United States Patent Office Patented Sept
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2,903,335 United States Patent Office Patented Sept. 8, 1959 2 In the customary production of plutonium by the bom 2,903,335 bardment of natural uranium with neutrons by a chain re SEPARATION OF FISSION PRODUCTS FROM action in a uranium lattice reactor, the reaction is usually PLUTONUM BY PRECIPITATION stopped when the concentration of the plutonium is still very small in relation to the U238, usually less than 1 Glenn T. Seaborg, Albany, Stanley G. Thompson, Richa percent by weight and often less than one or several parts mond, and Norman R. Davidson, Sierra Madre, Calif., per million parts of U238. Consequently the percentage assignors to the United States of America as repre of fission products is also very small, for example of the sentedSO by the United States Atomic Energy Commis order of 0.02 percent of the mass by weight. The sep O aration of the plutonium from the mass of uranium and No Drawing. Application December 4, 1946 from the fission products is therefore a very difficult Serial No. 713,906 process, not only because of the extremely minute con centrations, but also because of the dangerous radio 8 Claims. (C. 23-14.5) activity of the fission products present. 15 The separation of plutonium has been the subject of This invention relates to a method for the decontamina much investigation and as a result of such research, four tion of plutonium solutions. More specifically it is general types of chemical separation methods have been concerned with a method for the separation of plutonium developed: recovery based on different volatilities of from radioactive uranium fission products present in a plutonium compounds and corresponding fission product solution containing said products and plutonium. 20 compounds, absorption, solvent extraction, and carrier The designation "plutonium' or "element 94' as used precipitation methods. While some degree of success has throughout the present description refers to the trans been attained with each method, the precipitation method uranic element having an atomic number of 94. The ex has been found to be preferable. One embodiment of this pression "94289' means the isotope of element 94 having method is the procedure employing bismuth phosphate as an atomic weight or mass of 239. Similarly, the terms 25 a carrier for plutonium which depends on the fact that "element 93” or "neptunium' refer to the transuranic plutonium in a valence state of four forms an insoluble element having an atomic number of 93. phosphate and is carried down with the bismuth phos It is now known that the transuranic elements 94 and phate, but in a valence state greater than four is soluble 93 and also elements of lower atomic weight known as in a solution containing the phosphate ion. This process fission products may be produced by the bombardment 30 of uranium with neutrons. Natural uranium contains a is described and claimed in co-pending application U.S. large proportion of the isotope U238 and a smaller pro Serial No. 519,714, filed January 26, 1944, by Stanley G. portion (about A39 of 238) of the isotope U935. The Thompson and Glenn T. Seaborg granted as U.S. Patent reaction of U238 with slow neutrons produces the isotope No. 2,785,951 on March 19, 1957. U239 which undergoes beta decay with a half-life of 23 35 In accordance with the procedure therein described, minutes to 93289 which in turn decays with a half-life of neutron-irradiated uranium is dissolved in nitric acid or 2.3 days to 94239. preferably a mixture of nitric and sulfuric acids. The The slow neutrons also react with U235 to produce acidic solution thus formed contains uranium ions in auclear fission and two fragments called fission fragments. the hexavalent state, plutonium ions in the tetravalent These fission fragments are in general highly radioactive 40 state and the various fission products ions. If the plu and undergo beta disintegration, with gamma radiation, tonium is present in sufficient concentration, it may be into chains of two groups, a light group of elements with precipitated directly from solution as the phosphate. atomic numbers from 35 to 46 and a heavy group with Usually however the concentration of plutonium is so atomic numbers from 51 to 60, These elements either low that a precipitate of a plutonium compound will not alone or combined as compounds are known as fission form by itself and it is necessary to employ an auxiliary products. 