US2805985A - Electrodeposition of plutonium - Google Patents

Description

United States .Patet ()1 2,805,985 ELEcrRoDEPosrrIoN or PLUTONIUM Frederick J. Walter, Ames, Iowa, assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application November 29, 1945, Serial No. 631,773

1 Claim. (Cl. 204-15) This invention relates to a process for the separation of certain transuranic elements from dilute solutions thereof. More particularly it relates to a method for effecting such separation by means of electrolytically depositing plutonium from solutions containing that element.

It is known that plutonium can be produced in small quantities by the bombardment of natural uranium with neutrons. 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 that has an atomic weight or mass of 239. Similarly, the terms element 93 or Np refer to the new element known as neptunium having an atomic number of 93.

Natural uranium is composed of three isotopes, namely, U U and U the latter being present in excess of 99 percent of the whole. When U is subjected to the action of neutrons, a fourth isotope, U is produced having a half-life of 23 minutes and undergoes beta decay to Np which decays further by beta radiation with a half-life of 2.3 days to yield plutonium. In addition to the formation of the transuranic elements, neptunium and plutonium, there are simultaneously produced other elements of lower atomic weight known as fission fragments. These fission fragments are composed of two distinct element groups, i. e., a light and heavy element group. The light group contains elements having atomic numbers of between about 35 and 46 while the heavier group is composed of elements of atomic numbers varying between about 51 and 60. The elements of these groups as originally produced are considerably overmassed and under-charged, and hence are highly unstable. By beta radiation, however, they quickly transform themselves into isotopes of these various elements having longer half-lives. The resulting materials are commonly known as fission products.

The various radioactive fission products have halflives ranging from a fraction of a second to thousands of years. Those having very short half-lives may be substantially eliminated by aging the material for a reasonable period before handling. Those with very long half-lives do not have sufficiently intense radiation to endanger personnel protected by moderate shielding. On the other hand, the fission products having half-lives ranging from a few days to a few years have dangerously intense radiations which cannot be eliminated by aging for practical storage periods. These products are chiefly radioactive isotopes of Sr, Y, Zr, Cb, and Ru of the light group and Te, I, Cs, Ba, La, Ce, and Pr of the heavy group.

It may be readily seen that plutonium produced as generally set forth above is contaminated with considerable quantities of uranium and fission products. In fact, the plutonium constitutes only a very minor por tion of the irradiated mass, i. e., less than one percent 2,865,985 Patented Sept. '10, 1957 "ice thereof. In view of such a low concentration of plutonium in the irradiated metal, it becomes apparent that the procedure employed to recover that element must be highly eflicient in order to be at all practicable.

There have been devised a number of procedures for the removal and concentration of plutonium from extremely dilute solutions thereof. In general, such methods involve the formation of various insoluble compounds in said-dilute solutions capable of carrying plutonium in the reduced state. The carrier precipitate and plutonium thus obtained are then dissolved and the plutonium oxidized to P11O2++ in which state of oxidation it is soluble in the presence of said carrier. Under these conditions, the plutonium remains in solution and the fission products are removed when the carrier is added. Thereafter, the dissolved plutonium is reduced to a valence state in which it is carriable by the aforesaid carrier and removed from solution in the form of a carrier precipitate which may again be dissolved and the plutonium purified further if considered necessary or desirable by repeating the above cycle. This procedure, however, is obviously cumbersome and time consuming since it requires a number of such steps in order to effect a substantial removal of fission products and other impurities.

It is an object of this invention to provide simple and efiicient means of separating plutonium from aqueous media particularly from solutions containing impurities such as neptunium, uranium, and fission products.

It is a further object of this invention to provide simple and efiicient means of plating out a very thin adherent layer of plutonium from dilute solutions thereof.

It is another object of the present invention to provide a simple and eificient manner of removing plutonium from solutions Without the use of precipitating agents and without the accompanying difliculties of handling added impurities.

Other objects of this invention will be apparent from the description which follows.

In accordance with the present invention it has been found that recovery of the plutonium may be effected by the electrodeposition of plutonium in a mercury cathode. By proper control of the pH of the solution, the electrodeposition can be carried out from aqueous solutions, thus obviating the necessity of the use of anhydrous organic solvents or fused salt baths which are both expensive and diificult to handle.

It has been found in general that plutonium can be electrodeposited from its dilute aqueous solutions having a pH of about 5, with a potential of about 14 volts and a current density of about 0.44 ampere per square centimeter. The pH is preferably controlled by means of an acetate-sulfate buffer. The mercury cathode may be either a still pool of mercury or it may be a stirred or agitated body of mercury.

The process of the present invention is best understood by reference to the following example:

Example An aqueous solution containing plutonium, buffered to a pH of about 5 with sodium acetate and sulfuric acid, was electrolyzed for 2 hours with a current density of 0.44 amps/cm. at a potential of 14 volts. The cathode was a pool of mercury, and the anode a rotating platinum spiral. During the electrolysis, some grey insoluble sludge appeared in the electrolyte. conclusion the mercury layer was separated and Washed, and the washings were combined with the electrolyte for analysis. After vacuum distillation of most of the mercury, the remainder was dissolved and the mercury removed with hydrogen sulfide. The filtrate was ana- At the lyzed for plutonium activity, as was the electrolyte after filtering off the insoluble grey sludge. The plutonium Was precipitated with hydrofluoric acid using lanthanum fluoride asa carrier. The anode was. washed with.ni.- t'r'ic' acid and this solution was also analyzed for plutonium to detect any deposition'of activity on the anode. The analysis was carried out by measuring the alpha activity of the plutonium with a Geiger-Muller counter.

The results appear below:

Percent of total activity recovered 10 Activity recovered from Hg 1239 Activity recovered from electrolyte 6710 Activity dissolved from anode by 'HNOs 3.9

Activity in insoluble'sludge in electrolyte i 2.6

Activity .recoveredfrom all solutions byadditional' I precipitation' 2.6 Total recovery a 100.0

comprises carrying out the electrolysis at a current density of substantially 0.44 ampere per square centimeter in the presence of acetate-sulfate bufier while maintaining the pH of the solution at substantially 5 and using a stirred mercury cathode.

References Cited'in the file of this patent UNITED STATES PATENTS 1,970,973 Palmaer Aug. 21, 1934 2,323,042 Honsberg June 29, 1943 2,743,228 Boyer Apr. 24, 1956 OTHER REFERENCES.

Principles of Electroplating' and Electroforming, by Wm. Blum and G. B. Hogaboom, 2nd ed., McGra Hill Book Co., N. Y., 1930, pp. 246248.

Journal of Amer. Chem. Soc., vol. 57, pp. 440, 411, March 1935 (an article by A. V. Gro'sse).

Discovery of the Elements, by' Mary Elvira Weeks, pub. by Jour. of Chem. Education, Easton, PaJ, 5th edition, pp. 539, 540.

Uranium and Atomic Power, by Jack DeMent and H. C. Dake, Chem. Pub. Co., Brooklyn, N. Y., 1941, page 108. J l

MDDC'1657, The Electrodeposition of Plutonium by Oscar Cook. Pub. by Atomic Energy Comm, Oak Ridge, Tenn., pp. 1-11, date declassified Feb. 6 1948.