United States Patent im [in 3,13,439 Stein [45] Aug. 27, 1974
[54] ATMOSPHERE PURIFICATION OF XENON, 3,377,136 4/1968 Morrow 423/262 RADON & RADON DAUGHTER ELEMENTS 3,778,499 12/1973 Stein 423/210 3,784,674 1/1974 Stein 423/210 X [75] Inventor: Lawrence Stein, Downers Grove, 111. [73] Assignee: The United States of America as Primary Examiner—Earl C. Thomas represented by the United States Attorney, Agent, or Firm—John A. Horan; Arthur A. Atomic Energy Commission, Churm; James W. Weinberger Washington, D.C.
[22] Filed: Mar. 27, 1973 [57] ABSTRACT [21] Appl. No.: 345,419 A method of purifying an atmosphere of xenon, radon and radon daughter elements by passing the atmo- [52] U.S. CI 423/210, 423/249, 423/262, sphere containing these elements through a reaction 423/472, 176/37, 252/301.1 R bed of dioxygenyl hexafluoroantimonate, which oxi- [51] Int. CI BOld 53/34 dizes the xenon, radon and radon daughter elements [58] Field of Search 423/210, 262, 472, 249; to their respective fluorides which remain in the reac- tion bed and are thus removed from the atmosphere, 176/37; 252/301.1 R, 301.1 W and recirculating the purified atmosphere. The [56] References Cited method is also useful for separating the before-named elements from krypton. UNITED STATES PATENTS 3,185,548 5/1965 Fields et al 423/262 9 Claims, No Drawings 3,829,551 1 2 ATMOSPHERE PURIFICATION OF XENON, Radioactive noble gases are also found in the atmo- RADON & RADON DAUGHTER ELEMENTS spheres of nuclear reactor power facilities and nuclear reactor fuel reprocessing facilities. These gases gener- alI rnwTDAPTHAi nDirtK, ncTUc iw„cMTinM y consist of a number of isotopeP s of krypton and CONTRACTUAL ORIGIN OF THE INVENTION $ xe^on and range in half.,ife from sec0nds to years. The invention described herein was made in the Many of the krypton and xenon isotopes which have course of, or under, a contract with the UNITED short half-lives have radioactive daughter elements STATES ATOMIC ENERGY COMMISSION. having relatively long half-lives. In a boiling water reac- tor, the fission gases which have entered the water in BACKGROUND OF THE INVENTION 10 the reactor, either by diffusion through the fuel clad- This invention relates to a method for purifying an ding or through breaks in the cladding, are released at atmosphere of xenon, radon and radon daughter ele- the exit of the turbine condenser. These gases are nor- ments which may be contained therein and for the sep- mally discharged to the atmosphere after a short delay aration of these elements from krypton. time. In the pressurized water reactor, the gases are re- Radon is a heavy, radioactive, gaseous element 15 moved in the coolant loop and are stored for periods up formed by the alpha disintegration of radium. The most to months to eliminate the short-lived gases by radioac- common isotope, 222Rn, is an alpha emitter with a half- tive decay. In each case, the quantity of fission gas life of 3.8 days. Radon is inherently associated with ra- eventually released to the atmosphere depends upon dium in uranium ores and, with its short-lived radioac- the condition of the reactor fuel, and emissions could tive daughters, 2l8Po, 214Pb, 214Bi, and 214Po, constitutes 20 be increased greatly in the event of an accident such as a potential health hazard in uranium mining and ore- a fuel meltdown. Emissions from liquid-metal-cooled handling operations. The daughter elements are solids reactors, breeder reactors, and high-temperature-gas- and tend to be retained in the lungs, where they may cooled reactors will arise from the same general source ultimately cause cancer. but will vary as to nature and quantity from those ema- Forced ventilation is generally used to lower concen- 25 nating from light-water reactors, trations of the radioactive elements in the atmosphere Industrial sources of non-fission-product krypton and of a uranium mine, but it is often difficult to lower the xenon are liquid-air plants. Air contains only 1.14 ppm concentrations adequately in all parts of a mine by this krypton and 0.087 ppm xenon. The cost of obtaining method, since the gaseous radon diffuses continuously noble gases from liquidair plants is therefore high and from exposed veins of uranium ore and from piles of 30 the supply, particularly of xenon, is limited. Various the