CERTIFYING CONTAINERS FOR TRUPACT-II SHIPMENT USING FLAMMABLE GAS HEADSPACE MEASUREMENTS

Sinisa M. Djordjevic, L. Richard Spangler
Benchmark Environmental Corporation

Michael J. Connolly
Lockheed-Martin Idaho Technologies Company

ABSTRACT

Four U.S. Department of Energy-Carlsbad Area Office (DOE-CAO) initiatives in progress are designed to increase the portion of transuranic (TRU) waste that can be shipped in the Transuranic Package Transporter-II (TRUPACT-II). Although these initiatives will substantially increase the percentage of drums that can be shipped, a portion of the waste will still exceed current container decay heat limits and cannot be shipped using the current methodology required in the Safety Analysis Report for the TRUPACT-II Shipping Package.

The average concentration of flammable gases in TRU waste containers is approximately 0.05 percent based on headspace sampling of over 1,000 vented containers at the Idaho National Engineering Laboratory (INEL) and the Rocky Flats Environmental Technology Site (RFETS). These low concentrations indicate that actual levels of flammable gases are only a fraction of the permissible limits. The reason for this is that the decay heat limits are based on the assumption that the highest flammable gas generating material is receiving all the radiation and generating all the flammable gas. In reality, only a small fraction of the worst-case material will be irradiated. An alternate method of certifying containers that is not based on this extremely conservative assumption is needed.

This paper provides the rationale for sampling the waste container headspace for flammable gases as an alternative method of certifying TRU waste containers for shipment in the TRUPACT-II. This paper provides background information, a summary of the proposed methodology, and a scope of work. The scope of work includes methodology and code development, experimental testing to establish flammable gas leakage rates from TRU waste containers, analysis, documentation, and a presentation to the U.S. Nuclear Regulatory Commission (NRC). The proposed method will increase the potential for shipping more waste containers more promptly to the Waste Isolation Pilot Plant (WIPP) and would apply to all waste. The proposed methodology would reduce the number of TRUPACT-II shipments, thereby reducing risks and costs associated with waste repackaging or treatment and disposal.

INTRODUCTION

The TRUPACT-II shipping package is a reusable Type B shipping package designed for transporting contact-handled transuranic (CH-TRU) waste containers between DOE sites and the WIPP. Allowable waste containers are 55-gal. drums, standard waste boxes (SWBs), and ten-drum overpacks (TDOPs). Each CH-TRU waste container is designated by a TRUPACT-II shipping category based on container type (i.e., drum, SWB, or TDOP), the waste material type, and the packaging configuration (i.e., number and type of plastic confinement layers).

A primary transportation requirement for the TRUPACT-II is that the concentration of potentially flammable gases (i.e., hydrogen and methane) must not exceed 5 percent by volume within the waste and in the TRUPACT-II package during a 60-day shipping period. Decomposition of waste materials by radiation, or radiolysis, is the predominant mechanism of gas generation during transport. The gas generation potential of a target waste material is characterized by a G-value, which is the number of molecules of gas generated per 100 eV of ionizing radiation absorbed by the target material.

To demonstrate compliance with the flammable gas concentration requirement, theoretical worst-case calculations were performed to establish allowable flammable gas generation rates for each shipping category. Allowable decay heat (wattage) limits were then calculated for each shipping category by combining the allowable flammable gas generation rates with the G-value for the waste material with the highest potential for flammable gas generation. The decay heat limit for each shipping category was established by combining the allowable flammable gas generation rate and the flammable gas effective G-value.

The current approach is to calculate the decay heat of a waste container by using the isotopic inventory data from assay and the decay heats for each radionuclide. The calculated decay heat and the decay heat error are recorded in the data package for the container. If the sum of the calculated decay heat plus the error is below the decay heat limit for the shipping category of the container, the container can be shipped in a TRUPACT-II.

Based on existing allowable wattage limits, it is estimated that a large portion (approximately 34 percent) of the CH-TRU waste cannot be shipped. Four DOE-CAO initiatives in progress are designed to increase the portion of TRU waste that can be shipped, as summarized below:

1. The TRUPACT-II Gas Generation Test Program (GGTP) consists of controlled tests using actual containers of CH-TRU waste to determine gas generation rates of actual TRU wastes under simulated transportation conditions (2).
2. The TRUPACT-II Matrix Depletion Program (MDP) was established with the objective to investigate the phenomena of matrix depletion to support more realistic, age-dependent effective G-values (2). The MDP is comprised of experiments designed to examine the behavior of effective G-values over time for different waste materials and the effects of isotope, agitation, and heating. The experimental data will be evaluated in conjunction with waste container headspace gas sampling and theoretical and predictive modeling to formulate bounding effective G-values for each simulated waste material and time segment. The more realistic G-values are expected to increase container decay heat limits fivefold.
3. The Mixing of TRUPACT-II Shipping Categories Project extends the current TRUPACT-II calculational methodology to arrive at a decay heat limit for each payload container. Thus, payload containers of different shipping categories could be mixed as long as each container satisfied the new container specific wattage limit. The project's technical approach consists of developing a methodology, a computer program implementing the methodology, and documentation to support an application to the NRC for mixing shipping categories and raising the decay limits of individual containers by taking credit for the additional void volume of dunnage drums.
4. The objective of the Flammability Assessment Methodology Project is to increase the allowable concentrations of flammable volatile organic compounds (VOCs) and reduce the need for flame testing on drums by investigating the potential flammability of headspace gases within TRU waste drums (3). The approach includes experimental work to determine mixture lower explosive limits (MLELs) for the types of gas mixtures observed in TRU waste; a model for predicting the MLELs for mixtures of VOCs, hydrogen, and methane; and revised screening limits for flammable headspace gases based on measured drum headspace gas data and model predictions.

