US2865704A - Method of separating uranium, plutonium, and fission products by bromination and distillation - Google Patents

Description

2,865,704 METHOD OF SEPARATING URANIUM, PLUTONIUM AND FISSION m 7 n M 1 Y m A F a A J J 0 m m A Dec. 23,1958

PRODUCTS BY BROMINATION AND DISTILLATION 0 f WUKU BQQNR N K mvkamhmkk IN VEN TORS. flrfizar' flJzffey By Glenn T500601 METHOD OF SEPARATKN G URANIUM, PLUTO- NIUM, AND FISSEUN PRODUCTS BY BROMINA- TIUN AND DISTILLATIDN Arthur H. Jatfey, Chicago, TIL, and Glenn T. Seaborg,

Albany, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Application January 8, 1947, Serial N 0. 720,920

7 Ciaims. (Cl. 23-445) This invention relates to the treatment of compositions containing element 94 and more particularly relates to reaction and volatilization methods for separating element 94 and foreign products such as fission products and uranium from each other.

In this specification and claims the name of the element, unless otherwise indicated, designates generically the element either in its free or combined state. The designation plutonium or element 94" as used throughout the present description refers to the transuranic element having an atomic number of 94. The expression 94 means the isotope of element 94 having an atomic weight or mass of 239. Similarly, the terms element'93 or neptunium refer to the transuranic element having an atomic number of 93.

It is now known that the transuranic elements 94 and 93 and also elements of lower atomic weight known as fission products may be produced by the bombardment of uranium metal or uranium compounds with neutrons. Natural uranium contains a large proportion of the isotope U and a smaller proportion (about of 238) of the isotope U The reaction of U with slow neutrons produces the isotope U which undergoes beta decay, with a half-life of 23 minutes, to 93 which in turn decays with a halflife of 2.3 days to 94 g The slow neutrons also react with U to produce nuclear fission and two fragments called fission fragments. These fission fragments are in general highly radioactive, and undergo beta disintegration with gamma radiation into chains of two groups, a light group of elements with atomic numbers from approximately 35 to 44 and a heavy group with atomic numbers from approximately 51 to 58. These elements either alone or combined as compounds are known as fission products.

The half-lives of the various intermediate nuclei of these fission products range from fractions of a second to a year or more with several of the important species having half-lives of the order of a month or so. The dangerous radioactivity of fission products having a very short half-life may be eliminated by suitable aging of a few days. This aging period also serves to permit the transformation of the greater part of the 92 to 93 and of the 93 into 94 The fission products of very long half-lives have a minimum of radioactivity, but the fission fragments of intermediate half-lives, of the order of a month or so, for example: Sr, U (57-day halflife), Zr, Cb, and Ru of the group of atomic numbers from 35 to 44; and Te Te I Xe Ba (12 days half-life), La and Ce of -day and ZOO-day halflives, from the group of atomic numbers from 51 to 58 inclusive, make the mass extremely difiicult to handle without danger of exposure of personnel to gamma radiation,

In the production of plutonium by the bombardment of natural uranium with neutrons produced by a chain reaction in a uranium lattice reactor, the reaction is usually stopped when the concentration of the plutonium is still very small in relation to the U usually less than 1 percent by weight and often less than one or several parts per million parts of U Consequently the percentage of fission products is also very small, for example of the order of 0.02 percent of the mass by weight. The separation of the plutonium from the mass of uranium and from the fission products is therefore a very difiicult process, not only because of the extremely minute concentrations, but also because of the dangerous radioactivity of the fission products present. The concentration and separation of plutonium from the radio active fission products and uranium may be accomplished by the process of this invention.

As noted above, the very short half-life of neptunium (2.3 days) makes the chemical separation of neptunium from plutonium unnecessary since by suitable aging neptunium decays with beta emission to form plutonium. The reaction of the'uranium with neutrons may be carried out at such a slow rate of disintegration that a large proportion of neptunium decays to plutonium during the course of the reaction in the neutronic reactor, or the mass as removed from the neutronic reactor may be stored for a few days after the reaction to increase the 94 present in the mass, and ultimately all of the 93 present will be converted to 94 by radioactive decay.

