THE USE OF FILTERED BAGS TO INCREASE WASTE
PAYLOAD CAPACITY
Donald F. Dustin and Donald T. Thorp
Safe Sites of Colorado, L.L.C.
Michael A. Rivera
Los Alamos Technical Associates
ABSTRACT
For the past few years, the Department of Energy has favored the direct disposal of low-plutonium content residue materials from Rocky Flats rather than engage in expensive and time-consuming plutonium recovery operations. One impediment to direct disposal has been the wattage limit imposed by the Waste Isolation Pilot Plant on hydrogenous materials such as combustibles and sludges. The issue of concern is the radiolytic generation and accumulation of hydrogen and other explosive gases in waste containers. The wattage limits that existed through 1996 restricted the amount of plutonium-bearing hydrogenous materials that could be packaged in a WIPP-bound waste drum to only a fraction of the capacity of a drum. Typically, only about one kilogram of combustible residue could be packaged in a waste drum before the wattage limit was exceeded resulting in an excessively large number of drums to be procured, stored, shipped, and interred.
The Rocky Flats Environmental Technology Site has initiated the use of filtered plastic bags (called bag-out bags) used to remove transuranic waste materials from glove box lines. The bags contain small, disk-like HEPA filters which are effective in containing radioactively contaminated particulate material but allow for the diffusion of hydrogen gas. Used in conjunction with filtered 55-gallon drums, filtered bag-out bags were pursued as a means to increase the allowable wattage limits for selected residue materials. In February 1997, the Nuclear Regulatory Commission approved the use of filtered bag-out bags for transuranic waste materials destined for WIPP. The concomitant increase in wattage limits now allows for approximately four times the payload per waste drum for wattage-limited materials. The cost savings attributed to the improved efficiency of packaging combustible residues alone has been estimated at twenty-four million dollars. Rocky Flats is currently looking into extending the use of filtered bag-out bags to the direct disposal of other wattage-limited residues such as sludges and cemented materials.
INTRODUCTION
The Rocky Flats Environmental Technology Site near Denver, Colorado is currently storing a backlog of approximately 100 metric tons of plutonium- bearing residues. Residues comprise a category of materials with sufficiently high concentrations of plutonium that the recovery of that plutonium was, at one time, considered to be economically more favorable than the production of new plutonium in a reactor facility. With the change in mission for Rocky Flats from weapons production to environmental restoration and waste management, plutonium recovery operations will not be required for the foreseeable future. The problem remains, however, as to the ultimate disposition of the residue backlog.
Residues consist of a variety of materials that are by-products of nearly forty years of weapons production. Typical residues are incinerator ash, pyrochemical salts, combustibles (paper, cloth, and plastic), metal, glass, sludges, casting materials, insulation, firebrick, filters, and ion exchange resins. The average plutonium concentration in these materials is about 3 wt%. Approximately 45% of these residues have been determined to be hazardous as defined by the Resource Conservation and Recovery Act (RCRA) and, as such, are subject to regulation as mixed waste.
Combustible residues, for purposes of this report, are comprised of four types of materials: dry combustibles, wet combustibles, cartridge filter elements called Ful-Flo filters, and acid-contaminated glove box HEPA filters. They include paper, plastic, cloth, and wood and may be contaminated with a variety of chemicals including nitric acid, potassium hydroxide, carbon tetrachloride, and plutonium in a variety of forms but most likely as nitrates, oxides, hydroxides, and metal.
This report addresses the use of filtered bag-out bags as a means to achieve a more cost-effective disposal of combustible residues as transuranic (TRU) waste. Polyvinylchloride (PVC) bags are used to remove materials from glove box lines without subjecting operating personnel to contamination. Their use, however, imposes significant constraints on the amount of combustible material that can be emplaced in a 55-gallon TRU waste drum destined for the Waste Isolation Pilot Plant (WIPP). The topics that are discussed include the regulatory climate surrounding the disposition of residues, the perceived hazards associated with combustible residues as espoused by the Defense Nuclear Facilities Board (DNFSB), options for treatment and disposal of combustible residues, and the application of filtered bag-out bags to expedite that disposal.
BACKGROUND
Following a series of civil and legal actions that began in the late 1980s and continued through 1993, mixed residues at Rocky Flats are currently regulated as hazardous wastes by the Colorado Department of Public Health and Environment (CDPHE). Settlement Agreement and Compliance Order on Consent Number 93-04-23-01, issued in April 1993, directs that preparations be made to prepare mixed residues for off-site shipment and, once a repository is available, that shipment take place as expeditiously as possible (Reference 1).
