Jeanne M. Ball and Donald F. Dustin
Safe Sites of Colorado, L.L.C.

There are approximately 100 metric tons of residues at the Rocky Flats Environmental Technology Site containing approximately 3 metric tons of plutonium. The residues are byproducts of past plutonium operations--incinerator ash; pyrochemical salts; graphite; sand, slag, and crucible; and miscellaneous forms of combustibles, glass, metal, and sludges. In September 1993, a report was released (Reference 1) which identified concerns with the chemical stability of the residues and with the integrity of packaging. In May 1997, the Defense Nuclear Facility Safety Board published recommendation 94-1 citing a concern for the residue stability and requiring that the possibly unstable residues be processed within 3 years and all others within 5 years. A risk categorization scheme was developed which assigned a numerical risk to each residue type based on the probability and consequence of occurrence of failures associated with the hazards identified in the Reference 1 report and elsewhere.

The residues were ranked for priority of stabilization actions. Urgent concerns were resolved. All residue drums were vented to eliminate the potential for hydrogen and other explosive gas accumulation. Leaded rubber gloves and ion exchange resins were washed to eliminate the explosion potential. An aggressive characterization program was implemented to search for any additional safety or environmental concerns and to gain more definitive information concerning the choice of processes for stabilization and disposition of the residues.

A baseline plan and schedule were established for the residues. The plan called for minimal processing of the residues to resolve stability issues and the reconfiguration of residues to comply with safe storage criteria. The stabilized residues were to be repackaged to less than 200 fissile gram equivalent (FGE) and in accordance with all Waste Acceptance Criteria (WAC) for disposal at the Waste Isolation Pilot Plant (WIPP). In August 1996, Safeguards Termination Limits (STLs) were published (Reference 2) which established threshold plutonium concentrations, based on material attractiveness, above which the material was required to be safeguarded. Most of the residues were above the relevant STL. Since WIPP will not be safeguarded, materials destined for WIPP must be brought within the STL by reducing the plutonium concentration or reducing the attractiveness of the residue form. The baseline plan was adjusted to accommodate the STLs, and the site is implementing the processing of the residues in accordance with the plan.

Residue processing is on critical path to site closure. Initiatives are in work to simplify and accelerate the baseline plan. An STL variance request was approved for some calcium chloride salts when they are repacked to 200 grams. Those salts would have required a new aqueous processing system to comply with the STL. Miscellaneous combustibles and inorganics also received an STL variance. A variance request is pending which would remove major residue categories with less than 10 percent (at the 200-gram limit) plutonium from safeguards. A further initiative, called "Pipe and Go," supports the elimination of most processing by packaging the residues in a stout pipe container. The "Pipe and Go" proposal depends on added characterization to establish that residues are relatively benign and on testing and calculations demonstrating pipe integrity. If the STL variance request and "Pipe and Go" are approved, the residue disposition schedule will be significantly accelerated.

This paper summarizes the progress toward answering the two questions which drive the Residue Stabilization Program at RFETS:

1. Are the residues safe?
2. How can they be dispositioned?

The safety of the residues as presently stored is of highest concern. The early concerns regarding safety of the residues were validated. Elevated levels of hydrogen gas were found in drums of salt and sludges. The concerns about flammable gas accumulation were eliminated by drum venting. Other urgent safety concerns associated with resins and leaded gloves were mitigated. The characterization data to date has indicated that the vented drums are safe as they sit. Continued characterization is being performed to gain statistical confidence, and the continuing priority will be the identification and mitigation of emergent safety issues. This paper provides background on the safety issues and summarizes recent characterization data.

WIPP continues to be the answer to dispositioning the residues. The disposition plan calls for minimal processing and repackaging to meet safe storage and WIPP criteria. Due to the pressure to accelerate closure of RFETS, it is important to keep the residue flowsheet as simple as possible and to utilize existing nuclear facilities to the extent possible to process the residues. Modifications to those facilities are required. A constraint which complicates the flowsheet is the Safeguards Termination Limits (STLs). In order to eliminate safeguard protection of residues either plutonium will have to be extracted from the residues or they will need to be converted to less attractive materials. The residue processing and disposition plans, including schedule and cost, are summarized in this paper.

