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Microorganisms have been detected in TRU wastes, Pu-contaminated soils, low-level radioactive wastes, backfill materials, natural analog sites, and waste-repository sites slated for high-level wastes. Seventy percent of the TRU waste consists of cellulose and other biodegradable organic compounds. Biodegradation of cellulose under the hypersaline conditions such as in the WIPP repository can produce CO2 and methane gas, as well as affect the solubility of actinides. Microbially produced gases could have significant ramifications for the long-term stability of the repository (up to 10,000 years).” (A. J. Francis, Brookhaven National Lab, 2003)

A decade later, WIPP in May of 2014 at around 15 years…
WIPP 22 May 2014

Transuranic elements, such as plutonium, are radioactive metals and are subject to both corrosion and reactions with other metals, as well as changes due to radioactive disintegrations. Microbial corrosion is an important part of the reason that radioactive waste cannot be buried and facilities cannot be closed. https://en.wikipedia.org/wiki/Microbial_corrosion The metallic natural of transuranic radioactive waste means that it can corrode from inside out, as well as outside in.

Formerly buried radioactive waste, which blew up last November.
Nevada Department of Public Safety  Page III-8 State Fire Marshal Division  December 30, 2015 Beatty Incident Report
Low Level radioactive Waste (LLRW) Site near Beatty, Nevada Incident”, November 18, 2015, p. III-8, Nevada State Fire Marshal [1]

In the following article A.J. Francis (2003) also informs us that frighteningly “Free-living bacteria suspended in the groundwater fall within the colloidal size range and may have strong radionuclide sorbing capacity, giving them the potential to transport radionuclides in the subsurface“. There is also the risk of radiation-induced microbial mutations which may make them (more) virulent or resistant to antibiotics, unmentioned in the article.

From the Brookhavn [US] National Lab:
MICROBIAL TRANSFORMATIONS OF PLUTONIUM AND OTHER ACTINIDES IN TRANSURANIC AND MIXED WASTES July 2003. BNL-71559-2003-CP

A.J. FRANCIS Environmental Sciences Department Brookhaven National Laboratory, Upton, New York 11973, USA

The presence of the actinides Th, U, Np, Pu, and Am in transuranic (TRU) and mixed wastes is a major concern because of their potential for migration from the waste repositories and long-term contamination of the environment. The toxicity of the actinide elements and the long half-lives of their isotopes are the primary causes for concern. In addition to the radionuclides the TRU waste consists a variety of organic materials (cellulose, plastic, rubber, chelating agents) and inorganic compounds (nitrate and sulfate). Significant microbial activity is expected in the waste because of the presence of organic compounds and nitrate, which serve as carbon and nitrogen sources and in the absence of oxygen the microbes can use nitrate and sulfate as alternate electron acceptors.

Biodegradation of the TRU waste can result in gas generation and pressurization of containment areas, and waste volume reduction and subsidence in the repository. Although the physical, chemical, and geochemical processes affecting dissolution, precipitation, and mobilization of actinides have been investigated, we have only limited information on the effects of microbial processes.

Microbial activity could affect the chemical nature of the actinides by altering the speciation, solubility and sorption properties and thus could increase or decrease the concentrations of actinides in solution. Under appropriate conditions, dissolution or immobilization of actinides is brought about by direct enzymatic or indirect non-enzymatic actions of microorganisms. Dissolution of actinides by microorganisms is brought about by changes in the Eh and pH of the medium, by their production of organic acids, such as citric acid, siderophores and extracellular metabolites. Immobilization or precipitation of actinides is due to changes in the Eh of the environment, enzymatic reductive precipitation (reduction from higher to lower oxidation state), biosorption, bioaccumulation, biotransformation of actinides complexed with organic and inorganic ligands and bioprecipitation reactions.

Free-living bacteria suspended in the groundwater fall within the colloidal size range and may have strong radionuclide sorbing capacity, giving them the potential to transport radionuclides in the subsurface.

The actinides in TRU and mixed wastes may be present in various forms, such as elemental, oxide, coprecipitates, inorganic, and organic complexes, and as naturally occurring minerals depending on the process and waste stream. They exist in various oxidation states and the ones of concern are III (Am, Pu, U), IV (Th, Pu, U), V (Np), and VI (Pu, U).

Microorganisms have been detected in TRU wastes, Pu-contaminated soils, low-level radioactive wastes, backfill materials, natural analog sites, and waste-repository sites slated for high-level wastes1.

