November 19, 2019

Thorium - cleaner, safer & abundant

All discussions on nuclear energy revolve around uranium and plutonium, the stuff with which we can make atom bombs. But not too many outside atomic energy circles would have heard about their close cousin, thorium, which is another natural element that can be used like uranium to generate nuclear energy. In this article, we explore how Thorium can be a cleaner, safer and more abundant alternative.

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Elements, like hydrogen (H), helium (He), carbon (C), oxygen (O) all the way to uranium (U) that are found in the Earth’s crust are defined in terms of the number of protons in the nucleus (the atomic number) and the number of protons+neutrons (the atomic weight). Chemical properties are determined by the atomic number and elements that share the same atomic number but have different atomic weights are called isotopes of each other. Thus the common Carbon 12 and the relatively rare Carbon 14 are isotopes that both have 6 protons but have 6 or 8 neutrons respectively. The atomic weight determines the nuclear behaviour of atoms.  Under specific circumstances, certain  isotopes of some elements  disintegrate to generate isotopes of other, lighter elements and in the process release energy. This is called nuclear fission. The traditional route to generating nuclear energy involves the collision of a neutron with the nucleus of an isotope of uranium, namely U-235. This results in the fission or break-up of U-235 atom into barium-141 and krypton-92 atoms along with the release of three more neutrons and lots of heat. This heat is used to generate electricity but the three neutrons can  cause fission in three other U-235 atoms. If this happens too fast, we have a ‘military’ nuclear explosion but when carefully controlled, it leads to the steady release of immense amounts ‘civilian’ nuclear power.

Uranium and thorium can both be used to generate nuclear power but there are  fundamental differences in the way these two neighbouring elements in the periodic table can be used. To understand the difference we need to distinguish between elements being fissile and fertile. Atoms of  fissile elements like uranium 235 (U-235) split apart, or experience fission,  to give rise to ‘lighter’ elements and release energy as heat. Atoms of fertile elements like uranium 238 (U-238) can be converted to fissile elements like U235 and only then can be used to build atom bombs or generate nuclear power.

U-235 is the only fissile element that occurs naturally but it forms only 0.7% of uranium that is mined. Hence, the uranium ore, that is 99.3% U-238 needs to be processed to increase the concentration of U-235 to at least 5% before it can be used in nuclear reactors. This is a complex operation that needs sophisticated and expensive centrifuge technology. The U-235  in the enriched fuel undergoes fission, releases energy and neutrons. These neutrons can behave in two ways :  (a) Fast moving neutrons hit and split other U-235 in a chain reaction to continue generating energy, that unless curbed or moderated by eliminating neutrons, can create a nuclear explosion or (b) Slow moving neutrons are absorbed by the fertile U-238, that surrounds the U-235 fule, to create fissile plutonium 239 (Pu-239) plus many other trans-uranic, toxic and radioactive by-products.

Since U-235 is very scarce, the Pu-239 that is generated from abundant U-238 becomes the real nuclear fuel for the subsequent phase of operations.  This fissile Pu-239 can be used to both build atom bombs or generate nuclear power. But this Pu-239 has a problem. On being hit with neutrons, as in the case of  U-235 fission,  only 65%  of the Pu-239 undergoes fission and the other 35% ends up as useless Pu-240 that is highly radioactive and toxic that needs secure and expensive storage facilities.

Because of Pu-239’s one-in-three absorption problem, the reactor cannot make as much fresh radioactive fuel to replace what is being consumed. The ‘spent’ fuel cannot generate sufficient heat and has to be processed to remove the unusable and useless, but still radioactive parts. So a lot of uranium and plutonium is eventually not used but remains radioactive for thousands of years and leads to significant and dangerous pollution of the environment.

The chances of useful fission of Pu-239, as opposed to conversion to the useless Pu-240 can be increased if we stop slowing down the neutrons and allow them to travel fast -- as in fast breeder reactors. But fast neutrons can lead to run-away chain reactions and subsequent nuclear explosions and so are inherently dangerous. So the choice is between the danger of a nuclear explosion or that associated with generating mountains of toxic radioactive waste. This is why uranium based nuclear power is unpopular.

Fortunately, the use of thorium can remove both these problems.

Thorium 232 (Th-232) like U-238 is fertile but not fissile. However like U-238, Th-232 can absorb neutrons, fast or slow, and gets converted to Th-233 and subsequently to uranium 233 ( U-233) that is fissile, just like Pu-239 and can be used to generate more energy. So both U-233 and Pu-239 can be generated, or bred, in breeder reactors that create more fuel than what they consume. But unlike Pu-239 where only a fraction of Pu-239 undergoes fission and is used, the U-233 route is very efficient because almost all of it can be used. This means that almost all U-233 (more than 95% compared to 65% in Pu-239) is consumed to generate energy and so there is far less radioactive waste left over. Moreover, U-233 does not need fast neutrons for fission and so there is no need for the accident prone fast breeder technology. The far safer thermal breeder technology is good enough.

Th-232 is a replacement for U-238 but not the crucial U-235 that is required to kick start the process of fission of U-235 and generate the initial shower of neutrons that convert fertile Th-232 to fissile U-233. This no different from the enriched U-235 that is required to convert the fertile U-238 to fissile Pu-239. Except that Th-232 is far more abundant U-238.

If we leave aside the uranium found in sea-water, which is very difficult to extract, then the total quantity of thorium found in the earth’s crust is more than three times that of uranium. Even though not all of this is exploitable, this is still a bonanza because, unlike U-238 ( and Pu-239) most of the energy in  thorium (Th-233 / U-233) can be extracted and used. Hence thorium reserves will last much longer than uranium and for us in India, there is a double bonanza, because India has the maximum known reserves of thorium in the world.

Known and exploitable reserves of uranium ore across the globe are estimated to be about 7.6 million tonnes (MT) of which only 0.138 MT  (1.8%) is in India. Thorium has never been explored as intensively as uranium and estimates of reserves differ. 2005 IAEA estimates show global reserves of  2.8 MT out of which a good 21% or 0.519 MT is in India. 2011 USGS estimates put global reserves at 1.91 MT of which 50% or 0.963 MT is in India. The Government of India’s own estimates, as reported by the Prime Ministers Office in 2016 in Parliament state that our coastal areas have 11.935 MT of Monazite sands of which 9-10% ( ~ 1 - 1.1 MT) is thorium dioxide and that too on the easily accessible beaches of the Bay of Bengal. All estimates agree that India has the largest known reserves of thorium.

So as far as India is concerned, thorium is a nuclear fuel that (a) is safe, since it can be used with the thermal breeder technology that has a low risk of explosions (b) generates far less radioactive wastes so that  containment and management is much simpler and (c ) is readily and easily available without having to go begging to international regulators of non-proliferation.

Who could ask for anything more? Strangely enough, one could!

Thorium is also the preferred fuel for a radically different type of nuclear reactor -- the molten salt reactor (MSR) -- that has been proven to be far more safer than anything that we have today. MSRs have many benefits but the key is that if and when things go wrong, as in a power failure, the atomic ‘fire’ in the MSRs shuts down automatically. This is very different from the case of Chernobyl, Three Mile Island and  Fukushima where the failure of cooling systems led to an explosion or a meltdown. MSR is a stunning new technology that is inherently safer, but the key point is that MSRs are so thorium friendly that many people confuse one technology for the other even though they represent two completely different aspects of a new generation of safe nuclear technology.

So what is India doing with Thorium? We will explore that in the next article.

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