3.0 COMPARISON OF CONTACT-HANDLED AND REMOTE-HANDLED TRANSURANIC WASTES
The purpose of this chapter is to assess the similarities and differences between CH-TRU and RH-TRU wastes with respect to the LWA issues. However, this chapter does not compare the impacts of such waste on performance assessment. The initial step in identification of similarities and differences between CH-TRU and RH-TRU waste in the repository is to describe the inventory of each type of waste. By agreement with the State of New Mexico, no more than 250,000 cubic feet (7,080 cubic meter(s)) of RH-TRU waste may be disposed at WIPP [DOE and State of New Mexico, 1981]. This is slightly less than 5 percent of the total WIPP design capacity. The remaining 95 percent of the waste at WIPP will be CH-TRU. The primary difference between CH-TRU and RH-TRU waste is that RH-TRU waste contains enough gamma-emitting isotopes with relatively short half-lives to produce a radiation dose rate greater than 200 mrem/hr at the external surface of the waste container. The CH-TRU and RH-TRU wastes considered in this study are consistent with the DOE Order 5820.2A definition of TRU waste. A summary comparing CH-TRU and RH-TRU waste classification criteria for the WIPP inventory wastes is given below.
The Agreement for Consultation and Cooperation (C&C Agreement) with the State of New Mexico [DOE and State of New Mexico, 1981] restricts RH canisters to less than or equal to 23 curies/liter maximum activity (averaged over the volume of the canister). This limitation is also a statutory limitation in the LWA.
In the Final Environmental Impact Statement, Waste Isolation Pilot Plant [DOE, 1980], the DOE specified a maximum dose rate of 100 rem/hr, which included both CH-TRU and RH-TRU waste forms. To accommodate the higher dose rates that are characteristic of a portion of the RH-TRU inventory, the LWA included a provision (as derived from the C&C Agreement) to allow up to 12,500 cubic feet (354 cubic meter(s)) of RH-TRU wastes with limited dose rates of between 100 and 1,000 rem/hr. Restrictions are imposed on the total quantity of TRU waste to 6.2 million cubic feet (~176,000 cubic meter(s)) [Public Law 102-579, 1992] and total RH curies to 5.1 million curies [DOE and State of New Mexico, 1981].
The components of the TRU wastes to be regulated under the Resource Conservation and Recovery Act (RCRA) are anticipated to be very similar for CH-TRU and RH-TRU wastes. Hazardous constituents represented by EPA codes (codes assigned by EPA for each regulated hazardous waste) reported in the WTWBIR for the CH-TRU and RH-TRU inventories indicate that the CH-TRU hazardous waste constituents are inclusive of all the RH-TRU hazardous constituents [DOE, 1995e]. Treatment and processing programs are currently being developed by several of the generator/storage sites, which may also reduce the quantities of RCRA constituents in the RH-TRU inventory to be shipped to WIPP. The No-Migration Variance Petition currently being developed for disposal operations at the WIPP will further address the RCRA-regulated constituents of the RH-TRU inventory for the long-term period of performance.
CH-TRU and RH-TRU wastes are compared in two ways: (1) assessment of the waste form as received at WIPP during the disposal period pursuant to the waste acceptance criteria [DOE, 1991], and (2) assessment of the waste form under repository conditions as considered in the PA.
Data for comparison of the waste forms in the "as-received" condition are based on waste characterization information from the WTWBIR [DOE, 1995e]. The WTWBIR defines the waste inventory descriptions for CH-TRU and RH-TRU wastes for characteristics identified as potentially significant to PA. This information is discussed in section 3.1.1.
The second comparison assessment evaluates CH-TRU and RH-TRU wastes in the repository environment. The long-term behavior of each waste type can vary from the "as-received" condition due to chemical, biological, or radiological changes. This comparison is discussed in section 3.2.
3.1 "As-Received" Inventory Description and Comparison
This section provides a comparison of the CH-TRU and RH-TRU inventories in the form that these wastes will be received at WIPP (the "as-received" inventory). The primary governing documents are the WTWBIR and the WIPP WAC [DOE, 1995e; DOE, 1991]. Although sections of the WIPP WAC were meant to apply to activities planned in the WIPP Test Phase, the criteria throughout the WAC are applicable to both CH-TRU and RH-TRU waste in this study.
The three LWA issues applicable to "as-received" conditions are gas generation, flammability, and explosiveness because they apply to conditions prior to emplacement in the repository. These conditions and processes, as well as the "as-received" radionuclide inventory, are described below.
Gas Generation in "As-Received" Waste
All TRU containers received at WIPP are required to be vented to avoid gas buildup in the containers. The CH-TRU waste container venting requirements are defined in the WIPP WAC. The WAC states, "All waste containers, including any overpacks, shall be vented with filters . . . . The minimum number of filters shall be one per drum, two per overpacked experimental bin in a Standard Waste Box (SWB), and two per SWB." The WAC also states: "Any rigid drum liners used in the waste containers shall be filtered or punctured . . ." [DOE, 1991, p. 3-51].
