1. American Chemical Society Meeting - "Life cycle cost and energy balance of uranium recovered by a braid adsorbent system" paper by Erich Schneider
Life cycle discounted cash flow and inventory analysis methods are used to estimate the production cost and energy return on investment (EROI) of uranium recovered from seawater via a polyethelene-based braid type adsorbent. The estimates are built on original assessments of the cost and energy intensity of materials, capital equipment, labor and other inputs to the uranium production chain. If fresh adsorbent achieves a capacity of 2 grams of uranium (g U) / kg ads, as in trials off the coast of Japan, and the adsorbent may be reused 6 times with capacity degradation of 5% per recycle, the U production cost is estimated at $1230/kg U with a 95% confidence interval of [$1030/kg U, $1430/kg U]. If this uranium is used in a once-through fuel cycle, the EROI is found to be 22. Improving the capacity of the multi-recycled adsorbent to 6 g U/kg ads would reduce the cost by approximately a factor of three, as would attaining a very high capacity -- 20 g U/kg ads -- in a single-use adsorbent.
World Nuclear News - previous experiments have collected uranium from ocean currents by submerging long fibrous mats embedded with specially designed adsorbent compounds that chemically bind to uranium. After a few weeks in the sea, the mats are washed in mild acid to release the uranium and go on to be reused several times. Although these trials proved the principle of uranium extraction from seawater, the cost was prohibitively high - perhaps around $260 per pound. This compares badly to today's most economic mines on land, which produce uranium at around $20 per pound, while resources at higher costs up to about $115 per pound have already been identified that would last more than a century.
The latest research on seawater extraction was discussed at an American Chemical Society's (ACS's) meeting in Philadelphia and two groups presented new fibre technologies that stand to dramatically boost uranium recovery.
Conducting research for the US Department of Energy, Oak Ridge National Laboratory has worked with Florida firm Hills Inc to develop new adsorbent materials. Mats made from so-called 'HiCap' fibres, featuring high surface-areas, are irradiated and then reacted with chemical compounds that have an affinity for uranium. After an exposure period and extraction of uranium the mats require acid washing and conditioning with potassium hydroxide before re-use.
Oak Ridge said the fibres delivered five-times higher adsorption capacity, faster uptake and higher selectivity than the previous best. "These results clearly demonstrate that higher surface areas fibres translate to higher capacity," said Chris Janke, who led the project.
Another project presented at the ACS meeting concerned the use of fibres based on chitin - a long chain biopolymer that can be obtained from shrimp shells. Scientists at the University of Alabama led by Robin Rogers have been working to create a high-surface-area sorbent material from chitin resins sourced from the fishing industry. Rogers hopes the fibre may further help extraction of uranium from seawater.
The ACS summarised the session saying that the new techniques might reduce the cost of uranium from seawater to around $135 per pound. While this price remains uneconomic, the cost of nuclear fuel makes up only about 5% of the final cost of nuclear power. In this context, the feasibility of vastly increasing available supplies of uranium by tapping seawater, even at higher cost, assures nuclear power a feasible fuel supply for millennia to come. Further extensions to the resource timeframe could also be made by recycling uranium from used nuclear fuel, using advanced reactors that run on materials currently thought of as waste, or units that produce fissile fuel from non-fissile elements as they operate.
2. Enhancement of the extraction of uranium from seawater
The amount of dissolved uranium in seawater far exceeds the amount available in terrestrial ores. In addition to contributing additional uranium to the nuclear fuel cycle, retrieving uranium from seawater greatly reduces the technological complexity and environmental impact associated with conventional uranium mining and milling. Although several methods of extraction have been developed and tested over the past several decades, significant interest has been generated in developing adsorbent fabrics using radiation-induced polymerization after over a kilogram of uranium was obtained by Japanese groups using similar methods. Despite their success, a need remains to develop more efficient adsorbents to make this technology commercially viable.
