Development of Waste Unit for Use in Shallow Land Burial

摘要

A hexagonal waste unit has been developed for use in shallow land burial of low- and medium-level radioactive waste. The waste units used as overpack on empty standard 210 1 drums have been tested for tightness and mechanical resistance. Experimental burial of 21 empty full-size units has demonstrated the emplacement of the containers and the sealing of the crevises between them with molten bitumen. The development of the experimental burial with time is being followed. Three different conceptual designs for advanced burial systems using the hexagonal standard units are described. The outer barrier is a thick concrete structure covered by 2, 10 or 20 m soil, respectively. The waste units were cast from a normal high-quality concrete as well as from Densit, a new, very strong and impermeable type of concrete prepared by the combined use of silica-fume (microsilica) and a superplastizicer as additives. The migration of Cl/sup -/, Cs/sup +/ and tritiated water was found to be much slower in Densit than in normal concrete. In combination with leaching measurements for Cs/sup +/ from the same materials the results are used to present some theoretical considerations concerning transport through solution-filled pore systems as dependent on pore-size distribution, tortuosity, etc. A method based on neutron-activated cement cast in form of thin plates has been developed and used to study the dissolution chemistry of concrete. A preliminary model is presented. Indications for precipitation mechanisms were obtained. Densit was demonstrated to ensure a high degree of corrosion protection for steel reinforcement. The reason is mainly the high electrical resistivity combined with low diffusive transport in the material. The pozzolanic reaction results in somewhat lower pH in the pore water than in normal concrete, but the effect is not so pronounced that the passivation of steel reinforcement is endangered. (ERA citation 13:003072)