An object of this invention is to provide a simple and eflicient method of separating plutonium from uranium and radioactive fission products.

A further object of this invention is to provide a method of producing a pure and relatively non-corrosive plutonium halide suitable for use in the bomb reduction method of plutonium metal production.

An additional object of this invention is to provide a method for the selective separation of contaminating elements from plutonium, whereby individual, or groups of, contaminants may be separated from plutonium in a single cycle of operations.

Other objects and advantages of this invention will become apparent as the following detailed description progresses.

This invention is broadly a process for the separation of plutonium from uranium and fission products, which comprises the bromination of a mass consisting of plutonium and contaminants, the separation of contaminants which form more volatile bromides than plutonium bromide by the volatilization of the bromides of these contaminants from the plutonium, and the separation of plutonium from contaminants which form less volatile bromides by the volatilization of plutonium bromide from these less volatile bromides. Thus by the process of this invention plutonium can be separated from uranium and other contaminants and a substantially pure plutonium bromide obtained.

The volatilities of uranium, plutonium and the various fission products and of their bromides have been tabulated in Table I.

Temperatures are given for four vapor pressures, 10"*, 10- 10- and 1 atmospheres.-- Thus T T Tm-a and T are the temperatures in degrees Kelvir'i'at which the partial pressures 10- 10- 10- and l atmospheres respectively are obtained. These pressures are partial pressures of the species listed and are total pressures only when the species listed is the only gaseous species above the condensed phase. f

Estimated values and ones obtained by extrapolation from experimental data are given in parentheses.

Table 1 T Boiling Point at P atmospheres, Kelvln At. No. Element v. T- T10- Tia-2 Ti Bromine 331. 8 Krypton 120. 1 Rubidium- 496 561 650 952 900 l, 010 1, 150 1, 657 n 2, 100 2', 300 2, 700 3, 500 4 2,450 2, 700 3, 000 3, 850 Columbium 3, 340 3, 700 4, 120 5, 400 Molybdenum- 3, 000 3, 330 3, 750 5,077 lvlasuriumr 3,000 3, 300 3,700 5, 000 Ruthenium- 2, 900 3, 180 3, 500 4, 500 Antimony---" 1, 025 1, 160 1,340 1, 890

1ellurlum 758 825' 907 1, 130 Iodine"- 282 308 341 456 165 476 544 634 963 985 1,116 1, 293. 1, 911 Lanthanum 1, 800 2, 000 2, 250 3,000 Cerium 1, 700 850 2,100 2, 800 Uranium 2, 350 2, 580 2, 900 (3; 800) Plut0nlum 1, 950 2, 200 2, 500 3, 508

Bromides (935) 1, 039 1, 176 1, 625 1,300 plus or minus 100 2, 150 (1, 050) I (1,150) I (1,300) (1, 740) (900) plus or minus 200 1, 500) 31115 or minus 200 Disproportionates. 477 518 630 (430) 545 or minus 100 (620) (1, 200) (360) 408) 561 612 (610) (820) (270) 298 389 1, 007" 1, 139 1, 573 olus or minus 100. (2, 100) (1,220 (1,360) (1,850) 1,210 (1, 350 ,sao)

The vapor pressures :Of YBr UBr and PuBr over the range of temperatures and pressures shown .by Table I are graphically portrayedin the sole figure.

