Extraction of Plutonium from Lean Residues by Room-Temperature

Chapter 21

Emerging Technologies in Hazardous Waste Management Downloaded from pubs.acs.org by TUFTS UNIV on 11/07/16. For personal use only.

Extraction of Plutonium from Lean Residues by Room-Temperature Fluoride Volatility G. M. Campbell, J. Foropoulos, R. C. Kennedy, B. A. Dye, and R. G. Behrens Nuclear Materials Division, Los Alamos National Laboratory, Los Alamos, NM 87545 The use of dioxygen difluoride and krypton difluoride for the recovery of plutonium from lean residues by conversion to gaseous plutonium hexafluoride is being investigated. The synthesis of dioxygen difluoride in practical quantity has been demonstrated. Fluorination of plutonium compounds under ideal conditions supports the contention that a viable process can be developed. Application of the method to lean plutonium residues is in the early stage of development. The high cost and political sensitivity associated with the disposal of radioactive waste makes it imperative that the quantity of waste generated be reduced to the lowest reasonable level. The processing of plutonium residues by direct conversion to plutonium hexafluoride, which can be easily separated as a gas, was examined many years ago (1). The process was limited then to reaction of residues withfluorineat elevated temperature. Because of the corrosive nature of hotfluorine,there was a significant materials compatibility problem. Also, because plutonium hexafluoride becomes increasingly unstable at temperatures above 470° K, there was reduced reaction efficiency, and plutonium hexafluoride decomposition product deposits outside of the reaction zone. To some, the advantages of the process, including the reduction of the number of processing steps, space requirements, and the need for fewer chemical additives, outweighed the disadvantages. As the cost of waste disposal has sharply increased in recent times, the fluoride volatility process has gained new significance. To complement the high temperature process and overcome some of the perceived shortcomings, LANL has been examining the possibility of room temperature fluoride volatility. The compound dioxygen difluoride and its gas phase equilibrium product, the dioxygen monofluoride radical, have had the most emphasis in this study because of the potential for making it in sufficient quantity economically. They have been shown to be powerfulfluorinatingagents for the actinides (2,3). Krypton difluoride is believed to have desirable chemical properties (4), but is more difficult to produce in sufficient quantity at this time. LANL has found that the reaction with plutonium residues has a half time greater than 5 hours and a reaction efficiency at least as good as dioxygen difluoride. At room temperature the half time of dioxygen difluoride is a few seconds. 0097-6156/90A)422-0368$06.00A) © 1990 American Chemical Society

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The chemical reactions of interest in the dioxygen difluoride process are: F (g) + 02(g) - FOOF(g)

(1)

2

3 FOOF(g) + PuR(s) - PuF (g) + 30 (g) + R(s,g)

(2)

6 FOO(g) + PuR(s) - PuF (g) + 60 (g) + R(s,g)

(3)

6


6

2

2

where R(s) is a solid residue. The product R(g) represents gaseous compounds such as phosphorus pentafluoride, silicon tetrafluoride, and carbon tetrafluoride. The first reaction requires the input of energy either by photolysis, microwave excitation, or thermal heating. The others proceed spontaneously at room temperature. Kinetic studies (5,6) of the dioxygen difluoride, dioxygen monofluoride, oxygen system showed that there was an equilibrium, 2FOO(g)^FOOF(g) + 0 (g), 2

(4)

