What is the single greatest factor that prevents the large-scale deployment of thorium-fuelled reactors in India? Most people would assume that it is a limitation of technology, still just out of grasp. After all, the construction of the advanced heavy-water reactor (AHWR) — a 300 MWe, indigenously designed, thorium-fuelled, commercial technology demonstrator — has been put off several times since it was first announced in 2004.
However, scientists at the Bhabha Atomic Research Centre have successfully tested all relevant thorium-related technologies in the laboratory, achieving even industrial scale capability in some of them.
In fact, if pressed, India could probably begin full-scale deployment of thorium reactors in ten years. The single greatest hurdle, to answer the original question, is the critical shortage of fissile material.
A fissile material is one that can sustain a chain reaction upon bombardment by neutrons. Thorium is by itself fertile, meaning that it can transmute into a fissile radioisotope but cannot itself keep a chain reaction going.
In a thorium reactor, a fissile material like uranium or plutonium is blanketed by thorium. The fissile material, also called a driver in this case, drives the chain reaction to produce energy while simultaneously transmuting the fertile material into fissile material.
India has very modest deposits of uranium and some of the world’s largest sources of thorium. It was keeping this in mind that in 1954, Homi Bhabha envisioned India’s nuclear power programme in three stages to suit the country’s resource profile.
In the first stage, heavy water reactors fuelled by natural uranium would produce plutonium; the second stage would initially be fuelled by a mix of the plutonium from the first stage and natural uranium.
This uranium would transmute into more plutonium and once sufficient stocks have been built up, thorium would be introduced into the fuel cycle to convert it into uranium 233 for the third stage. In the final stage, a mix of thorium and uranium fuels the reactors.
The thorium transmutes to U-233 as in the second stage, which powers the reactor. Fresh thorium can replace the depleted thorium in the reactor core, making it essentially a thorium-fuelled reactor even though it is the U-233 that is undergoing fission to produce electricity.
After decades of operating pressurised heavy-water reactors (PHWR), India is finally ready to start the second stage. A 500 MW Prototype Fast Breeder Reactor (PFBR) at Kalpakkam is set to achieve criticality any day now and four more fast breeder reactors have been sanctioned, two at the same site and two elsewhere.
However, experts estimate that it would take India many more FBRs and at least another four decades before it has built up a sufficient fissile material inventory to launch the third stage. The earliest projections place major thorium reactor construction in the late 2040s, some past 2070. India cannot wait that long.
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