The United States is investigating the Yucca Mountain (YM) site in Nevada for the disposal of radioactive spent nuclear fuel (SNF)the primary waste from nuclear power plants. The SNF would be packaged and then emplaced 200 to 300 m underground in parallel disposal tunnels. The repository isolates the SNF from the biosphere until the radionuclides decay to safe levels. DU may improve the performance of geological repositories for disposal of SNF via three mechanisms:
- Radiation shielding for waste packages to protect workers
- Lowering the potential for long-term nuclear criticality in the repository
- Reducing the potential for releases of radionuclides from the SNF
Proposed YM Site in Nevada for SNF
General Characteristics of Repository Depleted Uranium (DU) Applications
The quantities of spent nuclear fuel that ultimately must be disposed of are sufficiently large that all the DU in inventory could be used for repository applications. The currently proposed Yucca Mountain repository, or any future repository, will be operated over a period of decades. As is true with any long-term human activity, the repository design will change with time. Thus, depleted uranium applications could be implemented when a repository starts operation or at a latter date.
The Yucca Mountain Project has conducted initial studies on disposal of DU as a waste in a separate area of the proposed repository. The studies indicate that the site would be acceptable for DU disposal; that is, long-term releases of DU from the repository would be very low. Studies are underway, as discussed herein, to beneficially use DU to improve repository performance by reducing the long-term release of radionuclides from the spent nuclear fuel.
The U.S. Nuclear Regulatory Commission (NRC) is responsible for licensing the proposed Yucca Mountain repository and assuring safe disposal of spent nuclear fuel and any other radioactive materials. It has concluded that (1) disposal of DU in a repository appears to be acceptable and (2) there may be benefits to use of DU in a repository. As a regulatory agency, no definitive conclusions on any disposal site characteristic is given until a licence is issued. The NRC has also concluded that DU oxide forms would be preferable given the geochemical characteristics of a repository. Our technical studies concur with this conclusion; thus, all repository applications use DU in the form of oxides.
Cross Section of Proposed YM Disposal
Tunnel Showing Potential Applications of DU
The cross section of a disposal tunnel in the proposed YM repository shows the potential applications for DU. The disposal tunnels would be more than 200 m underground. The SNF is in a waste package, which is supported by the invert. A drip shield is over the waste package to divert water from the waste package and thus extend its lifetime. The tunnel may be backfilled. Four potential DU applications are being investigated:
- DU Dioxide Fill. The uranium as uranium dioxide sand would fill the empty spaces inside the waste package.
- Cermet Waste Package. The waste package could be constructed of a uranium cermet.
- Invert. The invert could be constructed of uranium cermet or filled with an uranium oxide.