US2991150A - Purification of plutonium using a cerium precipitate as a carrier for fission products - Google Patents

Description

2,991,150 PURIFICATION OF PLUTONIUM USING A E- RIUM PRECWITATE AS A CARRIER FOR FIS- SION PRODUCTS Burt F. Fan's, Richland, Wash., and Carl M. Olson,

Newark, Dei., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Sept. 5, 1946, Ser. No. 695,048 1 Claim. (Cl. 23-145) This invention relates to a procedure for separating plutonium from extraneous matter such as substances of the kind present in neutron irradiated uranium as exemplified by uranium and especially fission products. More particularly this invention concerns a separation and concentration procedure involving the use of scavengers whereby improved decontamination may be accomplished,

As described herein, the isotope of element 94 having a mass of 239 is referred to as 94 and is also called plutonium, symbol Pu. In addition, the isotope of element 93 having a mass of 239 is referred to as 93 Reference herein to any of the elements is to be understood as denoting the element generically, whether in its free state or in the form of a compound, unless indicated otherwise by the context.

Elements 93 and 94 may be obtained from uranium by various processes which do not form a part of the present invention including irradiation of uranium with neutrons from any suitable neutron source, but preferably the neutrons used are obtained from a chain reaction of neutrons with uranium.

Naturally occurring uranium contains a major portion of U a minor portion of U and small amounts of other substances such as UX and UX When a mass of such uranium is subjected to neutron irradiation, particularly with neutrons of resonance or thermal energies, U by capture of a neutron becomes U which has a half life of about 23 minutes and by beta decay becames 93 The 93 has a half life of about 2.3 days and by beta decay becomes 94 Thus, neutron irradiated uranium contains both 93 and 94 but by storing such irradiated uranium for a suitable period of time, the 93 is converted almost entirely to 94 In addition to the above-mentioned reaction, the reaction of neutrons with fissionable nuclei such as the nucleus of U results in the production of a large number of radioactive fission products. As it is undesirable to produce a large concentration of these fission products which must, in view of their high radioactivity, be separated from the 94 and further as the weight of radioactive fission products present in neutron irradiated uranium is proportional to the amount of 93 and 94 formed therein, it is preferable to discontinue the irradiation of the uranium by neutrons when the combined amount of 93 and 94 is equal to approximately 0.02 percent by weight of the uranium mass. At this concentration of these substances, the concentration of fission elements which must be removed is approximately the same percentage.

A number of processes have already been proposed for accomplishing the separation and concentration of Pu. Certain of these processes are generically known as the bismuth phosphate type process and the wet fluoride type of process. These processes are the invention of others and the details of the processes are described in copending applications as for example application Ser. No. 519,714, filed January 26, 1944, now US. Patent 2,785,951, issued March 19, 1957, to be referred to hereinafter, which gives details relative to such processes. Consequently, all of the details of operation of the aforementioned processes are not described herein.

As indicated above, the materials containing Pu also hired rates Patent contain a substantial content of radioactive materials as exemplified by fission products.

However, it has been noted that, for example, while processes as exemplified by the bismuth phosphate process when first employed are particularly effective in eliminating a large amount of activity, that repetition thereof will not completely remove all of the activity. This presumably may be due to the fact that certain activities are not sensitive to, do not have an afiinity for, or otherwise are not affected by the bismuth phosphate carrier. Hence, repetition of cycles using the bismuth phosphate carrier does not entirely eliminate these specific activities. The same condition apparently also prevails with other carriers which in their initial use will carry substantial amounts of activity but which will not entirely eliminate the activity.

We have found that there are certain additions which may be employed in conjunction with by-product carrier precipitate formation for improving decontamination. That is, these added reagents not only increase the amount of fission product and the like activity carried down but apparently also function by carrying down certain specific activities whichhave heretofore presented difficulties of removal. The use of these materials is referred to herein as scavenger additions.

The meaning of the terms decontamination, scavenging, scavengers, product precipitate, by-product precipitate, bismuth phosphate process, extraction and other similar terms will be further apparent as the description proceeds.

Thisinvention has for one object to provide improvements in processes for the separation and recovery of Pu.

Still another object is to provide a method of treating materials containing P-u whereby better decontamination may be accomplished.

Another object is to provide a process for the separation and recovery of Pu which includes scavenging.

Still another object is to provide a procedure in the separation and recovery of Pu whereby specific activities may be removed.

