Decontamination zirconium

Uranium Purification. Subsequent uranium cycles provide additional separation from residual plutonium and fission products, particularly zirconium— niobium and mthenium (30). This is accompHshed by repeating the extraction/stripping cycle. Decontamination factors greater than 10 at losses of less than 0.1 wt % are routinely attainable. However, mthenium can exist in several valence states simultaneously and can form several nitrosyl—nitrate complexes, some for which are extracted readily by TBP. Under certain conditions, the nitrates of zirconium and niobium form soluble compounds or hydrous coUoids that compHcate the Hquid—Hquid extraction. SiUca-gel adsorption or one of the similar Hquid—soHd techniques may also be used to further purify the product streams. 

A decrease in the number of uranium and plutonium purification cycles from three to two, or even one, would be highly advantageous. First-cycle decontamination factors of uranium from neptunium and from the fission products ruthenium and zirconium must be significantly improved to realize such a decrease. 

More recently a flowsheet has been developed which employs 30% TBP/OK as the solvent.349-446 447 This involves the use of an acid feed to the first cycle to assist in zirconium decontamination and suppress hydrolysis. An acid-deficient partition cycle then follows in which the U-Th separation is effected. A pilot plant (JUPITER) has been constructed at Julich in Germany to process Th02/U02 fuel using this flowsheet. Although a complete separation of thorium, uranium and FPs is possible using TBP in the Thorex process,448 alternative approaches... 

A decrease in the decontamination performance of uranium and plutonium for fission products (mainly ruthenium, zirconium, cerium, and niobium)... 

Decontamination of equipment is identical to that described in the synthesis of calcium metal. A similar procedure can be used to produce powders of zirconium, chromium, vanadium, thorium, uranium, and other refractory metals from their oxides the reductant/metal oxide ratio, however, differs for each metal in order to avoid formation of oxygenated metal by-products and ensure maximum product purity. 

For stripping Pu from HDEHP, Fardy recommended an organic reducing agent (6). Our experiments prove that Pu(IV) can be quantitatively stripped by oxalic acid from HDEHP, while extracted zirconium remains in the organic phase. Usually, a decontamination factor of 200-300 for Zr/Pu can be obtained. 

The extraction of Zr" by pure TBP/OK phases increases rapidly with aqueous phase acidity so that Dzt for 19% TBP/OK increases from ca. 10 at 0.5 M HNO3 to ca. 10 at 13 M HN03- Satisfactory decontamination from zirconium can thus be achieved using aqueous phase acidities in the range 2-3 M where Dz, will be ca. 0.1. The extraction reaction was described by equation (158) but more recent work also indicates the presence of Zr(0H)2(N03)2 2TBP and Zr(OH)-... 

For the purpose of improving the decontamination factor (DF) of FPs from U or Pu in the reprocessing of highly irradiated fuels such as those from FBR, a modified method adding inactive zirconium or hafnium ion is proposed. The feasibility of this concept has been experimentally demonstrated by both batchwise extraction and process studies with miniature mixer-settlers. 

Zr-Al coprocess waste test, the feed, extractant, and scrub flows were 1, 0.5, and 0.1 mL/min, respectively. For the high sodium concentration waste, the feed, extractant, and scrub flows were 0.75, 1, and 0.25 mL/min, respectively. Samples of raffinate were drawn for analytical analysis approximately five hours after equilibrium had been reached. The resultant decontamination factors agreed reasonably well with our calculations. For the coprocess waste run, we expected an americium decontamination factor of 200. We purposely built in a large, overkillM in the sodium waste run by increasing the organic to aqueous flow rates. The sodium waste run produced a raffinate that, when calcined, would be well below the guideline for alpha-free waste with no allowance for decay. Analytical analysis of feeds and raffinates confirmed our batch results in that actinides were fractionated from major waste constituents such as aluminum, zirconium, sodium, and fluoride. 

The zirconium-hafnium decontamination factor is obtained from (4.48) and (4.49) with l3Hf=0.12 ... 

It is interesting to compare the decontamination obtainable for Pzt — 0.98 and EjF =1.0 with that obtainable with an infinite number of stages, conesponding to operation at the same zirconium recovery but at ( /F)min- From Eq. (4.35) ... 

Suppose that we wish to recover 98 percent of the zirconium and to obtain a zirconium-hafnium decontamination factor of 200. The limiting ratio of scrub to solvent, from... 

As an example of the use of these equations for an extracting-scrubbing cascade, consider the addition of a scrubbing section to the hafnium-zirconium separation example, which was first analyzed in Sec. 6.2 as a simple extraction problem. The modified flow sheet is shown in Fig. 4.17, and desired recoveries and decontamination are given in Table 4.7. 

