Transport of Technetium and Rhenium into Refractory Materials during Bulk Vitrification
- Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352 (United States)
- AMEC Earth and Environmental, Inc., GeoMelt Division, 1135 Jadwin Ave., Richland, WA 99352 (United States)
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) (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 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)
- Research Organization:
- WM Symposia, Inc., PO Box 13023, Tucson, AZ, 85732-3023 (United States)
- OSTI ID:
- 21208759
- Report Number(s):
- INIS-US-09-WM-06312; TRN: US09V1070079546
- Resource Relation:
- Conference: Waste Management 2006 Symposium - WM'06 - Global Accomplishments in Environmental and Radioactive Waste Management: Education and Opportunity for the Next Generation of Waste Management Professionals, Tucson, AZ (United States), 26 Feb - 2 Mar 2006; Other Information: Country of input: France; 16 refs
- Country of Publication:
- United States
- Language:
- English
Similar Records
Transport of Technetium and Rhenium into Refractory Materials during Bulk Vitrification
Progress Report on the Laboratory Testing of the Bulk Vitrification Cast Refractory
Related Subjects
36 MATERIALS SCIENCE
CERAMIC MELTERS
GRAPHITE
RADIOACTIVE WASTE PROCESSING
RADIOACTIVE WASTE STORAGE
RHENIUM
TANKS
TECHNETIUM
TEMPERATURE RANGE 0400-1000 K
TEMPERATURE RANGE 1000-4000 K
TRANSPORT THEORY
VITRIFICATION
WASTE FORMS
WASTE TRANSPORTATION