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Title: Transport of Technetium and Rhenium into Refractory Materials during Bulk Vitrification

Abstract

Bulk vitrification (BV) was selected as a potential supplemental treatment to accelerate the cleanup of low-activity waste (LAW) at the U.S. Department of Energy Hanford Site. In the BV process, low-activity waste, soil, and glass forming chemicals are mixed, dried and placed in a metal box lined with a castable refractory block (CRB). Electric current, supplied by two graphite electrodes in the box, melts the waste feed and produces a durable glass waste form. During engineering-scale (ES) tests of BV, a small fraction of radioactive technetium-99 (Tc) (and rhenium [Re], a nonradioactive surrogate) were transferred out of the LAW glass feed and molten LAW glass, and deposited on the surface and within the pores of the CRB. Tc is a primary risk driver for long-term performance of immobilized LAW; therefore, even small fractions of Tc present in a readily leachable form rather than immobilized in a glass matrix can impact long-term performance.

Authors:
; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
979528
Report Number(s):
PNNL-SA-48697
830403000; TRN: US1003199
DOE Contract Number:
AC05-76RL01830
Resource Type:
Conference
Resource Relation:
Conference: Waste Management '06: Global Accomplishments in Environmental and Radioactive Waste Management: Education and Opportunity for the Next Generation of Waste Management Professionals
Country of Publication:
United States
Language:
English
Subject:
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; EDUCATION; ELECTRIC CURRENTS; ELECTRODES; GLASS; GRAPHITE; PERFORMANCE; RADIOACTIVE WASTE MANAGEMENT; RHENIUM; TECHNETIUM; TECHNETIUM 99; TRANSPORT; VITRIFICATION; WASTE FORMS; WASTE MANAGEMENT; WASTES

Citation Formats

Bagaasen, Larry M., Brouns, Thomas M., Elliott, Michael L., Hrma, Pavel R., Kim, Dong-Sang, Matyas, Josef, Pierce, Eric M., McGrail, B. Peter, Schweiger, Michael J., Campbell, Brett E., and Beck, Andrew E. Transport of Technetium and Rhenium into Refractory Materials during Bulk Vitrification. United States: N. p., 2006. Web.
Bagaasen, Larry M., Brouns, Thomas M., Elliott, Michael L., Hrma, Pavel R., Kim, Dong-Sang, Matyas, Josef, Pierce, Eric M., McGrail, B. Peter, Schweiger, Michael J., Campbell, Brett E., & Beck, Andrew E. Transport of Technetium and Rhenium into Refractory Materials during Bulk Vitrification. United States.
Bagaasen, Larry M., Brouns, Thomas M., Elliott, Michael L., Hrma, Pavel R., Kim, Dong-Sang, Matyas, Josef, Pierce, Eric M., McGrail, B. Peter, Schweiger, Michael J., Campbell, Brett E., and Beck, Andrew E. Tue . "Transport of Technetium and Rhenium into Refractory Materials during Bulk Vitrification". United States. doi:.
@article{osti_979528,
title = {Transport of Technetium and Rhenium into Refractory Materials during Bulk Vitrification},
author = {Bagaasen, Larry M. and Brouns, Thomas M. and Elliott, Michael L. and Hrma, Pavel R. and Kim, Dong-Sang and Matyas, Josef and Pierce, Eric M. and McGrail, B. Peter and Schweiger, Michael J. and Campbell, Brett E. and Beck, Andrew E.},
abstractNote = {Bulk vitrification (BV) was selected as a potential supplemental treatment to accelerate the cleanup of low-activity waste (LAW) at the U.S. Department of Energy Hanford Site. In the BV process, low-activity waste, soil, and glass forming chemicals are mixed, dried and placed in a metal box lined with a castable refractory block (CRB). Electric current, supplied by two graphite electrodes in the box, melts the waste feed and produces a durable glass waste form. During engineering-scale (ES) tests of BV, a small fraction of radioactive technetium-99 (Tc) (and rhenium [Re], a nonradioactive surrogate) were transferred out of the LAW glass feed and molten LAW glass, and deposited on the surface and within the pores of the CRB. Tc is a primary risk driver for long-term performance of immobilized LAW; therefore, even small fractions of Tc present in a readily leachable form rather than immobilized in a glass matrix can impact long-term performance.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Feb 21 00:00:00 EST 2006},
month = {Tue Feb 21 00:00:00 EST 2006}
}

