Michael J. Connolly, PhD
Idaho National Engineering and Environmental Laboratory

Michael R. Brown
U.S. Department of Energy Carlsbad Area Office

Phil Gregory
Westinghouse Waste Isolation Division

Sinisa Djordjevic
Benchmark Environmental Corporation

Gene Mroz, PhD
Los Alamos National Laboratory

The Nuclear Regulatory Commission (NRC) has imposed a flammable (i.e. hydrogen) gas concentration limit on transuranic (TRU) waste transported using TRUPACT-II to minimize the potential for loss of containment during transport. This limit is set at the lower explosive limit (LEL) of 5% by volume for hydrogen in air. Accident scenarios and resulting safety analyses developed as part of the TRUPACT-II Safety Analysis Report for Packaging (SARP) require that this limit be complied with for a period of 60 days. TRUPACT-II SARP worst-case calculations and the current approach for demonstrating compliance with this requirement have resulted in approximately 35% of the waste stored at the INEEL, RFETS, and LANL and a significantly greater fraction at the SRS not being transportable using TRUPACT-II. Two options currently exist to address these rejected drums; gas generation testing as described in the TRUPACT-II SARP and waste form modification via repackaging and/or treatment. Given the extent of the problem and the cost associated with the two current options, more cost effective and efficient alternatives are needed.

The Mixed Waste Focus Area (MWFA) in conjunction with the National TRU Program (NTP) has initiated several activities specifically to address the hydrogen gas problem and to expand the TRUPACT-II waste envelope. Hydrogen gas build-up in TRU waste is the result of radiolysis of hydrogenous materials. The NRCs concern is hydrogen build-up and the potential for an explosion during transport. Activities currently funded by the MWFA and the NTP are designed to determine hydrogen "G" values that are more representative of actual TRU waste, develop an alternate method based on drum headspace measurements for demonstration compliance with the TRUPACT-II hydrogen gas generation requirement, and to evaluate hydrogen getter materials as a way of reducing hydrogen gas build-up within the TRUPACT-II. It is estimated that these activities will provide transportation relief for approximately 90% of the current TRUPACT-II flammable gas related drum rejections at the INEEL, LANL, and RFETS and result in a potential cost savings of ~$80M. The relief anticipated for the higher Curie loading Pu-238 waste stored at the SRS resulting from these programs in conjunction with a simple repackaging process could result in a potential cost savings of ~$600M.

The Transuranic Package Tranporter-II (TRUPACT-II) was developed for the U.S. Department of Energy (DOE)primarily for shipment of contact-handled transuranic (CH-TRU) waste from DOE generator/storage sites to the Waste Isolation Pilot Plan (WIPP). The TRUPACT-II may also be used for intersite and intrasite shipments of CH-TRU waste to facilitate consolidation of waste for characterization, certification, repackaging and/or treatment prior to disposal at the WIPP. TRUPACT-II is a reusable Type B shipping package that has been approved by The Nuclear Regulatory Commission (NRC).

The TRUPACT-II was designed in accordance with the requirements for Type B packaging found in Title 10, Code of Federal Register Part 71 (10CFR71). A certificate of Compliance (CofC) for theTRUPACT-II was granted by the NRC in 1989. The CofC specifies limits on the authorized contents, i.e. payload, in a TRUPACT-II to ensure safety during conditions of transport. These limits are based on results of testing and analyses as documented in the TRUPACT-II Safety Analysis Report for Packaging (SARP) as submitted by the DOE to the NRC.

The most restrictive limits and conditions imposed on the TRUPACT-II payload are the result of the NRCs concern with waste packages containing flammable mixtures of gases. Two primary limits, related to concentrations of flammable gases, imposed by the NRC are: (1) the concentration of flammable gases, i.e. hydrogen and methane, must not exceed 5 percent (by volume) in the payload during a 60 day shipping period; and (2) that the gas phase concentration of flammable volatile organic compounds (VOCs) in the payload must be less than 0.05% (500 ppm). Flammable gases are potentially generated during transport due to radiolysis of hydrogeneous materials and therefore the concentration at the end of the 60 day shipping period must be predicted. Flammable VOCs are not generated during transport and therefore the headspace payload concentration remains relatively constant and is approximately the same before, during and after transport.