Advanced adsorbent materials are being developed using polymeric substrates with high chemical stability, excellent degradation resistance and improved mechanical properties. Fabrics include polypropylene, nylon and advanced Winged FibersÔ from Allasso industries featuring extremely high surface areas for improved grafting density. Using the University of Maryland's 100 kCi Co-60 gamma source and 1-10 MeV electron beam linear accelerator, the various fabrics have been irradiated over a wide range of dose rates, total doses and temperatures and subsequently analyzed with EPR for determination of free radical concentration.
Also being utilized are innovative vinyl phosphonate monomers with high distribution coefficients and selectivity for uranium with excellent potential for free radical polymerization. Optimization of the grafting procedure involves precise control over reaction temperature, duration and methodology. Attachment of the chelating adsorbent to the substrate polymer is maximized by use of high monomer concentrations and quantified by determination of the grafting density of the sample. Grafted samples are subsequently analyzed for uranium adsorption with ICP-AES. Preliminary results with the new adsorbent fabrics have yielded distribution coefficients (kd) of around 1000. These results were obtained with real ocean water doped with approximately 10 mg/L uranium introduced in the form of uranyl acetate.
Current work includes optimization of irradiation conditions in addition to material characterization on the molecular level and analysis of the sample microstructure. Further testing in real seawater will be conducted to compare the selectivity of the adsorbent fabric towards uranium compared to that of other species, in addition to determining the loading and adsorption rates under various conditions such as pH, temperature and salt concentration. Experiments in seawater will also be performed to characterize the effects of organics on the adsorbent materials, test for durability and reusability and determine kinetics and efficiency of the uranium extraction as a function of the time of exposure to seawater in order to study the degradation of the sorbent in realistic environments.
3. Improving the chemistry of amidoxime-based uranium adsorbents (by Benjamin Hay, Oak Ridge National Lab)
The presence of 4 billion tons of uranium dissolved in Earth's oceans has motivated a sustained effort to develop technology to sequester uranium from seawater over the past 40 years. A demonstrably successful approach has involved the use of poly (acrylamidoxime) adsorbents, which are able to extract and concentrate 3 ppb levels of uranyl carbonate under actual marine conditions. This talk presents research progress toward increasing uranium uptake from seawater through chemical modification of amidoxime-based adsorbent materials. The research entails a combined theoretical and experimental approach to (i) better understand how current these adsorbents function, (ii) identify binding site architectures optimized for uranyl cation interaction using state-of-the-art de novo structure-based design methods, and (iii) synthesize, characterize, and evaluate performance of promising candidates in the laboratory.
4. Electrospun chitin nanofibers for uranyl absorbant materials
It has been recently shown that ionic liquids (ILs) allow the dissolution of biopolymers without the loss of the important high molecular weight of the natural polymer which leads to improved strength. Electrospinning polymers produces high surface area fibers which can be functionalized with selective ligands for preferential complexation of the uranyl ion. Here we will present our efforts to prepare electrospun nano and micron sized chitin fibers directly from the dissolution of shrimp shells in the IL 1-ethyl-3-methylimidazolium acetate. The results of this single step process suggest that chitin can be extracted with higher molecular weight and purity over current processes that result in chitin with a lower degree of polymerization. We will discuss the correlation of physical properties and electrospinning conditions with the surface morphology and size range of the fibers.
5. R and D towards recovery of uranium from seawater in Japan (by Noriaki Seko and Masao Tamada)
The recovery of uranium has been carried out by adsorption method. In this method, the adsorbent needs the high selectivity and capacity for uranium adsorption in the seawater. It was found that hydrous titanium oxide was noble material for the recovery of uranium from seawater. Then, screening researches were carried out to evaluate the many kinds of uranium adsorbents and concluded that the amidoxime was a promising functional group for recovery of uranium from seawater. We have developed the high performance adsorbent having amidoxime with radiation-induced graft polymerization and the collection of 1 kg uranium from seawater was demonstrated. In the recovery system, the stacks of fabric adsorbent were changed to a braid type adsorbent to improve the contact between the adsorbent and the seawater. The sea area being considered for the recovery of uranium was preliminary investigated on the conditions of seawater temperature and depth of the sea.
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