BharatNotes Thorium
noun (mass noun; chemistry)Usage in a UPSC answer
Given its vast monazite reserves along the coasts of Kerala and Tamil Nadu, India has anchored the third stage of its three-stage nuclear programme on thorium, betting that an indigenous breeder route to clean energy can simultaneously advance energy security and its climate commitments.
Synonyms
Word Family
thorite (n), thoriated (adj), thorium-232 (n), thoric (adj), thorianite (n)
Root
Named after Norse god Thor (god of thunder) by Berzelius (1832); -ium = standard chemical element suffix
Etymology
Named in 1832 by Swedish chemist Jons Jakob Berzelius after Thor, the Norse god of thunder; the mineral thorite in which it was first identified was found in Brevig, Norway.
Memory Hook
THOR-ium: think of the hammer-wielding Norse god Thor powering future reactors — a "thunderous" source of nuclear energy named in his honour.
How This Word Works in UPSC Writing
A weakly radioactive element that India's nuclear strategy is built around, because the country holds modest uranium but among the world's largest thorium reserves in the monazite sands of the Kerala, Tamil Nadu, Odisha and Andhra coasts. Homi Bhabha's three-stage programme was designed for exactly this resource position. Stage 1 burns natural uranium in pressurised heavy water reactors and yields plutonium. Stage 2 uses that plutonium in fast breeder reactors, which produce more fissile material than they consume and can convert thorium-232 into uranium-233. Stage 3 then runs on that uranium-233. The sequence cannot be short-circuited, because thorium is fertile rather than fissile and must absorb a neutron and transmute before it can sustain a chain reaction. The long-pending second stage opened when the 500 MWe Prototype Fast Breeder Reactor at Kalpakkam attained first criticality on 06-Apr-2026.
Don’t Confuse It With
Fertile material such as thorium-232 or uranium-238 cannot sustain a chain reaction by itself and must first absorb a neutron and transmute, whereas fissile material such as uranium-235, plutonium-239 or uranium-233 can sustain one directly. This is why thorium is properly called a resource rather than a fuel until it has been converted. A breeder reactor generates more fissile material than it consumes, which is what makes the conversion possible at scale. Monazite is the phosphate mineral in coastal sands from which thorium is extracted, and it is also a significant source of rare earths.
Hindi Meaning
थोरियम; उर्वर (urvar) for fertile and विखंडनीय (vikhandanīya) for fissile.
Common Questions
- Why is thorium called fertile rather than fissile?
- Because it cannot sustain a nuclear chain reaction on its own; it must first absorb a neutron and transmute into fissile uranium-233.
- Why did the Kalpakkam fast breeder reactor matter?
- Its first criticality on 06-Apr-2026 marked India's entry into Stage 2 of the three-stage programme, the stage that breeds the uranium-233 on which the thorium-based third stage depends.
Seen in UPSC Question Papers
- Prelims 2022 — Economic Geography
- Prelims 2020 — Nuclear Technology
- Prelims 2017 — Nuclear Technology
- Prelims 2016 — Nuclear Technology
Real UPSC previous-year questions whose text uses “Thorium” — proof this word earns its place on your list.
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