Annex 8. Evaluation of Cost of Seawater Uranium Recovery and Technical Problems toward Implementation

    The cost of uranium recovered from seawater was estimated to extract the technical problems in the practical application of the fibrous amidoxime adsorbent synthesized by radiation graft polymerization. Each cost of the adsorbent production, the dipping in seawater for uranium adsorption, and the uranium elution from the adsorbent was estimated in three different mooring systems of a buoy, floating body, and chain binding system. The recovery cost was estimated to be 5-10 times of that from mining uranium. More than 80% of the total cost was occupied by the cost for marine equipment for mooring the adsorbents in seawater, which is owning to a weight of metal cage for adsorbents. Thus, the cost can be reduced to half by the reduction of the equipment weight to 1/4. Improvement of adsorbent ability is also a problem for future research since the cost directly depends on the adsorbent performance.

I. Introduction

    Since the present scale of equipment in Japan for nuclear power electricity generation is 43 megawatts, nuclear power occupies about 34% of total electrical power generation. Although this requires 8,500 tons of uranium (U) fuel per year, this entire quantity is imported. It is estimated that the (known) total world uranium extractable ore resource is over 5 million tons. It is surmised that a shortage in supply will occur 50 to 60 years in the future upon consideration of the world-wide demand for nuclear power generation. However, the uranium resource is uniformly dissolved in seawater. Since the quantity of this resource is gigantic (4.5 billion tons), this resource is practically limitless with respect to world-wide demand. That is to say, if even a portion of the uranium in seawater could be used, not just Japan's, but the entire world's nuclear power generation fuel could be provided over a long time period.

However, the concentration of uranium in seawater is low at 3.3 mg per 1 m³ seawater (3.3 ppb). Although uranium is the 13th most abundant among dissolved metallic elements, concentration of uranium is 1/50th that of lithium (Li). Although research and development for recovery of this low-concentration element by inorganic adsorbents such as titanium oxide compounds, etc. has occurred since the 1960s in the United Kingdom, France, Germany, and Japan, the present status of all such research has been subsequent stoppage due to low recovery efficiency.

At the Takazaki Radiation Chemistry Research Establishment of the Japan Atomic Energy Research Institute (JAERI Takazaki Research Establishment), research and development have continued for the production of adsorbent by irradiation processing of polymer fiber. Adsorbents have been synthesized that have a functional group (amidoxime group) that selectively adsorbs heavy metals, and the performance of such adsorbents has been improved. Uranium adsorption capacity of this polymer fiber adsorbent is high in comparison to the conventional titanium oxide adsorbent. We have reached the point of being able to verify the attainment of 10-fold higher adsorption capacity on a dry adsorbent basis. This adsorbent can make practical use of wave motion or tidal power for efficient contact with seawater. This adsorbent has been used since 1996 in the actual marine environment by utilizing moored small-scale test equipment for recovery of trace metals, including uranium, from within seawater. As a result, it has become apparent that use of this adsorbent makes possible recovery of seawater uranium with higher efficiency than the earlier method.

Therefore with the goal of economically recovering seawater uranium, and with the object of identifying problems whose key may become required in the future, recovery cost estimates are made for various steps of the total process of recovery of uranium in seawater as shown in Figure 1.

Figure 1. Seawater uranium recovery process.

Recovery of uranium from seawater comprises three steps: an adsorbent synthesis step, a useful metal adsorption step, and a desorption-purification step. During the adsorbent synthesis step, adsorbent is synthesized from precursor material in the form of a cloth-like material (nonwoven) made by moderate binding-together of fibers (mainly polyethylene). The useful metal recovery step contacts seawater with amidoxime-type adsorbent so as to adsorb uranium present dissolved in seawater. After immersion for a prescribed time period in seawater, the adsorbent is pulled up. The adsorbed uranium is recovered during the desorption-purification step. Moreover, the adsorbent after desorption is regenerated by alkali treatment, and then the adsorbent used again in the next useful metal adsorption step. Adsorbent whose performance has declined is replaced by new adsorbent.

