US2994579A - Dissolution of a cerium-type plutoniumcontaining fluoride carrier - Google Patents

Description

2,994,579 DISSOLUTION OF A 'CERIUM-TYPE PLUTONIUM- CONTAINING FLUORIDE CARRIER Arthur C. Wahl, University City, Mo, assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Feb. 27, 1948, Ser. No. 11,860 7 Claims. (Cl. '2314.5)

The present invention is concerned with a method for separating plutonium from impurities normally associated therewith in neutron-irradiated uranium. More particularly it relates to an improvement in the lanthanum fluoride process for the decontamination and concentration of plutonium obtained from neutron-irradiated uranium.

In this specification and claims the name of the element is used to designate the element generically, either in its elemental state or combined in a compound, unless otherwise indicated by the sense in which it is used, or by a specific designation, such as metal or elemental.

Natural uranium is composed of three isotopes, namely, U U and U the latter being present in excess of 99% of the whole. When U is subjected to the action of slow or thermal neutrons a fourth isotope, U is produced, having a half-life of 23 minutes decaying by beta radiation to Np which in turn decays further by beta radiation with a half-life of 2.3 days to yield plutonium. In addition to the formation of P11 there are simultaneously produced other elements of lower atomic weight known as fission fragments. These fission fragments are isotopes of elements having atomic numbers between approximately 32 and 64. These isotopes as originally produced are considerably overmassed and undercharged and hence are highly unstable. By beta and gamma 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 half-lives ranging from a fraction of a second to thousands of years. Those having half-lives which are very short may be substantially eliminated by aging the material for a reasonable period before handling. Those with very long half-lives do not have sufliciently intense radiations 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 Te, I, Cs, Ba, La, Ce, Sr, Zr, and Ch.

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 portion of the irradiated mass, i.e., less than 1% 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 efficient in order to be at all practicable. In addition to the recovery of the plutonium, it is highly desirable to separate the fission products from the plutonium, since these have exceedtained are then dissolved and the plutonium oxidizedto the hexavalent state in which valence state it is soluble in the presence of said carrier. The carrier precipitate is then re-precipitated removing from the solution those fission products which originally may have carried with the plutonium, leaving the soluble hexavalent plutonium in the solution. 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.

One of the most eflicient carriers for triand'tetravalent plutonium known and used in the above type of process is lanthanum fluoride. Other fluorides of the cerium subgroup of rare earths, consisting of the elements of atomic numbers 57 to 62, inclusive may also be used but only the cerium and lanthanum fluorides have been widely used. There is one outstanding disadvantage, however, in the use of these fluorides in the above type of process and that is the extreme difliculty with which these fluorides are brought into solution. The usual procedure has been to fume the plutoniumcontaining lanthanum fluoride with fuming sulfuric acid or perchloric acid. This process, however, is diflicult to carry out. The process causes a great deal of corrosion upon the process vessels and results in a considerable health hazard.

It is an object of this invention to provide a convenient and efficient method of recovering plutonium from impurities commonly associated therewith.

An additional object of the present invention is to provide an effective method of dissolving a plutoniumcontaining carrier precipitate comprising the fluoride of a cerium sub-group rare earth.

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

Broadly, the present invention involves the treatment of a precipitate carrier for plutonium in the trior tetravalent state, said carrier comprising the fluoride of a cerium rare earth element containing plutonium and such other fission products as are carried by said carrier. The carrier precipitate is treated with an aqueous acidic solution in the presence of ferric ions whereby the fluoride carrier precipitate is dissolved. Following the dissolution step the plutonium may be oxidized to the hexavalent state, and the original carrier precipitate re-formed and separated from the solution, thus carrying with it the fluoride-insoluble contaminants. The plutonium may then be reduced to the trior tetravalent states and one or more additional cycles carried out whereby the plutonium is freed of fission products and substantially concentrated.

In its preferred embodiment, the procens of this invention comprises the treatment of a lanthanum fluoride carrier precipitate containing trior tetravalent plutonium produced in the first phase of the lanthanum fluoride plutonium separation process. In the first phase the neutron-irradiated uranium, containing plutonium and fission products, is dissolved in nitric acid and a lanthanum fluoride carrier precipitate is formed in the solution thus produced and is separated therefrom. The plutonium contained in this precipitate is primarily tetravalent but the precipitate thus formed may also contain some trivalent plutonium and those fission products which form insoluble fluorides. This precipitate is then treated with an aqueous acidic solution containing ferric ion. The acid may be any of the common inorganic acids but is preferably nitric, sulfuric, or hydrochloric acid. The normality of the acid in the solution is not critical and a 1' N acid solution has been found to he usually entirely adequate. This may, however, be increased should it be found by Patented Aug. 1, 1961 inspection that the precipitate fails to go into solution readily. The ferric ion should be present in the solution in excess of that stoichiometrically required to react with the fluoride of the mixed lanthanum-plutonium fluoride precipitate added to the solution. The ferric ion may be added as the soluble ferric salt, such as the chloride, sulfate, or nitrate. While the exact mechanism of the dissolving action of the aqueous acidic ferric solution upon the mixed lanthanum-plutonium fluoride precipitate has not been definitely proved, and the inventor does not wish to be bound by any theory advanced, it is believed that the fluoride ion enters into a very stable complex with the ferric ion. There are several chemical combinations possible and the complex ion formed may be any of these. A representative one is FeF In any event, the presence of the ferric ion in the dilute acid solution causes the mixed lanthanum-plutonium fluoride to dissolve easily in the solution. The stoichiometry of the reaction may be calculated upon the basis of the fluoride entering into combination with the ferric ion to form the complex ion mentioned above. Following the dissolution of the plutonium carrier precipitate of lanthanum fluoride, the plutonium may be oxidized to a hexavalent state by any suitable oxidizing agent such as KMnO argentic ion, silver catalyzed S ion, or K Cr O The lanthanum fluoride may then be re-precipitated by increasing the fluoride ion concentration in the solution. If alkali metal ions are present in the solution, for example potassium ions, a K FeF precipitate may be formed in addition to the lanthanum fluoride precipitate. The mixed LaF K FeF precipitate is quite effective in removing from the solution any of the radioactive contaminants which may have been carried with the original lanthanum fluoride carrier precipitate, thus leaving the plutonium in the hexavalent state uncontaminated by radio-active fission products. The hexavalent plutonium may then be reduced to the tetravalent or trivalent state and precipitated again with lanthanum fluoride to complete the cycle. This cycle may be repeated as often as desired to effect a substantial concentration of the plutonium. The advantages of this process are readily apparent. It avoids the use of large quantities of highly corrosive materials to eflect the dissolution of a lanthanum fluoride carrier and the small quantity of solvent required for dissolving the plutonium carrier permits the use of the cycle as a plutonium-concentrating step.

