Technical Report NTB 82-07

On the Theory of Transport in Fractured Media for the Safety Analysis of a Nuclear Waste Repository

New mixtures for solidification of borated waste concentrates have been developed under a Nagra contract by EIR in conjunction with staff of Beznau power station where the wastes are produced. The objective of the project was to find a solidification recipe which best enabled the wastes to fulfill the necessary conditions for geologic disposal with respect to their mechanical strength, water resistance, sulphate resistance, long-term stability and leachability.

Boric acid or borates in concentrates require additional calcium hydroxide in cements used for solidification so that precipitation of calcium metaborate which is necessary for hardening will take place. An initial scoping study was performed on inactive concentrates before the range of cements and additives was further reduced in a second series of experiments.

With active concentrates, 11 recipes were tested, including measurement of leach rates. Addition of Trass in the ratio to cement of 30:100 reduced caesium leach rates by around 2 orders of magnitude. The chosen mixture A.11. was then tested on a technological scale. In 200 drums at the Beznau facility mixing and stirring techniques leading to hardening in acceptable times were established. The hardening process and the temperatures were monitored. The strength, water resistance and sulphate resistance were measured for the mix from the drum.

Since the beginning of 1982 the method has been in use for borated concentrates at Beznau with the approval of the safety authorities.

The method is being patented; further details are available from the author.