An additional WAC requirement for CH-TRU waste is: "Any confinement layers . . . used in the waste containers shall be closed only by a twist and tape or fold and tape closure." [DOE, 1991, p. 3-51] This requirement precludes waste generators from using a heat-seal method for closing plastic bags containing TRU waste. A heat-sealed bag could potentially trap gas, whereas the taping method allows gas to dissipate.
The RH venting requirements are defined in the Remote-Handled Transuranic Waste Authorized Methods of Payload Control (RH-TRAMPAC) document, which is presented in Appendix 1.3.7 of the Safety Analysis Report (SAR) for the Remote-Handled Transuranic (RH-TRU) Waste Shipping Cask (Model NuPac 72-B) [VECTRA Technologies, Inc., 1994]. This document states: "Each Payload Container (RH-TRU Waste Canister) and any sealed containers (greater than 1 gallon in size) overpacked in a payload container to be transported in the 72-B Cask shall have one or more filter vents." [DOE, 1991, p. 3-51]
Because the CH-TRU and RH-TRU containers will be vented, most gas generated in the containers after receipt at WIPP is anticipated to dissipate. DOE will initially conduct headspace analysis as required for CH-TRU and RH-TRU waste to be shipped to WIPP. These analyses will confirm gas generation conclusions in stored containers [DOE, 1995f].
Flammability in "As-Received" Waste
In addition to requiring vented containers, the WIPP WAC does not allow free liquids in the waste containers. Section 3.3.2 of the WAC states the following for CH-TRU and RH-TRU waste:
"Liquid waste will not be emplaced in the WIPP. TRU waste for emplacement in the WIPP shall contain as little residual liquid as is reasonably achievable. All internal containers (e.g., bottles, cans, etc.) must be well-drained, but may contain residual liquids. As a guideline, residual liquid in well-drained containers will be restricted to approximately one percent of the volume of the internal container. In no case shall the total liquid equal or exceed one volume percent of the waste container (e.g., drum, SWB, or RH-TRU waste canister)."
Since there will not be significant quantities of liquids in the containers, there will be minimal hydrogen gas generated from alpha radiolysis of residual water (see section 3.2.1) and therefore no flammability concerns relating to water degradation.
Microbial degradation and alpha radiolysis of cellulose, rubber, and plastic are other mechanisms for hydrogen gas generation in "as-received" waste. These mechanisms have been evaluated as part of the transportation studies for CH-TRU and RH-TRU wastes. Because heat is associated with radiolysis, the thermal output (as measured in watts) of the waste containers is restricted [Nuclear Packaging, Inc., 1992; VECTRA Technologies, Inc., 1994]. However, very little heat is expected to be associated with TRU waste since the average RH-TRU waste heat output has been estimated at less than 1 watt for the current inventory projections.
The Department of Energy Waste Isolation Pilot Plant: Notice of Final No-Migration Determination (NMD) [EPA, 1990] requires that no container be placed in WIPP "if it contains flammable mixtures of gases in any layer of confinement, or mixtures of gases that could become flammable when mixed with air. To assure a sufficient margin of safety, EPA defines any mixture as potentially flammable if it exceeds 50 percent of the lower explosive limit (LEL) of the mixture in air." To ensure that this flammability restriction would be met during the proposed test phase, the NMD required headspace sampling of the void space of each waste container for hydrogen, methane, and volatile organic compounds (VOCs). If a container shows significant levels of flammable VOCs, sites must perform tests to determine if a flammable mixture can be formed with the air, or the waste must be treated to reduce the flammable concentrations.
The Transuranic Waste Characterization Quality Assurance Program Plan (QAPP) [DOE, 1995f] precludes emplacement of containers that may generate flammable gases. The QAPP requires that sites implement testing processes (headspace analyses) for waste that will be sent to WIPP. No container to be emplaced in WIPP may exceed 50 percent of the lower explosive limit in any layer of confinement for hydrogen and methane when potentially flammable VOCs as a class are greater than 500 ppm. This requirement will be applicable until DOE demonstrates that waste packages do not contain high concentrations of flammable gases. Although the NMD requirements have been developed by EPA for DOE's test program, headspace sampling is expected to remain a requirement for a statistical portion of the waste sent to WIPP for disposal.