The methodology developed and documented in the TRUPACT-II SARP for demonstrating compliance with the flammable gas limit is based on the gas generation potential of CH-TRU waste. CH-TRU waste is classified, in support of shipping in TRUPACT-II, into four major waste types (I, II, III and IV) based on physical and chemical characteristics and further divided based on bounding flammable gas generation potential. The gas generation potential of a target material, i.e. waste or packaging, is characterized by its G-value, which is the number of molecular or ionic products (usually gaseous) generated per 100 EV of ionizing radiation absorbed by the target material. Each CH-TRU waste container is assigned a shipping category, which is based on a combination of waste material type and packaging (number and type of plastic layers of confinement) of the waste materials within the waste container. To demonstrate compliance with the flammable gas requirement, theroretical worst-case calculations were performed using G-values to establish allowable wattage (decay heat) limits for each TRUPACT-II shipping category. The wattage limit represents the predicted balance point between gas generation potential of the waste and the resistance to flammable gas release to ensure that 5% flammable gas limit is not exceeded during transport.

Maximum allowable wattage limits for each shipping category are based on several conservative assumptions including: (1) initial G-values observed during irradiation experiments on materials found in TRU waste; (2) G-values are assumed to be independent of total dose received (i.e. no time dependence); and (3) decay energy is assumed to be deposited in the worst-case, i.e. highest G-value, waste or packaging material.

The assumptions regarding G-values are overly conservative because previous studies (1-7) have shown that G-values are dose (time) dependent and the majority of the DOE CH-TRU waste is between 5 and 23 years old. Given the limited alpha particle mean free path (~4.2 cm in air) coupled with the fact that for a significant fraction of the waste forms (e.g. glass, metal and graphite) the transuranic elements are in contact with non-hydrogenous waste materials, that the assumption that decay energy is deposited in the worst-case material (packaging material in this case) is unrealistic and overly conservative.

The current method for demonstrating compliance with the allowable wattage limits is based on a nondestructive assay (NDA) measurement to determine the mass of radionuclides and hence drum specific decay heat. NDA measurements have some amount (e.g. 20%-300%) of error associated with the measurement which must be included in the final value used in determining wattage limit (decay heat) compliance. This compliance approach methodology can be very conservative depending upon the waste type.

Taking into account the existing TRUPACT-II wattage limits and compliance methodology, it is currently estimated that a significant portion of the CH-TRU waste container inventory cannot be shipped. A joint effort conducted by the INEEL, LANL, and RFETS to determine the impact of existing wattage limits indicates that approximately 38 percent, see Figure 1, of the inventory that meets the WIPP Waste Acceptance Criteria cannot be shipped due to failure to meet TRUPACT-II wattage limits. A much higher percentage of the SRS Pu-238 waste cannot be shipped due to wattage limits. This is a result of the much higher specific activity (~x250) of Pu-238 compared to Pu-239.

Data collected at the INEEL and RFETS (8, 9) shows that a significant fraction of drums that exceed the allowable wattage limits have hydrogen concentrations less than 0.1%. However, data collected at the RFETS, LANL and SRS also shows that hydrogen can be present at concentrations greater than 5%. Data collected at the INEEL and RFETS (10, 11) shows that headspace concentration of flammable VOCs exceeds the 500 ppm limit in approximately 20% of the drums sampled. Therefore, there are three problem areas that need to be addressed: (1) predicted to exceed flammable gas limit when in fact is less; (2) exceeds the flammable gas limit; and (3) exceeds the flammable VOC limit.

Two options currently exist to address these drum rejection conditions; flammable gas generation testing as described in the TRUPACT-II SARP and waste form modification via repackaging and/or treatment to remove VOCs and/or hydrogenous materials. Given the extent of the problem, see Figure 1 and 2, and the estimated (~$80M-$600M) cost associated with the two current options, more cost effective and efficient alternatives are needed.

