The Possibility for Microbially-Influenced Degradation of Cement Solidified Low-Level Radioactive Waste Forms

Abstract

The Nuclear Regulatory Commission (NRC) regulations 10 CFR Part 61, “Licensing Requirements for Land Disposal of Radioactive Waste”, regulate the disposal of radioactive waste and provide, among other stipulations, that class B and C low-level radioactive waste (LLW) must be stabilised. This is intended to ensure that solidified waste does not structurally degrade and cause subsidence in the disposal unit’s cover system. It is reasoned that deterioration of the waste form could adversely affect the stability of the burial site and lead to the release of radionuclides to the environment. Because of its apparent structural integrity, cement has been widely used as a binder to solidify LLW. However, the resulting preparations called pozzolanic cements are susceptible to failure due to the actions of stress and environment.

This paper presents data from the literature that documents the significance of biologically mediated chemical attack on concrete, in general. Concrete is susceptible to aggressive reaction with acids (both mineral and organic) of natural and anthropogenic origin. If persistent, such reactions ultimately lead to structural failure. Groups of microorganisms have been identified that are capable of metabolically converting organic and inorganic substrates into organic and mineral acids.

The literature supports the conclusions that acid-producing bacteria of one type or another could be prevalent in all soils, even at depths expected for burial of LLW. Given the appropriate conditions of micro-environment and suitable substrate for growth, these bacteria will create conditions conducive to concrete deterioration. Growth substrates used by acid-producing bacteria (i.e., ammonia compounds, other reduced nitrogen compounds, sulfur and reduced sulfur compounds, reduced iron compounds, as well as organic carbon) could be naturally present in the disposal environment or be provided by the contents of the waste form.

Sufficient evidence is presented which demonstrates the potential for microbially-influenced deterioration of cement-solidified LLW. These data are the basis for the NRC initiating the development of appropriate tests to determine the resistance of cement-solidified LLW to microbiologically induced degradation.

Work supported by the U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research, under DOE Idaho Field Office Contract DE-AC07–761D01570.