ROCKY FLATS RESIDUE STABILIZATION
PROGRESS AND ACCOMPLISHMENTS
Henry F. Dalton
Rocky Flats Field Office
DOE
Gary M. Voorheis
Kaiser-Hill
ABSTRACT
Rocky Flats has made notable progress toward the stabilization of the Site's inventory of solid and liquid residues. Accomplishments include completion of residue drum venting, completion of the magnesium hydroxide precipitation of Building 771 uranium/plutonium solutions, removal of the highly enriched uranyl nitrate solutions (HEUN) from Building 886, residue leaded glove washing, solid residue characterization and startup of the salt stabilization process. Three other major solid residue stabilization processes are scheduled for startup in 1998, and all solutions are expected to be removed from Building 771 by the end of September 1998. In addition, a number of other important initiatives have been undertaken, such as the approval of the pipe overpack container for transuranic (TRU) waste shipping and storage. This container reduces the number of TRU waste drums generated from residue stabilization and reduces the risk from the post stabilization TRU waste storage until the TRU waste is shipped to the Waste Isolation Pilot Plant.
INTRODUCTION
When operations at the Rocky Flats Environmental Technology Site (RFETS) near Denver, Colorado were curtailed in 1989, the Site's special nuclear material inventory was stored in a manner that was considered unsatisfactory for long term storage. As a result of concerns with the storage of special nuclear material inventories at Rocky Flats and other DOE Sites, the Defense Nuclear Facilities Safety Board (DNFSB) issued Recommendation 94-1. When Recommendation 94-1 was issued, Rocky Flats had a solid residue inventory of approximately three metric tons plutonium contained in 106 metric tons of bulk material, a plutonium and plutonium/uranium solution inventory of ~29,000 liters, primarily in Buildings 771 and 371, and ~ 2700 liters of highly enriched uranyl nitrate (HEUN) solution in Building 886.
In response to Recommendation 94-1, the Department of Energy committed to processing all the solutions by June 1999 and stabilization of all solid residues by May 2002, with interim milestones established for the higher risk materials. The progress toward achieving these milestones and initiatives to reduce risks and improve schedules are provided in the following discussion.
SOLUTION STABILIZATION
Plutonium and Plutonium/Uranium Solutions in Buildings 771 and 371
The plutonium and plutonium/uranium bearing solutions in Buildings 771 and 371 would normally have been disposed through plutonium recovery or waste treatment operations prior to the 1989 curtailment. The presence of the solutions created an increasing safety risk due to the age and condition of the process equipment in Building 771, and to a lesser degree, in Building 371. These solutions were left in various stages of the recovery process equipment including tanks, piping and bottles. Approximately 11,600 liters were contained in the tanks that have a measurable inventory of solutions from various stages of the recovery processes. These solutions range from wastes to high plutonium concentration process feeds. Other tanks and process piping are considered operationally empty, but may still contain appreciable solution in the tank heels and in piping low points. The solution inventory for the operationally empty systems was originally estimated at 15,400 liters. In addition, a backlog of about 1400 liters of miscellaneous solutions was stored in 4 liter plastic bottles in drums at the time of the 1989 curtailment. These bottles have since been removed from the drums and are placed in gloveboxes where they can be routinely monitored for degradation of the plastic bottles until stabilized.
Solution Stabilization Processes
Four processes are being used to solidify the solutions in Buildings 771 and 371. The solutions containing less than 6 g/l plutonium content are being processed by direct cementation in Building 774. Feed for the direct cementation process consists primarily of the backlogged bottled solutions and solutions from Building 771 tanks with less than 6 g/l plutonium. In conjunction with treating the low concentration solutions, the direct cementation process is being used to accelerate the stabilization of residue ion exchange resins, one of the higher risk residues. The ion exchange resins are added as a minor (<1%) component in the drums of cemented plutonium solutions. Twenty drums of residue resin have been repackaged, with two drums completed in November 1997. Combining the ion exchange resin stabilization with the existing direct cementation process reduced the risk from these materials faster than originally planned. The cementation of the ion exchange resins is expected to be completed by the end of September 1998.
