Dr. Nayanmoni Saikia
With the increase in population, expansion of industry and the lifestyles of the present era the demand for electricity is far more than what it was a few decades ago. To cope up the growing need, different available methods have been tried for generating electricity out of which nuclear power plants is considered as one of the most efficient methods of producing a substantial amount of power. As a result the number of nuclear power plants around the world have dramatically increased to meet the demand. At present there are 451 nuclear reactors working all over the world and producing a total of 2476 TWh (trillion watt hour) of electricity. Presently India has 22 nuclear reactors in operation, producing a total of (~) 30,292 GWh of electricity while 6 more reactors are under construction.
Uranium is the solo fuel of every nuclear reactor. Uranium-235, a rare isotope of uranium ore that occurs at the level of 0.7 % (with the other 99.3 percent consisting of uranium-238) is used as the fuel for nuclear reactors. Uranium occurs naturally in soil, rock and water in low concentration, a few parts per million. The abundance of uranium is as common as tin or zinc in the earth’s crust. According to the OECD NEA & IAEA, Uranium 2016: Resources, Production and Demand (‘Red Book’), the total recoverable uranium resources on earth is 5.78Mt (Million tons). Out of this total recoverable uranium resources Australia has a substantial part (about 29%) of the world’s uranium, Kazakhstan 13%, Russia and Canada 9% each. India has about 2.3% (~ .13Mt) of the world uranium. The largest uranium reservoir in India is at Tummalapalle Mine, Tumalapalli Village, Andhra Pradesh. The Atomic energy commission of India in 2011 depicted that the Tummalapalle mine would be one of the largest reservoirs of uranium in the world.
In the 1930’s the international uranium mining industry started to extract radium from uranium ores. Radium is the fifth radioactive decay element in the U-238 radioactive decay chain. During that period, uranium mines were developed in the Northwest Territories, Canada and in Belgian Congo. In the refineries radium were extracted from the concentrated Uranium minerals and was used to make self-luminous paints for watches, aircraft instrument, dials and other instrumentation. The radium market later declined due to over-supply thereby resulting in shutting down of many radium mines.
The second stage uranium exploration started in 1944, driven by the newly developed atomic weapons. During the period 1944 to early 1960’s the demand for Uranium was high from the United States and European Military which in turn fostered the development of Uranium mines around the world. In between 1974-1983, Uranium mining was again noticed and was driven by the need for civil nuclear power among the different countries. The exploration efforts were relatively low during the period of 1985-2003. From 2004 to till to-day Uranium exploration has increased gradually and Australia produces highest amount of uranium followed by Kazakhstan, Russia and Canada. Australia itself produces nearly 29% of total uranium production all over the world.
Indian Atomic Energy program was started in 1967 with the establishment of Uranium Corporation of India (UCIL) under the Department of Atomic Energy. UCIL is responsible for uranium mining and uranium processing in different parts of India. Presently the firm operates mines at Jadugoda, Bhatin, Narwapahar, Turamdih, Banduhurang, Tummalapalle, Lambapur and Mawthabah in Meghalaya. There are many ways to extract uranium from the ore. The mining and milling process of uranium includes many steps starting from the underground mining or open pit mining. After mining, the ore material is milled near the mine and ground into small particles of uniform size. After grinding, uranium is extracted from the ore material by chemical process, known as chemical leaching. The final product after leaching is yellow in color called the yellow cake. The final step of uranium mining is the disposal and treatment of waste produce during the whole process.
Open-pit method and underground mining is the conventional methods for uranium mining. Open-pit method is applicable to concentrated shallow deposit while underground mining is applied in deeper deposits including vein type deposit. Open-pit mining is more economical than underground mining. Recently due to the discovery of alternative extraction methods, in which chemical solutions are injected into underground deposits to dissolve (leach) uranium from the ore, the conventional methods are become obsolete.
Milling is the process in which the mined ore is crushed and uranium is chemically extracted from the ore. It is further processed to a concentrated state which is called yellowcake. There are many ways of uranium milling, namely- acid leaching, alkali leaching and In-situ leaching (ISL). In-situ leaching is the most advanced method for uranium leaching from the uranium ore. In milling process significant amount of acids or bases are used to extract uranium from the ore. Based on the leaching agent, several post leaching steps of precipitation and purification need to be done to extract uranium complexes from the other metals.
The waste produced due to mining and milling of uranium extraction is called a tailing. Mining tailings are hard rocks and mine water. Hard rocks usually never contain uranium and may be used in the mines for some other purposes, like filling the pit, making embankments etc.. Mine water can be reused in the mine and the mill. Mine water may contain dissolved uranium, thorium or any other radioactive minerals.
Uranium mill tailing contains radioactive and non-radioactive materials which are solid residue after the uranium has been extracted. The mill tailing also contain a huge amount of waste liquid. The solid residue consists of mainly finely-ground bulk of the original ore and varieties of chemicals precipitated from the tailing liquids. These solid tailings are typically created in slurry form during the milling process and then deposited in an impoundment.
Nuclear energy is renewable energy which does not cause air pollution. It is therefore termed as clean energy. The low carbon emission from nuclear reactors makes nuclear energy one of the most favorable energy sources in this era of global warming. Many climatologists advocate for nuclear energy whereas many of them put their concerns on waste management from uranium mining. Many from the research community believe that due to the invention of new technology and cutting edge instruments, uranium mining and milling operations do not cause alarming concerns on health hazards and environmental degradation. But at the same time all of them put their concern on the management of the by-products of uranium mining and milling. This is because mill tailings contains heavy metal ore constituents and residual amount of radium. Radium is produced during the radioactive decay of thorium-232, uranium-235 and uranium-238. The radium (Ra-226) , in turn, decays to produce a radioactive gas known as radon (Rn-222) , which may be released to the environment. Radon being a radioactive element with longer half-life may enter our bodies during respiration and be deposited in our respiratory tract. Moreover since radon (Rn-222) is a noble gas, it does not react with other elements and may accumulate in our respiratory tract which in turn, according to research increases the possibility of risk of lung cancer.
Airborne dust, called as “Aeolian dust” can be created during the mining and milling process of uranium. The variance of concentration or the amount of such dust in atmosphere depends on the surface wind, practices of explosions and earthmoving and the preventive measures taken during the mining and milling processes. The size of such dust particles is mostly greater than about 0.2 micro- meter. Once these particles disperse into the atmosphere, it elevates the level of uranium series radio-nuclides and the potential inhalation and deposition in lung may lead to a radiation dose to the person.
Another important concern due to the uranium mining and milling is the contamination of surface and underground water. Uranium extraction units use enormous quantities of water for washing the ores. If the untreated effluents, slimes or tailings reaches nearby water sources, it will add chemicals, metals and radionuclides (Ra-226, Pb-210, U-238 and U-234) to those water sources. Dispersion of radionuclides through ground water system is another potential source of water contamination due to uranium mining. In this transport process relatively long half-life radio nuclides reaches to the ground water. These radio nuclides then incorporate in the traditional aquatic food stuff which in turn contributes to the radiation dose received by the public.
Radiation exposure is of greatest importance in the perspective of human health hazard not only for the occupational members of uranium mining but also the public around the mining sites. Thus, it is most important to evaluate the extent of radiation exposure the public has had via air, ground water and surface water and there is need to implement strict preventive measures to reduce the radiation levels to the internationally accepted normal radiation levels both inside as well as outside the mining area.
(The writer is teacher Department of Applied Physics, Brahmaputra College, Guwahati)