Still another object is to provide a scavenging method which may be used in conjunction with known processes for extracting and decontaminating materials containing Pu.

Another object is to provide a scavenging process which may be carried out in existing equipment without change along with known processes or with a minimum of change.

Another object is to provide a method of decontamination involving the use of cerium compounds.

Other objects Will appear hereinafter.

We have found that by supplementing by-product decontamination steps by means of cerium compounds as scavengers decontamination may be improved. In particular we have found that by means of the use of cerium scavengers in accordance with the present invention troublesome gamma activity present along with the Pu may to a large extent be eliminated. The cerium scavengers of the present invention may be added in various forms and ways as will be described in detail hereinafter.

The types of materials containing Pu which may be treated are any of the usual mixtures or solutions thereof encountered generally. For example, such materials are described in application Ser. No. 519,714 aforementioned. These materialswould, for example, comprise an inorganic acid solution containing the Pu, the radioactive materials Which it is desired to eliminate, and various other constituents. In general, the initial materials wonld have been subjected to conventional processing comprising extraction and concentration for the elimination of some of the extraneous substances. That is, a standard product precipitation would have been applied and the carrier containing Pu together with that radioactivity which had also been carried along would be redissolved and subjected to a conventional by-product precipitation for elimination of radioactivity. The scavenging of the present invention would usually be applied in conjunction with such a by-product precipitation for increasing the decontamination obtainable thereby.

An illustration of the types of carrier precipitates involved and related subject matter are described in application Serial No. 519,714, filed January 26, 1944, Thompson and Seaborg, and reference is made to that application for further disclosure, details thereof being omitted from the present disclosure except where necessary to an understanding of the present invention. As set forth in said application, it has been discovered that plutonium has more than one oxidation state, including a lower oxidation state or states referred to herein as Pu in which the element is characterized by forming insoluble phosphates and fluorides and a higher oxidation state or states referred to as Pu in which the element forms soluble phosphates and fluorides. As set forth in said application S.N. 519,714, plutonium in said lower oxidation state is in a valence state not greater than 4, and plutonium in said higher oxidation state is in an oxidation state greater than 4. When the Pu is in this higher oxidation state, by adding various ingredients to the liquid containing Pu a carrier precipitate may be formed This carrier precipitate, termed a by-product precipitate, will carry down fission products and the like extraneous matter leaving the Pu in solution in the liquid. The details respecting forming such by-product precipitates are not a part of the present application nor a limitation thereon and are disclosed in said application Ser. No. 519,714 and other copending applications.

For example, a standard bismuth phosphate by-product precipitation (i.e., as described in more detail in S.N. 519,714, Example III, to an acidic, aqueous solution containing the plutonium values maintained in Pu oxidation state, an excess of phosphoric acid is added, which in the presence of dissolved bismuth ions, throws down a bismuth phosphate precipitate, carrier precipitating therewith contaminating fission products and like extraneous matter, leaving the Pu in the supernatant liquid), alone may eliminate one half or more of the gamma activity dependent on the initial intensity and related factors. On the other hand, a comparable byproduct precipitation cycle conducted in the same manner but supplemented by a cerium compound as a scavenger and otherwise carried out in accordance with the present invention may permit the elimination of threefourths to substantially all of the gamma activity. In addition, by means of the scavenger action the substantially complete elimination of certain activity as for example that due to certain rare earth contaminants which are carried along with the Pu may be accomplished.

A still further understanding of our invention may be had by a consideration of the examples which follow. We have found that by supplementing the action of a by-product precipitation as for example BiPO, precipitation under oxidizing conditions, 'with cerium compounds as scavengers that decontamination may be improved. The scavenger may be added in various forms and ways as in the form of nitrates or oxides, or the scavenger may be made up as a preformed precipitate and added as a slurry.

Example I In accordance with this example, a BiPO by-product step as aforementioned was carried out. After the BiPO precipitate had formed, as for example about a half hour later, the scavenger material was added causing the forma tion of the scavenger precipitate. Both the BiPO and scavenger precipitates were separated, together by cel trifuging. However, filtering or other suitable separatory Example II In accordance with this example simultaneous precipitation was carried out as follows: The materials treated comprised a 10 N nitric acid solution containing Pu in the presence of fission products. The solution had been subjected to standard preliminary concentration cycles but still contained gamma activity. The solution was diluted to 5 N in accordance with usual practice. There was then added NaBiO plus a small amount of K cr O as oxidizing agent in accordance with usual practice. In addition, however, there was added Ce(NO Upon addition of phosphoric acid and dilution, all of the above-mentioned components precipitated. That is, the phosphoric acid was increased to .1 M and the nitric acid diluted to one normal. The precipitate was centrifuged and the centrifugate retained, together with the washings from the precipitate, as the centrifugate liquid contained the Pu in (0) condition. Thereafter, the liquid containing Pu may be treated by the fluoride process, or other procedure for concentrating the product.