Finally, the paper addresses initiatives undertaken by Safe Sites of Colorado (SSOC) to accelerate the residue program at savings in excess of $100M. One initiative is the request for variance from the STLs to greatly simplify the flowsheet. Another initiative aimed at accelerating the residue program is known as "Pipe and Go."

Byproduct materials from past RFETS plutonium processing were designated as residues, as opposed to TRU waste, if the plutonium concentration exceeded the economic discard limit (EDL) for the material of interest. The residues were set aside for subsequent processing for plutonium recovery. With the end of the Cold War, the EDL has no relevance, and the residues are now regarded as higher plutonium level TRU waste. The residues have been categorized into four "buckets," or groupings which have common disposition paths, with the following bulk weights:

• Ash--26 metric tons of incinerator ash, graphite fines, and sand, slag, and crucible from incineration and reduction operations.
• Salts--16 metric tons of electrorefining, molten salt extraction, direct oxide reduction, and miscellaneous other salts from pyrochemical operations.
• Combustibles--20 metric tons of paper, plastics, and coveralls which were the feedstock to incineration.
• Dry/Repack--40 metric tons of miscellaneous inorganics, i.e., metals, glass, etc., which require only repackaging, not processing, for disposition.

In March 1994, W.V. Conner published a report (Reference 1) citing safety concerns associated with the chemistry and packaging configuration of the residues. Conner was concerned about the potential of hydrogen and other flammable gases from radiolytic decomposition of plastic and other hydrogenous materials. He cited the potential of shock sensitive plutonium hydride production and noted the potential for container corrosion due to free acid and carbon tetrachloride. Conner's report cited the lack of definitive characterization information. In the absence of data, he derived the potential chemical reactions and emphasized the need for data to substantiate or dispel his concerns. The DNFSB published recommendation 94-1 in May 1994 citing similar concerns about residue safety and requiring the stabilization of "possibly unstable" residues within 3 years with stabilization of all residues within 5 years.

D.F. Dustin developed a risk categorization scheme (Reference 3) as a basis for specifying "possibly unstable" residues for processing priority. The scheme established eight safety parameters: (1) flammable gas formation, (2) shock sensitivity, (3) flammability, (4) pressurization, (5) container corrosion, (6) incompatible chemicals, (7) radiation exposure, and (8) toxic exposure. Based on process knowledge, Dustin derived the probability and consequence of the existence of each parameter for each material type. He applied a weighted numerical value to each material type and ranked the risk from high (Category 1) to low (Category 5).

Actions were taken to mitigate the risks of the Category 1 materials. Beginning in December 1995, all residue drums were vented to eliminate the potential for flammable gas generation. Organic nitrated ion exchange resins, which have an explosion potential when dry, were neutralized by washing in weak caustic. Most of the resins were then combined with liquid waste (from Building 771 tank draining) and processed by cementing in the Building 774 bottle box operation. Presently, 110 of the 171 batches remain to be cemented. Rubber glovebox gloves which had been in contact with nitric acid were also risk Category 1 due to the visible indication of a white powder in areas where the glove was cracked, exposing the lead shielding to nitric acid. There was concern that the powder could be a shock sensitive leaded compound lead fulminate. This concern was not validated. The gloves were bagged into a glovebox in Building 776 and wiped down with wet Kim-wipes and repackaged. These actions reduced the risk of all Category 1 items to Category 2. The risk Category 2 through 5 items were prioritized for processing . Processing plans and schedules were developed to support commitments to the DNFSB for meeting recommendation 94-1.