In accordance with one embodiment of this invention uranium metal, carbides, sulfides, oxides, peroxides or other compounds of uranium capable of reacting with the bromine or bromine derivatives to form uranium tetrabromide are irradiated with neutrons to obtain neptunium, plutonium and fissionproducts, and the reaction is terminated while a substantial proportion of uranium'remains unconverted.' This mass is then aged toconvert the neptunium to plutonium by nuclear disintegration. The mass is then treated with hydrogen bromide at an elevated temperature of, for example, the boiling point of YBr at the pressure used. As this temperature is approached, the following radioactive fission products are volatiliz'ed from the mass unchanged: bromine, kryptonflodine and xenon. The bromination of the mass at thistemperature and' pressure will'convert the following radioactive fission products to their respective bromides and they will be volatilized as shown inTable I and may be separated I from the mass: RbBr, ZrBr CbBr MoBr RuBr SbBr TeBr TeBr CsBr. and YBr and UBr It will be noted that both zirconium anduranium form several bromides'with varyingvolatilities. Zirconium tetrabromide, however, volatilizes at 630 K. under atmospheric pressure. Similarly uranium terabromide' volatilizes at 1060" K. under atmospheric pressure; By using an excessof hydrogen bromide thehigher bromides: of zirconium and uranium will be formed and will volatilize atthese relatively low temperatures.

Any iodine remainingin the mass will be volatili'zed with the hydrogen bromide-to form IBr, and thus may be separatedfrom the mass.

After the above bromides are substantially removed by volatilization, the temperature may be raised to the boiling point of-PuBr and the'brom'ination continued. In

closest to that of PuBr Y asesnoa 4; this range of temperature PuBr will form and vaporize from the remaining constituents of the mass. The constituents remaining should include BaBr SrBr LaBr CeBr and any radioactive fragments which do not form bromides. It is believed that this would include masurium since no masurium bromides are known and masurium has not been detected in the portions volatile at the above temperatures.

It may be desirable to broaden the PuBr fraction in the first cycle of vaporization so that it will include YBr LaBr and CeBr and this fraction can then be revolatilized with hydrogenbromide, and by suitable adjustment of theitemperatures and pressuresand volatilization through several cycles, the plutonium may be effectively separated from this fraction to yield substantially pure PuBr It is also contemplated that other methods of separation may be used to separate the radioactive fission products which form bromides with boiling points Thus Y, La, and Ce may be separated by adsorption or precipitation methods either before or after the separation of Pu from the other contaminants by the process of this invention.

It will be noted that the separation of plutonium by the process of this invention has certain great advantages over other types of separation. Thusthe radioactive fission products such as Xe, Kr, Br, and I, which volatilize or form bromides with low boiling points, are separated quantitatively by a single volatilization step. Uranium, which 'forms the greatbulk of a neutron-irradiated mass, may be very effectively separated from plutonium since the boiling point o'f'uranium tetrabromide is 725 C. lower than that of plutonium tribromide. And in addition, those radioactive fission products which are heavy gamma ray emitters, such as Zr,.Cb, Cs, and Ba, form bromides with volatilities greatly different from plutonium bromide and so may be easily separated from plutonium by volatilization of the bromides.

Although the process of this invention may be carried out at atmospheric pressure, it is to'be understood that this process may also be carried out at greater or less than atmospheric pressure by suitable adjustment of the temperatures used, as shown in Table I. Desirable changes in the temperatures and pressures involved in the use of brominating agents other than hydrogen bromide are also contemplated.

It will be understood that where a compound of urani um is irradiated which is not converted to the tetrabromide by treatment with hydrogen bromide, uranium and other compounds obtained after irradiation may be converted to compounds which are convertible to bromides. Thus uranium nitrate may beconverted to the oxide by heating and the oxide reacted with hydrogen bromide. Uranium metal, although it may be directly treated with hydrogen bromide, can be heated with oxygen to form U 0 which may be reduced to U0 with hydrogen. This compound can then readily be converted to UBL; by treatment with hydrogen bromide.

It is not necessary that thetreatment with hydrogen bromide be with anhydrous hydrogen bromide at elevated temperatures, although this is preferable since it then removes volatile radioactive fission products in the initial stage of the process. The treatment with hydrogen bromide may be with an aqueous solution of hydrogen bromide to obtain UBL, which may then be volatilized as set forth above.

