There are 4.5 billion tonnes of uranium in sea water, a thousand times more than what is known to exist in uranium mines
Uranium in trace quantities is present in soil, rock and water. Bounteous nature leaves about 4.5 billion tonnes of uranium in sea water, a thousand times more than what is known to exist in uranium mines. Since its concentration is extremely low (only one particle of uranium for 34 million particles of other elements), harvesting uranium from sea is a formidable task.
Japan developed a technology by using plastic sheets to which amidoxime, which is capable of selectively absorbing uranium from seawater, is grafted by high energy electron beam irradiation.
Scientists from the Desalination Division, Bhabha Atomic Research Centre recovered uranium at milligram levels from sea water using electron beam grafted amidoxime.
They developed a semi pilot scale facility to produce radiation grafted sheets of 1 metre X 1 metre size.
They collected about 800 microgrammes of uranium in five campaigns from CIRUS Jettyhead; about 1.8 milligrammes from the seawater intake and outfall canals at the Tarapur Atomic Power Station and around 200 microgrammes from Andaman and Nicobar Islands. Though these amounts are trivial, it gives confidence in the technology
Field trials carried out at the three locations gave concentration factors of 300, 600 and 700 for the submergence of the absorbent material for 12, 14 and 23 days respectively.
“What are the reasons for obtaining different concentration factors at different locations?” “The concentration factor depends on corrosion, bio-fouling and their combined effect on the adsorption kinetics. These may be different at different locations”, Dr P.K. Tewari, Head, Desalination Division, BARC responded to my query.
BARC scientists studied these factors and the mechanical properties of the materials used in the suspension assembly and the substrate. They established their compatibilities with seawater and process chemicals and the optimum submergence periods for various locations.
They also evaluated the potential of Polyhydroxamic Acid (PHOA) sorbent, for uptake of uranium from seawater. They obtained a concentration factor of over 190, when the resin, filled in a porous bag was dipped in seawater for a period ranging from 10-30 days.
BARC and the Commissariat a’ Energie Atomique (CEA), France, are collaborating to develop three innovative and efficient methods of uranium extraction from the concentrated brine rejected by integrated nuclear desalination systems, which both partners are currently developing.
The first method uses resin-grafted with calixarene (a synthetic material, indecently expensive!); magnetic separation is the second method and the third uses a canal system using absorbents.
These methods are highly selective but need further research and development.
Using three absorption cages, each of cross sectional area of 16 square metres and height of 16 cm and consisting of stacks of 52,000 uranium specific, non-woven sheets with a total mass of 350 kg, a Japanese group recovered more than one kg of uranium in terms of yellow cake during a submersion period of 240 days in the ocean.
Dr Masao Tanada of the Japanese Atomic Energy Agency hopes to get funding to construct an under-water uranium farm covering nearly 400 square miles that would meet one-sixth of Japan’s annual uranium requirements.
Tanada asserts that Japan’s nuclear power industry can harvest the 8,000 tons it needs annually from the Kuroshio Current that flows along Japan’s eastern seaboard.
Japanese researchers found out that they can harvest uranium from sea by cultivating genetically engineered gulfweed which will grow in sea at an unbelievable rate of two metres an year. The weed selectively soaks up heavy metals including uranium.
What will you do with possibly the millions of tons of grass left over after recovering uranium? Convert it to bioethanol! Gulfweed is an ideal non-food source of bio-ethanol. Gulfweed traps carbondioxide from sea.
Conventional uranium mining requires environmental restoration including long term tailings management. Uranium recovery from the sea does not leave any tailings. With superb green credentials, it is an environmental friendly process.
India has miles to go to reach kilogramme capacities of uranium. BARC has plans to upgrade the capacity.K.S. PARTHASARATHY
Raja Ramanna Fellow, Department of Atomic Energy