Given its limited reserves of natural uranium and its abundant supply of thorium, India has chalked out a unique three-stage nuclear program. In the first stage, pressurized heavy water reactors (PHWRs)--similar to those used in advanced industrial countries--burn natural uranium. In the second stage, fast-breeder reactors, which other countries have tried to commercialize without success, will burn plutonium derived from standard power reactors to stretch fuel efficiency. In the key third stage, on which India's long-term nuclear energy supply depends, power reactors will run on thorium and uranium-233 (an isotope that does not occur naturally).
Scientists and engineers at the Bhabha Atomic Research Centre, in Mumbai, have designed a novel advanced heavy water reactor to burn thorium. They say that because no reactor in the world today uses thorium on a large scale, they will be breaking new ground. The head of the Mumbai reactor design and development group, Ratan Kumar Sinha, spoke to IEEE Spectrum's Seema Singh in July about the challenges of and prospects for this new thorium reactor technology.
IEEE Spectrum: Why do you call this advanced heavy water reactor one of a kind?
Ratan Kumar Sinha: No reactor in the world utilizes thorium on a large scale. We are the first ones to design such a system, which we are validating through an experimental program. In April, we started a test reactor, which has a flexible configuration and allows use of a range of fuel materials; we can even physically shift the distance between fuel rods. Here we are able to simulate the reactor almost 100 percent.
Spectrum: What are the unique features in this reactor?
Sinha: While we have used the well-proven pressure-tube technology, we've introduced many passive safety features, a distinguishing one being the reactor's ability to remove core heat by natural circulation of coolant under normal operating and shutdown conditions. This eliminates the need for nuclear-grade circulating pumps, which, besides providing economic advantages, enhances reliability.
We have also introduced passive shutdown on the main heat transport system in case of a failure of the wired shutdown system. Using mechanical energy from the increased steam pressure, the system injects neutron poison into the moderator [that sustains the nuclear chain reaction]. There are several other safety features, which are important, because they allow the reactor to be built close to the population.
Spectrum: What will the fuel assembly look like once it is operational?
Sinha: This is a vertical, pressure-tube-type, heavy-water-moderated, and boiling-light-water-cooled natural circulation reactor. The fuel assembly is 10.5 meters in length and is suspended from the top in the coolant channel. The fuel cluster has 54 pins arranged in three concentric rings around a central rod. The 24 pins in the outer ring have thorium-plutonium as fuel, and the 30 pins in the inner and middle rings have thorium-uranium-233 as fuel. The plutonium pins are placed in the outer ring to minimize the plutonium requirement. The thorium provides 60 percent of the reactor's power.
Spectrum: Is it designed for a longer life than the present generation reactors?
Sinha: Yes, the reactor is designed [to last] 100 years. Present-generation reactors have a design life of about 40 years, and many of the reactors in the West have been extended beyond that. However, what goes inside the core of our advanced reactors will have a lifetime of [only] about 30 years, so the design includes replacement of the material twice in the life of the reactor, which can be carried out during normal annual shutdowns. The reactor is also designed for on-power fueling.
Spectrum: You said earlier that no reactor today uses U-233, but India has a reactor called Kamini running on this fuel.
Sinha: That's a small 30-kilowatt reactor in Kalpakkam [near Chennai in the southern state of Tamil Nadu] that uses U-233 not for power generation but for neutron radiography. It tests and evaluates the fuel from the fast-breeder test reactor at Kalpakkam. But our reactor will produce 300 megawatts of electricity and 500 cubic meters per day of desalinated water for its own purposes.
Spectrum: Since India is still some years away from using fast-breeder reactors as a source of U-233 for its third-phase nuclear program, what role do you see for the reactor you're designing?
Sinha: This reactor will provide a platform for further research and development. In nuclear technology, the biggest emphasis is always on proven concepts. We have to have the timely development of thorium-based technologies for the entire thorium fuel cycle.
The supply of uranium is not perpetual. With the rate at which nuclear programs are growing worldwide, it is projected that by 2028 any new power plant will not have a guaranteed lifetime of uranium supply. So, one has to go for recycling as well as thorium. I don't see any shortcut as such.
Spectrum: What was the trickiest problem you faced while designing this?
Sinha: There were several, but the perennial challenge was to match the reactor's physics requirements with heat-removal requirements from the core. Physicists wanted to bring down the moderator use as low as possible, which meant the reactor had to be made very compact, with fuel rods being placed as close to one another as possible. The fuel rod spacing had to be reduced from the standard 270 millimeters to 245, and finally to 225 µm--something not attempted anywhere before. And that tremendously improved the performance of the reactor.
Another innovation was in differentially enriching the fuel [that is, boosting its fissile content] at the top and bottom of the central rod. The upper half has 2.5 percent enrichment; the lower half has 4 percent enrichment. This caused the power to jump from 230 MW to 300 MW.
Spectrum: Even though thorium has always looked attractive theoretically, why hasn't the technology taken off yet? What are the impediments?
Sinha: There has been interest in thorium in some other countries because of its proliferation-resistant nature, but no other country had the problem of uranium supply like India. In other countries, the economics were not in favor of thorium, so uranium became the fuel of choice.
Spectrum: Why was thorium not economical?
Sinha: Thorium has a much lower neutron multiplication rate than plutonium, and hence you cannot achieve power levels in a reactor as high as with plutonium. When burned, thorium initially acts like a blotting paper for neutrons and keeps absorbing them. But this exercise also means it is getting enriched and converted into U-233, which will pay dividends later on. Once the energy generated has reached 40 000 megawatt-days per metric ton, U-233 starts contributing many more neutrons than what has been lost in absorption by thorium. So you tend to get economic benefits of thorium if you have a fuel that can run up to 40 000 MWd/t and beyond. But most early generation reactors had lower burn-up values of around 15 000 to 20 000 MWd/t. These have, of course, risen to about 40 000 MWd/t in recent time. So the world is now thinking of thorium.
About the Author
Seema Singh is a journalist based in Bangalore, India, who writes about science and technology. She is a regular news contributor to IEEE Spectrum.
To Probe Further
For a critical assessment of India's nuclear ambitions, see M.V. Ramana's ”More Missiles than Megawatts.”