Flammability is not a concern for "as-received" CH-TRU and RH-TRU wastes. There will be minimal residual water available from which alpha radiolysis can generate gas. The CH-TRU and RH-TRU safety analysis report restricts the wattage of the wastes to minimize packages that generate gas due to radiolysis of cellulose, rubber, and plastic. There are restrictions for flammable gases allowed to be packaged in the waste container pursuant to the NMD. CH-TRU and RH-TRU containers are required to be vented for an additional margin of safety. There is no significant difference between CH-TRU and RH-TRU waste in this respect.
Explosiveness in "As-Received" Waste
The WIPP WAC, Section 3.3.4, prohibits packaging explosives and compressed gases in TRU waste containers to be shipped to WIPP. The WAC requirement is stated as follows for CH-TRU and RH-TRU waste: "Transuranic waste shall contain no explosives or compressed gases. 49 CFR 173 Subpart C . . . defines explosives and 49 CFR Part 173 Subpart G defines compressed gases." [DOE, 1991, p. 3-29]. Additionally, DOE has conducted a chemical compatibility assessment of CH-TRU and RH-TRU wastes to ensure that a reaction will not occur as a result of incompatible waste [Nuclear Packaging, Inc., 1992; VECTRA Technologies, Inc., 1994].
Because of the WAC restriction on packaging explosives and compressed gas for shipment to WIPP and those discussed in the Flammability section, explosive gases will not accumulate in the waste containers. Since a buildup of explosive gas or the packaging of explosives in the waste will not occur, the potential for an explosive environment in "as-received" waste at WIPP is not a concern. There is no significant difference between CH-TRU and RH-TRU waste in this respect.
3.1.1 TRU Waste Inventory
The estimated TRU waste disposal inventory for WIPP, as defined in the WTWBIR, is presented in Table 3-1. This table compares the volumes of CH-TRU and RH-TRU wastes, grouped into final waste forms [DOE, 1995e].
In addition to showing the estimated disposal volume of each CH-TRU and RH-TRU final waste form, Table 3-1 shows the percentage that the final waste forms contribute to the CH-TRU, RH-TRU, and total TRU waste inventories. As can be seen in Table 3-1, the majority of the CH-TRU inventory is made up of combustibles, heterogeneous waste, metals, and solidified inorganics. The RH-TRU waste inventory is comprised primarily of heterogenous waste, solidified inorganics, and metals. In the RH-TRU inventories, generator/storage sites generally include the combustible waste in the heterogeneous waste category. Heterogeneous waste is a category that includes metal, inorganic non-metal, or combustible waste but is not dominant in any one of these types of waste. Combining the combustible and heterogeneous CH-TRU waste gives a more accurate comparison of the CH-TRU and RH-TRU inventories and shows that CH-TRU and RH-TRU waste volume percentages are similar. Since RH-TRU waste is only 5 percent by volume of the WIPP inventory, the last column of Table 3-1 shows that RH-TRU waste has very little contribution to the final waste forms in the entire WIPP TRU waste inventory.
In addition to the volumes of waste provided in Table 3-1, the WTWBIR also provides the densities (which can be converted to mass) for the TRU waste packaging materials [DOE, 1995e]. Conversion to mass provides for the measurement and comparison of materials on a total-quantity basis. The lead and steel in the RH canister and the steel in the RH shield plug make up almost 90 percent (by mass) of the RH-TRU inventory and 14 percent (by mass) of the total WIPP inventory. The steel in the CH-TRU packaging makes up 19 percent (by mass) of the CH-TRU inventory and 16 percent (by mass) of the total WIPP inventory. Because the amount of lead and steel associated with TRU waste packaging is included in the WTWBIR, it will be included in the evaluation of the long-term performance of the repository.
Many of the CH-TRU and RH-TRU waste streams are generated from similar processes and consist of similar waste materials with different concentrations of radionuclides. For example, an Oak Ridge National Laboratory (ORNL) waste stream for contaminated equipment, decontamination debris, or dry solids can be CH-TRU or RH-TRU waste; and in the WTWBIR, these streams contain identical densities for the waste material parameters. Only the radionuclide concentrations are different. Several of the Richland (Hanford) site waste stream descriptions and waste material parameter densities for two waste streams (one CH, the other RH) are identical, with the exception of the radionuclide concentrations [DOE, 1995e].
The TRU waste streams from the generator sites have been characterized in several ways, including process knowledge (review of available waste records and documentation), as well as sampling and analysis. The majority of RH-TRU waste is projected to come from ORNL and Hanford. DOE plans to process wastes not currently in conformance with WAC requirements in facilities such as the proposed Waste Handling and Packaging Plant (WHPP) at ORNL. Examples of procedures that might be used to process these wastes include evaporation, vitrification, and stabilization to immobilize radionuclides.
One conclusion of this document can be derived by examining Table 3-1. Table 3-1 shows that the final waste forms to be sent to WIPP are similar for CH-TRU and RH-TRU wastes. The table shows the percentages of each inventory relative to the final waste forms to be shipped to WIPP pursuant to the WTWBIR. This table indicates that each inventory includes similar types of materials in various quantities.