The MWFA in conjunction with the Carlsbad Area Office (CAO) National TRU Program (NTP) has initiated several programs to address the flammable gas and flammable VOC problems and to expand the TRUPACT-II waste envelope. These programs have been designed to be complementary to enable the maximum amount of CH-TRU to be transported using TRUPACT-II while minimizing the implementation complexity and cost and maximizing overall cost savings. Programs and activities that have been completed or are currently under development include: matrix depletion program, flammability assessment methodology program, headspace flammable gas sampling as an alternative method of certifying containers, and evaluation of hydrogen gas getter materials.

The matrix depletion program (MDP) was established with the objective to investigate the matrix depletion phenomenon and to establish a more realistic set of hydrogen G-values. These more realistic G(H₂) values would then be used to revise the current TRUPACT-II wattage limits. The MDP is composed of three separate activities (9); controlled simulated waste experiments, controlled real waste experiments, and mathematical modeling.

Previous studies (2-5) have shown that G (H₂) is reduced with increasing dose. The basis for this phenomenon is that the chemical composition of a hydrogenous material is altered due to interaction with alpha particles. Interaction of alpha particles with hydrogenous materials (e.g. cellulosics and plastics) results in reactions (free radical) that generate flammable gaseous products. Due to the short mean free path of alpha particles in air (~4.2 cm) and the waste matrix (~5x10^-3 cm), the matrix in the vicinity of alpha emitting radionucldes becomes depleted of hydrogen and subsequent alpha particle deposition results in lower hydrogen gas generation. Previous studies (2, 7, 13) have shown decreases of three to five fold for G(H₂) with increasing dose, see Figure 3.

Controlled simulated waste experiments have been designed to determine more realistic but bounding G (H₂) values. A total 60 to 74 test containers loaded with hydrogenous materials (polyethylene, polyvinyl chloride, cellulose, and cement) representative of CH-TRU waste will be spiked with oxides of plutonium blends dominated by either Pu-238 or Pu-239 will be tested. These experiments will also determine the effects of agitation and heating. Previous studies (2, 5, 7) investigated the effects of temperature and agitation and found no significant impact on radiolytic gas generation.

Data obtained from controlled real waste hydrogen gas generation studies will be used to verify that the simulated waste study G (H₂) are bounding and that assumption regarding energy deposition is unrealistic and overly conservative. Real waste gas generation data (8) collected at the INEEL and RFETS is shown, along with simulated waste data and TRUPACT-II SARP data, in Table 1. It can be seen from this data that; the simulated waste G (H₂) values are bounding and less than the TRUPACT-II values, and the energy deposition assumption is overly conservative.

The flammability assessment methodology program (FAMP) (11) was established to investigate the flammability of gas mixtures found in CH-TRU waste containers. The objective of FAMP was to provide a basis for increasing the permissible flammable VOC concentration limit (500 ppmv) in the TRUPACT-II. To meet the objective, the FAMP investigated the flammability of gases in CH-TRU waste; designed and tested a series of gas mixtures; evaluated and selected a model for predicting the gas mixture lower explosive limit (MLEL) in waste containers; developed screening limits for flammable gas and VOC concentrations; developed a strategy for determining flammability of gases in drums that do not pass the screening limits; and delineated the approach for determining acceptable gas generation rates, decay heat limit, and drum aspiration time requirements.

The FAMP was successful in measuring MLELs and using that data to evaluate several predictive models (11). This evaluation resulted in the selection of a model that was subsequently used to establish flammable VOC and flammable gas screening limits. These screening limits (700ppmv-11,000ppmv) are higher than the current limit and result in a reduction from 20% to less than 2.5% of the drums examined that would be considered flammable.

The current method for demonstrating compliance with the TRUPACT-II permissable gas generation rate is to determine the container isotopic inventory from NDA measurement and calculate the container specific decay heat. This decay heat, including the error, is compared to the TRUPACT-II decay heat limit. TRUPACT-II decay heat limits were determined by combining the allowed gas generation rate and the flammable gas G-value. The objective of the alternate method program was to develop a gas generation rate compliance method that is based on flammable gas headspace measurement.

