WSRC-MS-2001-00689
D. W. Bickley, T. C. Andes, A. S. Busby
Westinghouse Savannah River Company
Aiken, South Carolina 29808
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As Savannah River Site continues to be the primary receipt and storage facility for Aluminum based research reactor fuel, a number of facility improvements continue to be implemented which enhance the storage facilities while reducing the long term operating cost. The L basin facility improvements include projects which allow handling of the TN7/2 and LWT casks, a new sandfilter, and a project to modernize the cask handling cranes. Also, a project is under construction that will demonstrate the ability of a melt-dilute process to prepare research reactor fuel for disposal in a national repository. In an effort to provide the most cost effective long term storage of material, a project is underway to de-inventory the RBOF storage basin and provide storage for all future research reactor fuels in L basin.
The Savannah River Site continues to receive and store foreign and domestic research reactor (FRR/DRR) fuel in two underwater storage basins. The Receiving Basin for Offsite Fuels (RBOF) began receiving fuel in the early 1960s. As the RBOF basin neared capacity, L-Basin was modified to become the primary receiving facility in 1997. In order to continue to meet the long-term objectives of the FRR/DRR program, a variety of facility improvement projects continue to be implemented.
To reduce long-term facility operating cost, a project is underway to consolidate fuel storage into one facility. Plans are to de-inventory the RBOF storage basin by shipping fuels currently stored in RBOF to L-Basin by 2007. Transshipment of fuel is currently in progress. Project work is also ongoing to increase the storage capacity of L-Basin by designing and installing additional fuel storage racks. L-Basin has also been recently modified to allow handling of the TN7/2 and LWT casks which previously could only be unloaded in RBOF.
SRS continues to make improvement to its basin chemistry systems. A new sandfilter system is being designed for L-Basin which will maintain basin clarity. This system will feed the continuous deionizer system that was installed in 1996. Another project that has been recently completed is the modernization of the L basin overhead cranes that handle spent fuel casks.
Other significant activities include the construction of a project that will demonstrate the ability of a melt-dilute process in which fuel assemblies will be melted and mixed with depleted uranium to prepare the fuel for disposal in a national repository. Construction of this demonstration project has been completed and startup testing of the facility is in progress.
L Basin, which has served as the Savannah River Site’s primary FRR/DRR receipt and storage facility since 1997, has been designated to store the RBOF’s inventory of spent nuclear fuel (SNF). The de-inventorying of RBOF will allow SRS to reprogram operating funds into the Treatment and Storage Facility (TSF) to potentially allow for a more timely disposition of Al-based fuels. L-Basin will continue to meet the needs of the FRR/DRR program and is being modified to accommodate the additional inventory requirements. A de-inventory project has implemented to complete RBOF to L-Basin transfers by the end of 2007. Some particulars of the project are discussed below.
NFE Cask
The RBOF de-inventory project fuel consists of three major components. They include the shipment of Aluminum (Al)-based fuels, shipment of non-Al fuels, and shipment of processable fuels to the site’s chemical separations facilities for processing and disposal.
The bulk of the Al-based SNF was completed between February 1997 and May 2001 with the movement of approximately 3,800 Material Test Reactor (MTR) assemblies and 14 High Flux Isotope Reactor (HFIR) cores. At this time only 4 RHF assemblies remain. The MTR fuels were repackaged into new bundles in RBOF and transferred to L-Basin using the DOE owned Nuclear Fuel Element (NFE) casks (allowed for on-site shipments only). These bundles were stored in racks with the FRR/DRR fuels. The 14 HFIR fuel cores were also shipped via the NFE casks and stored in specially designed racks. With the design and installation of additional multi-purpose racks, the 4 RHF assemblies (similar in size to an outer HFIR assembly) will be transferred.
The first transfer of non-Al fuels began in March 2001 and currently about 12% of the approximately 800 fuel storage unit inventory has been moved. Non-Al SNF primarily consists of Zr or SST-clad power demonstration fuel assemblies irradiated before 1975. The transfer of this material presents significant challenges due its varied size, shape, condition, and material type. The movement of this fuel requires the development of Appendix A descriptions, criticality analysis of new shipping and storage configurations, and the cask studies for the varied fuel materials and conditions. About half of the SNF can be repackaged and bundled for storage in exiting DRR/FRR racks. The remainder will require new racks. Most fuel units will require new tool designs for handling purposes and some will require of special cask inserts.
The processable fuels to be shipped to the separations facilities primarily consists of fuels and material considered to be "at-risk". These fuels could impact basin operations because of their potential to release activity to the basin water. These fuels will be removed from RBOF by the end of 2004.
The L-Basin storage facility currently has storage capacity for approximately 9000 MTRE's and 60 HFIR cores. Due to the RBOF De-inventory effort along with the scheduled RRF receipts, L-Basin is expected to reach full capacity in FY2002. A project was started in FY2000 to design and install new fuel storage racks in L-Basin to provide adequate storage capacity through FY2009. This project will provide for storage of an additional 9000 MTRE's, 120 HFIR cores, and storage for approximately 800 storage units of SS/Zr clad type fuels which are not compatible with storage in MTRE type racks. The first rack installation was completed in August 2001. Future rack installations will occur in fiscal years 2002 through 2006. It is expected that even more racks will be required for fuel receipts in FY2010 and beyond.
