- What is Thorium?
Thorium is a radioactive element of atomic number 90 that is theoretically capable of being used as a fuel for generating large amounts of energy through fission. Thorium is estimated to be about three to four times more abundant than uranium in the Earth’s crust, and is chiefly obtained from monazite sands as a by-product of extracting rare earth metals (Scandium, Yttrium and fifteen lanthanides). Thorium-232 is the most stable and abundant isotope of thorium (thorium-227, 228, 229, 230, 231, 232, and 234) while the other natural isotopes occur only in traces.
- How does it work?
As stated earlier, Thorium is ‘theoretically’ capable of being used as a fuel, which means it cannot be a fuel all by itself. Unlike Uranium and Plutonium, Thorium contains very little, to no, fissile material and is incapable of initialising a chain reaction, which is needed in a nuclear reactor too keep the energy production going. It therefore needs a trigger- a source of neutrons – that start a reaction within Thorium and convert it into Uranium233, which is fissile and capable of running the reactor for a long time. When Th232 absorbs a neutron, it becomes Th233 which is unstable and decays into protactinium-233 which further decays into U233. This final product can then be used in reactors as a nuclear fuel, one that is fissile on its own. Thus, supplying Thorium with a continuous source of neutrons ensures the continuous production of U233, and this forms the basic idea behind Thorium breeding.
- What reactors can use Thorium?
Thorium can be used in six types of reactors – Heavy Water reactor, High Temperature Gas Cooled reactor, Light Water reactor, Fast neutron reactor, Molten Salt reactor and Accelerator Driven reactors. Out of these, the first four have entered operational service at some point of time, whereas, the last two are still conceptual. .
- Is it being used now?
Thorium found uses in many different kinds of test reactors in the early stages of nuclear power development but was first commercially used in HTGR in Germany (1985–1989) and US (1976–1989), where at both places the reactors had to be shutdown because of economic and maintenance issues. Currently many countries like US, UK, Germany, Brazil, India, China, France, Czech Republic, Japan, Russia, Canada, Israel and Netherlands are pursuing Research and Developmen in Thorium based reactors, primarily MSRs including Liquid Flouride Thorium Reactors (a type of MSR). 
- Why could it be useful?
Other than its abundance on Earth’s surface, there are many reasons that may make the Research and Development of Thorium fuel worth the efforts. 
- Possibility of breeding in a thermal reactor – The absorption cross-section for thermal neutrons of Thorium232 is nearly three times that of Uranium238. Uranium238 is much more abundant than Uranium235, which is the issotope that is used for nuclear fissiun. U235 and U238 are naturally found together (with about 0.7% U235, meaning it is often enriched to a higher precentage). Because of the higher cross section, a higher conversion of Thorium232 to useful fuel (U233) is possible than with U238 (to 239Pu). Thus, thorium is a better ‘fertile’ material than 238U in thermal reactors. This could enable ‘slow neutron breeder reactors’ which will be much safer than fast breeder reactors.
- Reduced production of minor actinides – In Th232–U233 fuel conversion, a much lower quantity of plutonium and long-lived Minor Actinides (Np, Am and Cm) are formed as compared to the 238U–239Pu fuel conversion, thereby minimizing the toxicity and decay heat of spent fuel.
- Burning of present Plutonium stockpile – The initial neutron source needed to initiate the transition to thorium could be done through the incineration of weapons grade plutonium (WPu) or civilian plutonium.
- Reduced nuclear proliferation – U233 is always contaminated with U232, which has daughter product Tl-208 emitting gamma radiation of 2.6MeV. This makes it easy to detect illegal storage and transportation and reduces the possibility of theft for weapons.
- Is there any catch?
Thorium certainly has great potnetial, but being an unnatural fission source (not capable of undergoing fission itself), it comes with its own set of challenges.
- One of the biggest challenges in developing a thorium reactor, is finding a way to fabricate the solid fuel economically. Making thorium dioxide is expensive, partly because of its very high melting point of 3,300° C.
- The options for generating neutrons needed to start the reaction, more often than not, come down to uranium or plutonium, so not completely irradicating these elements from the fuel.
- Making bombs using the fissile U233 is difficult but not impossible (operation Teapot). Although it will be much more expensive as separation of Pa233 is not easy while the reactor is running and any unwarranted diversion would not remain unnoticed, and because of the hard gamma from U232. Therefore, Th based reactors must be provided with the similar set of safeguarding rules and regulations as the current fleet.
- Presence of U232, which reduces proliferation risk in Th reactors, also makes the spent fuel handling and reprocessing more expensive due to strong shielding needed against the gamma radiation.
- Why hasn’t it been used before?
After weighing the pros and cons of Th as a fuel and realizing its potential, the first question is why wasn’t it used till now? Though there is no single answer for this, it is considered to be a result of the times (mid-twentieth century) when nuclear elements were being explored more because of their potential for weapons than for energy. The U-Pu chain was preferred because of the need to extract Pu and over the years the whole infrastructure for reactors got established with this cycle. Now, the knowledge and experience of thorium fuels and thorium fuel cycles are very limited, as compared to Uruanium Oxice (UO2) and Mixed Oxide (UO2 and PuO2) fuels, and need to be enhanced before large investments are made for commercial utilization of thorium fuels and fuel cycles.
- Why is India so interested in it?
Being endowed with a quarter of world’s Th reserves and a notable paucity of Uranium reserves, India has an understandable interest in developing Thorium based reactors. There is an active research & development programme in fabrication, characterization and irradiation testing of ThO2, ThO2-PuO2 and ThO2-UO2 fuels in the test and power reactors. 
- Fuel bundles containing high density ThO2 fuel pellets are being used in a few Pressurised Heavy Water Reactors (PHWRs) for flux flattening in the initial core.
- ThO2 pins and sub-assemblies are used as axial and radial blankets in Fast Breeder Test Reactor (FBTR) operating at Kalpakkam.
- Currently India is working on the design of an advanced heavy water reactor (AHWR), specially designed with thorium in mind. The Indian Molten Salt Breeder Reactor has also been conceptualised.
So, using Th fuel is really a trade-off between its desirable and undesirable properties and it comes down to the individual’s and a country’s needs as to what they prefer. The current times dictate the need for advanced reactors with higher efficiency and more safety. They can work well with U-Pu fuel and can be tested with Th-U fuel. Let research groups work on researching innovative fuel, material, etc. and commercial players work upon what they can deliver to us safely and economically. Until the Th fuel cycle establishes fully, focus should be more on what will make nuclear more efficient overall, and once this is achieved, we can move to a wider variety of fuels that have a great potential for the future.
- http://bit.ly/23MQYr9 (IAEA-TECDOC-1450 : Thorium fuel cycle — Potential benefits and challenges)
- http://bit.ly/1TI32p5 (IAEA-TECDOC-1319 : Thorium fuel utilization: Options and trends)