Economics of the use of uranium from seawater have been previously estimated by Nobukawa and Kitamura in 1993 and by Kato, et al. in 1999 using data for amidoxime-type adsorbent under development at the Takazaki Radiation Chemistry Research Establishment of the Japan Atomic Energy Research Institute. Within the present report, by assuming the use of the cylindrical fixed bed described in section III-2, three respective recovery systems are designed, and costs are estimated for synthesis of adsorbent, for the adsorption system, and for desorption-purification. Problems are identified for future research and development to improve economics.

II. Performance and Synthesis of Seawater Uranium Adsorbent

    The amidoxime-type adsorbent production process developed at the Japan Atomic Energy Research Institute is shown in Figure 2. The precursor material is a nonwoven mainly comprising polyethylene. This is the same type of material used as an oil barrier during an oil spill upon the ocean, etc. This nonwovern is irradiated with a high energy electron beam (2 MeV) and then undergoes graft polymerization with acrylonitrile (AN). The cyano group (-C=N) of this AN is chemically reacted with hydroxylamine for conversion into the amidoxime group.

Two of these amidoxime groups capture a single heavy metal ion. Uranium is present dissolved in seawater in the form of the uranyl tricarbonate ion. It is thought that this uranyl ion is captured by formation of a chelation bond with the amidoxime group. When the metal ion captured in this manner is immersed in an acidic solution, the metal ion desorbs from the amidoxime group, and the metal ion then becomes dissolved in aqueous solution. The amidoxime group from which the metal ion was desorbed mostly retains adsorption capacity, and again captures metal ions in seawater. Therefore repeated use is possible. The calculated stoichiometric chemical uranium adsorption capacity of this adsorbent is 500 g per kg adsorbent based upon the concentration of amidoxime groups. Results of beaker-scale tests using high uranyl concentration solutions have in fact indicated adsorption capacity roughly supporting this value.

Figure 2. Adsorbent production step by radiation-induced graft polymerization

During preliminary heavy metal recovery experiments from seawater, roughly 1 mm thick, 30 cm x ~ 15 cm sheets of adsorbent were stacked until roughly 1 kg was loaded into a 50 cm diameter cage constructed from metal mesh. This cage was moored by float and anchor at a water depth of 10 m (roughly 20°C water temperature) at 6 km offshore from Sekinehama of the Shimo-kita peninsula of Aomori prefecture. While the adsorbent was allowed to contact natural ocean current (averaging 0.1 m/s), the adsorbent was pulled up every 20 days, and the quantity adsorbed was measured. The quantity of uranium recovered from the adsorbent is shown in Figure 3. As indicated by this figure, the adsorbed quantity increased with days moored, and about 2 g had adsorbed per 1 kg of adsorbent at 60 days. Simultaneously vanadium (V) as a heavy metal was recovered at a loading 1.5 times that of uranium. In addition, 0.3 g of nickel (Ni), etc. was also recovered.

Performance of this adsorbent is greatly affected by seawater temperature. It is clear from experiments that metal adsorption rate increases roughly 3-fold above 10°C. Moreover, adsorption rate increases as the quantity of contacting seawater increases. For example, 3 g-U/kg-adsorbent was adsorbed during a 20 day time period according to data obtained at 25°C during lab seawater flow-through experiments. This performance of the adsorbent is based upon data obtained using adsorbent prepared from material with a 40 m fiber diameter of the nonwoven. During recent research and development, it has been confirmed that adsorption rate performance is improved roughly 2-fold by using nonwoven that has a small fiber diameter and thereby increasing specific surface area of the adsorbent 3-fold. However, it was observed that adsorption performance dropped when this adsorbent was used repeatedly. This decline was 20% after 5 uses. This decline is due to a drop in hydrophilicity of the adsorbent caused by the acidic solution used for desorption of uranium from the amidoxime group. Research is in progress to inhibit performance degradation. It is anticipated that performance degradation can be inhibited by improvement of the desorption method by use of an organic acid.

