Biodecontamination: microbially cleaning massive concrete structures

Decommissioning and decontamination (D&D) of nuclear facilities is a huge undertaking with equally large associated costs. Within the US Department of Energy alone, there are thousands of facilities currently identified for D&D and the list is growing daily. This equates to literally square miles of contaminated concrete surfaces within the DOE complex. The costs associated with these efforts are estimated in the tens of billions of dollars. The problem is not limited to the US.

The biodecontamination process developed by British Nuclear Fuels (BNFL) and the Idaho National Engineering and Environmental Lab (INEEL) is expected to require approximately six months to one year to remove the contaminated surface and can advantageously be applied during the Care and Maintenance phase of a D&D programme. A detailed assessment of the process in relation to the next best and base technologies was performed. The findings of this study recommended a large scale technology demonstration be performed to confirm the perceived cost and performance advantages that the biodecontamination process has over existing technologies.

A CONCRETE PROBLEM

Uncoated concrete has been used for the construction of ponds, canals, sumps and other structures within operating nuclear facilities. These concrete structures have served the purpose of containment, transport, and storage of liquid and solid radioactive materials. Use of the facilities has resulted in contamination of the concrete surfaces with radionuclides. Typically this contamination is securely fixed on the surface or within the first 1 or 2 mm.

Current methods for decontamination of concrete include physical and chemical removal. Such methods are costly, labour intensive, generate large volumes of waste, and pose potential risks to workers. The task of decontaminating concrete within the large number of buildings requiring decommissioning is enormous. The difficulty of the task is increased by the continuing demands to accomplish it within more restrictive limits of waste volume, cost, and environmental risk.

Many commercial technologies for concrete decontamination have been developed and applied. Traditional concrete decontamination methods included shot blasting, mechanical scabbling, detergent scrubbing, high pressure washing, chemical treatments, strippable coatings, clamshell scrapers, brushing, vacuuming and attacking cracks with jack hammers. However, the use of explosives, jackhammers, etc has been a problem because of high worker exposure to contamination suspended in the dust. It is evident from past experience that the primary decontamination methods used to date have been pressure-washing techniques and various types of scabbling.

A small number of innovative technologies that give promise of greater effectiveness/cost savings relative to technologies currently available for addressing the concrete decontamination problem through the D&D life cycle have been developed and are proposed for demonstration.

One such innovative approach to cleaning contaminated concrete is biological remediation or biodecontamination. It has been demonstrated that biological activity can promote degradation of concrete and the mechanism of degradation has been characterised as consistent with chemical degradation. Microbially-influenced degradation is the premise for biological approaches for removing contaminants fixed in surface layers of concrete structures.

A microbially mediated process implicated in the degradation of concrete was first reported by Parker in 1945,* when thiobacilli were isolated from corroded concrete. Much of the research that followed over the next five decades has focused on identifying and enumerating the organisms responsible for the degradation process. The bacterially mediated process appears to be an acid dissolution of the cement matrix of concrete resulting from the production of strong mineral acids by specialised microorganisms. This bacterially mediated process is referred to as microbially-influenced degradation (MID) of concrete.

Two groups of bacteria, generally thought to induce acid corrosion, are nitrifiers that oxidize inorganic nitrogen compounds such as ammonia to nitric acid and sulfur oxidizers that oxidize reduced inorganic sulfur compounds to sulfuric acid. Activity of both groups of bacteria has been related to biodeterioration of concrete. Nitrifying bacteria are usually implicated in concrete degradation in environments where sulfur oxidizers are not important because their substrate is lacking. A group of bacteria often associated with MID of concrete belong to the sulfur oxidizing genus Thiobacillus. Previous research conducted at INEEL showed that thiobacilli are aggressively involved in MID of concrete.

TECHNOLOGY DESCRIPTION

BNFL and INEEL are working jointly under a Cooperative Research and Development Agreement to develop a technology that utilises this naturally occurring phenomenon for removing surface material of radionuclide contaminated concrete. The technology can be described in three stages:

• Application of microbes and nutrients.