The process of the present invention may be further illustrated by the following specific example.

Example A l N nitric acid solution of 0.5 ml. total volume was prepared. To this solution was added a tetravalent plutonium tracer of 1350a counts per minute, and 0.1 mg. of Ce+ ion. The solution was then made 2 N in hydrofluoric acid and the mixed cerous and plutonium fluorides thus produced were removed from the solution by centrifugation. A second plutonium carrier precipitate was then formed in the supernatant liquid by adding 0.1 mg. La+ ion to the solution. This precipiate was removed from the solution by centrifugation, thus effectively removing all traces of plutonium from the solution. The two precipitates were then combined and washed twice with 0.1 N hydrofluoric acid solution. The combined precipitates were introduced into 0.5 ml. of 1 N H 80 solution containing 0.9 mg. of Fe and digested at 50 C.

for ten minutes whereby the precipitates were completely dissolved. The plutonium was then oxidized to the hexavalent state by introducing 2.5 mg. of K S O and a catalyst of 0.1 mg. Ag+ ion. The solution was heated on the steam bath for thirty minutes, cooled, made 1.5 N in hydrofluoric acid whereby a precipitate comprised of CeF LaF and K FeF was formed. This precipitate was separated from the solution by centrifugation. The hexavalent plutonium which had remained in the supernatant solution was then reduced with H and 02+ ion and a lanthanum fluoride precipitate was formed in the solution as described above. The precipitate was separated from the solution by centrifugation, thus removing the plutonium from the solution and completing the cycle.

It will be apparent to those skilled in the art that various modifications of the present invention exist.

What is claimed is:

l. The method of dissolving a plutonium-containing fluoride precipitate of a cerium sub-group rare earth, which comprises treating said precipitate with an aqueous acidic solution and ferric ion.

2. The process of claim 1 in which the cerium subgroup rare earth fluoride is lanthanum fluoride.

3. The process of claim 2 in which the cerium subgroup rare earth fluoride is cerous fluoride.

4. The method of dissolving the lanthanum fluoride carrier precipitate containing plutonium in a valence state less than +5, which comprises treating said precipitate with an aqueous inorganic acidic solution containing ferric ion.

5. The method of dissolving a lanthanum fluoride precipitate containing plutonium in a valance state less than +5 and a fluoride-insoluble fission product, which comprises treating said precipitate With an aqueous solution approximately 1 N in nitric acid and containing ferric ion in excess of that amount stoichiometrically required to convert the fluoride ion present to a complex ferric fluoride ion.

6. In a process for separating plutonium from uranium and a radioactive fission product contaminant, in which a precipitate of lanthanum fluoride containing plutonium in a valence state less than +5 is obtained, the improvement which comprises treating said precipitate with ferric ion in an aqueous acidic solution whereby said precipitate is dissolved, oxidizing said plutonium to the hexavalent state, treating the solution with excess fluoride ion whereby the lanthanum fluoride is re-precipitated, separating said lanthanum fluoride precipitate containing the associated radioactive fission product contaminant, reducing the hexavalent plutonium contained in the supernatant solution to the tetravalent state and separating said plutonium from the solution with a lanthanum fluoride carrier precipitate.

7. In a process for dissolving a lanthanum fluoride carrier precipitate containing plutonium in a valence state less than +5, the step which comprises contacting said precipitate contained in an aqueous medium with ferric 1011.

References Cited in the file of this patent CN-25ll, US. Atomic Energy Commission document dated Dec. 30, 1944, declassified Feb. 16, 1957; pages 810, 29 and 37.

Claims (1)

1. THE METHOD OF DISSOLVING A PLUTONIUM-CONTAINING FLUORIDE PRECIPITATE OF A CERIUM SUB-GROUP RARE EARTH, WHICH COMPRISES TREATING SAID PRECIPITATE WITH AN AQUEOUS ACIDIC SOLUTION AND FERRIC ION.