Although the process of this invention may be used asa complete method for the separation of plutonium from uranium and radioactive fission products, it also maybe conveniently used for the concentration and/or separation of plutonium from uranium or from radioactive fission products. Thus the bromination and volat'ilization of plutonium, uranium and radioactive fission 'product contaminants, either individually or in any suitable combination, may be used as a separate stepin, any other plutonium separation" process.

In accordance with another embodiment of this invention, neutron-irradiated uranium may be treated with bromine to separate plutonium from uranium. Thus when metallic uranium and plutonium are treated with bromine, the reactions are believed to proceed in the following manner:

Pu-l-Br (ca. 400 C.)=PuBr and 2UBr +Br (ca. 300 C.)=2UBr The bromine may be passed over the mass containing uranium and plutonium at a temperature of approximately 300 C. and the uranium tetrabromide which forms may be volatilized away and condensed as a distillate to separate the uranium from the plutonium. This may be repeated as often as necessary to increase the ratio. of separation.

In another embodiment of this invention and one which has been found to be a preferable method for separating plutonium from uranium, the oxides of plutonium and uranium may be reacted with hydrogen bromide to form their respective bromides and these separated by a fractional distillation. The reactions of these oxides are believed to be the following:

An experiment was carried outwith this method in which samples of plutonium and of uranium were mounted on platinum, ignited to redness and then counted. The samples were then placed in a suitable apparatus and treated with hydrogen bromide at various temperatures for 30 minutes, removed and again counted. Numerous runs were made at each temperature; the results are sum- A variation of the bromination of the oxides of uranium and plutonium is a method whereby the oxides are mixed with excess carbon and heated to red heat. A stream of N or other inert gas is bubbled through Br; and passed over the mixture. The resulting plutonium and uranium bromides may then be fractionally separated by condensation. This type of separation may be illustrated by the following reactions:

There are numerous other brominating agents which may be used in the process of this invention as well as bromine and hydrogen bromide which have been described above. For instance, an effective brominating agent has been found to be a sulphur bromine mixture, as shown in the following reactions:

and

The following is an example of this method of bromination it being understood that it is given only for purposes of illustration and that it is not intended to limit the invention to the details given in the experiment.

Bromine vapor at ca. 100 mm. pressure was passed over sulphur at C. The bromine dissolved in the sulphur to form red liquid sulphur bromide. The bromine-sulphur bromide mixture was passed through a quartz reaction tube containing the PuO .xI-I O at 800 C. The flow rate of the bromide was controlled at approximately 5 cc. per minute by a capillary at the outlet end of the system leading to a vacuum apparatus. Over a period of several hours 10 milligrams of Pu as PuO .xI-l O was converted to a green sublimate of PuBr which condensed at the outlet end of the reactor. The free energy of the reaction was calculated to be indicating that the sulphur-bromine mixture is more efiective than HBr as the brominating agent. Phosphorus pentabromide, phospohorus tribromide, aluminum bromide, antimony pentabromide, and antimony tribromide can also be used. The following equations serve to illustrate reactions involving several of these brominating agents:

The end product produced by the process of this in vention, anhydrous plutonium tribromide, is a blue-green crystalline compound having the following properties:

Melting point654 C.:4 Crystalline structure:

orthorhombic Four molecules per unit cell Lattice constantsa =l2.57i0.05 A. a =4.ll:0.03 A. a =9.l3::0.04 A. Calculated density-6.69

Experiments have been carried out to determine if plutonium forms bromides other than that of the trivalent plutonium, butit is believed from the results of these experiments that plutonium tribromide is the only one formed in reactions involving anhydrous reactants.

While there have been described certain embodiments of our invention, it is to be understood that it is capable of many modifications, and changes therefore may be made without departing from the spirit and scope of the invention as described in the appended claims in which it is the intention to claim all novelties and invention as broadly as possible in view of the prior art.