Figure 3. Quantity adsorbed of heavy metal in seawater by amidoxime type adsorbent (based on results of marine mooring tests in seawater at a temperature of 18°C to 20°C)

As described above, although the adsorbent can be said to be at the under-development stage with the targeted improvement still pending, a tentative cost estimate was made by assuming that the adsorbent had reached the point of practical use. For this estimate, adsorption was assumed at a coast that has a comparatively high current velocity in a warm current marine site in the vicinity of Japan. Adsorption performance of about 6 g U/kg adsorbent was assumed during mooring for 60 days. Since improvement of adsorbent performance can be anticipated based upon recent research results, a cost estimate is also indicated for the case in which adsorption performance reaches the predicted value of 10 g-U/kg-adsorbent.

III. Establishment of a Recovery System and Trial Calculation of Cost

1. Basic conditions of test

Basic conditions during recovery system testing are shown in Table 1. This assumes placement in a marine region of comparatively high current velocity and warm current in the vicinity of Japan. In consideration of actual adsorbent surface area, the quantity of adsorbed uranium was established to be 1,200 tons per year. This is over 1/10th that imported from abroad and is equivalent to that required for eight 1 megawatt nuclear power generation stations. A mooring time period of 60 days for the adsorbent was determined by two mutually related factors: the quantity adsorbed gradually reaches saturation as shown in Figure 3 as the number of days moored increases, and the costs of pulling up adsorbent and desorption-recovery drop as the number of days moored increases. Therefore an annual operating time of 300 days is assumed, and the per 1 year recovery frequency becomes 5. The use count (number of repeated uses) of adsorbent was set at 20 times. This was assumed since organisms become attached to the adsorbent, and handling becomes difficult.

2. Establishment of a Recovery System

A recovery system based upon this adsorbent uses ocean current to produce efficient contact between the adsorbent and a large volume of seawater. According to the basic conditions of Table 1, the required quantity of adsorbent (quantity at the time of mooring) becomes 40,000 tons, and the quantity exchanged due to adsorbent performance decline becomes 10,000 tons per year.

Adsorbent is used in the form of 15 cm wide strips of nonwoven sandwiching a spacer and coiled into a short cylindrical shape. This roll is loaded into a cage (adsorption bed = short cylindrical shape of 4 m diameter) as shown in Figure 4. A single adsorption bed is loaded with 125 kg of adsorbent. The quantity of adsorbed uranium per bed during 60 days is 750 g. These adsorption beds are strung and tied together by rope at roughly 0.5 m intervals to form 1 basic unit.

125 kg of adsorbent is loaded into a single adsorption bed. Specifically, the adsorption bed is a metal mesh container (cage), formed from stainless steel, that has specific a gravity of 7.8 and a mass of 685 kg. A 15 cm wide sheet of adsorbent (150 g/m²) is coiled so as to load 125 kg of adsorbent. A plastic mesh sheet is inserted between adsorbent windings as a spacer. The specific gravity thereof is 1.15 so total mass is 104 kg. Total bed mass becomes 914 kg. The weight in seawater becomes 611 kg, so the weight when pulled up becomes 1,161 kg.

Figure 4. Adsorption bed loaded with adsorbent

Three methods of mooring of the units comprising bound-together adsorption beds were investigated as shown in Figure 5: (a) the buoy method, (b) the floating body method, and (c) the chain-binding method. For the buoy method or the floating body method, 100 adsorption beds are connected together in series to form 1 unit. For the chain-binding method, 10 adsorption beds are connected together in series to form 1 unit. Desorption and purification after adsorption of uranium from seawater are the same for any of these recovery systems.

3. Production of Adsorbent

Although 40,000 tons of adsorbent must be produced beforehand prior to the start of uranium recovery, production then becomes 10,000 tons per year for replenishment during the time period of regular uranium recovery. We made a trial calculation of the cost of manufacture of 10,000 tons per year of adsorbent. Details of this calculation are shown in Table 2. Precursor material cost occupies a large proportion in comparison to production equipment cost. Even if we were to assume an increase in production equipment for annual production of 40,000 tons per year, the equipment cost increase would be held down to slightly more than 2-fold. From such estimates, production unit cost of adsorbent was estimated to be 493,000 yen per ton (493 yen/kg). The quantity of recovered uranium becomes 120 kg per 1 ton of adsorbent for the case of 20 reuses. Therefore the adsorbent production cost required for recovery of 1 kg of seawater uranium is estimated to be 4,100 yen/kg-U.