Q. 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? (UPSC Prelims 2024)
Answer: 2 and 3 only
Notes: The correct answer is [B] 2 and 3 only. Radioisotope Thermoelectric Generators (RTGs) are "nuclear batteries" that convert heat into electricity, but they function very differently from standard nuclear reactors.

• Statement 1 is Incorrect: RTGs are not fission reactors. A fission reactor works by triggering a chain reaction of splitting atoms (induced fission). In contrast, an RTG harvests heat from the natural radioactive decay of an isotope. It has no moving parts and does not involve a controlled chain reaction.
• Statement 2 is Correct: RTGs are ideal for spacecraft traveling to the outer solar system (like Voyager, Cassini, and New Horizons) or operating on planetary surfaces (like the Mars rovers Curiosity and Perseverance) where solar energy is too weak or unavailable. They provide a steady, long-lasting power source.
• Statement 3 is Correct: Plutonium-238 is the most common fuel for RTGs due to its high heat density and long half-life (87.7 years). While it is not the fissile material used in nuclear bombs (which is Plutonium-239), Pu-238 is a by-product produced in nuclear reactors during the process of weapons-grade plutonium production or through specialized irradiation of Neptunium-237.

The conversion of heat to electricity in an RTG is achieved through the Seebeck effect using thermocouples, making it a highly reliable power source for missions lasting several decades.