Example III Still another example is as follows: A cc. aliquot of a large extraction product solution was diluted to 5 N HNO and oxidized in normal manner with .01 M NaBiO An extraction product solution is the solution resulting from dissolving in a solvent a precipitate carrying Pu, together with the activity also carried therewith. After oxidation was complete, .001 M Na Cr O- was added as a holding oxidant. A by-product BiPO was precipitated in normal manner. Approximately 30 minutes after the BlPO4 precipitation, a solution resulting from dissolving about 2 grams of ceric ammonium nitrate in Water was added. The resultant precipitate was digested an additional 30 minutes, centrifuged, washed, and the effluent containing Pu reduced in the normal manner and further treated. The gamma activity in the materials of this example was reduced to a fraction of the value in its original solution.

From the foregoing it can be observed that we have provided a process whereby materials containing Pu contaminated by troublesome radioactivities may be treated to eliminate such activity. The Pu may then be processed by any conventional procedure to obtain it in a more concentrated form or in the form of its derivatives.

In general, the materials treated would have been subjected to preliminary processing as by the bismuth phosphate method or other known procedure for concentrating the Pu and eliminating a part of the activity. The details of the kinds and amounts of oxidizing agents, the addition of phosphoric acid reagent and similar details as set forth herein are set forth merely as guides and are not a limitation on the present invention. Such feature as oxidizing, forming by-product precipitates and the like per se are not a part of the present invention and are disclosed in other copending applications as aforementioned.

In operations of the present types, it is usually desired to reduce volumes to a minimum, hence the smallest amounts of reagents consistent with good operation would generally be used. As described, the use of up to about 2 grams of scavenger per liter of liquid treated is usually satisfactory on most materials processed by the present invention. However, the use of larger amounts of scavengers is not precluded. Usually as indicated in certain of the examples it is preferred to digest the precipitates from a quarter hour to several hours. That is the precipitates are held at the temperature of the reaction as, for example, between 35 C. and 100 C. for a period.

The use of a nitrate derivative of cerium has been de scribed in the above examples inasmuch as the solutions treated in accordance with the examples comprised nitric acid solutions. Hence, the use of the nitrate form would not involve the incorporation of a foreign acid radical. However, other 'forms of cerium as the halogen derivatives or organic derivatives may be employed. Hence, the exact source of the cerium scavenger addition is not a limitation of the present invention.

The extent of decontamination may be determined by measuring the gamma activity of the solution prior to the application of my scavenging process or other decontamination procedure thereto. After the precipitates carrying the activity have been removed the activity of the centrifugate may then be measured for determining the residual gamma activity. The scavenger treatment of the present invention may also remove other activity as, for example, beta activity.

It is to be understood that all matter contained in the above description and examples shall be interpreted as illustrative and not limitati-ve of the scope of this invention, and it is intended to claim the present invention as broadly as possible in view of the prior art.

We claim:

In a process for the decontamination of plutonium values, contained in an aqueous nitric acid solution together with contaminating uranium fission products and maintained in an oxidation state greater than 4, comprising precipitation in said solution, of a bismuth phosphate precipitate, by means of establishing therein a source of bismuth ions and an excess of phosphoric acid, to carrier precipitate therewith contaminating uranium fission products away from said plutonium values, the improvement step which comprises supplementing said precipitation by adding to said solution, after said bismuth phosphate precipitate has at least partially separated, a source of cerium ion, to form with said excess phosphoric acid, and precipitate, a precipitate nearly all of which consists of cerium phosphate, to thereby carrier precipitate contaminating uranium fission products from solution therewith, and removing the resulting fission-product-bearing cerium precipitate from its supernatant, plutonium-containing solution.

Smyth: A General Account of the Development of Atomic Energy for Military Purposes under the auspices of the United States Government, August 1945, page 100. Published by US. Government Printing Oflice.