A method based on sampling the waste container headspace for flammable gases, calculating the gas generation rate and comparing the rate to the existing TRUPACT-II limit was developed (14). The basis for the method is that at steady-state, the release rate of gases across each layer of confinement is the same and equal to the gas generation rate. Therefore, the gas generation rate at steady-state is equal to the product of the flammable gas concentration in the container headspace and the effective diffusivity of the container vent. This program developed the methodology, models and codes necessary to determine if the drum is at steady-state and for determining compliance with allowed rates.

The objective of the hydrogen gas getters evaluation program is to determine if getter materials can be used with CH-TRU waste and TRUPACT-II during transport to mitigate the accumulation of flammable gases to a concentration less than the permissable limit (5%). If getters are demonstrated and accepted by the NRC for use in TRUPACT-II, they will most likely be used with high Curie loading waste (e.g. Pu-238) in combination with repackaging to facilitate high gas transport rates with subsequent removal by the getter.

Getter material currently under evaluation (15) at LANL is an organic material [1,4-bis (phenylethynyl) benzene (DEB)] that is currently used in weapons applications. The DEB is mixed (3:1) with a carbon catalyst (5% Pd on carbon). The primary focus of the LANL studies is to assess the potential for gaseous components including VOCs and inorganic gases (e.g. HCl and CO), expected to be present in CH-TRU waste, to poison the DEB getter. This DEB/carbon catalyst getter material has been demonstrated to irreversibly react with hydrogen. One mole of DEB reacts with 4 moles of H₂ to generate the saturated alkane with a resulting sorption capacity of 240 to 330 cm³. If DEB is shown to be unaffected by these potential poisons, then the program will investigate approaches to deploy the getters to maximize its mitigating capabilities.

Table 2 shows the amount of relief anticipated for each of the programs, applicable to the DOE complex, if the data in Figure 1 is representative of the current stored volume. It is estimated, based on data collected to date, that the MDP will provide a minimum threefold increase in the allowable-wattage limit. This would provide relief for 50 to 70% of the drums that currently exceed the wattage limit. A combination of the VOC flammability assessment and alternate method programs is expected to provide relief for drums that have wattage values less than 10 times the allowed limit. This represents about 60 to 80% of the drums that exceed the current wattage limit. The MDP and the flammability assessment/alternate method programs would be used to address drums categorized into the two problem areas of: (1) predicted to exceed flammable gas limit when in fact is less; and (2) exceeds the flammable VOC limit.

Hydrogen gas getters would be used to address drums that exceed the flammable gas limit. In this application, it is anticipated that the gas generation rate is sufficient to require waste repackaging to reduce layers of confinement and the use of filtered bags. It has been estimated that with two layers of confinement, filtered bags and getter material in the TRUPACT-II inner containment vessel, that the maximum allowed wattage could be as high as 2.5 watts. This would enable more than 90% of the current high Curie loading waste to be shipped with less than a factor of three increase in volume due to waste repackaging.

Figure 4 shows a proposed methodology for demonstrating compliance with the TRUPACT-II flammable gas and VOC limits that incorporates all of the programs discussed in this paper. It is anticipated that this methodology will be incorporated into a future TRUPACT-II SARP revision for submittal to the NRC.

The programs that have been developed jointly by the MWFA and CAO/NTP are anticipated to expand the TRUPACT-II payload envelope and provide significant relief. It has been estimated that these programs will help fill the pipeline to the WIPP and provide for a cost savings of $80 to $600M. The current schedule is for these programs to be part of a TRUPACT-II SARP revision to be submitted to the NRC in late 1998 with subsequent approval and implementation 12 to 14 months after submittal.

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15. J. Weinrack, "Test Plan for Hydrogen Getters Project," Benchmark Environmental Corporation, Albuquerque, New Mexico, August 1997

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