Corrosion of steel mediated by sulphate-reducing bacteria, with reference to nuclear waste disposal
Of the multifarious economic activities of the sulphate-reducing bacteria, the best documented is that of steel corrosion. In the latter part of this century reports of SRB-induced corrosion have come from a variety of industries and much work has been done on practical as well as theoretical aspects of the problem. Despite this the corrosion mechanism has remained indeterminate. The bulk of the argument centres on the role of hydrogenase in the removal of hydrogen from cathodic sites on the steel surface, and whether this is important compared to cathodic depolarisation caused by biogenic ferrous sulphide. A relatively new industry where cognisably the SRB may manifest corrosion is the disposal of radioactive wastes, which concept entails the use of steel canisters. Many concepts for high-level waste disposal do envisage just this with burial in a mined repository, and backfilling with the clay mineral bentonite. For the UK concepts effort has been made to produce a safety assessment giving canister wall thicknesses to provide a steel canister of 500-1000 years integrity. The work effectuated for this thesis is of a dual nature. Firstly, consideration was given to the present theories of SRB-induced corrosion with experimentation aimed at deducing the most important mechanism(s) by a process of elimination. Secondly, and of equal emphasis, experiments were designed to examine what extra effect SRB may have on steel canister integrity under some conditions of high-level radioactive waste disposal, with a view to fitting any such extra effects into the safety analysis. SRB were found to be ubiquitous in proposed nuclear waste disposal sites examined for their presence. Laboratory corrosion studies using pure cultures of SRB (static and continuous culture) showed that corrosion of relevant mild carbon steel will be affected by levels of organic carbon, sulphate and phosphate which are unlikely to be present in significant quantities in a waste vault. Fe2+ availability was also shown to be a prerequisite to high corrosion rates. For high level radioactive waste the use of bentonite, a proposed backfilling material for waste canisters, containing SRB was shown to enhance corrosion of mild carbon steel. A model system to show this was set up using a bentonite clay/granitic groundwater mix in which SRB and steel coupons were present. Corrosion mechanisms which are likely to be involved include cathodic depolarisation and FeS production; it is not envisaged that the volatile phosphorus compound `identified' by Iverson as a corrosion product is involved. Genetic manipulation to produce hydrogenase-less mutants were unsuccessful during this study. It was aimed to produce such mutants to test the roles of the enzyme hydrogenase in rates of corrosion under repository conditions. Other factors (e.g. high pH) affecting the activity and effects of SRB on nuclear waste isolation materials were examined and are discussed for intermediate level radioactive waste disposal.