broken ore. methods have been developed for the treatment of Several methods have been developed for the re- these gaseous reactor wastes and the recovery of the moval of radon and its daughter elements from the at- fission product noble gases. Fixed adsorption beds have mosphere by contacting the atmosphere with various been used in a number of applications for the removal chemicals capable of reacting with the elements to re- ^5 Qf radioactive noble gases from dilute gas streams at move them from the atmosphere. room temperature. These adsorption beds may be ei- One of these methods uses a fluorinating solution to ther charcoal or molecular sieve. The advantages of the oxidize the radon and daughter elements to fluoride room temperature adsorption process are that it is sim- compounds which then dissolve in the solution. How- pie to operate and will accept very dilute feed material, ever, liquid fluorinating solutions pose several prob- 40 Disadvantages are the large volume of adsorbent re- lems. The liquids are highly corrosive and, in some in- quired and the potential fire hazard with charcoal. A stances, have high vapor pressures. Corrosionresistant low-temperature charcoal process can also be used for equipment is therefore required for contacting large noble gas recovery. It has the disadvantages of high op- volumes of air with the liquid phase. Additional equip- erating cost, requires pretreatment of the inlet gas to ment is also necessary to further purify the atmosphere 45 beds, and also requires extensive liquid nitrogen pro- (i.e., to remove any hazardous vapors picked up from duction capabilities and a steady flow and constant the fluorinating solution) before the atmosphere is re- composition of incoming feed gas. Low-temperature circulated. In another method, the atmosphere contain- adsorption beds are similar to room-temperature beds ing the radon and radon daughter elements is passed but are much smaller in size, since the noble gas capac- through a solid reaction bed of a fluorinating com- ity of adsorbents increases markedly as the bed temper- pound, as disclosed in applicant's copending applica- ature approaches liquid nitrogen temperature. Halo- tion Ser. No. 179,229 (70), filed Sept. 9, 1971. In this carbon solvents such as Freon have also been used for method, the elements are oxidized by the fluorinating selective adsorption of krypton and xenon. This compound to their respective fluorides and remain in 55 method utilizes the high solubility of krypton and the reaction bed; they are thus removed from the atmo- xenon in the solvent. In a continuous adsorption pro- sphere, which may then be recirculated. The fluorinat- cess, krypton and xenon are preferentially removed ing compounds disclosed therein are complex fluorides from an incoming gas stream by selective dissolution in formed by reaction of halogen fluorides and metal fluo- a Freon solvent at relatively low temperature and high rides such as ClF2SbF6, BrF2SbF6, BrF4SbFu, IF4 SbF6, 6Q pressure. and Br2BiF6. One problem with the use of the solid re- The recovery of xenon and krypton by treating gase- action compounds is that they release halogen fluoride ous radioactive wastes with fluorine is described in an vapors as reduction products in their reactions with ra- article entitled "Recovery of Xenon and Krypton in the don. These must be removed from the radon-free atmo- Treatment of Gaseous Radioactive Wastes" by J. Sliv- sphere before it can be recirculated. This removal re- 65 nik, Proceedings of Symposium on Treatment of Air- quires additional equipment and adds to the over-all borne Radioactive Wastes, Int. At. Energ. Agency, Vi- cost of radon and radon daughter element removal enna, 1968, pp. 315-321. In this study, an attempt was from the atmosphere. made to react xenon and krypton with fluorine at high 3,829,551 3 4 pressures and elevated temperatures. It was found that, thesis of Dioxygenyl Salts," Shamir and Binenboym, while the xenon would react with the fluorine, krypton Inorganica Chimica Acta, 2, 37, March 1968. The prod- would not, which permitted the separation of xenon uct obtained by photochemical reaction of oxygen, flu- from the krypton. However, fluorine is not a conve- orine and antimony pentafluoride is a white, crystalline nient reagent for this purpose, as it must be heated with 5 powder. the process gas and the excess fluorine must be re- The method of this invention is operable at tempera-