• Maintenance of microbial activity.

• Removal and packaging of surface material for waste disposal.

The process is a passive one that essentially leaves the bacteria to actively degrade the cement matrix until the concrete surface is loosened for removal to a desired depth. It is expected that the process will require 6-18 months for completion, depending on the depth and extent of contamination.

Application of the bacteria and nutrients can be conducted in a fraction of the time required to physically remove concrete surface material. The maintenance phase, which essentially consists of environmental control, requires only minimum attention, primarily to monitor progress. Removal of the degraded surface material again results in reduced labour due to the ease of removal of the already loosened material. In addition, the depth of removal can be controlled such that the waste volume is greatly reduced. These reductions in labour requirements have the potential to result in lower costs and exposure risks to personnel.

All phases can be conducted remotely if necessitated by high radiation fields. Laboratory and proof of principal demonstrations have been conducted and optimum conditions for maximum rate of material removal have been established. Moisture is necessary for bacterial action, although the process does not require saturation and therefore no liquid effluent or secondary waste stream is produced. Optimal temperatures are between 25°C and 30°C, however the process can continue at a reduced rate during sub-optimal conditions and can be used in shut down facilities that are not heated.

In controlled experiments, initiation and maintenance of MID bacterial communities on concrete surfaces was induced. Biofilm formation, bacterial production of sulfuric acid, and formation of calcium oxide dissolution products were demonstrated. In proof of concept demonstrations, concrete reactivity with biogenic acid was demonstrated to be as high as 20 times more efficient than mineral acid dissolution alone can account for.

It has been hypothesised that degradation is the result of microsite dissolution and subsequent weakening of the cement matrix between microsites. Prototype applications on both vertical and horizontal surfaces, verified that MID could be initiated and managed over a large surface area of contaminated concrete and also demonstrated that MID could be used to promote the removal of 2-4 mm of concrete surface.

Current efforts are focused on optimising and engineering the application system to provide bacteria and nutrients to the concrete surface in a more cost and time efficient manner. The application process will promote continued bacterial activity for the duration necessary to remove the desired depth of contaminated concrete material with minimal inputs to the system. The preferred method of application involves mixing the bacteria, nutrients, and reduced sulfur source in an inert matrix that is easily sprayed on the surface, is hydrophilic, and adheres to uncoated concrete. Additionally, efforts are underway to adapt and use currently available technology for removal of the concrete debris once the bacteria have loosened the surface. Design of a complete integrated system is expected to be ready for full scale, active demonstration by 1999.

A number of sites in the US are currently being reviewed to select the most appropriate demonstration opportunity for the fully developed process. Sites currently being reviewed include reactor decommissioning projects, the East Tennessee Technology Park (ETTP) and the US DOE Large Scale Demonstration and Deployment Project (LSDDP) initiative. UK based demonstration of the biodecontamination process is currently ongoing at the Sellafield Pile Chimney decommissioning project.

DISCUSSION

Biodecontamination is an innovative process that is currently being developed by the INEEL and BNFL to meet both the US DOE needs and the UK BNFL needs for cost effective concrete decontamination. Because the depth of removal of a contaminated surface can be controlled in the application of this technology, production of secondary waste is greatly reduced and the occurrence of airborne contamination is eliminated. Estimated costs for the process are considerably less than those for different scabbling methods. As methods for application are developed, it is thought that the process will be usable for decontamination of encumbered (fitting, conduit, piping, etc) floors and walls. Also, because of its “hands-off” operation, worker exposure to radiation and industrial accidents is expected to be greatly reduced.

Laboratory and proof of principal demonstrations have established that MID bacterial populations can be applied and maintained on large concrete surfaces and that their activity can be controlled to promote degradation for the purpose of decontamination. Systems for application, maintenance, and removal are currently being evaluated and optimised and an integrated technology is expected to be ready for demonstration in 1999.