Two projects have been completed which allow L-Basin to receive additional types of casks. The LWT and TN7/2 casks are too tall to be unloaded in the manner that most casks are handled in L-Basin. Casks are normally placed on the basin floor in an area with a water depth of fifteen and one half(15.5) feet. With the two taller casks, there was insufficient water shielding to unload the fuel assemblies underwater in this area.
The LWT Shielded Transfer System (STS) provides a means to unload fuel baskets remotely into a shielded cell. The shielded cell (or STS) contains a fuel grapple that operators use to remotely engage and lift fuel baskets into the STS. Once the fuel grapple and basket have been positioned into the STS, the system transports the fuel basket to an area where it can be lowered into the basin water. Once underwater, the fuel basket is connected to a portable fuel grapple that carries the basket to an unloading area. The LWT STS has been operational since 1998 and has unloaded approximately a dozen LWT casks.
LWT System
The modifications to unload the TN7/2 casks proved to be much simpler than the LWT since a deeper area existed in the unloading basin where this cask could be unloaded. However, this deep area of the pool could not be accessed by the fuel handling operators due to existing facility obstructions. The TN7/2 project provided a removable work platform that is installed over the deep section of the unloading basin for handing a TN7/2. The project also provided all necessary crane rigging and fuel handling tools needed to handle the cask and unload the fuel.
TN 7/2 Work Platform
Preparations are also in progress to receive the new TN-MTR cask in L-Basin once it has been licensed for shipment in the United States. The first scheduled shipment is in spring 2002.
A new sandfilter system is being designed for L-Basin. This sandfilter, which is used to maintain basin clarity, will replace the existing system that was installed in the early 1980s. The new sandfilter will be a skid mounted pressurized system consisting of two parallel flow paths at a nominal rate of 1000 gpm that will return filter effluent to the basin at less than 5 NTU turbidity. The nominal passable filter size will be 15 micron with 95 percent efficiency. This system is being designed to significantly reduce the maintenance cost associated with the old system and provide service through the duration of the FRR/DRR fuel storage program. The sandfilter feeds the continuous deionizer system that was installed in 1996. This system continues to maintain excellent basin chemistry with conductivity remaining below 10uS/ml.
New Sandfilter
L Basin contains two overhead cranes that handle spent fuel casks. These cranes were installed in the 1950s. Modernization projects have been recently completed on both cranes to improve crane performance and reliability.
The 120 Ton crane in L Stack Area is used to perform initial receipt and breakdown of the spent fuel cask prior to unloading in the basin. The crane is also used after cask unloading to support decontamination and re-assembly of the cask package for shipment off site. The upgrades performed to the crane were to modernize the electrical controls and power systems, and to replace the crane hoists and trolleys.
The 85/30 Ton crane used to handle casks in the spent fuel pool has also been modernized. The cranes electrical systems have been modernized by installation of variable frequency drives, new motors, and radio control capability.
The Spent Nuclear Fuel (SNF) Treatment and Storage Facility (TSF) Validation Test Program was developed for the purpose of validating the basic operation and concept of the full-scale TSF Melt and Dilute process. The intent of the TSF is to receive, treat, package, and store aluminum-based SNF for later disposal in a geologic repository. To validate the full-scale Melt and Dilute process, new process equipment has been installed in L-Area. All construction activities on the L-Area Experimental Facility (LEF) project have been completed and startup testing is in progress. Current plans call for the LEF to process at least six MTR uranium-aluminum (U-Al) SNF assemblies in order to validate Savannah River Technology Center (SRTC) tests that used unirradiated fuels and surrogate materials. The operation of the LEF provides operational experience with irradiated SNF. Lessons learned and experimental data acquired from the demonstration are incorporated into the final design of the full-scale TSF.
The SNF candidate assemblies melted within the LEF include both unirradiated and irradiated fuel, with both high and low enrichment. To demonstrate the proper operation of the furnace and the support equipment, initial melts will use trial aluminum and/or depleted uranium (DU) MTR type assemblies. To validate the dilution techniques, an unirradiated highly enriched MTR assembly will be melted with DU. Subsequent melts progress from low fission product inventory MTR SNF assemblies to high fission product assemblies. Candidate MTR assemblies are stored in the L-Basin pending individual transfer to the Purification Trailer Space for melting. The Melt and Dilute process seeks to convert and consolidate individual MTR SNF assemblies into solid, treated SNF ingots with less than 20% U-235 enrichment. The reduction in enrichment is achieved by adding DU. The SNF and DU is melted in an induction furnace with aluminum added to achieve a near eutectic mixture. The molten mixture is inductively stirred to ensure a homogenous composition. The furnace is expected to operate at approximately 850 degrees Celsius (ºC). The furnace used for the validation test was commercially procured and produces a treated SNF ingot approximately 6.75 inches in diameter by 8 inches in length weighing (including the crucible liner) about 80 pounds. The batching process for delivering DU to the LEF consists of preparing individual aluminum baskets at the SRTC, each with a lid containing approximately 1.25 kilograms (kg) of DU encased in aluminum. When scheduled for treatment, a selected stored fuel assembly from the L-Disassembly Basin is placed into an aluminum basket with the aluminum-encapsulated DU. The basket is transferred from the Disassembly Basin to the LEF for processing.