3.1.2 TRU Radionuclide Inventory
The WTWBIR was used to provide the TRU radionuclide inventories for this study. The initial activities for the radionuclides in both the CH-TRU and RH-TRU waste inventories are listed in Appendix B and have been normalized via activity decay estimates to December 1993. The RH-TRU radionuclide inventory in Appendix B was used to estimate an initial average heat output of less than 1 W per canister, much less than the 300 W allowed by the WIPP WAC. A 300 W heat output corresponds to a formation temperature increase of less than 10 degrees C [Molecke et al., 1993]. Therefore, a 1 W heat output is expected to cause a negligible temperature increase. The decay series for the CH-TRU and RH-TRU radionuclides considered in PA are shown in Appendix C. The WTWBIR indicates that Cesium-137 (Cs) and Strontium-90 (Sr) are the sources of most of the penetrating radiation. It is for this reason that Cs and Sr have been highlighted for comparison.
The activity for each radionuclide listed in Appendix B was calculated using the decay model ORIGEN2 over the period of zero to 10,000 years. Results of these calculations and an explanation of the ORIGEN2 calculation are presented in Appendix C.
Using the results presented in Appendix C, a plot was constructed to demonstrate that the activity of radionuclides in RH-TRU waste decreases to a small percentage of the total activity of the TRU inventory early in the post-closure period. Figure 3-1 shows the percent of the activity that is associated with RH-TRU waste versus time for up to 1,000 years. This figure demonstrates that the activity contribution of RH-TRU radionuclides to the total radioactivity in the repository decreases rapidly in slightly over 200 years from about 36 percent to 1 percent and then a very small increase begins after 300 years because of ingrowth from the decay of other radionuclides.
Important conclusions can be drawn from the information presented in Figure 3-1. These conclusions include the following:
Therefore, RH-TRU waste has an effect on the repository for only a short portion of the 10,000-year period of regulatory concern.
3.2 Comparison of LWA Issues During the Post-Closure Period
The next four subsections address the issues specified in the LWA for comparing CH-TRU and RH-TRU waste forms. The comparison of TRU wastes in these subsections considers the expected repository conditions and consequent behavior of the waste forms during the post-closure period of 10,000 years.
3.2.1 Gas Generation
The evaluation of gas generation from TRU waste is a necessary component in the comparison of CH-TRU and RH-TRU wastes because gas has the potential to directly affect the long-term performance of the repository. Because gas cannot transport significant quantities of radionuclides, brine is considered the primary transporting medium to be evaluated for both disturbed and undisturbed repository scenarios.
The dominant gas generation processes expected to occur in the WIPP repository include (1) corrosion of iron and aluminum alloys; (2) microbial degradation of cellulose, and perhaps rubber and plastic; and (3) alpha radiolysis of brine and residual water in the waste as well as alpha radiolysis of cellulose, rubber, and plastic. Gas generation is synergistically dependent on the conditions within the repository, with particular emphasis on the residual water content in waste at the time of emplacement and brine inflow from the surrounding formation. Laboratory gas generation studies have shown that the quantity of brine in the repository has a direct effect on the gas generation rates [Brush, 1995].
Gas Generation Mechanisms
From the standpoint of gas generation for the WIPP repository, the most important mechanisms are corrosion (specifically anoxic corrosion) of steels and other iron alloys, as well as aluminum alloys and microbial degradation (specifically anaerobic microbial degradation) of cellulosics, rubber, and plastic. Gas generation from alpha radiolysis is not as important as anoxic corrosion and anaerobic microbial degradation because results from radiolysis studies indicate that gas generation rates from alpha radiolysis are substantially lower than rates from anoxic corrosion and anaerobic microbial degradation [Brush et al., 1993]. A general discussion of the gas generation mechanisms is provided below. Detailed discussions of these complex mechanisms as they apply to this study can be found in Appendix D.
Anoxic corrosion of iron and aluminum alloys in TRU waste has the potential to consume water and produce hydrogen, assuming several repository conditions are present [Brush, 1995]. The primary conditions that must be satisfied for anoxic corrosion to occur are (1) sufficient quantities of brine from the surrounding Salado Formation enter the WIPP disposal rooms after closure and/or (2) initial water in the waste is available. Gas generation rates from anoxic corrosion for CH-TRU and RH-TRU wastes are similar because there are no significant differences between these waste forms that would directly influence corrosion.
