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Q31 (IAS/2024) Science & Technology › Space & Defence Technology › Space exploration missions Official Key

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 ?

Result
Your answer: —  ·  Correct: B
Explanation

The correct answer is option B (statements 2 and 3 only).

**Statement 1 is incorrect**: RTGs are not fission reactors, nor is the plutonium the type that is used for nuclear weapons.[1] Instead, RTGs harness the heat produced by radioactive decay rather than a nuclear chain reaction.[2]

**Statement 2 is correct**: Radioisotope thermoelectric generators (RTGs) have been the main power source for US space work since 1961.[3] For example, Cassini's science instruments and onboard systems was generated by three RTGs, known as [General Purpose Heat Source (GPHS) RTGs].[4]

**Statement 3 is correct**: Most RTGs use plutonium-238.[5] The high decay heat of plutonium-238 (0.56 W/g) enables its use as an electricity source in the RTGs of spacecraft, satellites, navigation beacons and so on.[6] Additionally, the documents indicate that it may be available within Europe as an unwanted by-product of the legacy nuclear fuel reprocessing cycle[7], which is associated with weapons-related nuclear programs.

Therefore, only statements 2 and 3 are correct.

Sources
  1. [1] https://science.nasa.gov/mission/cassini/radioisotope-thermoelectric-generator/
  2. [2] https://marspedia.org/Radioisotope_Thermoelectric_Generators:_Advantages_and_Disadvantages
  3. [3] https://www.sciencedirect.com/topics/earth-and-planetary-sciences/plutonium-238
  4. [4] https://science.nasa.gov/mission/cassini/radioisotope-thermoelectric-generator/
  5. [5] https://world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space
  6. [6] https://www.sciencedirect.com/topics/earth-and-planetary-sciences/plutonium-238
  7. [7] https://www.sciencedirect.com/topics/earth-and-planetary-sciences/plutonium-238
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Don’t just practise – reverse-engineer the question. This panel shows where this PYQ came from (books / web), how the examiner broke it into hidden statements, and which nearby micro-concepts you were supposed to learn from it. Treat it like an autopsy of the question: what might have triggered it, which exact lines in the book matter, and what linked ideas you should carry forward to future questions.
Q. With reference to radioisotope thermoelectric generators (RTGs), consider the following statements : 1. RTGs are miniature fission react…
At a glance
Origin: Mostly Current Affairs Fairness: Low / Borderline fairness Books / CA: 0/10 · 7.5/10
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A classic 'Definition + Application' S&T question. It punishes the lazy heuristic that 'Nuclear = Fission'. While standard books miss the specific RTG definition, the answer relies entirely on distinguishing passive radioactive decay (batteries) from active chain reactions (reactors). If you catch the error in Statement 1, the question solves itself.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
Are radioisotope thermoelectric generators (RTGs) miniature fission reactors?
Origin: Web / Current Affairs Fairness: CA heavy Web-answerable

Web source
Presence: 5/5
"RTGs are not fission reactors, nor is the plutonium the type that is used for nuclear weapons."
Why this source?
  • Explicitly states RTGs are not fission reactors.
  • Describes RTGs as 'nuclear batteries' but distinguishes them from reactors and weapon-grade plutonium.
Web source
Presence: 5/5
"RTGs ... harness the heat produced by radioactive decay rather than a nuclear chain reaction."
Why this source?
  • Contrasts RTGs with reactors by saying they harness heat from radioactive decay rather than a nuclear chain reaction.
  • Makes clear RTGs do not operate via fission chain reactions like reactors do.
Web source
Presence: 4/5
"Thus, 239Pu is warm to the touch, but 238Pu glows red hot. This characteristic is the basis for use of 238Pu in radioisotope thermoelectric generators (RTGs). Plutonium-238 decays by α-particle emission to"
Why this source?
  • Explains that Pu-238 produces heat through radioactive decay (α-particle emission), which is the basis for RTGs.
  • Supports that RTG heat source is decay heat, not an ongoing fission chain reaction.

Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 4: Earths Interior > Radioactive Decay > p. 58
Strength: 4/5
“• The high temperature below the crust is attributed to the disintegration of the radioactive substances. Nuclear decay happens primarily in the crust and the mantle.• Scientists believe that uranium is sufficiently concentrated at the base of Earth's mantle to ignite selfsustained nuclear fission, as in a human-made reactor. The new measurements suggest radioactive decay provides more than half of Earth's total heat.”
Why relevant

States that radioactive decay produces high temperatures in Earth's interior and contrasts that scientists consider the possibility of self-sustained nuclear fission 'as in a human-made reactor'.

How to extend

A student could use this to distinguish heat from natural radioactive decay versus heat from a self-sustained fission chain reaction (the latter being what a reactor does) and ask which process RTGs rely on.

Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > Atomic explosion (Nuclear fatlout): > p. 83
Strength: 5/5
“The nuclear arms use uranium-235 and plutonium-239 for fission and hydrogen or lithium as fusion material. Atomic explosions produce radioactive particles that are thrown high up into the air as huge clouds. These particles are carried to long distances by wind and gradually settle over the earth as fall-out or are brought down by rain. The fall-out contains radioactive substances such as iodine-131, etc.”
Why relevant

Identifies uranium-235 and plutonium-239 as materials used for fission in nuclear arms, implying that fission requires specific fissile isotopes and chain reactions.

How to extend

A student can check whether RTGs use those fissile isotopes and whether RTGs operate via chain reactions or simple radioactive decay.

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 9: Distribution of World Natural Resources > thorium > p. 40
Strength: 3/5
“Torium is a by-product of the extraction of rare earths from monazite sands. Torium was used for the breeding of nuclear fuel uranium. It is used as a nuclear fuel in aircraft engines. Torium is a very efective radiation shield. India's Kakrapara-1 reactor is the world's frst reactor which uses thorium. Australia, USA, and India have large deposits of thorium, followed by Canada, Brazil, South Africa and Turkey.”
Why relevant

Describes thorium and uranium as nuclear fuels and mentions use in reactors/engines, illustrating that 'nuclear fuel' usually refers to materials used in reactors (breeding, sustained reactions).

How to extend

Use this pattern to compare the isotopic fuel in reactors versus the isotope used in RTGs (if known) to see if RTGs use reactor-style fuel.

Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 9: Distribution of World Natural Resources > nucleaR eneRgy. > p. 23
Strength: 4/5
“Nuclear energy is a form of energy which uses nuclear reactions to produce steam to turn generators. Looking at the exhaustible nature of fossil fuels, nuclear energy development has become very vital for economic development. Te frst nuclear power station was built in Britain in 1956 at Calder Hall. At present, there are more than 400 nuclear power plants in the world. Most of them are in U.S.A., France, U.K. Germany, Russia, Japan, China, Sweden, Belgium, Switzerland, Italy, Australia and India. Te nuclear power stations are more economical which may soon overtake other forms of electricity generation. Although nuclear power will play a much more prominent role in future power supplies, it can never entirely replace conventional generation method.”
Why relevant

Defines nuclear energy as using nuclear reactions to produce steam that drives generators — describing the typical reactor-scale energy-conversion chain driven by controlled reactions.

How to extend

A student could contrast this reactor model (reaction→steam→turbine) with the RTG model (decay heat→thermoelectric devices) to test if RTGs match the reactor pattern.

NCERT. (2022). Contemporary India II: Textbook in Geography for Class X (Revised ed.). NCERT. > Chapter 5: Print Culture and the Modern World > Nuclear or Atomic Energy > p. 117
Strength: 4/5
“It is obtained by altering the structure of atoms. When such an alteration is made, much energy is released in the form of heat and this is used to generate electric power. Uranium and Thorium, which are available in Jharkhand and the Aravalli ranges of Rajasthan are used for generating atomic or nuclear power. The Monazite sands of Kerala is also rich in Thorium. Locate the 6 nuclear power stations and find out the state in which they are located.”
Why relevant

Explains that altering atomic structure releases energy used to generate electric power and names uranium/thorium as materials used for atomic power, reinforcing that 'atomic power' usually involves deliberate nuclear processes with specific fuels.

How to extend

A student might ask whether RTGs perform the same type of atomic alteration (sustained fission) as large power stations or rely on spontaneous decay of radioisotopes.

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Statement analysis

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Statement analysis

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