Actinides concentration

Sinha (60,61) has suggested that humic and fulvic acids play a major role in mobilising iron and transporting it from the soil to plant roots. At the normal soil pH it is believed that iron bound by the fulvic acid is partially hydroxylated as Fe(OH)2 (62). These complexes interact with phosphate to give an organicmetallic phosphate which may be taken up by plants (60). It has been suggested that the entire humic-iron-phosphate complex is taken up by the roots of plants and not just the iron and phosphate (60, 63). Jorgensen (64) has observed that soil humates suppress the uptake of Pb2+ into plants it is possible that they will also suppress actinide concentration in plants. 

It is significant that oat plants, which are known to contain the Fe3+ complexor, 2,4-dihydroxy-7-methoxy-l,4-benzoxazin-3-one (128), do not show any significant accumulation of plutonium, or the other actinides. It is possible that this complexing agent is located within plant cells which do not come into contact with the cation transporting mechanisms. Although there is evidence of the existence of microbial hy-droxamates in soil and that hydroxamates do become concentrated in plants (129), there has been no evidence presented yet that hydroxamates are the agents responsible for plutonium uptake into plants. On the other hand there is evidence that EDTA and DTPA can stimulate actinide concentration in plants (See Table 6). 

Sampling and Measurements. The determination of dissolved actinide concentration was started a week after the preparation of solutions and continued periodically for several months until the solubility equilibrium in each solution was attained. Some solutions, in which the solubilities of americium or plutonium were relatively high, were spectrophotometrically analyzed to ascertain the chemical state of dissolved species. For each sample, 0.2 to 1.0 mL of solution was filtered with a Millex-22 syringe filter (0.22 pm pore size) and the actinide concentration determined in a liquid scintillation counter. After filtration with a Millex-22, randomly chosen sample solutions were further filtered with various ultrafilters of different pore sizes in order to determine if different types of filtration would affect the measured concentration. The chemical stability of dissolved species was examined with respect to sorption on surfaces of experimental vials and of filters. The experiment was performed as follows the solution filtered by a Millex-22 was put into a polyethylene vial, stored one day, filtered with a new filter of the same pore size and put into another polyethylene vial. This procedure was repeated twice with two new polyethylene vials and the activities of filtrates were compared. The ultrafiltration was carried out by centrifugation with an appropriate filter holder. The results show that the dissolved species in solution after filtration with Millex-22 (0.22 ym) do not sorb on surfaces of experimental materials and that the actinide concentration is not appreciably changed with decreasing pore size of ultrafilters. The pore size of a filter is estimated from its given Dalton number on the basis of a hardsphere model used in the previous work (20). 

Currently proposed licensing regulations for geologic nuclear waste repositories require a performance assessment involving long-term predictive capabilities. Previous work (J- 5) has shown the importance of solubility controls for modeling maximum actinide concentrations in repository groundwaters. However, until reliable data are available on the actinide solid phases that may be present or that may precipitate in the environment, the solubility of solid phases such as hydrous oxides that have fast precipitation kinetics can be used to initially set maximum solution concentration limits. 

In laboratory tests using simulated HLW solution spiked with fission product tracers, Am and Cm, the denitration step proved to be a sensitive process, but Am/Cm recoveries of ca. 90% in the aqueous supernate could be realized under optimized conditions. Decontamination factors (DF) > 1000 for Zr, Nb, Mo, and 100 for Ru and Fe were obtained in the precipitation step. The solvent extraction cycle gave > 98% recovery of Am/Cm and DF > 10 for rare earths, Sr and Cs. Appreciable decontamination was also obtained for Zr/Nb (DF = 20), Ru (50), U (650), Pu (250), Np (800) and Fe (420). The ion exchange cycle served mainly for Am-Cm concentration and for removal of DTPA and lactic acid based on tests with europium as a stand-in for trivalent actinides, concentration factors of about 50 could be expected under optimized conditions. 

Feed Adjustment. The tantalum-lined evaporator used to collect the actinide product solutions from a series of double oxalate precipitation runs also serves to adjust the composite product to a feed solution for a series of oxide production runs. Excess acid is removed by boiling the solution slowly to near dryness. The temperature is held at greater than 119°C for 5 h or more during boiling to ensure the destruction of all oxalates. After approximate dryness has been reached, the evaporator is cooled, 0.01 M HNO3 is added to dilute the actinides to less than 10 g/L (usually to 10-15 L, total volume) and a sample is taken to determine the acidity (typically 0.05-0.10 M) and to verify that the actinides are still in solution. The adjustment is completed by the addition of acid and evaporation to give an actinide concentration of about 10 g/L and an acid concentration of 0.20-0.35 M at the final volume. 

The acid concentration of the feed solution is an important processing parameter. Acid concentrations in the range 0.01-0.70 M were investigated in the development tests. In each test, the curium sorbed on the resin was sufficient to produce acceptable oxide products. However, the acid concentration of the feed is maintained in the range 0.20 to 0.35 M in the production runs. In one of the earlier production runs at lower acidity, a precipitate formed in the feed solution. This was thought to be caused by an unknown contaminant, probably a phosphate species from an earlier solvent extraction step. In the production runs, the reduced actinide capacity of the resin is noticeable at the higher acidities. Convenient batch sizes and short loading times for the current scale of production are achieved with actinide concentrations of about 10 g/L, but actinide concentration is not considered an important variable. 