These initiatives are integrated in that data from one initiative support the objectives of the other initiatives. However, even if all the initiatives are approved by the NRC and incorporated in the TRUPACT-II compliance documentation, a portion of the waste will still exceed the revised container decay heat limits and cannot be shipped. The reason for this is the fact that the decay heat limits are based on the assumption that the highest flammable gas generating material is receiving all the radiation and generating all the flammable gas. In reality, only a small fraction of the worst-case material will be irradiated. The waste that exceeds the decay heat limits will have to be either treated to destroy the hydrogenous waste matrix or repackaged into many additional drums. Each remedy will be expensive and may increase the inventory of drums for transportation and disposal.

The average concentration of flammable gases in TRU waste containers is approximately 0.05 percent based on headspace sampling of over 1,000 vented containers at the INEL and the RFETS. These low concentrations indicate that actual levels of flammable gases are only a fraction of the allowable limits. However, due to the extremely conservative nature of the G-values used and the current method of comparing container decay heats to decay heat limits, these containers cannot be shipped.

This paper presents a methodology that provides an alternative method for demonstrating compliance with TRUPACT-II flammable gas requirements. The method is based on sampling the waste container headspace for flammable gases, calculating the flammable gas generation rate, and comparing the rate to the existing allowable flammable gas generation rate limit.

PROPOSED METHODOLOGY

The proposed methodology is based on sampling the waste container headspace for flammable gases as an alternative to demonstrating compliance with the flammable gas requirements (based on comparing container decay heats to decay heat limits). The drum headspace flammable gas concentration is used to calculate the actual drum flammable gas generation rate for direct comparison with the allowable flammable gas generation rate limit. The use of headspace gas sampling results to support certification efforts was proposed in several precedent-setting and related studies. These include:

• The derivation of TRUPACT-II aspiration times for containers that have been sealed for a certain period of time and then aspirated (1)
• As part of the TRUPACT-II MDP, a computer code has been developed as an extension of the TRUPACT-II aspiration model to support revised G-values. The code simulates the generation of flammable gas and subsequent transport across layers of confinement in a TRU waste drum. The code uses experimentally derived bounding effective G-values for each time segment, and actual TRU waste container decay heat or wattages to predict concentrations up to the age of the container when sampled (2).
• The GGTP will quantify the flammable gas generation rate in actual 55-gal. drums by combining the measured total gas release rate leaving the drum with the measured concentration of flammable gas in the total gas flow. The experimentally derived flammable gas generation rate is then compared to the corresponding limit for the waste container shipping category (2).
• Transient (i.e., time-dependent) VOC transport models were used to establish a drum age criterion (DAC) that each TRU waste drum must meet prior to sampling and analysis of headspace VOCs (4). The DACs for VOCs are analogous to the aspiration times for flammable gases. Once the DAC has been met, the headspace gas data can be used to predict the VOC concentration within each layer of confinement.

The proposed method is based on a parameter (i.e., flammable gas concentration) that can be measured with greater confidence (i.e., lower measurement error) than decay heat, the currently approved method. This provides the additional benefit that a lower error is used to define the upper confidence level with which to compare the allowable limit.

The basis for the method is the fact that at, steady state (i.e., the condition when concentrations are not changing with time), the release rate of flammable gas across each confinement layer is the same and equal to the flammable gas generation rate. Thus, the flammable gas generation rate can be directly calculated from the flammable gas release rate out of the drum. Specifically, the flammable gas release rate or generation rate is the product of the flammable gas concentration in the drum headspace and the effective diffusivity across the drum.

To apply this method, a drum must be at steady-state conditions. Drums are categorized as either 1) sealed and then vented; or 2) newly packaged and vented. To have reached steady state, a drum must have been aspirated or vented (condition of the drum with a punctured rigid liner and one or more carbon composite filters installed in the drum lid) for a sufficient period of time (referred to as the aspiration time). Aspiration times for containers stored in an unvented condition (i.e., in category 1) are tabulated by shipping category in aspiration tables contained in Appendix 3.6.11 of the Safety Analysis Report for the TRUPACT-II Shipping Package (1) and in the TRUCON document (5). The aspiration times were calculated using a computer code to simulate generation, accumulation, and transport of flammable gases across confinement layers. Based on the proposed methodology, a new set of aspiration time tables can be generated using the more realistic flammable gas generation rates.