A characterization program was developed by K.A. Phillips in December 1995 to gain definitive risk data on the residues. The first priority of the program was to determine if there were any further safety concerns which would indicate the need for immediate action. The plan also supports validation of the priority of processing established by Dustin. The program was also directed at verifying that the residue chemistry was compatible with the selected processes, and, finally, environmental data was to be collected to verify the RCRA status of the materials.

The program was developed to gain maximum information from a nominal amount of solid sampling. The solid sampling involved opening the drums or stand-alone cans, dumping the contents into a pan, documenting the visual condition, and taking samples to represent the range of heterogeneity in the container. Acceptance criteria were established for the parameters of interest, and pass/fail thresholds were developed to establish the presence of positive "hits" for the parameters. A statistical sampling plan was developed which provided 80 percent confidence that less than a specified number of "hits" existed in a population. The characterization effort also incorporated the results from drum filter testing, headspace gas sampling, and real time radiography.

To date, approximately 70 percent of the planned characterization is complete. The results to date validated the early concerns for hydrogen accumulation. Drums of salts and sludges were found with 20 to 60 percent hydrogen in the headspace. Fortunately, the radiolytic decomposition of plastic which generated the hydrogen also depleted the oxygen so the gas composition was not explosive. However, the data established the need to vent the residue drums which was accomplished in December 1995. Subsequent vent testing verified that the vents were functional.

The rest of the characterization data generally indicate that the residues are safe as they are presently stored. The packaging materials are generally in good shape with no indication of serious corrosion or breach of containment. There has been no indication of shock sensitivity or significant reactivity. The reactivity of pyrochemical salts is consistent with process knowledge, and no exothermic behavior has been exhibited below 60° C. Pressurization was indicated in three containers of pyrochemical salts which were packaged in nested produce cans. In each of the three cases, one of the produce cans was bulged. However, the cans did not appear to be pressurized as found indicating that the pressure had been relieved. This finding was consistent with testing which indicates that the produce cans leak.

The variety of packaging configurations was not expected. Approximately 25 different configurations were found including cans, bottles, cartons, clam shells, and Kraft tubes. This information was valuable as it related to the logistics of future processing. Some materials, for example material which was held up in the ducts, has indicated the presence of RCRA constituents which changed the management practices for those materials.

A baseline processing plan was established around October 1995 for processing of the residues to ensure a stable waste form and for repackaging the residues to conform with the Interim Safe Storage Criteria (Reference 4) and the WIPP Waste Acceptance Criteria. A key element of the plan was the pipe shipping component. The pipe component is a stout 0.69 cm walled stainless steel pipe, 15.2 or 30.5 cm inner diameter by 63.5 cm long. The pipe was developed to allow increasing the plutonium limit of TRUPACT-II shipments. The prior limit of 325 grams per shipment was based on the TRUPACT-II criticality limit. The pipe component was drop and fire tested at Sandia National Laboratories and was approved by the Nuclear Regulatory Commission for use with TRUPACT-II in February 1997.

The baseline plan for salts incorporated stabilization of the salts to oxidize reactive species such as sodium, plutonium, potassium, and calcium. The salts were to be brought to 900° C in a stationary pyrochemical furnace with addition of sodium carbonate with stirring. The oxidized salt was to be removed in its stainless steel can, placed in a convenience container, bagged out of the line, measured by calorimetry, and placed within the pipe component inside of the 55-gallon drum. The equipment was sized to meet the DNFSB commitments and 10,000 additional kilograms by September 30, 1999, and all 16,000 kg processed by May 2002. Ten new furnaces were constructed in four new gloveboxes in A Module of Building 707. In-line segmented gamma scanners (SGS) were incorporated for batching and six out of line calorimeters were acquired for final assay. Readiness preparations were completed for the salt oxidation process, and the system was started in January 1998.

