Development of a technical approach for assessing environmental release and migration characteristics of Hanford Grout
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
A Transportable Grout Facility is being constructed at the Hanford Site to immobilize low-level liquid radioactive waste in grout. This report addresses the grout and sediment testing methodology that is being developed at PNL to support assessments of the long-term performance of the disposed grout. Sediment is the soil that surrounds and underlies the disposed grout. A goal of these efforts is to certify tests for application at Hanford. An assessment of the long-term risks posed by grout requires data on the ability of grout to resist leaching of wastes contained within the grout. Additionally, data are needed on the ability of the sediments to retard the mobility of any wastes released from grout. The effects of aging on the ability of grout to retain waste must also be understood. Aging of grout can reduce or enhance the ability of the grout to contain waste. Credible predictive modeling of the fate of hazardous constituents in disposed grout for periods of up to 10,000 years would best be performed using comprehensive, coupled hydrologic and chemical reaction codes based on knowledge of the mechanisms that control waste release and mobility. It is not clear yet how soon such codes will be available or which types of waste disposal options they will apply to. In the interim we must be content with simpler and separate models that address individual reactions such as leaching and adsorption. One of these models, the Semi-Infinite Solid Diffusion Leach Model, is a popular release model used to describe the leaching of grouts and other cemented waste forms. Because others have found success in describing laboratory leach experiments with cemented waste forms using this leach model and because it appears likely to err on the conservative side for the Hanford application, we currently endorse the use of this model and its supporting experimental methodology for approximations of grout waste release rates. At the present time it is believed that the leachate from Hanford grout will not change significantly in its chemical nature once the major chemical reactions at the waste-form sediment interface are completed. Also, the range of sediments at Hanford through which the leachate will travel probably will not exhibit widely varying adsorption properties. These sediments are generally alkaline sands and silts containing little organic matter and have low-to-medium cation exchange capacities. Their interaction with the expected leachates from the Hanford grout should not appreciably affect the composition of the major constituents of the leachates. Therefore, the constant Rd adsorption model should be a useful first approximation of the adsorption processes likely to control trace concentrations of waste radionuclides and hazardous inorganic chemicals that may leach into the groundwater. Because the Rd approach is empirical, it does not lend itself to the identification of transport-controlling mechanisms, a key need for gaining credibility in longterm performance assessments. Despite its limitations, the Rd concept is believed to be a practical and useful tool for quantifying the interaction of Hanford grout leachate with Hanford sediments and assessing the mobility of waste species. Unlike waste-form leaching, the research of radionuclide adsorption does not have a programmatic focal point in which standardization of techniques and procedures is occurring. At present we recommend that sever a 1 different types of adsorption experiments be performed, including hatch and column tests. Both types of tests are needed to increase the probability that the deficiencies of each are addressed. The separation of the complex chemical interactions of grout, sediment and groundwater into simple leaching and adsorption processes for ease of experimentation and modeling is under question. Few experimenters have performed combined tests involving the waste form, sediment and leaching solution though such a combination represents the actual disposal scheme for Hanford grout. Consequently, investigations have been initiated at PNL that are intended to lead to the development of test procedures and methods of data analysis for such three-component tests. Until the controlling chemical processes are identified in the combined tests, detailed characterization of the starting materials (grout, sediment, and groundwater) and resulting products (leached grout, reacted sediment and leachate) is believed necessary. The combined tests should be used to evaluate the usefulness of the separate tests as well as to demonstrate the performance of disposed grout. Once controlling mechanisms are identified and coupled conceptual models and codes are available, many of the separate leaching and adsorption tests and detailed characterization of materials can be abandoned. Preliminary results of a combined test have been obtained and the test is continuing. In the test, a block of grout containing radioactive Hanford Facilities Waste (HFW) is supported on a layer of Hanford sediment inside a plexiglass cylinder (column). Additional sediment is packed around the grout block and fills the rema1n1ng void space at the top of the column. Hanford groundwater is pumped into the bottom of the column where it flows through the bottom layer of sediment, around the grout, and through the upper layer of sediment. The effluent is collected in a sealed container to minimize evaporation and loss or gain of CO{sub 2}. The effluent is analyzed for (a) Eh, pH, and alkalinity, (b) major cations and selected trace metals, (c) major anions, inorganic carbon and organic carbon, and (d) radionuclide content. The data collected to date show that the effluent is buffered at a pH between 8.0 and 8.8, whereas the effluent from a companion experiment (grout only, no sediment) rose quickly to the 11-12 pH range. The combined test appears to show net precipitation in the test column, whereas the grout-only test is showing net dissolution. The nitrite concentration rapidly and substantially increased in the grout-only test effluent but has not been measurable in the combined test effluent. The grout-only test effluent contained about five times the level of dissolved organic carbon that the combined test effluents showed. To date, no measurable quantities of radionuclides are present in the effluents from the combined test. The activity of {sup 137}]Ccs and {sup 85}Sr in the effluents in the grout-only test rose rapidly to steady-state values. Recause previous batch adsorption tests and a column adsorption test showed strong adsorption of these radionuclides, the radionuclide data to date are as expected. As part of the Hanford Grout Technology Program, numerous two-component leach tests have been underway since January 1qss. Two of these tests, the ANS 1n.1 test and the static test, are also showing leachates with distinctly different chemistries. A key to the differences seems to be the supply of HCO{sub 3}{sup -} or dissolved CO{sub 2} in the system. 8ifferences in pH were also observed. The pH of the ANS 16.1 system stabilized at approximately 8.5 whereas the pH of the static system rose to 12.0. At the higher pH, calcium and magnesium apparently precipitated, probably as carbonate minerals. It seems quite likely that grouts disposed in Hanford sediments will react with the carbonate-rich groundwaters to form calcite and carbonate-rich solids similar to those found in ancient artifacts from Cyprus and Greece. Whether these reactions would form protective layers on the grout that impede leaching is uncertain. Additional study is needed to explain the differences in these leach tests as well as the differences in the combined and grout-only tests. This report discusses numerous activites that will be performed in conjunction with the experiments just described. These activities include: 1) detailed mineralogic, radiochemical and total chemical characterization of the grout and sediment versus distance from the interface, 2) detailed analysis of solution data by equilibrium thermodynamic codes to identify possible solid phase solubility controls and 3) mathematical analyses of the combined test results by mass transport theory ("waste package") models deschbed in existing literature. Recommendations for future study include 1) additional development of combined tests under saturated and partially saturated conditions 2) an evaluation of whether CO{sub 2} gas will be supplied to the disposed grout monolith at a rate that maintains CO{sub 2} equilibrium conditions at the grout-sediment interface 3) development of methods to supply CO{sub 2} at an adequate rate should CO{sub 2} equilibrium prove to be a controlling factor and 4) testing of artificially aged grout for leaching properties. Until the mechanisms of leaching and the subsequent interactions with sediments are better understood, it is difficult to suggest a specific direction for the development of combined tests. It is our hope that the analyses yet to be performed on the separate leaching and adsorption tests and the combined test will provide knowledge on controlling mechanisms and shed light on what parameters should be considered as most important in designing experiments to provide the data upon which long-term performance assessments are based. In spite of the current lack of understanding of the interactions among grout, sediment, and groundwater, and considering that this effort was not started until January 1985, significant progress has been made toward the establishment of testing methodologies for assessing the long-term performance of Hanford grout.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC06-76RL01830
- OSTI ID:
- 1127224
- Report Number(s):
- PNL-5734
- Country of Publication:
- United States
- Language:
- English
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