Thus, six theoretical stages in the extracting section and four in the scrubbing section would result in higher values of zirconium recovery and hafnium decontamination than those specified. 

The Pu(rV) oxalate process achieves decontamination factors of about 3 to 6 for zirconium-niobium, 12 for ruthenium, 60 for uranium, and 100 for aluminum-chromium-nickel. As compared with peroxide precipitation, the oxalate process achieves less decontamination from impurities, but the solutions and solids are more stable and safer to handle. It is more suitable for processing solutions containing high concentrations of impurities that would catalyze peroxide decomposition. 

The other detrimental effect of TBP degradation is its complexing of zirconium. This increases the zirconium distribution coefficient and consequently decreases the decontamination coefficient. Moreover, solvent residual radioactivity is increased because of incomplete zirconium reextraction. Another and even more troublesome consequence of zirconium complexing is the formation of precipitates known as crud. This is a severe problem, particularly in mixer-settlers, and has led to a preference for pulsed columns or centrifugal contactors in the first extraction cycle when high-burnup fuel is to be processed. 

Siddall [SI 5] summarizes the effects of increasing radiation exposure on decontamination in the first Purex extraction contactor as shown in Table 10.16. The power density, or dose rate, also has an effect on solvent performance. Baumgartner [B5] cited experiments in which 1.2 Wh/liter, delivered to 20 v/o TBP in one pass through the HA and HS contactors, reduced the zirconium decontamination factor from 1000 to 10. 

The thorium nitrate solution from the dissolver will be about 9 Af in nitric acid. To obtain satisfactory decontamination of thorium from fission-product protactinium, ruthenium, and zirconium-niobium, it was found necessary to remove all of the nitric acid from the solution and make the solution around 0.15 Af acid-deficient in nitrate ion by converting a fraction of the A1(N03)3 to a water-soluble basic nitrate. This also converts the readily hydrolyzed nitrates of these fission products to basic nitrates that are less extractable than the species present in the acid dissolver solution. 

Figure 10.26 compares the low-concentration distribution coefficients of uranium, thorium, plutonium, protactinium, and the principal fission products. The spread between thorium and fission-product zirconium is greatest between 1 and 2 M HNO3, the range used in the decontamination step of the acid Thorex process. Because the distribution coefficient of protactinium is close to that of thorium, it is necessary to remove protactinium or complex it with fluoride or phosphate ion to prevent its extraction with thorium. 

CodecontaminatioiL The codecontamination section consists of the HA extraction section equipped with short-contact-time centrifugal contactors and the HS scrubbing section equipped with pulse columns. In the HA section, uranium and plutonium in the aqueous feed and reflux from the HS section are extracted into the organic stream containing 30 v/o TBP. In the HS section any ruthenium extracted by TBP is scrubbed into the aqueous phase with 3 M HNO3. Then any zirconium-niobium in the TBP is scrubbed with 0.3 M HNOa. Scrubbing is at 50°C to enhance decontamination of ruthenium. 

A technique for the separation of pertcchnetalc from mixed fission products by solvent extraction with TBP was described. The extraction was almost quantitative from a sulphuric acid solution. Sodium fluoride was used to provide the zirconium-niobium decontamination and a cation exchange column ensured the decontamination from metallic ions. Tc yields of 92 % were obtained [123. ... 

Impurities in the rare metals produced by the iodide process can generally be reduced to a few tens of parts per million or less, for each element, provided care is taken in the selection of materials of construction. This initial advantage over other processes arises from the fact that the rare metals are produced without direct contact with a crucible or other container. Elements with volatile iodides should clearly be avoided in locations where the temperature is appropriate for attack by iodine vapour. Similarly, the crude feed should be as free from such elements as possible. For example, the whole of the zirconium impurity in a crude titanium feed would be carried over into the product, and vice versa, since the iodide process is equally suitable for the impurity as for the rare metal being purified. A large fraction of the iron and aluminium would be transferred, but decontamination factors from other elements such as nickel, chromium, carbon silicon and nitrogen are usually of the order of 10 to 100. 

An important goal of the zirconium extraction example given in Problem 8.1.18 is to obtain a solvent extract stream containing zirconium substantially purified of hafnium. It is known that the value of k,i of Hf is 0.11. Determine the value of the separation factor for zirconium vis-a-vis hafnium in the extract stream for case (b) of Problem 8.1.18. This separation factor has also been called the decontamination factor (Benedict et al., 1981,... 

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