Conference:
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  • Bulk vitrification (BV) was selected as a potential supplemental waste treatment process to support the commitment for cleanup of low-activity waste (LAW) stored in large waste storage tanks at the U.S. Department of Energy's Hanford Site. In the BV process, LAW, soil, and glass-forming chemicals are mixed, dried, and placed within a castable refractory block (CRB) and sand, all within a metal box. Electric current, supplied through two graphite electrodes in the box, melts the waste feed and produces a durable glass waste form. During engineering-scale tests of By, a small fraction of radioactive technetium-99 (Tc) and rhenium (Re) (amore » nonradioactive surrogate) were transferred out of the LAW glass feed and molten LAW glass and deposited on the surface and within the pores of the CRB. Tc is a primary risk driver for long-term performance of immobilized LAW; therefore, even small fractions of Tc present in a readily leachable form rather than immobilized in a glass matrix can impact long-term performance of the immobilized waste. Laboratory and engineering-scale studies were undertaken to reduce or eliminate the readily leachable Tc in the CRB. These studies focused on 1) understanding the mechanisms of the transport of Tc/Re into the CRB during vitrification, and 2) evaluating various means of protecting the CRB against the deposition of leachable Tc/Re. The tests used either Re as a chemical surrogate for Tc, or Re and Tc together. A conceptual Tc/Re transport model was developed based on observed laboratory experiments to attempt to explain the transport behavior seen in engineering-scale tests. At temperatures below 650 deg. C, molten ionic salt (MIS) containing Tc and Re penetrates by capillarity from the feed into the CRB open porosity. At approximately 650 to 750 deg. C, the MIS decomposes through the loss of NO{sub x}, leaving mainly sulfate and chloride salts. The Na{sub 2}O formed during decomposition of the nitrates reacts with insoluble grains in the feed and the aluminosilicates in the CRB to form more viscous liquids that reduce further liquid penetration into the CRB. At 750 to 950 deg. C, a continuous glass phase traps the remains of the MIS in the form of inclusions in the bulk glass melt. At 950 to 1200 deg. C, the salt inclusions in the glass slowly dissolve but also rise to the surface. The Tc/Re salts also evaporate from the free surface of the glass melt that is rapidly renewed by convective currents. The vapors condense on cooler surfaces in the upper portion of the CRB, the box lid, and the off gas system. Results of the engineering-scale tests helped to validate the conceptual transport model of Tc/Re deposition and improved the understanding of likely mechanisms of deposition in the CRB. As a result, there is increased potential that Tc deposition can be controlled and reduced to ensure that the BV waste form will provide acceptable performance. (authors)« less
  • Bulk vitrification (BV) is a process that heats a feed material that consists of glass-forming solids and dried low-activity waste (LAW) in a disposable refractory-lined metal box using electrical power supplied through carbon electrodes. The feed is heated to the point that the LAW decomposes and combines with the solids to generate a vitreous waste form. However, the castable refractory block (CRB) portion of the refractory lining has sufficient porosity to allow the low-viscosity molten ionic salt (MIS), which contains technetium (Tc) in a soluble form, to penetrate the CRB. This limits the effectiveness of the final waste form. Thismore » paper describes tests conducted to develop a method aimed at reducing the quantities of soluble Tc in the CRB. Tests showed that MIS formed in significant quantities at temperatures above 300°C, remained stable until roughly 550°C where it began to thermally decompose, and was completely decomposed by 800°C. The estimated volume fraction of MIS in the feed was greater than 40%, and the CRB material contained 11 to 15% open porosity, a combination allowing a large quantity of MIS to migrate through the feed and penetrate the open porosity of the CRB. If the MIS is decomposed at temperatures below 300°C or can be contained in the feed until it fully decomposes by 800°C, MIS migration into the CRB can be avoided. Laboratory and crucible-scale experiments showed that a variety of methods, individually or in combination, can decrease MIS penetration into the CRB. Modifying the CRB to block MIS penetration was not deemed practical as a method to prevent the large quantities of MIS penetration seen in the full-scale tests, but it may be useful to reduce the impacts of lower levels of MIS penetration. Modifying the BV feed materials to better contain the MIS proved to be more successful. A series of qualitative and quantitative crucible tests were developed that allowed screening of feed modifications that might be used to reduce MIS penetration. These tests showed that increasing the specific surface area of the soil (used as the primary glass-forming solid in the baseline process) by grinding stopped MIS penetration nearly entirely for feeds that contained waste simulants with lower quantities of nitrate salts. Grinding soil significantly reduced MIS penetration in feeds with higher nitrate quantities, but it was necessary to add carbohydrates (sucrose or cellulose) to destroy a portion of the nitrate at low temperatures to reach the same low levels of MIS penetration seen for the lower nitrate feeds. Developing feeds to reduce MIS penetration in full-scale BV applications resulted in two additional refinements. Soil-grinding to the necessary levels proved to be difficult and expensive, so the fine soil was replaced with readily available fine-grained glass-forming minerals. Cellulose was shown to have less impact on dryer operation than sucrose and was chosen as the carbohydrate source to use in subsequent engineering- and full-scale tests.« less
