Apart from coal, thorium is the only abundant energy source in India. Recent developments at the Bhabha Atomic Research Centre (BARC) give hope that this mineral can be India’s answer to green energy.
BARC is working on several technologies for using thorium, of which two are noteworthy.
One is the ‘Indian high-temperature reactor’ (IHTR), which is designed to produce heat and, in turn, produce hydrogen through the ‘sulphur-iodine’ method. In this process, sulphuric acid decomposes into oxygen, sulphur dioxide and water under high temperature. When iodine is added to sulphur dioxide and water, you get hydrogen iodide, which again splits into hydrogen and iodine at high temperatures. The fuel for this reactor is a mixture of uranium-233 dioxide and thorium dioxide. Uranium-233 does not occur naturally and is obtained by the mutation of thorium in reactors.
The other important thorium technology at BARC is the ‘ Indian molten salt breeder reactor’ (IMSBR). This 5-MW pilot project in Visakhapatnam is cloaked in secrecy, with BARC declining to share information about it.
Its design has two interesting features. First, it pulls more uranium-233 from thorium and is, therefore, considered better than the fast-breeder reactors like the one under construction (for decades) in Kalpakkam, near Chennai. The IMSBR is also a ‘breeder reactor’, which means it produces more that it consumes.
This happens when you place uranium-233 inside the reactor core and a blanket of thorium around the core. When neutrons bombard the fissile nuclei of uranium-233, energy is produced, but some of the neutrons hit the thorium nuclei and convert it into uranium-233. Thus, even as uranium-233 is burnt inside the reactor, more of it is produced (from thorium) outside the reactor.
However, there is a problem. Thorium does not mutate into uranium-233 straightaway; it first turns into an isotope of an element known as protactinium. The isotope, protactinium-233, turns into uranium-233. Meanwhile, during its brief life, protactinium-233 is also bombarded by neutrons and it partly absorbs the neutrons to become uranium-234, which is not fissile and therefore useless. The longer protactinium tarries in the reactor, the lower the output of uranium-233 from thorium.
BARC’s design, on the other hand, features a fluid fuel instead of the solid fuels used in conventional reactors. Liquid fuels can be pumped in and out unlike solid fuels, which stay in place until they burn out completely. In the IMSBR, the fuel is a liquid thorium-based salt. It can be pumped out to ensure that the decay of protactinium-233 into uranium-233 happens outside the reactor, leading to a higher output than in conventional reactors. There are a few more advantages, too. These, as BARC scientist IV Dulera notes in a paper, include continuous removal of xenon and krypton, resulting in improved neutron economy, negative fuel salt reactivity coefficient and other technical benefits.
BARC has developed an appropriate salt for this purpose (LiF-CaF -ThF –UF) which is circulated by pump through the reactor core. (Elsewhere in the world, researchers are coming up with better salts, which are chloride-based rather than fluoride-based (such as NaCl-ThCl4-PlCl3).
BARC has also developed other things such as pumps, valves, flow meters and heat exchangers. It has also concocted an alloy for the pipes conducting the molten salt.
The second interesting feature of the IMSBR is how it produces electricity. It does away with the conventional, inefficient Rankine cycle, where water boils to steam and turns the turbine. Instead, the IMSBR is to be married to the highly efficient Brayton cycle, where supercritical carbon dioxide (namely, carbon dioxide that is in a state between liquid and gas at a certain temperature and pressure) is used to drive the turbines, thereby cutting water use and producing a lot more electricity.
In response to an emailed query from Quantum, Dr Soumyakanti Adhikari, Head, Scientific Information Resource Division, BARC, said: “The MSBR and HTR are promising technologies among other available options and constitute a part of our continuing developmental endeavour. Both these systems and the associated fuel cycle aspects are in exploratory evaluation and initial assessment stages.” The aim is to gain a sound understanding of the challenges inherent in these reactors and fuel systems, and the likely solutions to them, he said.
When the IMSBR demonstration plant comes up at Visakhapatnam, it would mark a significant step in India’s green energy journey with the entry of the thorium cycle. It appears as if the electricity generating IMSBR and the hydrogen producing IHTR could be the vehicles taking India closer to its net-zero target. Both use thorium — which is abundant in India and, yet, intriguingly absent in its green transition narrative.
