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India's Thorium Dream

Most of the world's reactors use uranium - so why is India so keen on Thorium?

Nuclear reactors utilise the heat energy released from the fission of the nucleus of a large atom. The most common fuel used in most commercial reactors is enriched uranium, which is 10% uranium 235, and 90% uranium 238. Most of the energy is won from the 235. The greatest part of the fuel is therefore not exploited, but nevertheless ends up as highly dangerous nuclear waste, which causes enormous headaches for its handling and disposal.

There has been a great deal of interest, therefore, in finding alternatives to U-235. One possibility is the breeder reactor, so-called because it 'breeds' a fissile fuel, plutonium, from non-fissile uranium-238. Yet this wonder technology has been tried and so far found to be impracticable.

India has been pursuing an alternative, motivated by the fact that it has no uranium resources of its own, but it does have ample resources of thorium: a quarter of the world's resources! And anyway, there is four times as much thorium in the Earth's crust than uranium, so it would be a good idea for everywhere. So what is holding it up?

India has had a steady increase in demand for electrical power right through the 20th century, yet it does not possess any natural energy resources, except dirty coal.

In the 1950s, it began to investigate nuclear energy as a possible alternative to the burning of polluting coal, or the importation of costly oil and gas. Since it began its nuclear programme, it has been aware that importing uranium was going to have political and economic issues which made it vulnerable.

It does, however, have a potential solution in thorium - a slightly lighter radioactive element, which undergoes a natural decay with a half-life of 14 billion years - the age of the universe! This makes it much less naturally fissionable than uranium-235 (703 million years), or even sluggish uranium-238 (4.5 billion years).

However, if a neutron were caused to impact the nucleus of a aleepy thorium atom, the situation would be very different. Abundant Thorium-232 becomes rare Thorium-233, which then beta decays to Protactinium-233, before settling on Uranium-233. And U-233 is a possible nuclear fuel, which packs quite a punch!

${\table {232};{90}}$Th $ + {\table {1};{0}}$n $ → {\table {233};{90}}$Th + γ    $ {→}↖{β-} $   ${\table {233};{91}}$Pa     ${→}↖{β-} $   $ {\table {233};{92}}$U

So, where is the problem?

One obstacle has been getting that magic bullet, the neutron, to impact the thorium nucleus - at least in a sufficiently controllable manner that a sustainable chain reaction ensures to safely generate enough U-233 for the purposes of power generation. This hurdle has held up development for half a cuentury.

The Nuclear Power Corporation of India (NPCIL) is building an Advanced Heavy Water Reactor (AHWR), designed to generate 300 MW. The hope is that the technology will be safer and cleaner than conventional nuclear plants - so safe, in fact, that such reactors can be built within large metropolitan areas.

Cleaner because thorium reactors produce much smaller quantities of radioactive waste, in particular the highly dangerous plutonium by-product of U-238-containing fuel. Thorium decay products have much shorter half-lives. On the one hand, this ensures the waste will lose its radioactivity on a shorter timescale, but on the other, any accident would release far more radioactive material into the environment - not a popular risk management scenario.

India's pursuit of thorium has left its nuclear capacity trailing behind other developing nations, like China, which has embraced uranium. India produces just on 5 GW from nuclear power, which is just over 2% of the nation's demand for electricity. China, in comparison, has more than 15 GW, even though its nuclear programme started a third of a century after India's.

The promise of thorium joins other nuclear energy hopefuls for the future of nuclear power. These include self-sustaining uranium-238 breeders, and fusion. As nuclear countries around the world watch in dismay their waste stockpiling, and their political liabilities mounting, hopes rest on a breakthrough, which may come from India.

AHWR, Advanced Heavy Water Reactor, reactor vessel head, the business end of the reactor

Article by Andrew Bone, January 19, 2016

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