Seventy percent of the TRU waste consists of cellulose and other biodegradable organic compounds. Biodegradation of cellulose under the hypersaline conditions such as in the WIPP repository can produce CO2 and methane gas, as well as affect the solubility of actinides. Microbially produced gases could have significant ramifications for the long-term stability of the repository (up to 10,000 years). Little is known of microbial degradation of organic constituents in TRU and mixed wastes and its implications on gas generation and the fate of actinides i.e., mechanisms of the dissolution and precipitation of actinides. Chelating agents are present in TRU and mixed wastes because they are widely used for decontaminating nuclear reactors and equipment, in cleanup operations, and in separating radionuclides. Plutonium forms very strong complexes with a variety of organic ligands. Naturally occurring organic complexing agents, such as humic and fulvic acids, and microbially produced complexing agents, such as citrate, and siderophores, as well as synthetic chelating agents can affect the mobility of Pu in the environment. Biotransformation of actinide-organic complexes should result in the degradation of the organic ligand and precipitation of the actinide. Key microbial processes involved in the mobilization or immobilization of selected actinides of interest is summarized in Table 1. Among the actinides, biotransformation of uranium has been extensively studied, whereas we have only limited understanding of the microbial transformations of other actinides such as Th, Np, Pu, and Am present in TRU and mixed wastes1, 2.
Table 1. Biotransformation of Selected Actinides in TRU and Mixed Wastes.
The effects of microbial activities on TRU and mixed waste and their potential for treatment of certain waste forms to stabilize the actinides and reduce the volume of the waste have not been fully exploited.
Fundamental understanding of the mechanisms of microbial transformations of several chemical forms of actinides under various microbial process conditions such as aerobic, anaerobic (denitrifying, fermentative, and sulfate reducing) and repository relevant conditions would pave the way to assess the microbial impact on TRU wastes and the long-term behavior of actinides in the environment. In this paper biotransformation of actinides, in particular Pu under various microbial process conditions is presented.
References
1. Francis, A.J. 2001. Microbial transformations of plutonium and implications for its mobility. In “Plutonium in the Environment” A. Kudo, (Ed) Elsevier Science Ltd., Co., UK. Pp 201-219.
2. Neu, M.P., C.E. Ruggiero, and A.J. Francis. 2002. Bioinorganic Chemistry of Plutonium and Interactions of Plutonium with microorganisms and Plants. In “Advances in Plutonium Chemistry 1967-2000” D. Hoffman (Ed), pp 169-211. Published by American Nuclear Society, la Grange Park Illinois and University Research alliance, Amarillo, Texas.
………………………………………
This work was performed under the auspices of the U.S. Department of Energy.
Submitted to The Plutonium-Futures Conference, Albuquerque, NM, July 2003
https://www.bnl.gov/isd/documents/25649.pdf (Emphasis our own).

Don’t think that survival of microbes in radioactive waste is a good thing either. Besides corrosion and possible transport of radionuclides to the surface, we don’t want mutant microbes. Unlike humans, microbes have very short life cycles and mutation can give them advantages against antibiotics and human immunity. Most likely nuclear reactors and waste have been breeding grounds for more genetically fit and resistant microbes. Some microbes have natural resistance to radiation. Useful bacteria need to stay as they are, as well.

According to Los Alamos National Nuclear Lab: “Animal studies have found that a few milligrams of plutonium per kilogram of tissue are lethal. Plutonium is more dangerous when inhaled than when ingested. When inhaled, plutonium can pass into the bloodstream moving throughout the body and into the bones, liver, or other body organs. Plutonium that reaches body organs generally stays in the body for decades and continues to expose the surrounding tissue to radiation and thus may cause cancer.http://periodic.lanl.gov/94.shtml

El Hajj et. al. (2010) found that the corrosion rate of steel more than doubled under microbial conditions, compared to blank sterilized runs, in a simulation of a clay stone geological repository. They speak of the plausibility of sulfate-reducing bacteria (SRB) “growth during the construction and operational phases” and “their survival at least temporarily after the disposal closure if water is available, which may cause fast corrosion of the steel containers under disposal conditions” (Abstract)….
SRB may survive to severe conditions of temperature, pressure, or lack of water but their growth is conditioned by the presence of enough liquid water for nutrients transport and bacterial cell development. In the case of deep geological repository, where high amount of hydrogen is expected to be produced via steel corrosion,” p. 252 H. El Hajj et al. / Physics and Chemistry of the Earth 35 (2010) 248–253
https://www.researchgate.net/profile/Abdesselam_Abdelouas/publication/222966383_Microbial_corrosion_of_P235GH_steel_under_geological_conditions/links/545897e00cf2cf5164825201.pdf?origin=publication_detail
L Basin cobwebs Savannah River site microbial
L Basin microbial cobwebs Savannah Nuclear River site
http://srnl.doe.gov/news/releases/nr14_srns-cobwebs.pdf

Many thanks to Leuren Moret for pointing out the issue of microbial corrosion based on her familiarity with this problem at Yucca Mountain and WIPP, when she worked at Sandia Labs. She says that there was serious corrosion of waste containers within 5 years, but did not specify if the waste containers were buried and in closed, high humidity rooms, or open and ventilated.

