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Review of concrete biodeterioration in relation to nuclear waste

Review of concrete biodeterioration in relation to nuclear waste

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Abstract

Concrete structures can be used to store radioactive waste and radionuclides from nuclear operations. Concrete structures used for radioactive waste contain microbial contamination. Previous research has shown that this can lead to concrete degrading and structural destruction.

This literature review reviews the current research in this area and focuses on the specific parameters that can be used to model and predict the fate of concrete structures used for the storage or disposal of radioactive waste. Concrete biodegradation rates vary depending on the environment. This highlights the need to understand the bioavailability of key compounds involved with microbial activity. To allow microbial growth, pH and Osmotic Pressure must be within a specific range. Concrete biodegradation can also be predicted by considering carbon flow and availability.

It is possible for concrete structures containing radioactive material to be degraded by microbial activity. Concrete biodegradation rate and extent are affected by many physical, chemical, and biological parameters. These parameters are important to consider for modeling activities. There are also possible mitigation options that could be used to minimize concrete biodegradation.

 

Temperature

The environment contains a continuum containing organisms that have different growth rates and are dependent on temperature. Microbial activity can be described as following Arrhenius-type laws. These laws generally describe a doubling in growth rate for every 10 degrees Celsius increase in temperature. This is until thermal activation occurs. Each microorganism is different because they have different enzymes that work under different conditions. Thermal inactivation can occur in any species. However, other species will adapt to higher temperatures. Temperatures between 10 and 45 degrees Celsius will be common throughout the year in the vadose zone and surface environments. This is where LLW is stored and reflects sunlight aboveground. In summer, soil surface temperatures will be higher than exposed concrete. If you look at the entire microbial community, activity will likely increase as a result of higher temperatures in the 10-45 degree Celsius temperature range.

 

Water availability

With the understanding that water availability is restricted in terrestrial systems, microorganisms may be considered aquatic organisms. Water availability is expressed in water activity (aw), which is the ratio of air pressure in equilibrium with pure water. The w values of water activity range from 0 to 1. Most agricultural soils are between 0.9 and 1. Microbial activity is generally found at a w value between 0 and 1, with most agricultural soils between 0.9 and 1.

 

Because of the robust and resilient nature of the microbial activity, most "harsh” growth conditions outside of concrete containment structures will not stop but reduce growth. It is possible to maintain soils in the vadose area at aw levels below 0.7, but this would likely result in very dry conditions. Therefore, growth appears to be most likely and almost unavoidable on the concrete's outside. This assessment shows that concrete's surface is the most susceptible to microbial activity. Concrete exposure to the environment and microorganisms will result in concrete biodegradation. Concrete silos and concrete casks above ground will experience dramatic swings in water availability. Many microorganisms will be affected by drying, but not destroyed. Many microorganisms become dormant when they are exposed to extremely dry conditions. This can be caused by a variety of mechanisms including sporulation. While the information provided above can provide some guidance on limits to microbial activity, it is important to consider the adaptive and robust nature of microorganisms.

 

Studies that involved 3 climate areas, including regions with stable temperatures of 20 to 23 degrees Celsius and extremes in humidity, and another with high humidity and low average temperatures (often below freezing) did not show significant differences in concrete biodegradation. This could be due to the environmental buffering and/or metabolic and physiological resilience of microorganisms under different conditions. Different conditions might select for microorganisms that have similar optimal metabolic rates. This illustrates how microbial communities may have overlapping growth rates despite being exposed to different environmental conditions.

pH

Environments with a neutral pH are ideal for microbial activity. However, the pH range for growth is generally between 1 and 10, with some exceptions. In general, fungi are more active in low pH environments than bacteria. The average internal pH for microbes is 7. External pH values can often be attributed to microbial activity, such as the production of waste products. Biofilms are formed by microbial communities on surfaces. Depending on their physiological status and the number of microbes present, they can buffer pH values upon contact with liquids as well as external liquids.

 

Eh

The soil's redox potential is the ratio of oxidizing to reducing conditions. It can be controlled with chemicals in the soils. Anaerobic soils are more likely to experience oxidizing conditions than oxygenated soils. In terms of microbial activity, soils with oxidizing conditions favor aerobic microbial growth and respiration while soils that are anoxic favor anaerobic growth or fermentation. Certain metabolic functions and therefore specific types of microbes may be favored by soil redox conditions. The soil's redox condition and related biogeochemical activities. This review will concentrate on key geochemical and biological parameters that seem to play important roles on concrete's outer surface as it relates to concrete biodegradation at the interface of concrete structures exposed to the air or in the vadose zone. Specific microbial physiologies will be further examined to accelerate these biogeochemical interactions. This literature is intended to assist in the development of models to predict the rates and degree of microbial activity in specific biogeochemical niches, as well as the overall microbial ecology. The main sources of organic matter in the environment are detritus (such leaf litter and other decaying biomass) as well as root exudates. Organic carbon can also be contributed to by cellular biomass and the waste products of autotrophs. Subsurface microorganisms can grow by consuming nutrients from the detritus decay process and then transferring them through the vadose zone to saturated areas (aquifers). This step-wise process of organic carbon from plant detritus leads to a decrease in O 2 levels in the vadose, upper aquifer, and subsurface microorganisms' ability to grow. This contributes to the energy input that drives biogeochemical and other changes due to microbial activity. While the inorganic transformations may not directly benefit from the activity of the respiratory and fermentative microbes, it is crucial for the whole process.

 

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