RH-TRU corrodible metals (i.e., RH-TRU iron, aluminum, and waste packaging) will contribute 6 percent by mass without the shield plug and 31 percent by mass with the shield plug to the total corrodible metal content (i.e., all TRU iron, aluminum, and waste packaging) of the repository [DOE, 1995e]. However, if sufficient brine is available, microbial degradation will produce carbon dioxide and/or hydrogen sulfide (in addition to other gases) that could potentially passivate steels and other iron-base alloys and thus prevent additional hydrogen production and water consumption from anoxic corrosion of these waste metals. Further, small amounts of brine could initiate anoxic corrosion, which will produce hydrogen, consume water, increase the pressure, and perhaps slow or prevent additional brine inflow or even cause brine outflow, thus impeding additional anoxic corrosion and hydrogen generation. Thus, the availability of water in the WIPP repository may limit anoxic corrosion and therefore hydrogen generation, regardless of the quantity of CH-TRU and RH-TRU steels and other iron-base alloys and packaging materials included in the WIPP inventory [Brush, 1995]. The DOE is also currently evaluating alternatives for the RH-TRU shield plug, which could lower the mass contributed by RH-TRU corrodible metals from 31 percent to 6 percent of the total corrodible inventory.
The comparison of CH-TRU and RH-TRU corrodible metal waste indicates that RH-TRU iron-base alloys, aluminum-base alloys, and the waste packaging materials could contribute from zero to 31 percent to the total gas from corrodible metal waste (i.e., amount of gas from all TRU corrodible metal waste and packaging materials) in the WIPP repository. The range of RH-TRU gas generation potential from corrodible metals will be dependent on the amount of brine present and the passivation of steels and other iron-base alloys from microbial degradation products. To obtain the lower end of nearly zero percent gas generation from RH-TRU metal waste, the amount of brine would be severely limited to the extent that no TRU metal waste would corrode. To reach the higher extreme of 31 percent, sufficient brine must be available to react with all CH-TRU and RH-TRU corrodible metal wastes, and passivation of the steels and other iron-base alloys must not occur. Therefore, depending on the total amount of brine present and the potential for the passivation of the steels and other iron-base alloys, 6 percent (with the exclusion of the RH-TRU shield plug) or up to 31 percent (with the inclusion of the RH-TRU shield plug) of the total gas from corrodible metal waste could be generated from the RH-TRU corrodible metal waste inventory.
Anaerobic microbial degradation of cellulosics, rubbers, and plastics in the TRU waste has the potential to produce a variety of gases (carbon dioxide and/or hydrogen sulfide in addition to other gases), assuming several repository conditions are present. One of the primary conditions for the generation of gases from anaerobic microbial degradation is the presence of sufficient quantities of brine or water vapor for diffusive transport from the waste or rock in which the microbes occur [Brush, 1995]. Estimates of microbial gas production are dependent on how much microorganisms will degrade cellulosics, rubbers, and plastics in the waste that is to be emplaced in the WIPP [Brush et al., 1991]. Gas generation rates from anaerobic microbial degradation for CH-TRU and RH-TRU wastes are similar because there are no distinguishing attributes of these waste forms that would directly influence anaerobic microbial degradation.
The comparison of CH-TRU and RH-TRU organic wastes indicates that the inclusion of RH-TRU cellulosic, rubber, and plastic wastes in the WIPP repository could contribute approximately 1 percent by mass to the total organic content (i.e., all TRU cellulose, rubber, and plastic) of the WIPP repository [DOE, 1995e], therefore contributing from zero to 1 percent of the total gases from all TRU organic waste materials in the WIPP repository. To reach the 1 percent, sufficient brine must be available to interact with all CH-TRU and RH-TRU organic waste. Therefore, depending on the total amount of brine present, up to 1 percent of the total gas generated from all TRU organic waste could be produced from the RH-TRU organic waste inventory.
Radiolysis by alpha particles is not expected to be a significant mechanism for gas generation in the WIPP repository [Brush, 1995]. Alpha radiolysis of the water in the waste and brine could consume water and brine, producing hydrogen and oxygen. A variety of gases can also be produced by the alpha radiolysis of cellulosics, rubbers, and plastics in the waste [Molecke, 1979]. Conclusions from experiments conducted at Argonne National Laboratory East indicate that alpha radiolysis of WIPP brines will produce hydrogen and oxygen at rates much lower than the expected gas production rates for anoxic corrosion and anaerobic microbial degradation [Brush et al., 1993]. Additional evaluation of alpha radiolysis at Sandia National Laboratories suggests that gas generation from alpha radiolysis of cellulosics, rubbers, and plastics will be minimal from the standpoint of long-term gas production in the WIPP repository [Brush, 1995]. Further, because molecular dissociation caused by beta and gamma radiation will be insignificant in a repository for TRU waste [Brush, 1995], these types of radiation have not been considered in this evaluation of gas generation. In consideration of the CH-TRU and RH-TRU waste inventory, the amounts of gas generated from alpha radiolysis is anticipated to be minimal. Therefore, based on the estimated production of gas from alpha radiolysis that could occur in the WIPP disposal rooms, alpha radiolysis from RH-TRU waste will be a minor contributor to the production of gas from all TRU waste.
