After a long interval, attention has again turned to harnessing of nuclear energy.
The price rise in oil and gas markets, realisation that these resources may not last long and warning signs of climate change have each contributed in some measure to this. There still prevail, however, proliferation fears and concerns about long-term management of radioactive wastes, including plutonium in spent fuel.
It is in this context that the use of thorium, instead of uranium, in reactors holds promise.
Although it is not fissionable by itself, thorium generates a form of uranium that is a good fissile material. But, that is not amenable for use in nuclear weapons. Use of thorium greatly minimises generation of long-lived radioactive wastes. These features have led to a sort of resurgence of interest in the design of thorium-fuelled reactors.
The aim of every reactor designer is to make the reactor work with a minimum amount of fissile material and to make it very safe under all foreseeable conditions. For this, the designer seeks to know in detail how the neutrons interact with the various materials in the reactor.
This includes the materials introduced in the reactor initially as well as those that are produced during operation. Information of this kind is referred to simply as nuclear data.
Some of the nuclear data is obtained by mere calculations. Some are produced in laboratory experiments with neutron sources and some are generated by measurements in assemblies of fissile materials that are similar to reactors.
The reactor designer considers these data useful only if they have been properly validated using operating reactors as a benchmark.
Safe and efficient design of nuclear reactors also requires complex calculations. The success of these calculations is measured by the extent to which the results are proven by measurements in the reactors after they begin operation.
Aiming for the sun: India joins the world
The International Atomic Energy Agency has had a longstanding programme for compilation of validated nuclear data as well as calculative methods for reactor design purposes. The IAEA makes them available for interested users in member States. With over 400 power reactors, an equal number of research reactors and a large number of submarine reactors built so far, there is no dearth of data for uranium-fuelled reactors.
About 15 years ago, the IAEA began a project to establish similarly well-validated nuclear data for thorium-fuelled reactors. India was one of six countries that took an active part in this effort. As early as in the 1970s, Indian scientists had generated data about the interaction of neutrons with thorium. This also forms part of the world nuclear data on thorium.
Several reactors operating with thorium fuel were built in the 1970s. These reactors used a mixture of thorium and highly enriched uranium. By the end of the 1980s, they were closed down with plentiful availability of uranium. India is now perhaps the only country to have an operating reactor -- Kamini, the mini reactor at Kalpakkam -- using a form of uranium derived from thorium along with plutonium.
India is also the only country now with an ongoing programme to build a power-generating reactor -- Advanced Heavy Water Reactor, AHWR -- that will be fuelled with thorium and plutonium. The AHWR is designed to have many advanced safety features.
Indian interest in thorium stems from the fact that our thorium resource is five times as much as our uranium resource. There is now considerable interest worldwide in the use of thorium reactors, also as a means of utilising the large quantities of plutonium that have accumulated in some countries.
When participating in the IAEA programme for generating nuclear data for thorium, scientists from Bhabha Atomic Research Centre pointed out some of the limitations of the existing data and offered to refine it through measurements in the research reactor Kamini at Kalpakkam.
This has now been done and the Indian data from Kamini has been adopted as a benchmark.
Any further refinement of nuclear data for thorium by any group can be tested against the Kamini data to determine its acceptability. It must be said that Kamini is only one of many benchmarks available for design of thorium reactors. It is a benchmark for a reactor using ordinary water as a moderator and coolant and uses uranium as well as plutonium as fuel.
A large assembly of fissile material-bearing thorium is being set up by BARC at Trombay to simulate the core of a large reactor such as the AHWR. Such an assembly is known as a critical facility. When completed, measurements will be undertaken in this facility to verify the design of the AHWR.
There is a long history in the Department of Atomic Energy of working with whatever nuclear data is available from open sources and refining it to suit our purposes. Many years ago, when the Tarapur reactor was commissioned, the American supplier of the reactor had quoted a prohibitively high cost for providing the necessary guidelines for shuffling the fuel around in the core and for replacement of spent fuel with fresh fuel.
These operations have to be done with great care to preserve the safety status of the reactor and to extract the maximum energy from the fuel. And the guidelines can be worked out only with reliable nuclear data and elaborate computational schemes.
BARC scientists accepted the challenge and successfully worked out a proper scheme.
With the confidence derived from this test, they were later able to work out similar schemes for the heavy water reactors. Likewise, at Kalpakkam, the design of the Fast Breeder Test Reactor was worked out for a unique mixture of uranium and plutonium where the plutonium content is high. More recently, the Kalpakkam group has worked out the design of the Prototype Fast Breeder Reactor, PFBR, and the refuelling schemes for it.
L V Krishnan is former director, Safety Research and Health Physics Group, Indira Gandhi Centre for Atomic Research, Kalpakkam.
