Biofractionation and bioseparation of elements

A very little understood application of bioseparation involves using micro­organisms to discriminate radioisotopes by size. So far, this application has remained conceptual due to limited understanding on the structure and function of organisms that are suspected to achieve biofractionation (Molokwane and Chirwa, 2009). In the latter study, Molokwane and Chirwa observed with a modest degree of certainty that microbial cells previously isolated from a high radiation-exposed facility accumulated C-14 while growing on a C-14/C-12 carbon matrix from powdered nuclear graphite. The experiment was conducted in a closed loop chemostat system equipped with biofilters for collection of suspended matter for analysis. The observed metabolic activity in the cells indicated that the process was possible under very low dissolved oxygen, suggesting that the microorganisms preferred inorganic forms of carbon as the primary carbon source. Bacteria that utilize inorganic carbon sources such as CO2 and HCO3- as primary carbon sources — known as autotrophic organisms — favour anaerobic conditions for growth. However, in this preliminary study, the amount of C-14 remain­ing in solution which could be required to draw a mass balance on C-14 in the system was not measured. These preliminary results on C-12/C-14 bioseparation hold promise for future HTGR nuclear reactors that produce large amounts of low radiation level waste as expired nuclear graphite. Success in the above process is also important for the decontamination and recovery of nuclear graphite from decommissioned plants for reuse in new reactors.

15.9 Sources of further information and advice

Chow B. G. and Jones G. S. (1999) Managing Wastes With and Without Plutonium Separation (Santa Monica, Calif.: RAND, 1999). The reprocess­ing LLW figure also uses data from Groupe Radioecologie Nord Cotentin, Inventaire des rejets radioactifs des installations nucleates, vol. 1, July 1999, p. 19.

International Union of Pure and Applied Chemistry (IUPAC), 1984. Makhijani A., Hu H. and Yih K. (1995). Nuclear Wastelands: A Global Guide to Nuclear Weapons Production and Its Health and Environmental Effects, Cambridge, Mass.: MIT Press, Massachusetts USA.

United States Nuclear Regulatory Commission (http://www. nrc. gov/waste).

[1] Estimating required heights and diameters for the columns. The height determines how much of the resin capacity is utilized before the con­taminant concentration in the effluent becomes unacceptable. The diam­eter mainly determines the liquid throughput for a given utilization of the resin capacity.

• Channeling of the liquid through the ion exchange bed. Larger diameter columns can suffer from channeling if the feed is not adequately distrib­uted across the column cross-section. Channeling can be exacerbated by fissures in resin beds caused by it swelling and shrinking.

• Feed distributor design. A well-designed feed distribution device can mitigate channeling at least to some degree in a large diameter column.

• Deciding on upward or downward flow of liquid through the bed. Up-flow velocities are limited by the need to prevent fluidization of

[2] A mixer compartment where the two liquids are mixed to form a disper­sion. Mixing is typically achieved by a mechanical impeller with the motor and gearbox mounted outside the biological shielding for ease of maintenance. Mass transfer is primarily achieved in the mixing compartment.

• A settler compartment where the two liquids are separated by gravity and exit through a system of weirs. The weir system can be designed for the separated liquids to flow to the succeeding contactor under gravity without the need for inter-stage pumping.

Mixer-settlers’ continued application in nuclear facilities is warranted on the basis of their mechanical simplicity and industrial pedigree and their important chemical engineering attributes are summarized in Table 3.2. Mixer-settlers are well-established equipment for liquid-liquid extraction unit operations for processing radioactive material and are employed in currently operating commercial UNF recycling plants (e. g. THORP, Sellafield, UK; UP3, Cap La Hague, France; and the Rokkasho Reprocessing Plant, RRP, Rokkasho Mura, Japan) mainly for purifying uranium product streams in which the fissile material content is sufficiently low that nuclear criticality can never be reached even within the non-ever-safe geometry of these contactors.

[3] Solvent extraction cycles are dedicated to virtually complete recovery of the uranium and plutonium products with a high degree of purifica­tion from the other constituents of used nuclear fuels, particularly with

[4] Dissolution of the solid used nuclear fuels to produce liquid feed to the separation (solvent extraction) operations. This portion of the process, typically termed headend operations, is an important consideration in determining and controlling the underlying chemistry of the subsequent separations processes. Obviously, the characteristics of irradiated fuel vary widely with a variety of interrelated factors, e. g. fuel type (metallic, UO2, MOx, etc.); reactor type (PWR, LWR, BWR, Fast, etc.); burnup; and enrichment.

Notes: (1) The repository addressed by the GNEP program was Yucca Mountain.

[6] Transuranic elements (of concern): Pu — plutonium, Np — neptunium, Am — americium and Cm — curium.

[7] Non-TRU: Waste containing no more than 3700 becquerels (100 nanocuries) of alpha-emitting transuranic isotopes per gram of waste and half-lives greater than 20 years.

[8] the need to decrease HLW acidity resulting in an increase of volumes of feed solutions;

• the use of complexones and salts of organic acids for stripping of actinides and lanthanides, which makes further handling of the strip product more difficult;

• insufficient mutual purification of Cs-Sr and An-Ln fractions.

Additionally, the use of this system is complicated by the fact that the syn­thesis and purification of phosphorylated PEGs is a rather sophisticated

[9] The reduction potentials of elements in LiCl-KCl eutectic salt into liquid cadmium alloy were derived when the concentrations of the elements in the salt and in liquid cadmium were the same (0.001 mole fraction).

[10] SFCs/Na = DCs/DNa, where DM is the distribution ratio of the extracted alkali cation M.

[11] A DIAMEX raffinate is the genuine highly active solution issued from the imple­mentation of a DIAMEX process, co-extracting trivalent actinides and lanthanides from a PUREX raffinate (issued from the reprocessing of a dissolver liquor by TBP solvent to recover uranium and plutonium).

[12] surrogate, spiked, or genuine PUREX raffinates, as well as well as PUREX concentrates (Serrano-Purroy et al., 2005a, b, Modolo et al., 2007a), as the feeds;

• DMDOHEMA dissolved at 0.65 M in HTP, as the solvent;

• mixer-settlers, centrifugal extractors, rotating ‘Couette-Taylor’ effect columns, and pulsed columns, as the laboratory scale contactors.

Oxalic acid and A-(2-hydroxyethyl)ethylenediamine-A, A’,A’-triacetic acid (HEDTA) were added to the feeds and the scrubbing solutions to limit the extraction of Mo and Zr on the one hand, and Pd on the other. The DIAMEX flowsheet tested in 2005 at the CEA Marcoule (France) on a genuine PUREX raffinate (31 litres, from the reprocessing of 15 kg of a 52 GWd/t UOX type spent nuclear fuel: 6.5 years of cooling), in three

[13] SANEX, for Separation of ActiNides by Extraction. The objectives of the SANEX process are to recover more than 99.9% of the An(III) in a purified product stream containing less than 5 wt.% of Ln(III).

[14] Dry impregnation — this is the most often used method wherein the extractant, or extractant diluted with an appropriate organic diluent, is

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