The baseline plan for the ash materials--incinerator ash, graphite fines, and sand, slag, and crucible--incorporated thermal stabilization to oxidize reactive species such as plutonium, calcium, and magnesium. The material was to be batched into flat pans for heating to 500° C for 2 hours in air in muffle furnaces. The oxidized ash was to be placed in a metal can and packaged in the pipe within a 55-gallon drum for shipment to WIPP. The equipment was sized to meet the DNFSB commitments of 4000 kg of graphite fines and sand, slag, and crucible to be processed by May 1998 and all ash processed by May 2002. Six new muffle furnaces were planned to be placed in existing gloveboxes in E Module of Building 707. Two new SGS units are planned. The ash process is in construction with completion scheduled for March 1998.

The safety concern for combustibles is the potential for formation of explosive nitrated organics from the exposure of organic combustibles to nitric acid. Simulation testing failed to generate an explosive material, and the safety concern associated with combustibles is highly conservative. The plan calls for shredding the combustibles and rinsing with dilute sodium hydroxide followed by filtration and addition of a drying agent. The combustibles which have been exposed to organics, such as the carbon tetrachloride and machine oils from Building 707, will be heated in a vacuum to evolve the organics. The organics will be thermally desorbed. The combustibles will be packaged in filtered plastic bags within drums. The wattage limit of 0.08 watts/drum will be met before the drum volume becomes limiting. Two aqueous and one organic processing lines are planned for Room 3701 of Building 371. The room contained radioactively cold equipment planned for past residue processing. The new process utilizes some existing lines and added equipment. The combustibles processing lines are in construction with start of operations expected by April 1, 1998.

The Dry/Repack "bucket" contains miscellaneous glass, metal, and other inorganics which contain no safety risk associated with material stability. No processing of the materials is planned. They will be size reduced with tools and equipment, as required, and repackaged to facilitate safe storage, transportation, and to meet WIPP Waste Acceptance Criteria. The pipe component will be incorporated for the higher plutonium loading, but the leaner materials will be packaged to meet packaging requirements within drums. A repack line is planned for D Module of Building 707. Construction is in process with start-up scheduled for April 1998 to meet the commitment of all 40,000 kg prepared for shipment by May 2002.

The baseline processing plans were based on the premise that the residues, when reconfigured to meet the Interim Safe Storage Criteria and WIPP Waste Acceptance Criteria, were TRU waste and would not be subject to safeguards control. This is a key point because WIPP will not be safeguarded and materials shipped to WIPP must previously be removed from safeguards control. In the Reference 2 Department of Energy memorandum, the status of non-SNM plutonium containing materials was clarified. Threshold limits of plutonium concentration were set, based on the attractiveness of the material, above which the material was required to be safeguarded. The STLs and present average plutonium concentrations for RFETS residues are compared in Table I.

For materials exceeding the STL, there are two options. The plutonium could be separated from the matrix to bring the matrix within the STL for disposal at WIPP. The plutonium would be dispositioned along with other RFETS special nuclear material. Alternatively, the material could be reduced in attractiveness by making the plutonium "practically irrecoverable" and raising the STL.

Task teams were formed by DOE-RFFO and by DOE-Headquarters around each of the major residue categories to establish flowsheets which would most expeditiously bring the residues into STL compliance. Each of the "bucket" processing plans was rebaselined with adjusted schedules and budgets to incorporate the revised flowsheets.

The salt team considered options including blending to the STL and aqueous dissolution and salt distillation to separate the actinides. Salt distillation was selected based on development work underway at Los Alamos National Laboratory. The oxidized salt is brought to molten temperature, around 950° C, and vacuum is applied to selectively volatilize the salt from the actinide heel into a separate cooled chamber. The salt is packaged for disposal at WIPP, and the heel is packaged for disposition with SNM.

Distillation was incorporated into the baseline plan. Since the salt has to be oxidized prior to distillation, to eliminate the carryover of volatile plutonium, the plan is to continue with the baseline oxidation processing, and instead of shipping the salt to WIPP, stage the salt for subsequent distillation. Eight to ten distillation units are planned for installation in B Module of Building 707. Demonstration and testing (D&T) is underway at Los Alamos National Laboratory to set the distillation parameters, and a design contract for the RFETS units has been placed. Distillation start-up is scheduled for October 1999. One complication is that the oxidant choice for salt oxidation--sodium carbonate--causes foaming in the subsequent distillation step so part of the D&T effort was to revise the oxidant choice.