Solutions in Building 771 containing chloride, uranium, and plutonium/uranium mixtures with a total actinide concentration greater than 6 g/l were solidified using a magnesium hydroxide precipitation process in 4-liter beakers. The precipitate from this process was filtered and dried on a hot plate, resulting plutonium oxide/magnesium uranate suitable for interim storage. The filtrate from the process was bottled and transported to Building 774 for disposal via the direct cementation process. Processing of the ~300 liters of solution scheduled for magnesium hydroxide precipitation was completed in March 1997. This process will be kept available to solidify any uranium or high chloride concentration solutions that may be generated from tapping and draining residual solutions held up in piping and the operationally empty tanks.
The Caustic Waste Treatment System (CWTS) in Building 371 was upgraded and the process modified for production scale magnesium hydroxide precipitation of solutions with plutonium concentrations greater than 0.004 g/l from Building 371 tanks and piping. The precipitate from this process is filtered through a series of filters, dried on a hot plate to an oxide and placed in interim storage. The filtrate is collected, sampled, if the plutonium concentration is below the Building 374 waste transfer limit of 0.004 g/l, transferred to Building 374 and stored until the solution can be processed through carrier precipitation or by another waste treatment process. If the filtrate plutonium concentration does not meet the Building 374 limits, it is then recycled through CWTS.
The carrier precipitation process in Building 374 will be used to eliminate the filtrates resulting from the CWTS process. In addition, some of the solutions removed from the operationally tanks and piping in Building 371 may meet the plutonium concentration limits for carrier precipitation and will be processed directly through this process. The carrier precipitation process involves mixing the solution with potassium hydroxide, if required, ferrous sulfate, calcium chloride, magnesium sulfate, and a flocculating agent. The plutonium is precipitated into the resulting sludge and stored until an approved treatment process is available. The filtrate from the carrier precipitation process is concentrated and the evaporator bottoms will be converted to low level saltcrete waste.
As of January 1998, approximately 9600 liters of solution has been drained from process tanks, 12,000 liters of solution from backlog bottles and tanks have been stabilized, and the residue ion exchange resins have been repackaged and stabilized pending final disposal through the bottle box cementation process.
One of the recent successes of the solution stabilization program is the accelerated removal of the high plutonium concentration solutions from Building 771 for blending with Building 371 solutions and processing in CWTS. This modification advances the removal of the Building 771 tank contents by about 4 months, eliminates one of the processes originally planned for processing these solutions, and allows redirection of resources to the accelerate draining of residual solutions in the tanks and piping and other risk reduction activities in Building 771. In addition, the operating limit for feed solutions to CWTS was recently increased from 6 g/l to 25 g/l plutonium. This process improvement will further accelerate the stabilization of the high plutonium concentration solutions. Removal of the high level solutions was completed in December 1997 and removal of all solutions in Building 771 is expected to be completed in September 1998.
Highly Enriched Uranyl Nitrate Solution in Building 886
The HEUN solution in Building 886 had been used in criticality experiments starting in mid-1960s. The solution was stored in 9 Raschig ring filled tanks and ranged in concentration from 120 to 370 g/l uranium. The HEUN solution was drained from the tanks to a bottling station, transferred into 10-liter critically safe bottles, packaged in shipping containers, and transported to Erwin, TN where it was converted to uranium oxide. Offsite shipment of the HEUN to Erwin, TN was completed in November 1996 and the conversion to uranium oxide was completed in May 1997. The uranium oxide will be shipped to Oak Ridge, TN.
Once the HEUN solution was drained, the Raschig rings were removed from the tanks and a pit area and packaged as low-level waste for disposal at the Nevada Test Site. Building 886 is now ready for decontamination and decommissioning.
SOLID RESIDUE STABILIZATION
Solid residues were generated at Rocky Flats as a by-product of plutonium operations since the early 1950s and were recycled to recover the plutonium until operations were curtailed in 1989. The residue inventory is stored in approximately 8000 - 0.21 m3 drums and other containers and consists of a variety of plutonium bearing solid materials, such as incinerator ash, pyrochemical salts, glass, paper, plastic, metal, refractory materials, ion exchange resins and filters. The materials were segregated into common residue types based on chemical and physical characteristics and were given unique identifiers known as Item Description Codes (IDCs). The approximately 100 IDCs that are in the solid residue population have been further categorized into four processing groups based on similar chemical and/or physical properties and the planned stabilization process. These four processing groups are known as Salt, Ash, Combustibles and Dry/Repack.
