Plutonium peroxide

J. A. Leary, Studies on the Preparation, Properties, and Composition of Plutonium Peroxide, LA-1913, Los Alamos Scientific Laboratory, N.M., 1955. 

Impure plutonium oxide residues are dissolved in 12M HN03-0.1M HF under refluxing conditions, and then the plutonium is recovered and purified by anion exchange. Plutonium is leached from other residues, such as metal and glass, and is also purified by anion exchange. The purified plutonium eluate from the anion exchange process is precipitated with hydrogen peroxide. The plutonium peroxide is calcined to the oxide, and the plutonium oxide is fluorinated. The plutonium tetrafluoride is finally reduced to the metal with calcium. 

Fluorination and Reduction. Until recently, plutonium dioxide from the calcination of plutonium peroxide was contacted with HF in a rotary hydrofluorinator. The reaction converted Pu02 to PuFt, according to the following equation ... 

Precipitation Processes. Plutonium peroxide precipitation is used at Rocky Flats to convert the purified plutonium nitrate solution to a solid (14) the plutonium peroxide is then calcined to Pu02 and sent to the reduction step. The chemistry of the plutonium peroxide precipitation process is being studied, as well as alternative precipitation processes such as oxalate, carbonate, fluoride, and thermal denitration. The latter method shows the most promise for cost and waste reduction. 

Plutonium Peroxide Precipitation Review and Current Research... 

A prime responsibility of the Rocky Flats Plant since it was built by the Atomic Energy Commission some 25 years ago has been the recovery and purification of plutonium. This recovery and purification has been done using an aqueous process. One of the major steps in that process is the precipitation of plutonium peroxide. This step converts the plutonium from an aqueous to a solid form for further processing and conversion to metal. 

Plutonium peroxide was investigated by Hamaker and Koch, (0 Hopkins, (2) and Koshland, et al. (3) in the 19 0 s and Leary (h) in the early 1950 s. This work showed that the composition of the precipitate varied and often incorporated anions from the solution from which it was precipitated. These investigations, as well as other work (5), also showed that the peroxide precipitate exists in both hexagonal and cubic crystalline forms. Although both forms are compounds of Pu(lV), they have slightly different O /Pu ratios (6). The cubic form can be colloidal and therefore is less suitable for process application than the hexagonal form. The acidity of the solution has an effect on the... 

There are some data that indicate sulfate ion aids in the formation of a more easily filtered plutonium peroxide precipitate. Ganivet (7) found, for instance, that peroxide precipitated from a nitric acid medium containing sulfate had better settling characteristics than precipitates from comparable solutions to which no sulfate was added. However, sulfate in the precipitate is undesirable, because of the corrosive effects it can have on processing equipment. Mainland, et al, (8) showed that by careful control of the precipitation parameters, it was possible to effectively precipitate peroxide in the absence of sulfate. 

The solubility of plutonium peroxide in nitric acid media ( .,is a function of the acid concentration as well as the concentrations of H2O2 and foreign ions. Typical solubilities range from 7 to 1190 mg plutonium per liter as the acid concentration varies from 1.2 to 5.2N (2). 

These investigations, as well as others over the past three decades at Rocky Flats and at other sites, indicate that there are a number of individual variables that affect the physical and chemical characteristics of plutonium peroxide. The work in this area that is available in the unclassified literature has been summarized by Cleveland (10). A review of the literature indicates a lack of any planned investigative program that encompasses all of the variables. This type of investigation is important, especially from a production process viewpoint, for it would verify the relative importance of the individual variables, as well as any significant interactions between them. 

Based on experience at Rocky Flats and information from the literature, six of the variables deemed most important in the precipitation of plutonium peroxide were selected for investigation. These six, and the ranges over which they were investigated, are identified in Table I. 

VARIABLES AND LEVELS USED IN INVESTIGATING THE PRECIPITATION OF PLUTONIUM PEROXIDE... 

A MFi 1 terometer11 was used to measure the fi 1 terabi 1 ity of the plutonium peroxide precipitate. This equipment, designed especially for this work, consisted of a calibrated tube (from a 100 ml buret) attached to a funnel-type support containing a wire screen. A Millipore filter (Solvinert, 1.5ym mean pore size) was supported by this screen. The tip of the funnel extended down into a 500 ml Erlenbeyer filter flask and a constant vacuum of 5.08 cm of Hg was applied through the side arm of the flask. 

In a run, for example, in which the filtration time for water was 33 seconds and for plutonium peroxide 96 seconds, the relative filtration time is 2.91. 

The volume of the plutonium peroxide slurry varied slightly depending upon the experimental conditions. This variance ranged from 23.k to 31.6 ml with an average of 30.1 ml. (These are buret readings and they do not include the volume of the section of the Filterometer containing the filter). To correct for this variation in volume, the relative filtration time calculated above was multiplied by the factor... 

At the end of the digestion time the plutonium peroxide slurry was transferred to the Filterometer. The volume of the slurry was measured to the nearest milliliter and the time required for filtration to the nearest second. The volume of the filtrate was also measured to the nearest milliliter and then diluted with an equal volume of concentrated HNO3. This HNO3 dissolved any plutonium peroxide present and prevented any post precipitation. The filtrate was then analyzed coulometrically for plutonium. From the filtration time, slurry volume, and the plutonium analysis data, the relative filtration time and the... 

The objective of this investigation was to identify those variables that have an effect on the precipitation of plutonium peroxide, rank these variables in order of their effect, and then select levels for each of the variables that will give an optimum precipitation of plutonium peroxide. 

The identification of those variables that have a major effect on plutonium peroxide precipitation was done in two ways. The first way used t-test values associated with each variable. The comparative magnitude of these values indicates the relative importance of the variable. The second way involved a subjective evaluation of the relative importance of each of the variables based on a visual comparison of the graphs constructed from the experimental data (Figures 1 through 12 plus a couple of dozen other comparable graphs that could not be included in this paper because of space limitations). The results of the subjective evaluation indicate that only the nitric acid concentration and the rate of hydrogen peroxide addition have a major effect on the relative filtration time. The other four variables influence the... 

Based on results obtained in this investigation, the levels shown in Table IV are recommended for the six major variables in the plutonium peroxide precipitation process. These levels were selected to give the best compromise between the fastest filtration time for the plutonium peroxide precipitate and the lowest concentration of plutonium in the filtrate. 

RECOMMENDED LEVELS FOR MAJOR VARIABLE IN PLUTONIUM PEROXIDE PRECIPITATION... 

The slowest rate of H2O2 addition investigated gives a plutonium peroxide precipitate with the fastest filtration time. The rate of addition has very little effect on the plutonium concentration in the filtrate. 

The recommended levels for each of the six variables are based on experiments in which the plutonium peroxide precipitations were made by a batch process rather than by a continuous process. However, the recommendations would be applicable, basically, to either process. 

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