D.F. Williams

The Influence of Lewis Acid/Base Chemistry on the Removal of Gallium by Volatility from Weapons-Grade Plutonium Dissolved in Molten Chlorides

Abstract It has been proposed that GaCl3 can be removed by direct volatilization from a Pu-Ga alloy that is dissolved in a molten chloride salt. Although pure GaCl3 is quite volatile (boiling point: 201°C), the behavior of GaCl3 dissolved in chloride salts is quite different because of solution effects and is critically dependent upon the composition of the solvent salt (i.e., its Lewis acid/base character). In this technical note, the behavior of gallium in prototypical Lewis acid and Lewis base salts is contrasted. It is found that gallium volatility is suppressed in basic melts and promoted in acidic melts. These results have an important influence on the potential for simple gallium removal in molten salt systems.

Evaluation of Process That Might Lead to Separation of Actinides in Waste Storage Tanks Under Alkaline Conditions

This study addresses the physical-chemical processes that might naturally or inadvertently occur and that would lead to a separation of the poisoning nonfissionable actinides (232Th, 238U) from the fissionable ones (239Pu, 235U) by selective dissolution and redeposition over a prolonged storage of the waste. Of the various chemistries that were evaluated, carbonate complexation reaction is the most plausible means of achieving the separation of these actinides. Carbonate ions (formed by the dissolution and hydrolysis of atmospheric CO2) can selectively dissolve the actinide oxides through the formation of soluble carbonate complexes, which could result in the separation of poisoning actinides from the fissionable ones. The concentrations of these soluble carbonate species are dependent on the pH, temperature, and other ions; therefore, changes in any of these parameters over time—especially cyclic changes (daily or seasonal)—could cause a selective dissolution and redeposition of the more soluble species away from the less soluble ones. Detailed calculations using the stability constants for the carbonates have shown that the most likely pH range for this process to occur is pH = 10–11. Increased solubility through reaction with organic complexants such as EDTA was also considered, and while it presents a situation similar to carbonate complexation and similar potential for autoseparation of the actinides in the waste tanks, it would require the uncontrolled dumping of large amounts of complexants into the storage tanks.

A descriptive model of the molten salt reactor experiment after shutdown: Review of FY 1995 progress

During FY 1995 considerable progress was made toward gaining a better understanding of the chemistry and transport processes that continue to govern the behavior of the Molten Salt Reactor Experiment (MSRE). As measurements in the MSRE proceed, laboratory studies continue, and better analyses are available, our understanding of the state of the MSRE and the best path toward remediation improves. Because of the immediate concern about the deposit in the auxiliary charcoal bed (ACB), laboratory studies in the past year focused on carbon-fluorine chemistry. Secondary efforts were directed toward investigation of gas generation from MSRE salts by both radiolytic and nonradiolytic pathways. In addition to the laboratory studies, field measurements at the MSRE provided the basis for estimating the inventory of uranium and fluorine in the ACB. Analysis of both temperature and radiation measurements provided independent and consistent estimates of about 2.6 kg of uranium deposited in the top of the ACB. Further analysis efforts included a refinement in the estimates of the fuel- salt source term, the deposited decay energy, and the projected rate of radiolytic gas generation. This report also provides the background material necessary to explain new developments and to review areas of particular interest. The detailed history of the MSRE is extensively documented and is cited where appropriate. This work is also intended to update and complement the more recent MSRE assessment reports.

Laboratory tests in support of the MSRE reactive gas removal system

The Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory has been shut down since December 1969, at which time the molten salt mixture of LiF-BeF{sub 2}-ZrF{sub 4}-{sup 233}UF{sub 4} (64.5-30.3-5.0-0.13 mol%) was transferred to fuel salt drain tanks for storage. In the late 1980s, increased radiation in one of the gas lines from the drain tank was attributed to {sup 233}UF{sub 6}. In 1994 two gas samples were withdraw (from a gas line in the Vent House connecting to the drain tanks) and analyzed. Surprisingly, 350 mm Hg of F{sub 2}, 70 mm Hg of UF{sub 6}, and smaller amounts of other gases were found in both of the samples. To remote this gas from above the drain tanks and all of the associated piping, the reactive gas removal system (RGRS) was designed. This report details the laboratory testing of the RGRS, using natural uranium, prior to its implementation at the MSRE facility. The testing was performed to ensure that the equipment functioned properly and was sufficient to perform the task while minimizing exposure to personnel. In addition, the laboratory work provided the research and development effort necessary to maximize the performance of the system. Throughout this work technicians and staff who were to be involved in RGRS operation at the MSRE site worked directly with the research staff in completing the laboratory testing phase. Consequently, at the end of the laboratory work, the personnel who were to be involved in the actual operations had acquired all of the training and experience necessary to continue with the process of reactive gas removal.

Laboratory tests using chlorine trifluoride in support of deposit removal at MSRE

Experimental trials were conducted to investigate some unresolved issues regarding the use of chlorine trifluoride (ClF{sub 3}) for removal of uranium-bearing deposits in the Molten Salt Reactor Experiment (MSRE) off-gas system. The safety and effectiveness of operation of the fixed-bed trapping system for removal of reactive gases were the primary focus. The chief uncertainty concerns the fate of chlorine in the system and the potential for forming explosive chlorine oxides (primarily chlorine dioxide) in the trapping operation. Tests at the MSRE Reactive Gas Removal System reference conditions and at conditions of low ClF{sub 3} flow showed that only very minor quantities of reactive halogen oxides were produced before column breakthrough. Somewhat larger quantities accompanied breakthrough. A separation test that exposed irradiated MSRE simulant salt to ClF{sub 3} confirmed the expectation that the salt is basically inert for brief exposures to ClF{sub 3} at room temperature.