The LWR being built with Russian cooperation at Kudankulam in Tamil Nadu.
NUCLEAR power reactor: This is a source of heat, which is produced when the uranium atom splits (fission). The heat produces steam, which drives the turbine in a nuclear power station to generate electricity.
A nuclear power reactor can use different types of fuel to generate electricity. The fuel can be natural uranium, enriched uranium, plutonium-uranium oxide/carbide and, thorium-uranium-233. A nuclear power reactor may use heavy water or light water as coolant and moderator. Liquid sodium is also used as a coolant.
Light Water: Ordinary water, which humans consume, is light water. It consists of two atoms of hydrogen and one atom of oxygen –H2O.
Heavy Water: In heavy water, or –D2O, the two hydrogen atoms are replaced by two atoms of deuterium. The technology of making heavy water is difficult to master. India has six heavy water plants: at Manuguru in Andhra Pradesh, Tutico rin in Tamil Nadu, Thal in Maharashtra, Vadodara and Hazira in Gujarat and Rawatbhatta in Rajasthan. India is self-sufficient in the production of heavy water. It has exported heavy water to South Korea, China and the U.S.
The physical and chemical properties of light water and heavy water are similar, but their nuclear properties are totally different.
Types of reactors: Depending on the choice of fuel, moderator and coolant, different types of reactors are built. They include Light Water Reactors (LWRs), which are also called Boiling Water Reactors (BWRs); Pressurised Heavy Water Re actors (PHWRs); Gas-Cooled, Graphite-Moderated Reactors (GCR); and Advanced Gas-Cooled Reactors (AGR). Other types of reactors include Fast Breeder Reactors (FBRs), which breed more fuel than they consume, and thorium-fuelled reactors.
India has 15 indigenously built PHWRs operating in different parts of the country and two imported LWRs at Tarapur, Maharashtra. India is building a Prototype Fast Breeder Reactor (PFBR), which can generate 500 MWe, at Kalpakkam in Tamil Nadu. Two LWRs from Russia are under construction at Kudankulam, Tamil Nadu.
The indigenous PHWRs use natural uranium as fuel and heavy water as coolant and moderator. The LWRs in India use enriched uranium as fuel and light water as both coolant and moderator.
The working of a PHWR: In a reactor, nuclear fuel is needed to create fission to generate heat to produce steam. This steam drives the turbine that generates electricity. PHWRs use natural uranium as fuel. Natural uranium contains two kinds of isotopes – uranium-238 and uranium-235. It contains 99.3 per cent of uranium-238, which is not a fissile material, and only 0.7 per cent of uranium-235, which is a fissile material. So it is the less abundant uranium-235 isotope that splits and generates energy.
When a uranium-235 atom is struck by a slow neutron, it splits into two or three neutrons. The splitting (fission) is accompanied by a tremendous release of energy in the form of heat, radioactivity and two or three fast neutrons. The slowing down of neutrons is achieved by a “moderator”, which is the heavy water. These fast neutrons, which fly out of the split atom at great speed, are made to slow down so that there is a high probability of their hitting other U-235 atoms, which, in turn, release more energy and further sets of neutrons and cause fission. Attainment of self-sustained splitting of uranium atoms is called a “chain reaction”. At this stage, the reactor is said to have attained “criticality”.
Heavy water is used as a moderator because one cannot afford to lose neutrons, and the deuterium in it has a tendency to absorb fewer neutrons.
Enriched uranium: If the non-fissile U-238 is removed from natural uranium, then the U-235 concentration will go up. This is known as uranium enrichment. This is done by a series of chemical and physical processes. In India, enrichment of uranium is done at the Rare Materials Plant, Ratnahalli, near Mysore.
If one were to use enriched uranium as fuel, the availability of neutrons is high enough to generate electricity and one can use light water as moderator and coolant.
A model of the AHWR at BARC, Trombay.
Reprocessing: The spent fuel, which comes out of the operation of a nuclear power reactor, will contain some U-235, which may not have undergone fission. The U-238, which is not fissile, decays over a period of time, and it contain s plutonium. Spent fuel is essentially a mixture of all these. If this spent fuel is dissolved in a series of chemical processes, U-238, U-235 and plutonium can be separated. This process of recovering plutonium, which is a man-made fissile material, is called reprocessing. The rest becomes low-level, intermediate-level and high-level radioactive waste.
Closed and open fuel cycles: The advantage in reprocessing/recovering plutonium is that plutonium and uranium can be used in FBRs to generate 50 times more electricity than the once-through route. Adopting the reprocessing route to ge nerate more electricity is called the closed fuel cycle. If spent fuel is not reprocessed to obtain plutonium, it is called the open fuel cycle.
The Department of Atomic Energy, keeping in mind the modest nature of the reserves of natural uranium available in India, has gone for the closed fuel cycle route. Thus, India’s nuclear power programme consists of three stages and reprocessing runs like a thread through these stages.
In the first stage, the PHWRs use natural uranium as fuel and heavy water as moderator and coolant. In the second stage, the plutonium reprocessed from the PHWRs and uranium will be used as fuel in the breeder reactors. The breeder reactors also use thorium as a blanket around their fuel core of uranium and plutonium oxide. This thorium in the breeder reactors is converted into the fissile U-233. In the third stage, thorium, which is abundant in India, and U-233 will form the fuel in new-generation reactors to generate even more electricity.
Source: S.K. Malhotra, Head, Environment and Public Awareness Division, DAE, and NPCIL publications.