What is claimed is:

1. The method of separating plutonium from uranium and radioactive fission products, which comprises reacting a neutron-irradiated uranium mass with a brominating agent at an elevated temperature to form the bromides of uranium, plutonium and such fission products as form bromides, distilling the uranium bromide, volatile fission products, and volatile fission product bromides from the mass by subjecting the mass to a temperature between 777 and 1467 C. and a pressure substantially equal to the vapor pressure of YBr at said temperature and then distilling the plutonium bromide from the mass by subjecting the mass to a temperature greater than 1512 C. and pressure less than the vapor pressure of PuBr at that temperature, and separately collecting the resulting dis tillate.

2. The process of claim agent is bromine.

3. The process of claim agent is hydrogen bromide.

4. The process of claim agent is AlBr 5. The process of claim 1 wherein the brominating agent is a mixture of sulfur and bromine.

6. The method of separating PuBr; and UBr which 1 wherein the brominating 1 wherein the brominating 1 wherein the brominating comprises distilling the URI- from a in'r'i-xturetof PuBr, and UB'Q by subjeetingi the-mixture total temperature higher than 392 C-.-and: pressurefless th'an the; vapor pressureofUBr; at said temperature, and then distilling the plutonium bromide from the massrby subjectingthe mass to a temperature higher thanrS-IZ" C. and-pressure less than the vapor pressure ofPuBr lat saidwemperature, and separately collecting the distillate.

7. A method off separatingv uraniumand plutonium from neutron-irradiated uranium 'mass,..which comprises treating said mass with hydrogensbromidel at a temperaturaofi approxin-r'ateliya40w e-a anrl-npressure; of: approximately 1t0T -statmosphe1iesi: whereby: UB1; is; ,vaporized from thehmass, selectively cond'ensingzsaid UBrgyapor, heating said neutronvir-radiated masst to approximately 100 I Chundema pressure of 10- atmospheres, and selec. tively condensing the PuBr vapor thus. formed.

References Cited in the file ofthis patent UNITED STAT-ES PATENTS

Claims (2)

1. THE METHOD OF SEPARATING PLUTONIUM FROM URANIUM AND RADIOACTIVE FISSION PRODUCTS, WHICH COMPRISES REACTIN A NEUTRON-IRRADIATED URANIUM MASS WITH A BROMINATING AGENT AT AN ELEVATED TEMPERATURE TO FORM THE BROMIDES OF URANIUM, PLUTONIUM AND SUCH FISSION PRODUCTS AS FORM BROMIDES, DISTILLING THE URANIUM BROMIDE, VOLATILE FISSION PRODUCTS, AND VOLATILE FISSION PRODUCT BROMIDES FROM THE MASS BY SUBJECTING THE MASS TO A TEMPERATURE BETWEEN 777* AND 1467*C. AND A PRESSURE SUBSTANTIALLY EQUAL TO THE VAPOR PRESSURE OF YBR3 AT SAID TEMPERATURE AND THEN DISTILLING THE PLUTONIUM BROMIDE FROM THE MASS BY SUBJECTING THE MASS TO A TEMPERATURE GREATER THAN 1512*C. AND PRESSURE LESS THAN THE VAPOR PRESSURE OF PUBR3 AT THAT TEMPERATURE, AND SEPARATELY COLLECTING THE RESULTING DISTILLATE.

7. A METHOD OF SEPARATING URANIUM AND PLUTONIUM FROM NEUTRON-IRRADIATED URANIUM MASS, WHICH COMPRISES TREATING SAID MASS WITH HYDROGEN BROMIDE AT A TEMPERATURE OF APPROXIMATELY 400*C. AND PRESSURE OF APPROXIMATELY 10-4 ATMOSPHERES WHEREBY UBR4 IS VAPORIZED FROM THE MASS, SELECTIVELY CONDENSING SAID UBR4 VAPOR, HEATING SAID NEUTRON-IRRADIATED MASS TO APPROXIMATELY 700*C. UNDER A PRESSURE OF 10-4 ATMOSPHERES, AND SELECTIVELY CONDENSING THE PUBR3 VAPOR THUS FORMED.

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