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ENERGY/M S S Murthy on a hybrid nuclear reactor
that makes nuclear power safer and cheaper
Hybrid n-reactors: Cheap and safe
The nuclear route to electricity generation as practised currently has become somewhat controversial due to safety and environmental reasons. Nuclear engineers have been trying innovative ideas to make nuclear power much safer and cost-effective. One of these is a hybrid reactor known as the Accelerator Driven Sub-critical System
(ADSS).
The essence of the current 400 and odd nuclear power plants is the controlled fission chain reaction in natural Uranium. This produces heat which is used to generate steam to drive the turbines. In order that the chain reaction becomes self-sustainable, a critical mass of nuclear fuel must be employed in these reactors. They are referred to as critical reactor systems (CRS). The most challenging aspect of operating a CRS is to precisely control the number of neutrons available for fission at any time. Any lapse in this can be catastrophic. It can lead to a critical accident in which the nuclear fuel may melt down and release radioactivity to the environment. Globally, 10,000 reactor-years of operation have witnessed two such major accidents.
Natural Uranium used as fuel in CRS consists of only 0.7 percent Uranium-235, which is fissionable. In some reactors this is slightly enriched. Nevertheless, the bulk is Uranium-238, which is not fissionable under the conditions of use. During the operation of the reactor, the non-fissile bulk parasitically absorbs neutrons and gets converted to heavier transuranic elements, which are collectively called actinides. This includes plutonium, which is one of the most dangerous materials. As they accumulate in the fuel, the reactor efficiency is reduced. Therefore, they have to be taken out and separated from the unburnt fuel at regular intervals. Many of the actinides are highly radioactive and have very long half-life. These have been accumulating over the past fifty years. They have to be safely contained for thousands of years to prevent any harm to the environment. No safe way of managing them is yet available. ADSS promises to provide answers to these questions.
What is an ADSS?
Unlike a conventional critical reactor system, ADSS works on a sub-critical fuel mass. Though fission chain reaction can be initiated in a sub-critical fuel assembly, it cannot be sustained since there are not enough neutrons. However, by continuously supplying neutrons from an external source, the system can be made to sustain a fission chain reaction.
To generate a copious supply of neutrons, recourse is taken to another type of nuclear reaction. When a high-energy particle such as a proton strikes a metal target, the target nuclei undergo a nuclear upheaval called spallation, generating a large number of high-energy neutrons. These can be directed to drive the fission chain reaction in a sub-critical reactor. Since each fission reaction releases about 200 million electron volt energy, the overall energy produced in the system is several times higher than the input proton energy. Hence, it is also called an energy amplifier. More importantly, the reactor, being sub-critical, is inherently safe. It cannot undergo a critical accident.
ADSS would not have been attractive only on these counts. To be economically viable it should have more attributes.
ADSS as a Breeder
Alternatively, the excess spallation neutrons can be made to breed a special type of nuclear fuel- Uranium-233. If Thorium-232, which occurs in nature in abundant quantities, but is non-fissile, is mixed with the sub-critical fuel in an ADSS, it absorbs the excess neutrons to produce protactinium. Protactinium is a short-lived radioactive element, which on decay becomes Uranium-233, which is fissile. Uranium-233 can be separated from the spent fuel and used as ‘seed’ fuel in other ADSS systems by mixing with appropriate quantity of Thorium.
Conceptually, an ADSS consists of an accelerator to generate an intense beam of protons. The end of the proton beam accommodates a spallation target of heavy metal like Lead, Tungsten, etc. Surrounding the spallation target is a blanket consisting of sub-critical fuel mass (generally Uranium-233) mixed with actinide waste as in an incinerator or Thorium as in a breeder. As in CRS, the heat generated by the fission reactions is removed by a primary heat transfer system to a steam generator, which turns the turbine. The reactor power is mainly controlled by controlling the proton beam from the accelerator. Turning off the proton beam shuts down the reactor safely.
Before an ADSS can go commercial, however, the accelerator technology has to be mastered. Preliminary experiments have shown that, to be economically viable, the accelerator must be capable of producing a continuous beam of protons of energy 1 GeV and current 10 to 15 milliamps. Such an accelerator has not yet been built anywhere in the world. It requires super conductor technology. However, there is enough experience with smaller accelerators. Alternatively, cyclotrons of a particular design called "Sectored Cyclotrons" can be adopted to accelerate the protons to the required energy.
Various design concepts are under study in different countries, depending on their national interests.
Indian programme
India has the world's largest deposits of Thorium in the beach sands of Kerala, but very limited natural uranium deposits. Indian design envisages a blanket of Thorium mixed with Uranium-233 fuel driven by a sectored cyclotron to produce about 300 MW electricity and to breed sufficient fuel to run more such systems. This essentially cuts down to a single stage the original three-stage program of Thorium utilisation. India also has experience in the construction and operation of cyclotrons, though of smaller capacity, at the Variable Energy Cyclotrons, Calcutta and the Centre for Advanced Techno-logy, Indore. A prototype Uranium-233 reactor has also been tested at the Indira Gandhi Centre for Atomic Research at Kalpakkam, Tam-ilnadu. Scientists at the Bhabha Atomic Research Centre, Mumbai have developed another design called ‘one way coupled fast-thermal ADSS’. It has the advantage that a given reactor power can be achieved by a much lower proton beam current."
Last December, Chairman, Atomic Energy Commission announced the formation of a high power committee consisting of the Department of Atomic Energy, Department of Science & Technology, and others to study these new concepts and the possibility of upgrading the technologies for the ADSS requirements. Rupees 100 crore has been provided in the Tenth Plan. It is estimated that ADSS may become a reality in the next 15 to 20 years.
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