Evaluation of Exothermic Reactions from Bulk-Vitrification Melter Feeds Containing Cellulose

Abstract

SummaryThe baseline bulk-vitrification (BV) process (also known as in-container vitrification ICV™) includes a mixer/dryer to convert liquid waste into a dried, blended feed for vitrification. Feed preparation includes blending liquid low-activity waste (LAW) with glass-forming minerals and cellulose and drying the mixture to a suitable dryness, consistency, and particle size for transport to the ICV TM container.This drying process is conducted under vacuum in the temperature range of 60 to 80°C. The nominal melting process starts with a refractory-lined metal box that is partially filled with feed. The refractory lining is referred to as the castable refractory block (CRB). Heat is applied in the form of electrical power through two graphite electrodes. Initially, the electrical current is carried through a graphite starter path, but as the feed is heated, it forms a molten glass that becomes the electrical conductor. The feed subsides as the glass is formed, allowing more feed to be added to the top of the melt in a feed-while-melt operation that continues until the box is filled to the desired level with glass.A full-scale (FS) test (FS-38C) was conducted in CY06 (Witwer et al. 2007), which evaluated using a thick cold cap of feed to condense volatile contaminants and increase the single-pass retention of those contaminants in the BV melt. Using a thick cold cap resulted in high single-pass retentions and significant quantities of molten ionic salt (MIS) outside the CRB panels, which is thought to be related to the thick cold cap.The main problem with MIS outside of the CRB panels is that radionuclides in the LAW are also initially salts and are concentrated in the MIS. If the MIS migrates to cooler regions of the ICV™, the salts either do not decompose, or they partially decompose to form glasses with poor durability. This is undesirable as the unreacted salts and low-durability glasses are susceptible to leaching after disposal. The significant quantities of MIS in the FS-38C test are also thought to be related to the high concentration of lowmelting nitrates in the Tank 241-S-109 simulant (S-109) used for the test.Because Tc is carried into the CRB by MIS, decreasing MIS migration into the CRB would proportionally decrease the concentration of the soluble Tc in the refractory lining. Current activities are exploring several methods for controlling MIS migration but focus mainly on methods to decrease the MIS mobility within the BV feed. These studies find that the MIS mobility is decreased by 1) increasing the specific surface area of solids and 2) adding organic carbon to denitrate the feed and reduce the amount of MIS. Using solids with fine grains or grinding the existing solids (soil) reduces migration by bonding free MIS to solid grains by capillary forces. Adding organic carbon decreases the amount of MIS in the feed by destroying its main components, nitrates and nitrites, at temperatures below 300°C. iv However, because of potential increased reactivity at off-normal conditions, the project pursue...

Thermal Flammable Gas Production from Bulk Vitrification Feed

“…As in previous work (Scheele et al 2008), the initial reaction is endothermic and begins essentially immediately and is likely the loss of the 4 wt% water. It is followed by a small exothermic reaction at 290°C and two stronger exothermic reactions beginning at 300 and 380°C.…”

“…For the isothermal testing, we used the ARC, used in our earlier thermal sensitivity studies (Scheele et al 2008), in its isothermal mode. The ARC is an adiabatic calorimeter originally developed by Dow Chemical to determine safe process operating temperatures for tested materials.…”

“…We represent and repeat the results of our studies on cellulose's thermal reactivity in a BV feed (Scheele et al 2008) here to assist the reader.…”

Thermoanalytical investigation of the reactions causing the transuranic waste drum breach that occurred in the Waste Isolation Pilot Plant

“…This article provides results from our probative set of thermoanalytical experiments performed in support of the Technical Advisory Team's assessment to identify the internal waste reaction(s) that under WIPP conditions could have led to the drum breach and radionuclides release. Using CCPS recommendations [17,18] and our previous experience investigating reaction thermal sensitivities of 1) cellulose and different nitrate salt mixtures [5,6,8,20,21] and 2) amine-based complexants and nitrate mixtures [22] as our investigative foundation, we used simultaneous thermogravimetric and differential thermal analysis (TG/DTA) and accelerating rate calorimetry (ARC) to determine the thermal sensitivities of a selected set of waste constituents, simulated remediation products, and simulated remediation wastes. The simulant compositions were based on the multi-component composition of the mixture of the original Los Alamos liquid and salt TRU waste, remedial additives, and other materials that were put into the drum.…”

Waste Isolation Pilot Plant Technical Assessment Team Report

“…For example, Beitel investigated the explosivity and combustibility of various organics with nitrate-rich Hanford salt cake waste and nitrate salts Beitel 1976a;Beitel 1976b;Beitel 1976c]. Scheele and coworkers investigated the reactions of amine-based complexants, carboxylate-based complexants, cellulose, and sucrose with nitrate and/or nitrite salts and simulated Hanford nitrate-rich wastes [Scheele 1995;Scheele 2005;Kim 2005;Scheele 2008], and the Bureau of Mines investigated the reactivity and explosivity of amine-and carboxylate-based complexants with nitrate and/or nitrite salts [Turner 1994b;Turner 1994a]. ww Findings from these studies revealed that mixtures of nitrates and organics having many constituents can have lower onset temperatures than that of pure mixtures indicating the possibility of synergistic and catalytic effects of the constituents.…”