  • Bulk vitrification (BV) is a process that heats a feed material consisting of glass-forming solids and dried low-activity waste (LAW) in a disposable refractory-lined metal box using electrical power supplied through carbon electrodes. The feed is heated to the point that the LAW decomposes and combines with the solids to generate a vitreous waste form. However, the castable refractory block (CRB) portion of the refractory lining has sufficient porosity to allow the low-viscosity molten ionic salt (MIS), which contains technetium (Tc) in a soluble form, to penetrate the CRB. This limits the effectiveness of the final waste form. This papermore » describes tests conducted to develop a method aimed at reducing the quantities of soluble Tc in the CRB. Tests showed that MIS formed in significant quantities at temperatures above 300 deg. C, remained stable until roughly 550 deg. C where it began to thermally decompose, and was completely decomposed by 800 deg. C. The estimated volume fraction of MIS in the feed was greater than 40%, and the CRB material contained 11 to 15% open porosity, a combination allowing a large quantity of MIS to migrate through the feed and penetrate the open porosity of the CRB. If the MIS is decomposed at temperatures below 300 deg. C or can be contained in the feed until it fully decomposes by 800 deg. C, MIS migration into the CRB can be avoided. Laboratory and crucible-scale experiments showed that a variety of methods, individually or in combination, can decrease MIS penetration into the CRB. Modifying the CRB to block MIS penetration was not deemed practical as a method to prevent the large quantities of MIS penetration seen in the full-scale tests, but it may be useful to reduce the impacts of lower levels of MIS penetration. Modifying the BV feed materials to better contain the MIS proved to be more successful. A series of qualitative and quantitative crucible tests were developed that allowed screening of feed modifications that might be used to reduce MIS penetration. These tests showed that increasing the specific surface area of the soil (used as the primary glass-forming solid in the baseline process) by grinding stopped MIS penetration nearly entirely for feeds that contained waste simulants with lower quantities of nitrate salts. Grinding soil significantly reduced MIS penetration in feeds with higher nitrate quantities, but it was necessary to add carbohydrates (sucrose or cellulose) to destroy a portion of the nitrate at low temperatures to reach the same low levels of MIS penetration seen for the lower nitrate feeds. Developing feeds to reduce MIS penetration in full-scale BV applications resulted in two additional refinements. Soil-grinding to the necessary levels proved to be difficult and expensive, so the fine soil was replaced with readily available fine-grained glass-forming minerals. Cellulose was shown to have less impact on dryer operation than sucrose and was chosen as the carbohydrate source to use in subsequent engineering- and full-scale tests. (authors)« less
  • Radioactive wastes at the Hanford Reservation will be separated into high-level and low-level waste streams for processing. The low-level wastes will be vitrified. Volatile components and technetium compounds may pose potential problems in the vitrification process. In this paper, the chemistry of technetium species is described.
  • Volatile loss of technetium (Tc) during vitrification of low-activity wastes is a technical challenge for treating and immobilizing the large volumes of radioactive and hazardous wastes stored at the U.S. Department of Energy's Hanford Site. There are various research efforts being pursued to develop technologies that can be implemented for cost effective management of Tc, including studies to understand the behavior of Tc during vitrification, with the goal of eventually increasing Tc retention in glass. Furthermore, one of these studies has focused on identifying the form or species of Tc and Re (surrogate for Tc) that evolve during the waste-to-glassmore » conversion process. This information is important for understanding the mechanism of Tc volatilization. In this paper, available information collected from the literature is critically evaluated to clarify the volatile species of Tc and Re and, more specifically, whether they volatilize as alkali pertechnetate and perrhenate or as technetium and rhenium oxides after decomposition of alkali pertechnetate and perrhenate. The evaluated data ranged from mass spectrometric identification of species volatilized from pure and binary alkali pertechnetate and perrhenate salts to structural and chemical analyses of volatilized materials during crucible melting and scaled melter processing of simulated wastes.« less