45 insoluble "carrier' such as bismuth phosphate to effect removal of the plutonium from the solution. This pre The half-lives of the various intermediate nuclei of cipitation is called the product precipitation. The carrier these fission products range from fractions of a second to is believed to act either by the incorporation of plutonium a year or more with several of the important species hav ions into the carrier crystal lattice, by surface adsorption ing half-lives of the order of a month or so. The dan 50 of plutonium ions, or by a combination of both. Cer gerous radioactivity of fission products having a very tain of the uranium fission products ions in the solution, short half-life may be eliminated by suitable aging of a chiefly the zirconium and columbium, are isomorphic few days. This aging period also serves to permit the with the plutonium ions and will be precipitated or transformation of the greater part of the 92239 to 93339, carried out of solution with the plutonium. The uranium and of the 93289 into 94239. The fission products of very 55 ions and the bulk of the fission products ions are quite long half-lives have a minimum of radioactivity, but the soluble under the conditions employed and remain in fission fragments of intermediate half-lives, of the order solution. The phosphate precipitate containing the plu of a month or so, for example: Sr, Y (57 day half-life), tonium and isomorphic radioactive fission products is Zr, Cb, and Ru of the group of atomic numbers from 35 dissolved in nitric or other suitable acid and the plu to 45; and Tel 27, Te129, I131, Xel33, Ba (12 days half-life), 60 tonium ions are oxidized to the hexavalent state with Lalao, and Ce of 20 day and 200 day half-lives, from the sodium bismuthate, sodium dichromate, or other suitable group of atomic numbers from 51 to 60 inclusive, make oxidizing agents. Thereafter the fission products are pre the mass extremely difficult to handle without danger of cipitated either directly or by carrier technique with exposure of personnel to gamma radiation. A prime ob bismuth phosphate and the hexavalent plutonium ions ject of the decontamination of plutonium is to remove remain in solution. This precipitation step is called the these radioactive fission products so that the plutonium 65 by-product precipitation. The hexavalent plutonium ions may be handled without the massive shielding necessary in solution may then be reduced with hydrogen peroxide, to protect operating personnel from radioactivity. Con ferrous ammonium sulfate, or like reducing agents to the sequently it has been found convenient to measure the tetravalent state and the process repeated. extent of decontamination of plutonium by measuring the While such a procedure functions to separate plutonium amount of radioactivity present after the separation has 70 from impurities, the very low concentrations of plu been effected. tonium and fission products and large amount of solu 2,903,385 3 4 tions used make a quantitative separation by this method and greatly increasing the overall efficiency of the separa very difficult and require that the oxidation-reduction tion process with which it is used. cycle be repeated until the yield is satisfactory and the Although single scavenging agents have been used in radioactivity reduced so that the product may be handled some separation processes and do constitute an improve without excessive shielding. The disadvantages of this ment over the normal separation process, it has been procedure are especially noticeable in connection with found that by the use of the process of this invention the the separation of plutonium from the phosphate-insoluble decontamination factor is increased by 1.5 for beta radia radioactive fission products, such as zirconium and co tion and 2-5 for gamma radiation over that obtained lumbium, since these elements tend to be closely asso with a single scavenging agent. ciated with plutonium and are strong gamma emitters. O As used with the bismuth phosphate separation method, It is, therefore, apparent that any method whereby the the process of this invention comprises the addition, to removal of the aforementioned radioactive fission prod the solution containing plutonium ions in a hexavalent ucts could be substantially increased in one cycle would state and radioactive fission products contaminants, of constitute a much desired improvement over the present zirconium and cerium compounds which are soluble in existing methods. 15 the acidic solution. The bismuth phosphate precipitate is A principal object of this invention is to provide an then formed in the solution in the usual manner, and improvement on plutonium separation processes em since both cerium and zirconium phosphates are in ploying a by-product carrier precipitation step, whereby soluble, the two types of cations will form precipitates the separation of radioactive fission product contaminants, with the phosphate ions present and may be separated in said by-product carrier precipitation step, can be sub 20 from the solution with the main by-product carrier stantially increased in a single decontamination cycle. precipitate of bismuth phosphate. It is believed that In accordance with the present invention it has been the greatly increased factor of decontamination obtained discovered that this object may be achieved by the use by the use of the process of this invention is due to a of a scavenger precipitate of mixed cerium and zirconium combination of several factors, including the surface compounds.