moved afterwards. tures from about 0°C. to about 125°C., above which
SUMMARY OF THE INVENTION temperature 02SbF„ begins to decompose. At these temperatures, the xenon, radon and radon daughter el- I have found a method for purifying an atmosphere 10 ements will react immediately with the hexafluoroanti- of xenon, radon and radon daughter elements by pass- monate compound. No reaction of krypton at 485 to ing the atmosphere containing these elements through 70Q mm pressure was observed from 23° to 150°C. a reaction bed of 02SbF6, whereby the xenon radon ,n the method of thjs invention> the atmosphere to be and radon daughter elements are oxidized to their re- fied .g d th h a reaction bed Qf Q SbFs in spective fluorides, which remain on the bed and are ° , , , .£. , . , n , , . , . powder form, so that any xenon, radon and radon thereby separated from the atmosphere, and recirculat- ,, . ' ^ . ' , ... „ , ,, in the urified atmos here daughter elements in the atmosphere will contact the mTheemethdd of Suction is also useful for the bed and wil1 be oxidized thereby to their respective flu- separation of xenon from krypton by passing an atmo- orme compounds which will remain on the bed and sphere containing these elements through a reaction 20 thus be removed from the atmosphere. If .t is desired to bed of 02SbF6, whereby the xenon is oxidized to separate the xenon from krypton, the atmosphere XeFSb2Fu and remains on the reaction bed while the containing these elements is passed through the bed, krypton passes through the bed. whereby the xenon will be oxidized to its respective flu- A distinct advantage in the use of the method of this orine compound and remain on the bed and the atmo- invention over the prior art methods for purification of 25 sphere containing the krypton, now purified of xenon, the atmosphere is that the compound of this invention will pass through the bed. has a sufficiently low vapor pressure that further purifi- The oxidized xenon and radon can be readily recov- cation of the atmosphere is not necessary before it can ered from the reaction bed and separated from the be recirculated. In addition, the reaction of the noble radon daughter elements by hydrolyzing the reaction gas with the compound results in the release of oxygen 30 bed containing the noble gas compounds. The reaction rather than halogen fluoride, thus again eliminating the bed will thus be reduced and dissolved, as will the necessity for additional atmospheric purification prior radon daughter element fluorides, while the xenon and to its being recirculated. radon will be released as gases and may be easily col- It is one object of this invention to provide an im- lected. proved method for purifying an atmosphere of radon 35 if the atmosphere from which the xenon, radon and and radon daughter elements. radon daughter elements are to be removed contains It is still another object of this invention to provide any moisture, it will be necessary to dry the atmosphere a method for purifying an atmosphere of xenon, radon before it is contacted with the reaction bed, since water and radon daughter elements. vapor wjn decompose the dioxygenyl hexafluoroanti- It is a further object of this invention to provide an monate and release the radioactive noble gases. This improved method of purifying an atmosphere of xenon, drying step may be accomplished by passing the humid radon and radon daughter elements by passing the at- atmosphere through a desiccant such as anhydrous cal- mosphere through a reaction bed of a material having dum sulfatCi magnesium perchlorate or silica gel, or by a low vapor pressure, thereby eliminating the necessity , the air th h refrigeration coils to remove the for additional atmospheric purification water by condensation. It is still another object of this invention to provide n ' . . . ,. , , .. . ., , r c • . . c j Raman spectral studies have shown that the xenon a method of purifying an atmosphere of xenon, radon , „ . f, t-+0. .. and radon daughter elements whereby the reaction Product l\XfSbf" ' a,1:2 ™ dlfluonde - anti- mo com le xenon ,s add d rproduct of thes! elements with the oxidizer is oxygen.
