecting grouts made of cement and fly-ash having maximum diameters of twenty micrometers. INTRODUCTION
In France, the National Agency for Radioactive Waste Management (ANDRA) is evaluating the possibilities of the disposal of long-life radioactive waste in different types of crystalline and sedimentary formations. In an underground repository, the containment system consists of the following three barriers : waste packaging, engineered barriers, and geological formations on the site. The engineered barriers constitute a back-up system that seals the repository. They include buffer materials, drift backfill, and plugs of the access shafts. Of the various materials under consideration, fly-ash from power plants has been studied by ANDRA. For some time, this material has been used in the mining industry to seal abandoned shafts. The technique, successfully implemented by Charbonnages de France (the French Coal Mining Company) (CdF) and Mines de Potasse d'Alsace (MDPA), consists of placing a plug of fly-ash at the bottom of the shaft-lining [1]. The fifty-meter-high plug starts twenty meters beneath the base of the lining. The sealing of the shaft is provided both by the upper part of the plug in the lining and by the lower part of the plug below the base of the lining. ANDRA is studying the adaptation of this principle to seal an underground radioactive waste repository, primarily access shafts and possibly main drifts. The aim of this work is to find a fly-ash-based material, for sealing a repository, that has a hydraulic conductivity of the same order of magnitude as that of the host geological formation (10-o m/s). Fly-ash is a material with low hydraulic conductivity, essentially composed of spherical grains with a granulometry of 0.5 to 200 micrometers [2]. ANDRA, with the assistance of In
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