Method to Reduce Molten Salt Penetration into Bulk Vitrification Refractory Materials
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. This 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.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 979523
- Report Number(s):
- PNNL-SA-58723; 830403000; TRN: US1003196
- Resource Relation:
- Conference: WM 2008: Phoenix Rising: Moving Forward in Waste Management, Paper No. 8275
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CARBOHYDRATES
CARBON
CELLULOSE
CRUCIBLES
ELECTRODES
GRINDING
LINERS
MODIFICATIONS
MOLTEN SALTS
NITRATES
POROSITY
SACCHAROSE
SOILS
SPECIFIC SURFACE AREA
TECHNETIUM
VITRIFICATION
WASTE FORMS
WASTE MANAGEMENT
WASTES
Vitrification
Waste
Molten Ionic Salt
Glass