Maintaining Chemically Reducing Waste-Package Conditions
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Maintaining Chemically Reducing Waste-Package Conditions Charles W. Forsberg and Leslie R. Dole Oak Ridge National Laboratory Oak Ridge, TN 37831, U.S.A. ABSTRACT Most fission products and actinides in spent nuclear fuel (SNF) are trapped in the uranium dioxide (UO2) crystal structure and cannot escape until the UO2 is oxidized. Oxidation destroys the crystal structure, exposes the radionuclides to groundwater, and creates the potential for dissolution. For a repository in an oxidizing geochemical environment, the potential exists to delay the release of radionuclides for hundreds of thousands to millions of years by using a combination of excess depleted uranium dioxide and iron in the waste package (WP) to locally maintain chemically reducing conditions. This approach limits the rate of oxygen transport to the SNF after WP failure. Alternative methods of oxygen transport to the SNF were investigated, including transport by groundwater, diffusion through air, and diffusion through water. INTRODUCTION It has been proposed [1] that spent nuclear fuel (SNF) storage, transport, disposal, and multipurpose casks be constructed of a depleted uranium dioxide (DUO2)Bsteel cermet. Cermets consisting of DUO2 particulates embedded in a continuous-steel phase (Figure 1) replace the steel components (structural shell and the basket) in the cask. For the multipurpose-cask application, the SNF would be loaded at the reactor and the cask would then be used for storage, transport, and disposal. For all waste package (WP) applications, the cermet cask would become the inner container of the WP, with an overpack of a corrosion-resistant metal. Cermets are a means of making a ductile form of DUO2 suitable for use as a cask material of construction. Cermet casks have several attractive features. The DUO2 cermet is an excellent shielding material and may be the best shielding material that could be used in a repository. Highperformance shielding materials maximize the SNF cask capacity for any given weight and size limits. Cermets are used in armor; thus, they offer the potential for better assault protection of the SNF. The use of DUO2 provides a method for beneficial use of the 500,000 tons of excess depleted uranium (DU) in inventory. In the repository, the iron and DUO2 may potentially reduce and delay release of radionuclides from the WP. The DUO2 can be incorporated into a cermet or be used as a particulate fill [2] for the void spaces in and around the SNF assemblies within the WP. Several potential mechanisms may improve repository performance. The DUO2 and iron can act as sacrificial reagents to maintain chemically reducing conditions next to the SNF. Most radionuclides in SNF are incorporated within the SNF UO2 crystal structure and cannot be released until the UO2 is oxidized. The DU in and around the SNF saturates the groundwater and thus eliminates the concentration gradient for dissolution of uranium. The DUO2 and iron oxide degradation products chemically and physically sorb various radionuclides and delay their r
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