_ , „„ _ . n achieved by shaking equimolar mixtures of the gases Rn(bg+ 202 SbF6 (,) - RnF+Sb2Fn (,) + 202(g). wkh q^ powde= in>yrex buIbs containing stirring The following examples are given as illustrative of the vanes. The mixtures were shaken intermittently, by invention and are not to be taken as limiting the scope hand, at room temperature with excess amounts of the of the invention as defined by the claims. 5 powder over periods of 3-24 hours. Residual gases EXAMPLE I were then analyzed mass spectrometrically. The final . . , . , ^ ^ gas mixtures were found to contain less than 2 percent Flow experiments were carried out with 02SbF6 pow- xenon der and samples of air (0.33-0.761 at standard temper- ' , . „ , „, . . r v , . , , ,, . ,, / It can be seen that the method ocf the invention is ature and pressure) which had been artificially contam- io , . ;...... ~ , , - , , inated with 222Rn and >33Xe. In each experiment, a ra- h,8hly effectlve for the removal of xenon' radon and don-air mixture or a xenon-air mixture was passed radon daughter elements from an atmosphere contami- through a glass U-tube packed with the powder, then nated Wlth these radioactive noble gases and for the through a trap cooled with liquid nitrogen to condense separation of these gases from krypton. any unreacted radioisotope. The distribution of the ra- 15 h is to be understood that the invention is not to be dioisotope was afterwards determined by measuring limited by the details given herein but that it may be the y-emission of the U-tube and the cold trap. (The modified within the scope of the appended claims. distribution of l33Xe was determined immediately; the The embodiments of the invention in which an exclu- distribution of 222Rn was determined after 3 hours, sive property or privilege is claimed are defined as fol- when 222Rn and its -/-emitting daughters 214Pb and 214Bi 20 lows: were known to be in radioactive equilibrium.) In three 1. A method for purifying an atmosphere of xenon, experiments with the radon isotope and with a bed of radon and radon daughter elements comprising: pass- powder 5.0 cm long and 6.3 mm in diameter, all of the ing the atmosphere containing the xenon, radon and radon was absorbed. In five experiments with the xenon radon daughter elements through a reaction bed of isotope and with a bed of powder 6 5 cm long and 5 5 25 o2SbF6 whereby the xenon, radon and radon daughter mm in diameter 67-100 percent of the xenon was ab- elements are oxidized to their respective fluorides sorbed. The results are given in the table below...... , , T , ^ , " which remain on the bed and are thereby separated from the atmosphere, and recirculating the purified at-
,0 mosphere. REMOVAL OF RADON AND XENON FROM JU 2. The method of claim 1 wherein the temperature is AIR WITH OjSbFe AT 23-25 Cone, in Average Amount of trom 0 to about 125 C. Radio- Air Flow Rate Radioisotope 3. The method of claim 2 comprising the additional Isotope (mci/1) (ml/min) Removed (%) r 0 step of drying the atmosphere containing the xenon, 222Rn 13 12 100 35 radon and radon daughter elements before passing the do. 15 15 100 do 24 12 100 atmosphere through the reaction bed. 4. A method of separating xenon from krypton com- mXe 9.8 15 67* do. 7.9 13 100 prising: passing an atmosphere containing xenon and do 2.0 14 100 krypton through a reaction bed of 02SbF6 whereby the do 3.2 13 98 40 do 4.3 14 100 xenon is oxidized to XeFSb2Fu and remains on the bed while the krypton passes through the bed, contacting •Flow rate poorly controlled at start of experiment. the bed with water, thereby hydrolyzing the bed and re- leasing the xenon and collecting the xenon. EXAMPLE II ^^ 5. The method of claim 4 wherein the temperature is Equimolar mixtures of krypton and xenon were from 0 to about 125 C. partly separated on transit through U-tubes packed 6. The method of claim 5 comprising the additional with 02SbF6. The mixtures were passed through beds of step of drying the atmosphere containing the xenon and the compound 5.8-6.0 cm long and 5.5 mm in diameter krypton before passing the atmosphere through the re- at 23°-25° and at total pressures of 20-160 mm. Gases 50 action bed. emerging from the beds were collected and analyzed 7. In the method of purifying an atmosphere of radon mass spectrometrically. The first samples had the fol- and radon daughter elements by passing the atmo- lowing range of composition: 45.5-49.6 percent 02, sphere containing said elements through a reaction bed 12.7-16.2 percent Xe, and 37.7-38.3 percent Kr. The of a fluorinating compound whereby the radon and percentages of oxygen fell and the percentages ofkryp- 55 radon daughter elements are oxidized to their respec- ton and xenon rose in succeeding gas samples as the tive f,uorides and remain on the bed and are thereb Q2SbF6 was depleted. (The color change of the powder, ted from the atmosphere, and recirculating the from white o yellow could be used to indicate the ified at here, the improvement wherein the flu- amount depleted, as there was a sharp interface be- r . r . , _ „ na n tween colored zones.) Each time that gas was admitted, 60 °" " 8 compound has the formula 02SbF6. 8 Th e method of c,aim 7 wherem the a blue-green fluorescence was noted in the G2SbF6. The " 0 temperature is source of this emission is not known at present, but it "om ® to a^out C. is probable that an excited xenon, oxygen, or ozone 9- The method of claim 8 comprising the additional species is formed in the reaction. step of drying the atmosphere containing the radon and 65 radon daughter elements before passing the atmo- EXAMPLE III sphere through the reaction bed. More complete separation of krypton and xenon was * * * * *