Furthermore, the combination of aging and of valence fluctuations (Janoschek, 2015, http://advances.sciencemag.org/content/1/6/e1500188) appears to mean that no one can ever really know what’s going on with plutonium waste, although some governmental researchers stay busy trying to prove otherwise, their work appears unconvincing. And, even they admit lack of understanding: Brandon Chung et. al. (2015) in “Self-Irradiation Effects on Metallurgical Properties of Plutonium Alloys” note that “The plutonium alpha-decay leads to the formation and accumulation of numerous defects and transmutations, which affect metallurgical properties… There remain, however, a number of outstanding questions regarding the role of impurities and defects on the fundamental aging process of plutonium… This work reveals that we continue to need better understanding of integrated contribution age-related defects to metallurgical property changes“. Their work is sponsored by USDOE- Lawrence Livermore National
Pu Valences

According to Janoschek, et. al. (2015) “because the various valence configurations imply distinct sizes of the Pu ion, the valence-fluctuating ground state of Pu also provides a natural explanation for its complex structural properties and, in particular, the large sensitivity of its volume to small changes in temperature, pressure, or doping.” (M. Janoschek, P. Das, B. Chakrabarti, D. L. Abernathy, M. D. Lumsden, J. M. Lawrence, J. D. Thompson, G. H. Lander, J. N. Mitchell, S. Richmond, M. Ramos, F. Trouw, J.-X. Zhu, K. Haule, G. Kotliar, E. D. Bauer, “The valence-fluctuating ground state of plutonium“. Sci. Adv. 1, e1500188 (2015). http://advances.sciencemag.org/content/1/6/e1500188.full.pdf+html )

Pu metal ring LANL
Apart from corrosion, plutonium changes as it undergoes radioative disintegrations:
Transmutation, Helium Bubbles, and Voids.
The transmutation products resulting from α-decay—radiogenic helium and other actinide atoms—can also affect bulk properties. After 50 years of storage, weapons-grade plutonium will have grown in the following amounts of transmutation products: approximately 2000 atomic parts per million (ppm) helium, 3700 ppm americium, 1700 ppm uranium, and 300 ppm neptunium. After that length of time, a piece weighing 1 kilogram will contain nearly two-tenths of a liter of helium measured at standard conditions. Said differently, after 50 years of decay, the accumulated helium in plutonium would generate a pressure of 3 atmospheres in an equivalent empty volume! The most important concern about the in-growth of actinide transmutation products in plutonium is their potential effect on the delicate balance of phase stability. Under equilibrium conditions, the addition of americium makes fcc δ-plutonium more thermodynamically stable, so several thousand parts per million of americium should help to further stabilize δ-phase alloys. Uranium and neptunium, on the other hand, reduce δ-phase stability. In both cases, however, the conditions present after the α-decay events are far from equilibrium and, hence, may not have the expected effects at ambient temperature. The accumulation of radiogenic helium could affect the properties of plutonium substantially. Helium has extremely low solubility in metals because it does not bind. It diffuses rather easily through the lattice (as easily as vacancies) until it becomes trapped in one of the vacancies. The helium-vacancy clusters can migrate and coalesce, potentially forming into helium bubbles, which cause swelling. It is well known that less than 100 ppm of helium in fcc stainless steel can cause swelling or dramatic embrittlement. Wolfer concludes, however, that macroscopic swelling in plutonium as a result of helium bubbles is very unlikely at ambient temperature. The combination of irradiation-induced lattice damage and the presence of helium atoms can cause void growth and bulk swelling without the presence of helium bubbles. Void growth is an obvious potential consequence of vacancy migration and clustering. Void swelling has been found to be a serious problem for materials in the irradiation environment of reactors. Reliable predictions of void swelling in plutonium are not currently possible because of a lack of fundamental knowledge of intrinsic properties such as the relaxation volumes for vacancies and interstitials. Wolfer’s best estimate is that void swelling is expected to begin for δ-phase plutonium alloys in the temperature range of –30°C to 150°C after 10 to 100 years (for typical plutonium-239). Once void swelling begins, he estimates that the swelling rate will be approximately 1–2 percent per 10-year lifetime for plutonium-239. This value is similar to that found for other fcc metals. Compared with the range of swelling resulting from helium bubble formation, void swelling is the more important concern. Number 26 2000 Los Alamos Science “Aging of Plutonium and Its Alloys”
http://permalink.lanl.gov/object/tr?what=info:lanl-repo/lareport/LA-UR-00-2619

Note 1:
Nevada Department of Public Safety Page III-8 State Fire Marshal Division December 30, 2015 Beatty Incident Report – “Low Level radioactive Waste (LLRW) Site near Beatty, Nevada Incident 30 day Report Radiation Control Program November 18, 2015https://web.archive.org/web/20160412235048/http://dps.nv.gov/uploadedFiles/dpsnvgov/content/media/SFM-BeattyIncidentReport.pdf (The Nevada State Fire Marshal said that the immediate cause of exploding waste at Beatty was sodium metal reacting with water and producing hydrogen gas and sodium hydroxide, but the report notes the high level of corrosion which allowed the water to meet the sodium metal)