Corrosion Engineers and Nuclear Waste Disposition
Corrosion Engineers and Nuclear Waste Disposition

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Published: 2006

Total Pages: 4

ISBN-13:

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More and more articles appear in the press daily about the renaissance of nuclear energy. Even many former opponents of nuclear energy are now convinced that nuclear energy is more environmentally friendly than burning fossil fuels. Nuclear energy does not release carbon dioxide to the atmosphere and therefore does not contribute to the global warming problem. But nuclear energy produces spent fuel or nuclear waste. Spent fuel is radioactive and requires thousands of years of isolation from plants, animals and humans. Every country currently studying the option for disposing of high-level nuclear waste has selected deep geologic formations to be the primary barrier for accomplishing this isolation. It is postulated that by the very nature of these geological sites, they will contain the waste for long time, limiting the spread of radionuclides, for example, through water flow. The release of radionuclides to the environment can also be delayed by the construction of engineered barrier systems between the waste and the geologic formation. Corrosion engineers are participating in the design and the performance prediction of the engineered barriers. The principal engineered component in this multibarrier approach is the container for the waste. Beyond the metallic containers, other engineered barriers could be added to attenuate the impact of the emplacement environment on the containers. The containers will probably be concentric double walled vessels of dissimilar metals. Each vessel would have a specific function. For example, the inner container may be designed to shield radiation and provide structural support to facilitate the safe handling and emplacement operations. This inner container may be over-packed with a corrosion-resistant outer layer. The design of the different containers for nuclear waste would vary according to the nature of the geologic formation at the site of the repository. The most common host rocks for nuclear waste repositories in the world are clay, basalt, tuff and granite. The groundwater associated with the containers should all be relatively benign to most materials because of their low ionic strengths, near neutral pH, and low concentrations of halide ions. The corrosiveness of these waters could increase if significant vaporization occurs due to heating from radioactive decay during the early times of emplacement. Many different alloys are currently being studied for the container materials, including carbon steel, stainless steel, titanium, copper, and nickel alloys. Twenty years ago, most of the designs for repositories specified life spans of 300 to 1000 years. Today, some designs are considering lifetimes as high as 1,000,000 years. This requirement has created a difficult problem for engineers to solve. The unique aspect of this problem is associated with making predictions about the corrosion behavior of container materials for extended periods of time. Many of the alloy systems being considered have been in existence for less than 100 years. The understanding and methodologies of existing corrosion engineering remain largely oriented to traditional problems involving less corrosion resistant alloys and corrosion behavior over time periods less than decades. Predicting the behavior of engineering alloys over geologic time periods poses new challenges but at the same time offers new opportunities for the development of corrosion science. Even though nuclear energy has been in use for half a century, no country currently operates a nuclear waste repository. The current renaissance of public interest in nuclear energy may put some pressure on the development of the repositories. Building a nuclear waste repository is a long process since the whole society needs to be ready for it, and nowadays this society may extend beyond country lines. Corrosion engineers can help.

Corrosion of Spent Nuclear Fuel
Corrosion of Spent Nuclear Fuel

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Published: 2003

Total Pages: 29

ISBN-13:

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The successful disposal of spent nuclear fuel (SNF) is one of the most serious challenges to the successful completion of the nuclear fuel cycle and the future of nuclear power generation. In the United States, 21 percent of the electricity is generated by 107 commercial nuclear power plants (NPP), each of which generates 20 metric tons of spent nuclear fuel annually. In 1996, the total accumulation of spent nuclear fuel was 33,700 metric tons of heavy metal (MTHM) stored at 70 sites around the country. The end-of-life projection for current nuclear power plants (NPP) is approximately 86,000 MTHM. In the proposed nuclear waste repository at Yucca Mountain over 95% of the radioactivity originates from spent nuclear fuel. World-wide in 1998, approximately 130,000 MTHM of SNF have accumulated, most of it located at 236 NPP in 36 countries. Annual production of SNF is approximately 10,000 MTHM, containing about 100 tons of ''reactor grade'' plutonium. Any reasonable increase in the proportion of energy production by NPP, i.e., as a substitute for hydrocarbon-based sources of energy, will significantly increase spent nuclear fuel production. Spent nuclear fuel is essentially UO2 with approximately 4-5 atomic percent actinides and fission product elements. A number of these elements have long half-lives hence, the long-term behavior of the UO2 is an essential concern in the evaluation of the safety and risk of a repository for spent nuclear fuel. One of the unique and scientifically most difficult aspects of the successful disposal of spent nuclear fuel is the extrapolation of short-term laboratory data (hours to years) to the long time periods (103 to 105 years) as required by the performance objectives set in regulations, i.e. 10 CFR 60. The direct verification of these extrapolations or interpolations is not possible, but methods must be developed to demonstrate compliance with government regulations and to satisfy the public that there is a reasonable basis for accepting the long-term extrapolations of spent fuel behavior. In recent years ''natural analogues'' for both the repository environment (e.g., the Oklo natural reactors) and nuclear waste form behavior (e.g., corrosion and alteration of uraninite, UO{sub 2+x}) have been cited as a fundamental means of achieving confirmation of long-term extrapolations. In particular, considerable effort has already been made to establish that uraninite, UO{sub 2+x}, with its impurities, is a good structural and chemical analogue for the analysis of the long-term behavior of the UO2 in spent nuclear fuel. This proposal is based on the study of uraninite and the naturally occurring alteration products of UO{sub 2+x} under oxidizing and reducing conditions. The UO2 in spent nuclear fuel is not stable under oxidizing conditions. In oxic solutions, uranium has a strong tendency to exist as U{sup 6+} in the uranyl molecule, UO2{sup 2+}. Uranyl ions react with a wide variety of inorganic and organic anions to form complexes. Throughout most of the natural range of pH, U{sup 6+} forms strong complexes with oxygen-bearing ions like CO32−, HCO3−, SO42−, PO43−, and AsO43−, which are present in most oxidized stream and subsurface waters. In arid environments, the U{sup 6+} ion can precipitate as a wide variety of uranyl oxide hydrates, uranyl silicates and uranyl phosphates. This is well demonstrated in experimental work, e.g., in long term drip tests on UO2 and is confirmed by natural occurrences of UO2 in which a wide variety of uranyl phases form as alteration products. The most striking feature of these studies is the very close parallel in the paragenetic sequences (i.e. phase formation sequence) between the very long term (10 year tests) and the young (therefore, low-Pb uraninites) of the Nopal I deposit in Mexico.