Also in 1993, the DNFSB began investigating the safety of residues being stored at Rocky Flats. Their conclusions, published in DNFSB Recommendation 94-1 in May 1994, stated that possibly unstable residues should be stabilized to eliminate potential flammability, pyrophoricity, and reactivity concerns and packaged to ensure safe storage until such time as they could be shipped off site (Reference 2). Residues believed to present high risks to operating personnel were to be stabilized within three years while the stabilization and/or repackaging of lower hazard materials was to be completed in seven years.
Combustible residues were determined to be among the materials that represented a relatively high risk to plant workers. That determination was based on the knowledge that approximately 60% of combustible residues originated in former process lines where concentrated nitric acid was used extensively. The intimate contact of nitric acid (an oxidizer) and cellulosic or polymeric materials (potential fuels) could result, over time, in the generation of reaction products that were readily flammable. Thus, the elimination of the potential oxidizer/fuel mixtures was established as a primary stabilization goal for this category of combustible residues. A second hazard associated with all combustible residues is the generation of hydrogen and other flammable gases as a result of radiolysis of hydrogenous materials. Therefore, the stabilization process eventually undertaken would have to address this hazard as well.
A second category of combustible residues included those materials that originated in process lines where carbon tetrachloride and other halogenated organic solvents were in contact with plutonium metal. This mixture of materials could result in a strongly exothermic reaction that was, again, viewed as a potential fire hazard. The elimination of any vestigial halogenated organics and the passivation of any remaining plutonium metal was a parallel objective for the stabilization of combustible residues. The organically-contaminated combustible residues were also subject to the same hazard associated with the radiolytic generation of flammable gases as were aqueous-contaminated combustibles.
TREATMENT AND DISPOSAL OPTIONS
The development of potential treatment options to stabilize combustible residues was focused on two sets of requirements. The first set of requirements was embodied in the WIPP Waste Acceptance Criteria (WIPP/WAC) which defined the material form and packaging requirements for all TRU waste to be disposed of in WIPP (Reference 3). Since WIPP is envisioned as the ultimate repository for all residues, any treatment process undertaken should produce WIPP-certifiable wastes without the need for additional processing at some later date.
The second set of requirements was published as the Interim Safe Storage Criteria (ISSC) which was developed by DOE in response to the DNFSB Recommendation 94-1 (Reference 4). The ISSC defined material form and packaging requirements for plutonium-bearing materials that would be stored at the various DOE sites until such time as WIPP opened.
For the aqueous-contaminated combustibles, a process involving neutralization and washing of nitrates from the matrix was developed. After introduction into the glove box line, the combustible residues are to be shredded, batched, and washed with dilute caustic to neutralize nitric acid. A second washing with water is intended to remove the bulk of the residual nitrate from the mixture. The product of the two-stage washing is then to be vacuum dried, removed from the glove box line, and packaged for disposal at WIPP as TRU waste
A two-step process was developed for the organic-contaminated combustibles. After the residues materials are shredded and batched, carbon tetrachloride and other solvents are to be removed via low-temperature thermal desorption. Any plutonium that still exists in the metallic form is to be passivated with low-pressure saturated steam in a pressure vessel (the same vessel used for the low temperature thermal desorption). The product of this process is then removed from the glove box line and packaged for disposal at WIPP as TRU waste.
The two processes described above represent the minimum but sufficient degree of treatment necessary to satisfy stabilization requirements for interim safe storage and to meet the WIPP/WAC requirements for disposal. The disadvantage of this processing scheme is, however, that most, if not all, of the plutonium remains with the combustible matrix in both cases. Combustibles represent a category of TRU wastes that are wattage limited. That is, the amount of plutonium in a waste drum is limited so that the accumulation of radiolytic gases such as hydrogen in the void space of any waste package does not exceed 5 vol%.
The TRUPACT-II Safety Analysis Report for Packaging (SARP) establishes the requirement that the waste form shall not accumulate a flammable or explosive atmosphere inside the TRUPACT-II or in any layer of confinement in the TRUPACT- II during shipment (Reference 5). To meet this requirement, the waste generator must either analyze the material being prepared for WIPP to determine its shipping category, and thus its watt limit, or perform real time gas generation testing to verify that the generation rate meets the TRUPACT-II requirements.