Newly packaged vented drums consist of drums and associated rigid drum liners vented at the time of packaging. The time to reach steady state (also referred to as the aspiration time) in these containers can be estimated using a slightly modified version of the computer code used for the drums stored in an unvented condition. If a drum is sampled after the aspiration time has elapsed, the drum is at steady state and the drum headspace flammable gas concentration directly provides the flammable gas generation rate. If the drum flammable gas generation rate is less than the allowable TRUPACT-II flammable gas generation rate limits listed in the TRUCON document (5), the drum can be safely transported.

IMPLEMENTATION

The scope of work includes methodology and code development, analyzes, experimental testing to establish flammable gas diffusivities from TRU waste containers, documentation, and a presentation to the NRC. If the NRC approves the proposed methodology, significant changes will have to be made to the current Safety Analysis Report for the TRUPACT-II Shipping Package (1) and the TRUCON document (5). The specific activities necessary to present a defensible methodology to the NRC are summarized below:

• Finalization of a methodology to implement the use of headspace flammable gas sampling as an alternative method of certifying TRU waste containers. The key to implementing this method is a thoroughly documented rationale that will convince the NRC to approve an amendment to the TRUPACT-II Certificate of Compliance.
• Development of computer software to ascertain when a container has attained steady state with respect to flammable gas concentrations. The software will be a graphical user interface (GUI) application implementing the methodology written in the Java language. The software will iteratively establish new aspiration times based on sequential analysis of the container headspace flammable gas concentration. Once the aspiration time has been satisfied, the software will calculate the flammable gas generation rate and compare the value to the allowable flammable gas generation rate. The application will be designed to facilitate data input, perform a series of checks to eliminate invalid input, provide for software revision control, and facilitate the certification process.
• Preparation of the Software Quality Assurance Procedure; the Methodology and Software Requirements, Design, and Implementation Documentation; the Software Verification and Validation Plan; the Software Verification and Validation Report; the Software User Documentation; Software Configuration Log; and the Software Abstract Form
• testing to establish effective flammable gas diffusivity across drums. The effective diffusivity across drums is the sum of the diffusivities across the carbon composite filter in the drum lid and the effective diffusivity across the gasket in the drum lid. The diffusivity across the drum filter has been established through earlier experimentation. The effective diffusivity across the drum lid gasket will be established through experimental testing by measuring the decrease in flammable gas concentration in the drum as a function of time.
• Performing analyzes using the computer software to derive aspiration times for both sealed and newly packaged containers, and preparation of a final report documenting the results of analyzes
• Presentation of the methodology to and negotiation with the NRC

CONCLUSIONS

Initial analyzes applying the proposed methodology on the over 1,000 drums sampled as part of the TRU Waste Characterization Program indicate that over 99 percent of the containers would comply with allowable flammable gas generation rates. In summary, the proposed methodology will provide the following benefits:

• Reduce the number of TRUPACT-II shipments, thereby reducing the risks and costs associated with transportation
• Increase the amount of shippable waste
• Reduce risks and costs associated with waste repackaging or waste treatment
• Use a parameter associated with a lower measurement error than decay heat
• Could be used to reduce the currently lengthy aspiration times for containers
• Could be applied to all wastes including both inter- and intrasite shipments and final shipments to the WIPP
• Complements other ongoing initiatives and provides a method of addressing the fraction of waste that still cannot be shipped after implementation of the current initiatives

REFERENCES

1. NRC DOCKET NO. 9218, "Safety Analysis Report for the TRUPACT-II Shipping Package. Revision 14," NRC Docket No. 9218, U.S. Nuclear Regulatory Commission, Washington, D.C. (1984).
2. M.J. CONNOLLY, S.M. DJORDJEVIC, V. BANJAC, AND C.A. LOEHR, "TRUPACT-II Matrix Depletion Program Test Program," INEL95/0360, Idaho, Idaho National Engineering Laboratory, Idaho Falls (1995).
3. LITCO, "White Paper on Assessing Flammability of Gases in Transuranic Waste Containers," Lockheed Idaho Technologies Company, Idaho Falls, Idaho (1995).
4. M. J. CONNOLLY, M.J., S.M. DJORDJEVIC, K.J. LIEKHUS, C.A. LOEHR, AND L.R. SPANGLER, "Position for Determining Gas Phase Volatile Organic Compound Concentrations in Transuranic Waste Containers," INEL95/0109, Revision 1. Idaho National Engineering Laboratory, Idaho Falls, Idaho, (1995).
5. DOE, "TRUPACT-II Content Codes (TRUCON)," Rev 8, DOE/WIPP 89004, Waste Isolation Pilot Plant, U.S. Department of Energy, Carlsbad, New Mexico (1994).