Evaluation of the primary gas generation mechanisms indicates that the brine available in the disposal environment is the determining factor relative to the quantity of gas generated during the compliance period. The amount of corrodible metals and organic materials present in the TRU waste inventory indicates a high probability of gas generation in the WIPP disposal rooms, provided there is sufficient brine available. The amount of gas generated from RH-TRU corrodible and organic waste forms in the WIPP repository can range from approximately zero to 31 percent of the total gases generated from TRU waste. Only about zero to 1 percent of this gas can be attributed to organic wastes. The zero to 31 percent range could be significantly narrowed to about 6 percent if a shield plug made from a non-corrodible material were used as an alternate. In addition, the amount of gas generated from RH-TRU waste is expected to be smaller because of the small amount of RH-TRU organic waste, the limited time frame in which anoxic corrosion is likely to occur, and the potential passivation of iron-base metals due to microbial degradation.
3.2.2 Flammability and Explosiveness
The term "flammable" can be defined as the ability of a material to generate a sufficient concentration of combustible vapors to be ignited and produce a flame. An explosion of gases, on the other hand, is simply the very rapid combustion of flammable vapors. An explosion typically occurs when flammable gases are ignited in a confined space and pressure cannot freely dissipate. The terms flammability and explosiveness can be considered synonymous for purposes of this study.
The U.S. Department of Transportation (DOT) defines a gas as being flammable if, when mixed with air at a concentration at or below 13 percent by volume, it forms a flammable mixture, or its flammable range in air is wider than 12 percent regardless of the lower limit [49 CFR Part 173]. The flammable range is defined as where combustion is possible for a given gas mixture if a credible ignition source exists. This range is the same for both the flammability and explosiveness potential of a given gas mixture. Combustion can occur in this range because the optimal fuel-to-air (flammability) ratio exists (expressed as a percentage of fuel in air) to allow for ignition and sustained combustion.
The lowest concentration of fuel in the flammable range is termed the Lower Explosive Limit (LEL). Concentrations less than the LEL are not flammable because there is too little fuel. The highest ratio that is flammable is the Upper Explosive Limit (UEL). Concentrations greater than the UEL are not flammable because there is too much fuel displacing oxygen (resulting in too little oxygen). Fuel concentrations between the LEL and UEL are optimal for starting and sustaining combustion.
For a flammable gas to be ignited, three conditions must be met. The flammable gas must be present in flammable concentrations, an oxidant such as oxygen must be present (the repository is expected to be anoxic shortly after closure), and a source of ignition must be available. For this study, the availability of an ignition source is not being evaluated.
There is a potential for the collection of flammable mixtures of hydrogen, methane, and oxygen in the room headspace above the waste during the post-closure period [Slezak and Lappin, 1990]. The primary source of flammable gas in the repository will be from corrosion of iron- and aluminum-based metals and microbial degradation of cellulose, rubber, and plastic in the waste and waste containers (see section 3.2.1). Alpha radiolysis is expected to be a minor contributor to gas generation (see section 3.2.1).
The waste material parameters in TRU waste that generate gas as they corrode or degrade include primarily iron, aluminum, cellulose, rubber, and plastic. The steel in waste packages and shield plugs can also contribute to flammable gas generation. A side-by-side comparison reveals that the gas generating waste material parameters and packaging materials in RH-TRU waste could contribute only a minor portion of the total flammable gas potential compared to CH-TRU waste. This conclusion can be derived by examining the percentages by mass for the gas generating waste material parameters and packaging from the total TRU waste inventory for both CH-TRU and RH-TRU wastes. These percentages are presented in Table 3-2.
The waste material parameters and packaging materials can be collectively grouped into a category called gas generation materials. This category can then be normalized to the total of gas generation materials in the entire TRU inventory for WIPP to give an estimate of how much the CH-TRU and RH-TRU inventories individually contribute to the flammable gas potential. Table 3-2 shows that when the shield plug is included in this estimate, the CH-TRU and RH-TRU inventories contribute 87 and 13 percent, respectively. If the shield plug is not included, then the results are 94 and 6 percent, respectively.
RH-TRU waste is not unique compared to CH-TRU waste in terms of its potential to generate flammable gas because both inventories consist of similar gas generating materials. The quantities of these materials in RH-TRU waste, however, are much less than that for CH-TRU waste. Therefore, it can be concluded that the RH-TRU waste inventory will have only a small contribution to the total flammability and explosiveness potential in the repository.
