Chennai: India drew its three-stage nuclear power programme because of insufficient uranium reserves estimated in the region of 73,000 tonnes for the overall power generation plan. Curiously, all the three are in operation, though in different stages.
To understand better the linkage between the three stages, one should know about the fuels. The materials that one comes across as fuels for atomic power plants are U235, U238, U233, plutonium and thorium.
Says Dr S M Lee, director, Indira Gandhi Centre for Atomic Research (IGCAR): Natural uranium has two isotopes: U238 (99.27 per cent) and U235 (0.72 per cent). Only U235 can sustain a fission chain reaction and is called fissile material. Two other fissile materials that can fuel nuclear reactors are plutonium and U233, which are manmade and not available in the nature.
Plutonium is created when U238 is irradiated in a nuclear reactor where it absorbs neutrons and part of it is transmuted into plutonium. Similarly, U233 is created when thorium is irradiated in a nuclear reactor, where it absorbs neutrons, which transmutes a part of the thorium into U233.
Thus U238 and thorium are also valuable nuclear resources, called fertile materials, as they can be converted into fissile material for fuelling nuclear reactors and generate power.
With the available uranium reserves around 20,000 mw can be generated for 30 years. In order to utilise the abundance availability of thorium (3.6-lakh-tonne reserves) and to be able to generate nuclear power beyond 30 years, India drew up a three-stage atomic power programme.
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The first stage is the setting up of Pressurised Heavy Water Reactors (PHWR). India has 12 such reactors and several more are being planned. The readily-available natural uranium in India will be entirely committed to these reactors over their lifetime, says Lee. Besides generating power, PHWRs will convert a part of U238 in natural uranium into plutonium, which is separated from the spent fuel in plutonium reprocessing plants.
The second stage of fast breeder reactors (FBR), which will come into operation largely from 2020, will be fuelled by plutonium (obtained from the first stage PHWRs) and U238. The fast breeder reactors will fission plutonium for power production and at the same time breed more plutonium from the U238.
The surplus plutonium from each FBR can be used to set up additional FBRs and grow the nuclear capacity in tune with Indias needs. The uranium resource position is such that with FBRs a nuclear electric base of as much as 350 GW appears technically feasible.
Consolidation and further growth of the nuclear electric base is planned to by means of thorium breeders, which will form the third stage of the programme, which is several decades away. But this will set the stage for adequate power for the next couple of centuries. As mentioned earlier, thorium when irradiated in a nuclear reactor gets converted into U233, a fissile material.
The third stage use of thorium for power generation has already begun. A prototype 30 kv reactor at IGCAR called Kamini for thorium separation, irradiation in reactor, reprocessing for separation of U233, neutron radiography and activation analysis have been commissioned successfully.
Following the success Bhabha Atomic Research Centre is developing an advance heavy water reactor that will use both thorium U 233 and thorium plutonium-mixed oxide as fuel. It also incorporates several advanced safety features, such as heat removal by natural circulation. Thorium fuel bundles have also been successfully used in the PHWR for flux flattening. Construction is expected to start during the 10th Plan period.