The ash team considered options including blending to the STL and cementation or vitrification to reduce material attractiveness. Vitrification was selected. The parameters for frit type, ratio, temperature, and time are being established through D&T at Savannah River Site and Pacific Northwest National Laboratory. Preliminary indications are that incinerator ash must be calcined prior to vitrification to prevent separation of the ash from the frit. The D&T effort is targeted at providing RFETS the vitrification process design to support the April 1998 start of operations, but that milestone is considered to be at risk.

The combustibles team established that the baseline process, with nominal blending, was adequate to meet the STL. Some improvements, such as a dryer, were added to the baseline. The combustibles start-up schedule remains at April 1998. The Dry/Repack "bucket" received STL waivers; therefore, the start-up schedule of April 1998 is not affected.

Alternatives to the baseline are being considered to decrease the cost and accelerate the schedule of the Residue Stabilization Program. One initiative, promoted by SSOC, is to request variances from the STLs to allow specified residue categories, packaged to Interim Safe Storage Criteria and WIPP Waste Acceptance Criteria, to be exempted from safeguards requirements and shipped to WIPP. The basis for the exemption requests is that residue categories with less than 10 percent plutonium, when packaged to the 200 FGE limit, cannot roll up into Category II quantities of SNM. The Level IV equivalent controls are in place for wastes at RFETS, and the materials will be controlled to prevent diversion until interment at WIPP, 0.5 miles underground.

A precedent is in place for this approval. In July 1997, STL variances were approved by DOE-Office of Safeguards and Security for specific populations of direct oxide reduction and other calcium chloride salts, inorganics, and combustibles. The salt variance was of particular concern because calcium chloride salts are not amenable to salt distillation. A separate aqueous processing circuit would have been required in Building 371. Approximately 800 kg of calcium chloride salts were not submitted for variance because of higher levels of plutonium, and those salts will likely be processed at Los Alamos National Laboratory. The approval of inorganics and combustibles precluded the requirement for batching to the STL. The expected savings resulting from the variance is around $36M.

A subsequent variance request was submitted for major residue categories with average plutonium concentrations less than 10 percent. Approval would eliminate the need for salt distillation and ash vitrification, resulting in an expected $40M savings and significant schedule acceleration.

A follow-on initiative, proposed by SSOC, is called "Pipe and Go." The concept is based on the proven integrity of the pipe shipping container and the characterization data to date which indicate that the residues are relatively benign. The proposal is that the pipe be credited with the mitigation of any untoward event resulting from residue storage and shipment such that stabilization processing is not required. The materials which are intended for use with the pipe--salts and ash--would be repackaged into the pipe for disposal at WIPP. The combined savings of the STL variances and the "Pipe and Go" initiative are estimated at approximately $100M due to the avoidance of stabilization processing as well as the processing of the actinide heel resulting from distillation.

1. W.V. CONNER, "Evaluation of Residue Drum Storage Safety Risks," RFP-4826, EG&G Rocky Flats, Inc. (1994).
2. E.J. McCALLUM, Department of Energy Memorandum for Distribution, "Additional Attractiveness Level E Criteria for Special Nuclear Material (SNM)," Department of Energy, Office of Safeguards and Security, NN-51, Germantown, Maryland (July 1996).
3. D.F. DUSTIN, "Hazard Categorization of Plutonium-Bearing Residues at Rocky Flats," RFP-5169, Safe Sites of Colorado, L.L.C. (1996).
4. C.B. CURTIS, The Under Secretary of Energy Memorandum, "Criteria for Interim Storage of Plutonium-Bearing Materials," Department of Energy, Washington, DC (October 1995).

BACK