The solubility of radionuclides in TRU wastes determines their potential mobility as solutes in WIPP brines. The mobilized radionuclides, for the purposes of transport modeling, are called the radionuclide source term. The most reasonable mechanism for potential release of radionuclides to the accessible environment is dissolution in brine followed by some means of transport. The mobile concentrations that may potentially leave the repository are a key factor in the determination of compliance with 40 CFR Part 191.
There are several major parameters that influence the amount of radionuclides dissolved in the WIPP brine [Novak et al., 1995]. These parameters include:
The radionuclide solubilities for CH-TRU and RH-TRU wastes will be the same because the parameters mentioned above, which directly influence the solubility, will not be unique to either CH-TRU or RH-TRU waste forms. Therefore, the difference in the potential inventory concentration of radionuclides relative to CH and RH is dependent on the increased quantity of certain radionuclides present in the RH-TRU wastes. Since strontium and cesium are initially present in RH-TRU in quantities greater than in CH-TRU waste, the determination of the influence of RH-TRU on the potential inventory concentration is governed by the availability of Sr and Cs.
For the major radionuclides in the CH-TRU and RH-TRU inventory, the potential for these radionuclides to cross the regulatory boundary and reach the accessible environment will be controlled by the degree to which they can become soluble in WIPP brines, which is the transport mechanism, and by the degree to which the radionuclides are retarded during transport. There must be interaction of the brine and the radionuclide source terms in order for transport of the radionuclides to potentially occur.
The extent to which radionuclides become dissolved in WIPP brine will be limited by two significant factors:
These expected outcomes lead to the conclusion that the chemical dissolution of radionuclides in the total expected brine volume will not be substantially altered by the presence of the RH-TRU waste inventory. After 200 years, the CH-TRU waste inventory will be the primary contributor to the radionuclide source term.
In conclusion, the RH-TRU waste inventory will not significantly add to the radionuclide source term of WIPP. This can be concluded from (1) the rapid decay of the Sr and Cs radionuclides initially present in the RH-TRU wastes are available for only a limited period of time for brine interaction; and (2) the RH-TRU waste package and configuration, which will limit brine accessibility to the waste during the period when RH radionuclides are present in high concentrations.
3.2.4 Brine and Geochemical Interactions
The magnitude of the radiation dose rate at the surface of the waste packages is the primary characteristic that can affect brine and geochemical interactions with respect to comparing the similarities and differences between CH-TRU and RH-TRU wastes. Radiation at the surface of the waste packages will be a result of gamma emissions. The TRU waste to be emplaced in WIPP will emit three primary radiations: alpha, beta, and gamma. Only gamma radiation can penetrate the waste package and be absorbed by the surrounding formation, while alpha and beta radiation will be shielded by the waste contents and waste packaging. The result of absorbed radiation in the formation can be an alteration in the chemical structure of rocksalt as well as a heating of the formation.
The maximum surface dose rates allowed for WIPP waste packages are listed below:
Waste Type / Dose Rate (rem/hr)
CH / 0.2
RH-1 / 100
RH-2 / 1,000
For gamma radiation, rem/hr is approximately equivalent to rad/hr. RH-TRU waste has two maximum limit values because 95 percent of RH-TRU waste by volume cannot exceed 100 rem/hr, while 5 percent is allowed to have up to 1,000 rem/hr [Public Law 102-579]. For purposes of this evaluation, these two categories have been referred to as RH Type 1 and Type 2, respectively.
Rocksalt is known to suffer radiation damage for absorbed doses in excess of 10,000 rad [Hull, 1987]. A result of this damage is an increase in the hardness, embrittlement, and cleaving characteristics of the rocksalt [Levy, 1983]. In addition, when dry gamma irradiated rocksalt is dissolved in water, hydrogen gas is released with a corresponding increase in the pH (alkalinity) of the aqueous solution [Jenks and Bopp, 1977; Panno and Soo, 1984]. When brine and rocksalt are irradiated together, the brine becomes more acidic, while the dissolution of the rocksalt produces a basic solution as is the case with dry irradiations [Panno and Soo, 1984]. When rocksalt is exposed to damaging doses of radiation, changes occur in its chemical structure. These changes take the form of induced defects within the lattice or crystalline structure of sodium chloride. These defects include the formation of F-centers, which are then followed by the formation of sodium metal colloids.
An F-center is a defect that causes rocksalt to become amber in color. This can occur when radiation energy causes a chloride ion to vacate its position in the lattice structure of sodium chloride. These vacant positions are then subsequently filled by free electrons that become trapped in these positions by electrostatic forces. F-center formation is known to occur for radiation doses exceeding 10,000 rad along with temperatures ranging from room temperature (25 degrees C) up to about 300 degrees C [Levy, 1983; Hull, 1987].
