LONG-TERM PERFORMANCE ISSUES OF NUCLEAR-WASTE PACKAGE MATERIALS The longevity of manufactured materials in the repository environment over such long periods of time is subject to significant uncertainty. At the same time, the prediction of material performance is essential in the development and use of waste packages (waste forms and waste containers). In the absence of a good mechanistic understanding of a material’s performance and data that span a wide range of the expected performance and physicochemical conditions, extremely conservative assumptions need to be considered. Many of the performance predictions rely on data collected over a relatively limited range of test conditions; thus, extrapolation of these data requires good mechanistic understanding.16,26 Without proper data support, any benefits that the waste forms or container might provide could be ignored; hence, it is highly desirable to improve the predictability of the materials performance. This also requires demonstration of quality control of the product. Various technical issues must be addressed in the assessment of the long-term performance of the waste package in a geologic repository.27,28 As all components of a waste package may be altered in time within the repository environment, the environment for a waste package (both internal and external) must be well characterized. A demonstrated understanding of factors that might affect long-term service behavior is required for the characterization of materials for the waste-package components. These factors include variations in characteristics such as chemical composition, stress state, microstructure, fabrication or production history, and thermodynamic phase equilibria. Various interactions may be expected from gaseous or aqueous media that are in contact with the materials of the waste package. For metallic containers, various forms of corrosion that result from interactions with water and oxygen are important, as are the effects of hydrogen, which may result from radiolysis of water and vapor or galvanic coupling with borehole liner or container support structures. The environment may produce hydrostatic or lithostatic pressure, which may alter the stress state in waste-package components. Radiation will change the environment and create species with the potential for accelerated degradation of the waste-package components. Microbial species, if they are present in significant quantities, have the potential for interactions with the waste-package materials.29 The service life of the waste package must be determined based on the consideration of these interactions between the environment and the waste-package components, including joints, seals, and welds. For details on the experimental programs specific to Yucca Mountain, refer to Reference 21.

New Nuclear Power Technologies Introduction: There are many new waste disposal technologies which could prove to be somewhat of a solution to the problem of nuclear waste. Reprocessing, The Missing Step: Although not a new technology, reprocessing can be part of the solution to nuclear waste. When nuclear power was first developed, it was assumed that spent nuclear fuel would go through a process called reprocessing. In reprocessing, one of the major transuranic wastes, 239Pu, is extracted from the spent fuel rods. This 239Pu (plutonium-239) is fissile and can be reused in power plants. The advantages of this process are somewhat obvious: The volume of waste is lessened and more fuel is created for nuclear reactors. However, as with all things, politics can get in the way. In the US plutonium reprocessing was banned because the recovered 239Pu is weapons grade material. If, after reprocessing, the fuel is stolen, it could be used by anyone to construct a nuclear weapon. As of a few years ago, the ban against reprocessing in the US was lifted, but there are still no operating reprocessing plants in the US because of the heavy regulations and the anti-nuclear sentiment of the general public. There are a few countries which do reprocessing, however. France, for instance, regularly reprocesses its spent fuel. High Temperature Breeder Reactors: Many of us are familiar from television (and hopefully not from real life experience) of the bar-room game in which a very large man holds a mug of beer on top of his head and challenges people to punch him. If his opponent punches him hard enough, the beer falls off and spills all over the man holding it. The harder the punch, the better chance that the beer will fall off and the puncher will win. Also, the bigger the man is who is getting punched, the harder the punch must be to knock the beer down. You might be wondering why we are talking about a bar-room game. Think of the guy holding the beer as an atom and the guy punching as a neutron. The transuranic elements are bigger than uranium and generally don't fission (get their beer knocked off) in a regular reactor. The neutrons aren't excited enough (don't punch hard enough) to induce fission in them. However, if they are placed in a high-temperature reactor in which the neutrons are much more excited (and carry more punch), there is a much better chance that they will fission. In a reactor being developed by Argonne National Laboratory in the US, almost 100% of the transuranic nuclear wastes produced through neutron capture can be caused to fission. Generally, the fission products created have shorter half-lives and are not as dangerous. This reactor, dubbed EBR-II, uses liquid sodium as a coolant, which means that the internal reactor temperature is much, much hotter than that of a normal PWR reactor, which uses water as a coolant. Another advantage of EBR-II is that its fuel is not weapons grade quality. When the transuranic wastes are separated from the other wastes in the spent fuel rods, the resultant mix of isotopes can not be used in a bomb. Thus, the mix can be used as fuel for EBR-II without a chance of it getting stolen by a terrorist group for use in an explosive device. Breeder reactors "breed" fuel. That is, they are designed to create 239Pu from 238U through neutron capture. This "waste" can then be used as fuel.