The analytical wattage limits on a drum of TRU Waste is a function of the following variables:
For combustibles, the established wattage limit was about 0.02 watts per drum. In other words, only about 1 kilogram of combustible materials could be packaged in a WIPP-acceptable drum. The total number of primary waste drums expected to be generated as a result of treating and repackaging 700 drums of combustible residues was about 16,000 drums. The logistics of having to store a large volume of product drums as compared to the input or current volume of stored materials was daunting. In addition, with an expected certification cost of $2000 per drum, there was sufficient economic motivation to pursue an alternative form of either treatment (to separate the plutonium from the hydrogeneous material) or packaging (to allow a higher waste loading per drum).
FILTERED BAGS AS A POTENTIAL SOLUTION
Since diffusivity is a factor in the wattage limits, any means to increase the diffusivity of the layers of confinement was a mechanism to increase the wattage limit by minimizing the accumulation of hydrogen. Use of filters was identified as a mechanism to increase the diffusivity through bags.
A possible solution to the large increase in the number of waste drums appeared in the form of a specialized waste bag used at the Savannah River Site (SRS) and the Paducah Gaseous Diffusion Plant for the disposal of combustible wastes. Low-density, compressible wastes (such as combustibles) are packaged in large PVC bags that contain a disk-like HEPA filter attached to the wall of the bag. When the waste bag is full, it is sealed, and then a vacuum source is attached to the HEPA filter. The contents of the bag are evacuated by the removal of air from inside the bag using the vacuum pump. Particulate contamination remains inside the bag being retained by the HEPA filter. The result is a waste package having far less volume than the bag as originally filled.
For the Rocky Flats situation, the volume of combustible waste was not at issue. At only 1 kilogram of combustibles allowed per drum, drums would be shipped to WIPP perhaps only 10% full with the remaining volume of the drum being void space. However, the HEPA filter installed in the SRS bags was viewed as a potential diffusion pathway for radiolytic hydrogen. More waste material, containing more plutonium, might be able to be packaged in a waste drum while maintaining the hydrogen concentration below 5 vol% in any container. As a result of this proposal, the wattage limit for packaging configurations utilizing vented bags was increased by a factor of four over the limits for comparable configurations using unvented bags.
To implement the use of vented bags, the Department of Energy Carlsbad Area Office (CAO) was provided with hydrogen diffusion and dimensional data for the SRS HEPA filter media. CAO then calculated a selection of wattage limits based on the layers of confinement and the increased diffusion rate for hydrogen through the filters. The initial results of the calculation were incorporated into the Transuranic Package Transporter Content Codes (TRUCON) document (Reference 6), and new TRUCON waste codes for combustibles were generated and forwarded to the Nuclear Regulatory Commission for approval. This approval was granted in February 1997, and Rocky Flats TRUCON Codes utilizing the vented bag technology have been developed and forwarded to CAO for incorporation into the TRUPACT-II SARP.
With the raising of the wattage limits from 0.02 to about 0.08 watts per drum, the number of primary waste drums expected from the treatment and repackaging of combustible residues dropped from 16,000 to 4,000. The number of drums generated now was comparable to the number expected from any of the matrix destruction or plutonium recovery processes. In addition, cost savings approximating 24 million dollars could be realized in drum certification costs alone without having to resort to more cumbersome and costly methods of treatment.
With the higher wattage limits in hand, the Rocky Flats Residue Stabilization organization developed a total packaging configuration for the disposal at WIPP of stabilized combustible residues. A schematic of this configuration is shown in Figure 1. The product of either the aqueous or organic stabilization process, as described above, is a batch of nominally 1 kilogram of shredded material contained in a porous mesh Nylon sack, similar to the hop sacks used in the brewing of beer. In the case of the aqueous process, the shredded material may contain up to 30 wt% of absorbed moisture even after vacuum drying but no free liquids. Two of these 1-kilogram batches are to be combined in a single bag-out operation utilizing a bag-out bag fitted with HEPA filters.
Fig. 1. Combustible residue packaging configuration (not to scale).
As an extra measure of precaution against loss of contamination control, Rocky Flats operating procedures prescribe placing the bag-out bag into a second bag, made from polyethylene and known as a "bomb bag." This bomb bag will also have HEPA filters installed. A layer of absorbent such as absorbent pads made from commercially available "superabsorbent" materials will be placed between the bag-out bag and the bomb bag to prevent the accumulation of free liquids during storage.
The bomb bag and its contents will then be placed in a 12-inch diameter by 12-inch high stainless steel can having a threaded lid which has been fitted with yet another HEPA filter. A stainless steel can is utilized because the ISSC requirements mandate the use of two metal contamination barriers for stored residues, and plastic bags, vented or not, do not qualify as credited contamination barriers. Plastic bags, however, may still be used as a means of controlling the spread of contamination during operations. The stainless steel can also contains a sheet of absorbent material to collect free liquids that may vaporize, condense, and collect at the bottom of the can. The filled can then undergoes non-destructive assay using a segmented gamma scan can counter.