Sodium metal colloids are formed as a result of F-center formation and cause the rocksalt to become purple-to-blue-to-black in color with increasing radiation dose [Jenks and Bopp, 1977]. The vacancies caused by the F-centers are filled with electrons that then combine with sodium metal ions to form the uncharged sodium atom. Since chloride ions have been displaced from their lattice positions, the sodium metal atoms tend to aggregate into colloids.
Besides radiation induced effects, heat can also have effects on brine and rocksalt geochemistry. Evaluations involving the heating of unirradiated rocksalt and its subsequent dissolution indicate that pH shifts similar to those for dry irradiated rocksalt occur for tested temperatures as low as 40 degrees C [Panno and Soo, 1984]. In addition, rocksalt heated to high temperatures (about 190 degrees C) has resulted in the release of hydrogen chloride gas [Pederson, 1985]. Another possible effect of heat is its tendency to mobilize brine. Brine filled inclusions are known to migrate up a thermal gradient [Olander, 1984].
As mentioned earlier, absorbed gamma radiation doses of a magnitude in excess of 10,000 rad are required to cause F-center formation and subsequent sodium metal colloid growth. If these defects occur, then the potential exists for localized pH shifts, which could result in a slight increase in the corrosivity and gas generation potential of the repository environment. Also, heating effects could contribute to the corrosivity of this environment.
The genesis of such an environment could occur if sufficient brine were to become available in the near-field formation surrounding the waste packages. Under this condition, brine would migrate into the near-field formation because of a thermal gradient. The accumulation of brine would dissolve irradiated rocksalt, producing corrosive gases and a corresponding increase in brine pH.
To evaluate whether the emplacement of CH-TRU and RH-TRU waste in WIPP will result in sufficient gamma radiation to generate defects, the analysis in Appendix E was conducted. From this analysis a plot of the average absorbed dose in ten half-thicknesses (one half-thickness is approximately 2 inches) for rocksalt as a function of time was estimated and is shown in Figure 3-2 for both CH-TRU and RH-TRU waste. The estimate is based on the maximum surface dose levels allowed for each type of waste as presented above. In addition, the radionuclide half-life for Cs-137 was used in the estimate since it is the most abundant gamma-emitting radionuclide in the WIPP TRU inventory.
As Figure 3-2 shows, the potential exists for the formation to receive absorbed dose levels in excess of 10,000 rad as a result of gamma radiation from both CH-TRU and RH-TRU wastes. Radiation levels of this magnitude would occur in about the first 100 years following waste disposal and could be sufficient to cause defects in the formation in the immediate vicinity of the waste packages. However, these dose levels are small compared to HLW.
Heat generated in the repository because of RH-TRU waste emplacement has been evaluated by SNL in nonradioactive in situ tests [Molecke et al., 1993]. These tests have demonstrated that at a maximum allowable heat output of 300 W a temperature rise of less than 10 degrees C at the top of the borehole-air interface could be expected. This small temperature rise is insignificant compared to the high temperatures required to cause the release of hydrochloric gas and significant brine migration. CH-TRU waste is expected to cause negligible temperature rises in the surrounding formation [Sandia WIPP Project, 1992].
As indicated by the discussion above, both CH-TRU and RH-TRU waste could potentially deposit sufficient radiation in the formation to alter the chemical characteristics of the waste package environment. The degree to which this can occur is dependent on many uncertain factors, one of which is the proximity of the waste packages to the formation during the first 100 years after closure (after this period, dose rates are expected to be so low that little additional radiation and heat would be imparted to the formation), the presence of mineral impurities in rocksalt (impurities tend to suppress colloid growth [Levy, 1983]), the amount of brine available in the near-field formation, and the probability that waste packages are, and will be, packed to the maximum dose rates allowed.
In conclusion, the radiation from both CH-TRU and RH-TRU could have a small effect on brine and geochemical interactions in the WIPP disposal system. The result is a potential increase in the corrosivity of the waste package environment for both CH-TRU and RH-TRU wastes. The contribution of gas generated by this corrosion process is expected to be negligible compared to other corrosion processes because radiation doses will be relatively low, thus irradiating very little rocksalt. Further, the initial thermal gradient is expected to be small, resulting in little brine migration, and the initial heat will rapidly reduce to ambient temperatures in about 200 years because of radionuclide decay.
In conclusion, CH-TRU and RH-TRU wastes are essentially the same with respect to processes and mechanisms associated with gas generation, flammability and explosiveness, solubility, and brine and geochemical interactions. The processes that generate CH-TRU and RH-TRU waste are similar and include the same materials, with the exception of greater concentrations of short-lived radionuclides in the RH-TRU waste. These radionuclides exhibit high decay rates, so that the RH-TRU radionuclide inventory will begin to resemble the CH-TRU radionuclide inventory within a few hundred years. Therefore, the CH-TRU and RH-TRU radionuclide inventories will demonstrate the same behavior after the first few hundred years.
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