(374d) Disposition of Surplus Highly Enriched Uranium through Downblending and Purification for Placement in the Nuclear Fuel Cycle

The global disposition of surplus highly-enriched uranium (HEU) has become one of the most pressing initiatives of our time.    International cooperation has achieved new heights to evaluate and implement options to ensure that this surplus of fissile material is not used in rouge nuclear weapons.  In the
United States, government agencies such as DOE-NNSA/GTRI are moving forward with programs to safely disposition such materials.  The disposition programs also offer recycle of a valuable resource, provide the end users with lower cost commercial reactor fuel, and reduce the threat of nuclear proliferation.

As part of the Tennessee Valley Authority (TVA) Blended Low Enriched Uranium (BLEU) project, Nuclear Fuel Services, Inc. (NFS) in Erwin, Tennessee is downblending 23 MT HEU in the form of alloy, metal and oxide to achieve a low-enriched uranyl nitrate (LEUN) stream that meets all critical specifications for commercial nuclear fuel reactors.  The distribution of types of HEU feeds accepted in the TVA downblending program is exhibited in Figure 1.  Downblended U from this program is now fueling TVA reactors. The program, established by a DOE/NNSA - TVA Interagency Agreement, provides for the downblending of 40 MT of HEU to a Low-Enriched Uranium product suitable for use in the TVA Browns Ferry Nuclear Units 2&3.  This 40 MT also includes another 17 MT HEU downblended at the Savannah River Site (SRS).  The LEUN from both NFS and SRS is then converted to reactor-grade UO₂ powder by Areva-NP in
Erwin, Tennessee.   This cooperative effort by the team of DOE/NNSA, TVA, Areva-NP, and NFS garnered the 2005 Platts Global Energy Engineering Award, and the 2004 DOE Secretary of Energy Award.  As of this date, over 30MT of HEU out of the original 40 MT has been downblended to LEU. 

Figure 1 - Distribution of BLEU HEU Feeds

NFS is also undertaking the downblending of an additional of 17.4 MT HEU for the DOE Reliable Fuel Supply (RFS) Program.  NFS has already received metal, oxide, and assorted unirradiated reactor fuel assemblies for dissolution and downblending to achieve about 290 MT of 4.95% LEU that can be made available as a reliable fuel supply inventory for commercial LWRs. The product from downblending is LEUN solution that achieves ASTM specifications for commercial nuclear fuel and is subsequently transported to the Westinghouse Columbia, SC fuel fabrication plant for conversion to UO₂.  The general classification of the HEU material in the RFS program is shown in Figure 2. 

Figure 2 - Distribution of RFS HEU Feeds

The BLEU Preparation Facility (BPF) at NFS has capability to selectively: blend, process, recover, blend, and purify HEU materials from various sources to achieve HEUN feed that will produce a conforming LEU product (even those sources containing significant soluble and insoluble impurities).   NFS deploys a robust purification system to remove radiological and chemical impurities in order to meet strict product specifications. 

The DOE also possesses a variety of small quantity HEU streams with various physical, chemical, and radiological attributes.  To address such HEU streams, the NFS CD-Line, an extension of the BPF downblending process, has been designed for the purpose of addressing these more challenging HEU forms.  This system has the flexibility to address a variety of feed streams including, but not limited to: UF₄, UO₂F₂, UF₆, filter ash, and other miscellaneous HEU-bearing scrap.   HEU values recovered from this process can be returned to the client for further storage and disposition or purified and downblended for conversion into high purity uranyl nitrate solution for further processing to UO₂.  Figure 3 is an example of miscellaneous scrap destined for processing in the CD-Line.

With startup in late 2008, the CD-Line processing facility will be used to disposition a variety of HE U-bearing scrap materials as well as HE uranium hexafluoride (UF₆).   The highest enrichment UF₆ (≥ 94% enriched) will be converted to solid U₃O₈ and packaged for shipment to the DOE customer, with the lower-enrichment UF₆ and various the scrap materials to be processed to recover residual U. 

Figure 3 ? HEU Scrap

The scrap includes alumina trapping materials and alumina standards, process gas, and miscellaneous scrap including U-bearing parts.  After dissolution, the resulting uranyl nitrate solution will be pumped to the BPF for purification via liquid-liquid solvent extraction, followed by downblending to 4.95% ²³⁵U enrichment.    

The CD-Line has the capability of recovering purified U from almost all chemical and physical forms of legacy HEU.  The facility can convert these compounds to a form that can be fed into the existing BPF for purification and downblending.  The process design for this UF₆ / non-UF₆ recovery system was validated through extensive NFS flowsheet testing using HEU specimens from the actual containers. 

The DOE will also release for processing additional legacy HEU over the next 10 years.  These HEU forms include fluoride-bearing solids, U oxides, U-Zr, U-Al, U-Mo, and C-coated UC₂.  Many of these materials are also cladded in Al, Zr, stainless steel, or in a graphite matrix.  The CD-Line is designed to allow for future processing of these small and varied streams. The design employs modular, skid-mounted equipment for quick installation and removal, as the starting U types change.