Salt Residues
The Salt residues consist of about 16,000 kg bulk of pyrochemical salts remaining from direct oxidation reduction (DOR), salt scrub, electrorefining (ER) and molten salt extraction (MSE) processes. The DOR process converted plutonium oxide to plutonium metal buttons used as feed in the weapons fabrication. The ER and MSE processes were used to purify recycled plutonium metal by removing trace metallic impurities and americium, respectively.
The salts will be stabilized by undergoing a pyro-oxidation process, using an oxidant such as a metal carbonate in a salt melt to oxidize any residual plutonium, sodium, calcium, potassium or magnesium metal that could potentially be in the salt residues. After stabilization, the salts will be either packaged as transuranic (TRU) waste once domestic safeguards are terminated or placed in interim storage until subsequent processing can be performed, if required, to terminate domestic safeguards. Calcium chloride based salts, such as DOR salts, that require further processing will be packaged and shipped to Los Alamos National Laboratory for dissolution and recovery of the plutonium. The remaining non-calcium chloride based salts, primarily the ER and MSE salts, will undergo a distillation process to separate the plutonium oxide from the salt matrix, pending the Record of Decision (ROD) for the Residues Environmental Impact Statement (EIS). The plutonium oxide from this process will then be packaged to meet DOE-Standard-3013 and the salt will be disposed as TRU waste.
Salt oxidation operations were started January 16, 1998 and approximately 65 kg of salt have been stabilized as of January 31, 1998.
Ash Residues
The residues in the Ash category consist primarily of incinerator ash, sand, slag and crucible (SS&C), and graphite fines and account for approximately 29,000 kg of the bulk residue inventory. The incinerator ash residues resulted from the combustion of feed materials during operation of the incinerator in Building 771. SS&C residues were generated in Building 771 during the plutonium metal reduction operations and consist of crucible pieces, magnesium oxide sand, and calcium fluoride/calcium oxide slag. Graphite fines resulted from the mechanical cleaning of graphite molds to remove the mold coating and any plutonium imbedded on the graphite surface.
The graphite fines and incinerator ash will be stabilized by agglomeration with glass in Building 707. The process consists of mixing the residues with a glass frit at a pre-determined ratio and heating the mixture at 700 - 1300 degrees C to produce a glass matrix. The agglomerated material will then be packaged and certified for disposal as TRU waste. Startup is expected to occur in June 1998.
The SS&C residues were originally slated to be stabilized using the same agglomeration process as the ash and graphite fines. However, the preferred option for this material is to send the SS&C to the Savannah River Site for processing in F Canyon. The SS&C will be size reduced, segregated as necessary, and packaged to meet the F Canyon processing requirements, starting in March 1998. Packaging and shipping is anticipated to start in May 1998, after issuance of Residue EIS ROD.
Combustible Residues
The Combustible residue group is comprised primarily of wet combustibles, dry combustibles, filter media, and plutonium fluoride residues. The Combustible group contains about 22,000 kg bulk.
The wet combustibles were generated as part of routine glovebox cleanup operations from gloveboxes that contained processes using water, nitrate or organic solutions. Dry combustibles were also generated from cleanup activities in gloveboxes that did not contain liquids. The filter media residues include polypropylene filters used in the processes to filter particulate from aqueous and organic solutions and High Efficiency Particulate Air (HEPA) filters made of fiberglass filter media in plywood or metal frames that were used to remove particulates from the building ventilation systems. Nitrate contaminated materials will be processing through shredding, agitating with water or caustic, and dried. Organic contaminated materials will be shredded, undergo thermal desorption and steam oxidation, and dried. Dry combustibles and HEPA filters will be repackaged. The output these process streams will be packaged and certified as TRU waste. Combustible processing is schedule to start in June 1998.
