Limitations of the NRC method of microbial stability evaluation of waste forms

摘要

In this study a critical examination of the current Nuclear Regulatory Commission (NRC) protocol for waste forms microbial stability evaluation is carried out using Tuskegee cement/cobalt chloride waste form and Thiobacillus thiooxidans (T. thiooxidans) as models. Two major limitations were identified in the current NRC protocol that makes it impossible to accurately use it to evaluate the stability of cement-based waste forms to microbial degradation. One of the limitations is the use of a T. thiooxidans culture that is significantly lower in pH in comparison to the control medium, which could lead to an overestimation of the degradative effect of T. thiooxidans. The other limitation is the use of a culture that is substrate limited, which could result in an underestimation of T. thiooxidans's capability. A radical change in approach to waste forms microbial stability evaluation is being suggested to ensure an accurate estimate of T. thiooxidans involvement in waste form degradation. The approach involves the use of a two-stage system with the first stage serving as an inoculation stage and the second stage serving as the evaluation stage. At both stages the experimental and control samples are fed with solutions of similar pH, thus eliminating the pH differential which is a major weakness in the NRC protocol.