that produced dioxygen monofluoride in the gas phase. The reaction rate was very temperature sensitive with an activation energy of 13 kcal per mole. It was found that dioxygen monofluoride reacted to produce plutonium hexafluoide much more efficiently when the plutonium residue was spread over a metal surface. Thefirstgas circulating loop was designed to optimize thefluorinationreaction in light of the information gainedfromthe kinetic studies. It was operated at a high flow rate so that dioxygen difluoride passed quickly from the supply reservoir to the gas-solid reactor. The gas-solid reactor had a large volume so that the gas reactant remained in contact with the solid residue for several seconds. The solid residue was spread over a metal surface (metal matrix). After demonstrating that the metal matrix reactor operated efficiently, the information obtained was used to adapt the reaction to afluidizedbed. It was believed that a fluidized bed would be more convenient for use in a production mode. To compensate for the slowerflowrate used influidization,and the lack of a metal catalyst, the amount of dioxygen difluoride (as opposed to dioxygen monofluoride) reaching the reaction zone was optimized. This required additional cooling of the gas stream and control of the oxygen pressure. A severe test of plutonium extraction was made by removing it as plutonium hexafluoride from incinerator ash. Fluorination of the ash at elevated temperature was shown to result in the formation of nonvolatile plutoniumfluorides.When the untreated ash wasfluorinatedat room temperature, volatile plutonium hexafluoride was formed. Experimental. The apparatus used in carrying out the gas-solid reactions involving the fluorination of plutonium residues was enclosed in a glove box designed for the safe handling of plutonium. Figure 1 is a simplified schematic of the equipment used. Although nickel or aluminum were the preferred materials of construction for handlingfluorinatingagents, stainless steel was found to be perfectly adequate for many uses at room temperature. Type 316 stainless steel was used in this case. The dioxygen difluoride was made outside of the glove box and cryopumped to a receiving reservoir inside the glove box. The receiving reservoir was one component of a gas circulating loop.

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Metal Matrix Reactor. To take advantage of the effect of a metal surface, the first reactor used was a stainless steel cylinderfilledwith compacted aluminum foil balls, Figure 2. The cylinder containing the aluminum balls accepted 13 liters of gas whenfilled.The balls formed a matrix for support of the solid reactant. The solid reactant was distributed evenly throughout the reactor. The 13 liter volume of the reactor was the largest component of the 18 liter gas circulating system. At a circulation rate of one liter per second, the gas entered the reactor about 2 seconds after vaporization in the dioxygen difluoride reservoir and then spent 13 seconds in the reactor. The circulation rate was controlled by throttling a bypass valve around the bank of three model 601 Metal Bellows compressors. The rate of dioxygen difluoride addition to the fluorine and oxygen carrier gas mix was controlled by adjusting the temperature of the dioxygen difluoride reservoir. Fluidized Bed Reactor. Thefluidizedbed reactor consisted of a tapered aluminum cylinder with a 1.9-cm-diam base opening to a 3.8-cm-diam at the top of the 30.5-cm-long reaction cylinder. The reaction cylinder opened into a 10-cm-diamfilterassembly. The gas entered through a nickel frit at the base of the reactor, mixed with the solid reactant in afluidizedstate and exited after passing through particulatefilters.The optimum circulation rate was about 1 standard liter per minute. The linear gas velocity through the bottom frit was 30 cm/s. The pressure drop across the bed was nominally the weight of the solid reactant per unit cross sectional area. The measured pressure drop across the bottom frit andfluidizedbed was about 60 torr in most of these experiments. The dioxygen difluoride reservoir was located as close as possible to the entrance of thefluidizedbed to minimize the gas travel time. At liquid nitrogen temperature no dioxygen monofluoride radical was present under the conditions used, but was formed in the gas phase after evaporation of dioxygen difluoride. The rate at which dioxygen monofluoride was formed increased with temperature. To maintain a high ratio of dioxygen difluoride to dioxygen monofluoride in thefluidizedbed reactor, the carrier gas stream was precooled by a second liquid nitrogen trap before it entered the dioxygen difluoride reservoir. In addition to retarding the formation of dioxygen monofluoride from dioxygen difluoride (as a consequence of the lower temperature), precooling the gas limited the oxygen pressure in the gas stream. An equal mixture of oxygen and fluorine has a vapor pressure of only about 160 torr at liquid nitrogen temperature. This favorably effects the equilibrium [Eq. (4)]. Chemical Reactants. The dioxygen difluoride was prepared in a separate operation by reacting thermally excitedfluorineatoms with oxygen at a cold interface. Within a year LANL expects to have the capability of producing a kilogram of dioxygen difluoride per day by this method* (T. R. Mills, personal communication, April 1989). The plutonium tetrafluoride used in these experiments was the unreacted portion of the material used for the thermal generation of plutonium hexafluoride for another project. The powder density was about 1.3 g/cc. Particle size ranged from 25 to 125 microns. The plutonium dioxide (not generated by the incineration of contaminated waste) was produced by the calcination of plutonium oxalate precipitate.

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INTRODUCTORY LINE

Extraction of Plutonium from Lean Residues by Room-Temperature

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