Nuclear fuel assemblies used by nuclear power plants typically have a life span of about 3 years. Once the fuel of the assembly has been exhausted, the assembly must be removed from the reactor that powers the nuclear plant. Although no longer useful for generating power, the fuel assembly is still highly radioactive and must be stored for hundreds or even thousands of years before the material is no longer a threat to the environment.
Spent fuel assemblies are presently being stored in large pools or water basins at nuclear reactor sites. One of the primary methods of expanding existing storage capacity of water basins at nuclear reactor sites is spent fuel rod consolidation. A fuel assembly includes a series of long metal clad fuel rods typically comprised of uranium oxide. The fuel rods are held in the required geometry to be usable within a nuclear reactor by metal hardware spaced between and along the length of the rods of a given assembly. Fuel rod consolidation is a process that involves dismantling the fuel assembly and rearranging the spent fuel rods into an even closer-packed geometry within a storage canister. Typically, all of the fuel rods from two spent fuel assemblies can be loaded into a single storage canister that has the same overall external dimensions of a spent fuel assembly prior to its dismantlement. Such an assembly can be safely stored in the water basins at nuclear reactor sites. Except for the metal hardware left over from the rod consolidation operation, the method has the potential to double existing storage capacity at reactor sites that have pools with sufficient structural strength to safely support the added weight. Rod consolidation is relatively low-cost alternative to providing additional storage pools.
The non-fuel hardware left over from rod consolidation includes end fittings, rod spacer grids, and control rod guide thimbles for pressurized water reactor spent fuel. For boiling water reactor spent fuel, the hardware includes tie plates, rod spacer grids and flow channels. The radioactive isotopes of most concern for storage and handling which are most commonly formed in the hardware by radiation in the reactor are cobalt-60, niobium-94 and nickel-59. The degree of radioactivity of the waste hardware is typically much less than the degree of radioactivity of the spent fuel on a unit volume basis. However, because of the penetrating power of cobalt-60 gamma radiation, it requires comparable radiation shielding. The lower amount of radioactivity means that less heat is generated by the spent fuel hardware and that a greater amount of non-fuel hardware could be stored safely in a given volume than that for spent fuel.
The current method used for volume reduction of non-fuel hardware has been low pressure compaction with hydraulic presses. This method can typically reduce the volume of the hardware from six pressurized water reactor fuel assemblies to the volume inside a single canister the same external dimensions of a typical spent fuel assembly containing consolidated fuel rods. Similarly, the hardware from five boiling water reactor fuel assemblies can be compacted into the volume of a single canister of the same size as a typical spent fuel assembly. The larger hardware volume associated with boiling water reactor fuel assemblies is due to an additional hardware component not present in pressurized water reactors. Collectively, the present overall achievable storage capacity volume reduction factor for fuel and hardware is about 1.4. In other words, 1.4 spent nuclear fuel assemblies including the associated hardware can be consolidated and stored in the same volume as one fuel assembly in the configuration it is received within the nuclear reactor.
Alternate methods have also been evaluated for processing non-fuel hardware to increase its density and correspondingly reduce its storage volume requirements. Two methods under consideration are shredding/super-compaction and melting. Shredding would be a one-step process that allows metal pieces to fit inside canisters and increases their bulk density. Super-compaction provides greater volume reduction of shredded non-fuel hardware without extensive treatment. It is similar to low pressure compaction except that it uses pressures of about 60 MPa (as opposed to about 7 MPa presently used) that increases the final bulk density to about 50% to 80% of theoretical maximum. Present low pressure compaction achieves about 25% of theoretical maximum density. Shredding and super-compaction are discussed in Ross et al., "Treatment Alternatives for Non-Fuel-Bearing Hardware", January, 1987, and is incorporated herein by reference.
The melting alternative provides the highest possible volume reduction of waste radioactive material to a bulk density near theoretical maximum. At this density, the non-fuel hardware will occupy only about 2%-5% of its volume in the original spent fuel assemblies. In other words, a twenty to forty-fold reduction of volume is theoretically possible. However, heating of the metals to very high temperatures under controlled and radiation-shielded conditions would be necessary. An article authored by Westsik et al., "Induction Melting for Volume Reduction of Metal Tru Wastes", 1986, identifies induction melting as the preferred process for volume reduction of radioactive metal waste material. The article discloses induction melting of waste material into a graphite crucible inside of a large vacuum chamber. However, high stainless steel content of the melt is corrosive to graphite crucibles. The Ross et al. article identified above also discusses melting as a possible alternative for consolidation and compacting of non-fuel hardware at pages 6.4-6.6.
Page A.9 of Ross et al., Evaluation of Alternative Treatments for Spent Fuel Rod Consolidation Wastes and Other Miscellaneous Commercial Transuranic Wastes, 1986, discloses the possibility of using a plasma arc in a melter system. The published paper Moriyama et al., Volume Reduction of Non-Combustible Radioactive Solid Wastes by Plasma Melting also discloses melting of waste with a plasma gun.
The present invention is an improvement over the above teachings. The invention arose primarily from the needs and concerns associated with consolidating non-fuel hardware of spent nuclear fuel assemblies. It will be appreciated by those skilled in the art that the concepts of the invention could be employed for consolidating many other radioactive waste materials for storage.