Plutonium fluoride residues will be calcined as necessary to meet shipping container requirements, packaged, and shipped to the Savannah River Site for processing in F Canyon. Preparation, packaging, and shipment of the plutonium fluoride is scheduled to start in June 1999.
Dry/Repack Residues
The Dry/Repack residues consist of approximately 39,000 kg bulk of inorganic materials such as metal, glass, refractory materials, and graphite. Size reduction is required for some of these materials and all must be repackaged to meet TRU waste certification requirements. Dry/Repack operations are currently scheduled to start in June 1998.
Residue Risk Mitigation and Characterization Activities
In addition to the preparation activities supporting startup the residue stabilization processes, a number of risk mitigation activities have been planned and executed to reduce or better characterize the hazards associated with the residues during pre-stabilization storage.
The potential for drum pressurization and hydrogen gas accumulation were deemed to be the highest risk issues for the stored residues. Drum venting to eliminate these potential hazards was initiated in Fiscal Year 1995 (FY 95) and completed in early FY 96. The venting process consisting of punching a hole in the drum lid and existing liner, withdrawing a gas sample in some cases, and replacing the old drum lid with a new filtered lid. In FY 97, a monitoring and surveillance program was initiated that tests the drum filters. The program's primary focus is on testing those drums that contain residues with the greatest potential for hydrogen gas generation and/or corrosion that may plug the drum filter.
As mentioned in the previous discussion on the solution stabilization program, the residue ion exchange resins are being immobilized in the Building 774 direct cementation process in conjunction with the solution stabilization activities. This combined stabilization effort started in FY 96 and accelerated the treatment of one of the higher risk residue materials by taking advantage of an existing process rather than waiting for the installation of the process equipment originally planned for the ion exchange resin stabilization.
Another residue category that was judged to be a higher potential hazard were the drums containing leaded rubber gloves, due to a concern that shock sensitive species may have been formed due to the exposure of the lead and organic components of the gloves to nitric acid. All eleven drums of residue leaded rubber gloves were stabilized by rinsing and repackaging in FY 96.
The residue characterization program was initiated in FY 96 with the primary objective of identification and mitigation of any imminent safety concerns. In addition, the characterization program also provides data on storage container and material conditions, verifies process knowledge, and supports stabilization process development. Sampling priority was given to those residues that were considered to present the greatest risk, primarily due to pyrophoricity, shock sensitivity, flammable gas buildup, ignitability or loss of containment concerns.
Currently, 550 samples have been taken and analyzed. The information gathered to date supports the position that the residues are safely stored while awaiting processing and/or repackaging. There has been no evidence of material spontaneously igniting in an ambient environment and no indication of shock sensitive materials. In general, the drums and containers do not show compromised containment, although there is degradation of the plastic barriers in some cases. The sampling program will be completed in FY 98.
Other Initiatives
The Pipe Overpack Container was approved for use in the TRUPACT-II shipping container by the Nuclear Regulatory Commission in February 1997. The Pipe Overpack Container optimizes shipments to WIPP by increasing the TRUPACT-II criticality limit from 325 fissile grams equivalent (FGE) to 2800 FGE and provides additional shielding for high americium content wastes. The container has been subjected to additional testing that supports its use as an on site TRU waste storage container for highly dispersible TRU waste from the Residue Stabilization program, thus reducing Site risk while the TRU waste is awaiting shipment to WIPP.
Use of the Pipe Overpack Container, in conjunction with characterization data that demonstrates that the residues are low risk, is being evaluated as an alternative to some of the planned stabilization processes. This initiative could potentially reduce the residue stabilization schedule in Building 707 by approximately 18 months to 2 years and result in a program savings of approximately 60 million dollars, with a total Site cost savings potential of up to 130 million if other stabilization activities in Building 707 are accelerated or relocated.
CONCLUSION
Significant progress toward stabilizing the solid and liquid residue inventory at Rocky Flats has been made to date, particularly in the areas of solution stabilization and risk mitigation for the solid residues. To summarize, as of January 31, 1998:
The residue stabilization program is on the critical path for Rocky Flats closure. Activities that accelerate the residue stabilization schedule, such as the use of the Pipe Overpack Container mentioned above, or possibly shipping some of the residue inventory to another Site for treatment are being actively pursued.
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