AN ASSESSMENT OF CARBON STEEL OVERPACKS FOR RADIOACTIVE WASTE DISPOSAL
The report summarizes the results obtained at Harwell in the second phase of a project evaluating the corrosion behaviour of high level waste overpacks in geological disposal. It has concentrated on the use of carbon steel in granitic and argillaceous environments, and has aimed at estimating the corrosion allowance required to achieve a 1000 year overpack life. Experimental and mathematical modelling studies have indicated that 200 mm of steel should be more than sufficient to prevent overpack penetration by general or localized corrosion. In fact there are sound mechanistic reasons to regard this value as an overestimate which will be reduced by further refinement of the models, and recommendations are made of what further work is needed if this is to be achieved. A theoretical assessment of the possible effects of microorganisms on overpack corrosion has concluded that such species are likely to be found in repositories, but that only a fraction of their population should be corrosive towards carbon steel. Making the pessimistic assumption that all organic carbon in a 500 mm bentonite backfill is utilized by corrosive sulphate reducing bacteria, it has been estimated that this will result in an additional metal loss of 13 mm. One form of corrosion which cannot be dealt with by the corrosion allowance approach is stress corrosion cracking, since even at the lowest reported propagation rates, a metal thickness exceeding 3 m would be penetrated in 1 000 years. It has been concluded that the possibility of stress corrosion cannot be dismissed, but, because the process requires a certain minimum stress level before it can occur, it should be possible to avoid the problem by giving the overpacks a stress relief heat treatment. Tests in a model environment have shown that a heat treatment designed to reduce fabrication stresses to 50% of the yield strength should be sufficient to prevent cracking. It is recommended that this conclusion be substantiated by scaled-up experiments with model overpacks. The report draws further attention to degradation by hydrogen embrittlement. It is considered that the low strength carbon steels which are favoured for overpack construction should be resistant to such embrittlement, but because of the high activity of hydrogen which may develop at the overpack surface due to transport limitations, it would be prudent to examine the process more closely.
Bibliographic Reference: EUR 10437 EN (1986) FS, 71 P., BFR 250, EUROFFICE, LUXEMBOURG, POB 1003
Availability: Can be ordered online
Record Number: 1989124110500 / Last updated on: 1987-01-01
Available languages: en