When a choice has to be made as sometimes occurs when the breakthrough loading raffinates are recycled to the feed tank, the acid concentration is held in the desired range, and the actinide concentration is allowed to decrease. Process operation and product quality are unaffected. 

Smith DD, Black SC. 1975. Actinide concentrations in tissues from cattle grazing near the Rocky Flats Plant. Report ISS NERC-LV-539-36. 

Four types of colloids were considered in the WIPP program intrinsic actinide colloids, mineral colloids, microbes, and humic acid colloids (US DOE, 1996). Intrinsic actinide colloids, consisting of polymerized hydrated actinide hydroxides, are not stable in the neutral to moderately basic pH conditions expected in the WIPP, and were assumed not to contribute to the total actinide concentrations in solution. Mineral colloids are destabilized and tend to flocculate in the high-ionic-strength WIPP brines (Kelly et al., 1999). In the performance assessment calculations for the WIPP, a highly conservative value of 2.6 X 10 mol actinide per liter, for each actinide, was assumed to be bound to mineral colloids and to contribute to the mobile fraction. Actinides sorbed onto microbes and humic acids were estimated to contribute significantly to the concentration of mobile actinides in WIPP brines as discussed above (Section 9.06.3.2.2). 

Figure k illustrates in schematic form the precipitation-ion exchange process devised and developed for reducing the actinide concentration of the PRF salt waste, when solidified, to or below 10 nCi/g. This process involves (a) Addition of... 

A litre of waste shown in Table I, when calcined, results in the production of 250 g of total solids. The plot shown in Figure 1 describes significant actinide concentrations in ICPP calcine from a typical raffinate as a function of time. Though uranium and neptunium are present in the calcine, their contribution to alpha activity is not significant. It is apparent that separation factors of xlOO must be attained for the Pu and Am to... 

Figure 3. Concentration ratios (CR) for Pu and Am. CR is defined as the simple ratio of actinide concentration for plants vs. soil on a dry weight basis. Sites where data sets have been collected are NTS (Nevada Test Site, Nev.), SR (Savannah River, S. C.), OR (Oak Ridge, Tenn.), RF (Rocky Flats, Col.), Enewetak Atoll, South Pacific. Arrow denotes approximate value used in the LMFBR Program Impact Assessment (2).

With a specific HLW concentrate volume of 600 liters/MT of heavy metal, the total fission product concentration will be on the order of 50 g/liter and the actinide concentration on the order of 10 g/liter. 

The diversity of reactions which actinides can undergo in natural waters is pres ted schematically in Figure 22.9. Complexation by anions such as hydroxide, carbonate, phosphate, humates, etc. determine the species in solution. Sorption to colloids and suspended material increases the actinide concentration in the water while precipitation of hydroxides, phosphates, carbonates, and/or sorption to mineral and biological material limit the amount in the solution phase. 

Muscatello et al. (117) discuss the use of hollow fiber ILMs containing blfunctlonal organophosphorus extractants to remove americium and plutcnlum from nltrate-nltrlc acid waste streams. A reduction In the actinide concentration In a waste stream would allow disposal of the stream as a low-level waste. Partial neutralization of the nitric acid In the waste stream was necessary to obtain high (>9M9) removal of the Am(III). 

We evaluated removal performance by contacting a weighed amount of the sorbent with a measured volume of solution in a shaker bath at 25 2 °C. For titanium-containing sorbents, we add the sorbent to provide the equivalent amount of titanium as that from 0.4 g/L MST. For non-titanium materials, we added the sorbent to provide the same number of equivalents as that provided by 0.4 g/L MST. Typically, we sampled the batch-contact test bottles after 4, 24 and 168 hours and measured solution phase strontium and actinide concentrations after removing sorbent solids by filtration. 

Plutonium metal is often purified by electrolytic refining the plutonium sample is immersed in a molten chloride salt under an inert atmosphere, comprising the anode in an electrolytic cell. Liquid Pu metal is collected on the surface of a tungsten cathode and drips off into a collector. Transition-metal contaminants remain in the residue of the anode, and rare earths and other actinides concentrate in the molten salt. The yield of purified Pu metal can be as high as 97%. Zone melting is also used to purify metallic plutonium the plutonium is fabricated into a bar along which a high-temperature zone is passed. As the melt zone is moved... 

Benjamin, T. M., J. H. Jones, W. R. Heuser, and D. S. Burnett, Laboratory actinide partitioning whidockite/liquid and the influence of actinide concentration levels, Geochim. Cosmochim. Acta. 47, 1695-1705, 1983. 

At higher actinide concentrations and in the absence of complexing agents precipitation of amorphous An(IV) hydroxides with subsequent aging to oxides occurs (Duffield et al. 1991, Morss 1986). 

---