A maximum of two filled and assayed cans will be selected to be placed into a single 55-gallon TRU waste drum. The selection of which cans are placed into a given drum will be based on achieving the maximum plutonium loading possible while still remaining within the established wattage limit. A third sheet of absorbent material will be placed in the bottom of the drum to prevent the accumulation of free liquid. The packaged drum of stabilized combustible residues meets all ISSC requirements for interim storage and all WIPP/WAC requirements for ultimate disposal. Once the drum has met all certification requirements, it will be transported to WIPP in a TRUPACT-II shipping container.
DESCRIPTION OF THE FILTERED BAG-OUT BAG
The Rocky Flats Plant Standards provide the specifications for bag-out bags used for plutonium operations. A standard bag-out bag is 10 inches in diameter, 8 feet long, and fabricated from 10-mil PVC. It is closed at one end and, at the open end, incorporates an elastic shock cord by which the bag may be securely affixed to a bag-out ring on a glove box. The bags are 8 feet long so that a single bag may be used for more than one bag-out operation. After the first bag-out operation, the closed end of the bag is sealed by twisting and taping the plastic. The taped area is cut with a knife leaving a "pigtail" closure for the next bag cut.
Modifications to the standard bag include only the installation of two HEPA filter disks every 27-1/2 inches along the length of the bag. Therefore, every section of bag cut from the original length can be ensured of having at least two HEPA filters to allow for the diffusion of hydrogen out of the bag. A bag-out bag with the HEPA filters installed and attached to a mock-up glove box is shown in Figure 2. The HEPA filters will be installed in standard bag- out bags by the filter vendor, so that the operating staff have only to install the bags on to the glove boxes according to current procedures. They will not be responsible for the installation of the filters into the bags--an operation that is subject to rigorous quality controls since the bags provide the only means of contamination control and worker protection during bag-out operations.
Fig. 2. Filtered bagout bag attached to mock-up glove box.
Filters will be installed in the bomb bags in much the same manner. Standard bomb bags are fabricated from 3-mil polyethylene and are intended for single use only. Hence, only two HEPA filters will be installed in each bag.
The HEPA filters to be installed in either the bag-out bags or the bomb bags consist of a 3/4-inch diameter carbon composite filter element 1/4 inch thick. The filter element is held in place by two plastic interlocking, gasketed retaining rings, one on each side of the PVC bag. The filter assembly is shown in Figure 3. As an added precaution to prevent the retaining rings from becoming disengaged during handling and creating a contamination pathway, they are further held in place by PVC tape. The specification for the performance of the filter element is a flow rate of 200 cubic centimeters of air at one inch water column differential pressure. This is the same flow rate specified for filters used as vents on drums and other containers such as the stainless steel inner contamination barrier can. Particulate retention is at least 99.97% with materials 0.3 microns in diameter.
Fig. 3. Bagout bag filter assembly.
CONCLUSIONS
The treatment and disposal of combustible residues has presented regulatory, programmatic, and operational issues for Rocky Flats for several years. One impediment to efficient and cost-effective disposal of combustibles has been the WIPP requirement limiting the wattage or amount of plutonium allowed per drum. These limits translate into a loading of about 1 kilogram of material per drum and the ultimate generation of many thousands of drums for interim storage and ultimate disposal.
An adaptation of a type of bag developed at the Savannah River Site for the minimization of waste volume to the packaging of Rocky Flats combustible residues will result in a significant reduction in the number of TRU waste drums generated. These bags contain disk-type HEPA filters which retain particulate contamination but allow for the diffusion of hydrogen out of individual packages of waste. The use of filtered bags allows for increased wattage limits and will result in a four-fold reduction in the number of drums and a concomitant four-fold reduction in storage, certification, transportation, and disposal costs. Cost savings by avoiding the certification of 12,000 drums alone amount to an estimated 24 million dollars.
The use of filtered bags has been incorporated into the processing plans for the stabilization of combustible residues at Rocky Flats. Stabilization of selected combustible residues is scheduled to begin in April 1998 and will continue through May 2002 in accordance with the objectives established by the DNFSB. The use of filtered bags allows for the stabilization of combustible residues to proceed via simple, straight-forward treatment processes and the avoidance of unproven or otherwise unacceptable techniques. By employing more efficient processes, the expectation of completing the residue stabilization mission by the established milestone date of May 2002 is more readily achievable.
REFERENCES