UPSC Prelims 2024 Q31: Radioisotope Thermoelectric Generators — Explained
With reference to radioisotope thermoelectric generators (RTGs), consider the following statements:
1. RTGs are miniature fission reactors.
2. RTGs are used for powering the onboard systems of spacecrafts.
3. RTGs can use Plutonium-238, which is a by-product of weapons development.
Which of the statements given above are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Answer: (b) 2 and 3 only
Statement-wise explanation
- 1: incorrect — An RTG is not a reactor of any kind. It has no chain reaction, no fission, no control rods and no moving parts. It produces heat from the natural radioactive decay of an isotope such as plutonium-238. Thermocouples then turn that heat into electricity using the Seebeck effect. NASA states plainly that RTGs generate power through decay, not fission.
- 2: correct — RTGs supply electricity to a spacecraft's instruments, computers, heaters and radios, especially where sunlight is too weak or unreliable for solar panels. According to NASA and the US Department of Energy, more than two dozen US missions have flown with radioisotope power since 1969. These include Voyager 1 and 2, Cassini, New Horizons and the Curiosity and Perseverance rovers. The Multi-Mission RTG (MMRTG) gives about 110 watts of electricity when newly fuelled.
- 3: correct — Plutonium-238 is the standard RTG fuel. It has a half-life of about 88 years, and it gives off alpha particles, which are easy to shield. The US made it at the Savannah River Site in South Carolina from the 1960s to the late 1980s, in reactors built for the nuclear-weapons programme. That is why it is called a by-product of weapons development. Pu-238 is not itself weapons-grade: bombs use Pu-239, and NASA notes that Pu-238 'would not work well as fuel in a nuclear reactor'.
How to eliminate
Statement 2 is the anchor. Anyone who has read about Voyager or the Mars rovers knows spacecraft carry nuclear power sources, so 2 is almost certainly correct. That leaves only the options containing 2. Now test Statement 1 using the name itself: 'radioisotope' points to decay, and 'thermoelectric' points to heat being turned into electricity. Neither word suggests splitting atoms. A 'miniature fission reactor' would be a different device, a space reactor such as the old US SNAP-10A or the Soviet BES-5. Once 1 is dropped, only the pair 2 and 3 is left, so you do not even need to be sure about Statement 3. The fact to remember: RTG = radioactive decay + thermocouples, no fission, fuelled by Pu-238.
Know more
How an RTG works. A radioisotope with a suitable half-life decays naturally and heats up. One junction of each thermocouple touches this hot fuel. The other junction faces cold space or a planet's atmosphere. The temperature difference drives a current (the Seebeck effect, discovered in 1822). Nothing moves, so there is nothing to wear out. Some of Voyager's RTGs, launched in 1977, still work. Output falls slowly as the fuel decays and the thermocouples age.
Why plutonium-238? It has a half-life of about 88 years, so its power lasts for decades. It gives about 0.5 W of heat per gram. It emits mostly alpha particles, which need very little shielding. It is packed as a ceramic plutonium dioxide that breaks into large chunks, not fine dust, if there is an accident. After US production stopped in the late 1980s, the US bought Pu-238 from Russia. Since 2011 it has restarted production at Oak Ridge and Idaho National Laboratories.
| Feature | RTG | Fission reactor |
|---|---|---|
| Energy source | Natural radioactive decay | Controlled chain reaction |
| Typical fuel | Pu-238 | U-235 / Pu-239 |
| Moving parts / control rods | None | Yes |
| Can be switched off | No, decays continuously | Yes |
| Output | Tens to hundreds of watts | Kilowatts to gigawatts |
Related devices. A radioisotope heater unit (RHU) makes about 1 W of heat but no electricity. It keeps instruments warm. Reports say Chandrayaan-3's propulsion module carried two RHUs built by BARC. Betavoltaic 'nuclear batteries' are a different technology again: they turn beta radiation directly into a small current.
Where RTGs are used. Mostly on deep-space missions, where solar power is too weak, and on missions that must survive long nights, dust storms or polar shadow.
In the news
Nuclear power for spacecraft was in the news in 2023–24. Chandrayaan-3's propulsion module was reported to carry BARC-built radioisotope heating units, and NASA's Dragonfly mission to Titan, which will run on an MMRTG, was moving towards confirmation.
NCERT Concepts to Revise
- Nuclear energy: Statement 1 depends on separating heat from splitting nuclei (fission) from heat released by natural radioactive decay.