📖 Chapter 6 of 11 · Green Energy - Advance 1 · Class 8
Chapter 6: What Is Nuclear Energy, and Is It Clean?
In this chapter
- Atoms, uranium and fission — energy from the tiniest things.
- How a nuclear power plant works, and how it is kept safe.
- India’s three-stage plan and the thorium treasure.
- The honest questions: accidents and waste. Meet Aditi and Murugan.
The Big Idea
Everything is made of atoms. The centre of an atom, the nucleus, holds enormous energy. In a nuclear power plant, heavy uranium nuclei are split apart — this is fission — releasing heat. The heat boils water, and the steam spins a turbine, just like in other power stations. Nuclear plants release almost no carbon dioxide while running, and they work day and night. But they need very strict safety, and their used fuel stays radioactive for a very long time.
Did You Know?
India has 24 nuclear reactors with about 8.8 GW of capacity, making around 3% of the country’s electricity. The government aims for 100 GW of nuclear capacity by 2047. In April 2026, the Prototype Fast Breeder Reactor at Kalpakkam, Tamil Nadu, started its chain reaction for the first time — a key step in India’s long-term nuclear plan.
Story Time — Aditi and the Badge That Counts
Aditi, age 13 · Palghar district, Maharashtra
Aditi’s mother is a safety officer at the Tarapur Atomic Power Station, where India’s first nuclear power reactors began working in 1969. Aditi’s class came to the plant’s information centre for a science visit.
Her mother showed them a small badge clipped to her uniform. “This is a dosimeter. It measures how much radiation I receive. Everyone who works near the reactor wears one, and the readings are checked regularly.”
“Isn’t radiation dangerous?” asked Aditi’s friend Kabir. “Large doses are,” she said. “But we all receive small amounts every day from rocks, soil, the air and even bananas. The plant is designed so that people living nearby receive only a tiny extra amount, far below safe limits.”
On a model, she pointed out the layers of protection: the fuel sealed in metal tubes, the thick steel reactor vessel, and the massive concrete containment building. “We call it defence in depth. If one layer fails, the next one holds.”
On the bus back, Kabir said, “I still feel a bit nervous about nuclear.” Aditi thought for a moment. “That’s okay. My mother says being careful is the whole job. The people who work there take the danger more seriously than anyone.”
Story Time — Murugan’s Question at the Meeting
Murugan, age 12 · A fishing village near Kudankulam, Tamil Nadu
From Murugan’s beach, you can see the round domes of the Kudankulam Nuclear Power Plant, India’s largest, built with Russian cooperation. Some years ago, after the 2011 accident at Fukushima in Japan, many people in nearby villages protested. They were worried about safety and about their fishing.
This year, a group of scientists came to Murugan’s school for a question-and-answer session with families. Murugan raised his hand. “What happened at Fukushima, and could it happen here?”
A scientist answered honestly. “At Fukushima, a huge earthquake and tsunami knocked out the power that ran the cooling pumps, and the reactor cores overheated. Since then, plants around the world, including in India, have added extra backup power and cooling. Kudankulam also has safety systems that can cool the reactor even without electricity. No design is perfectly risk-free, which is why regulators inspect the plants.”
An old fisherwoman stood up. “And the hot water that goes back into the sea?” “It is monitored,” the scientist replied, “and we share the results. You are right to ask.”
Walking home, Murugan’s mother said, “I didn’t agree with everything, but I’m glad they listened.” Murugan nodded. “Maybe asking good questions is also part of keeping everyone safe.”
Fission and the Chain Reaction
When a slow neutron hits a uranium-235 nucleus, the nucleus splits into two smaller atoms, releasing a lot of heat and two or three new neutrons. If those neutrons split more nuclei, a chain reaction follows. In a power reactor, the chain reaction is kept exactly steady: on average, each fission causes just one more.
- Control rods absorb neutrons. Pushing them in slows or stops the reaction.
- A moderator (such as heavy water in India’s PHWR reactors) slows neutrons so they split uranium more easily.
- A power reactor cannot explode like a nuclear bomb — its fuel is far too weak for that. The real danger is overheating, which is why cooling must never fail.
- Completely splitting one kilogram of uranium-235 releases about as much energy as burning roughly 3,000 tonnes of coal.
How a Nuclear Power Plant Works
The reactor heats a coolant. Through a steam generator, that heat boils separate water into steam, which spins the turbine. A condenser cools the steam back into water using sea or river water, which is why many plants are near the coast.
- Clean side: almost no CO₂ or smoke while running; steady power day and night; very little land.
- Hard side: high building cost and long construction; strict safety needed; rare but serious accidents (Chernobyl 1986, Fukushima 2011).
- Waste: used fuel stays radioactive for thousands of years. India reprocesses used fuel to recover useful material — a “closed fuel cycle” — and stores the remaining waste under strict control.
- India’s three stages: (1) heavy-water reactors using natural uranium; (2) fast breeder reactors that make more fuel than they use; (3) reactors using thorium, of which India has large reserves.
🔢 Numbers Corner
Energy density: one fission releases about 200 million electron-volts — millions of times more than burning one atom of carbon. Fully splitting 1 kg of uranium-235 would give about as much heat as roughly 3,000 tonnes of coal. Real reactor fuel holds only a small share of U-235, but even so, a truckload of fuel can run a reactor for many months.
Myth or Fact?
Myth: A nuclear power plant can blow up like an atom bomb.
Fact: No. Power-reactor fuel is not nearly concentrated enough for a nuclear explosion. Accidents come from overheating and the release of radioactive material — serious, but a different kind of danger.
🔬 Try it in the Green Energy Lab
Experiment with these ideas in Lab 4: Chain Reaction Controller.
Solved Examples
Work it out first, then check the answer.
1. What is nuclear fission?
Show answer
Answer: The splitting of a heavy nucleus, such as uranium-235, into two smaller nuclei when hit by a neutron, releasing heat and more neutrons.
2. How do control rods keep a reactor safe?
Show answer
Answer: They absorb neutrons. Inserting them reduces the number of fissions; fully inserting them stops the chain reaction.
3. Why do nuclear plants still need turbines?
Show answer
Answer: The reactor only makes heat. That heat produces steam, and the steam must spin a turbine and generator to make electricity.
4. Give one strong advantage and one serious challenge of nuclear power.
Show answer
Answer: Advantage: steady, low-carbon power day and night. Challenge: long-lived radioactive waste and the need for very strict safety.
5. Why is thorium important for India?
Show answer
Answer: India has large thorium reserves but limited uranium. The third stage of India’s nuclear plan aims to use thorium for fuel in the long term.
Check Your Understanding
A. Fill in the blanks
- Splitting a heavy nucleus is called nuclear .
- Rods that absorb neutrons are called rods.
- India’s first nuclear power station is at .
- A badge that measures radiation dose is a .
- The third stage of India’s nuclear programme aims to use .
B. True or False
- A nuclear reactor releases heat that makes steam. ( ___ )
- Nuclear plants produce lots of smoke while running. ( ___ )
- Used nuclear fuel stays radioactive for a very long time. ( ___ )
- A power reactor can explode like an atom bomb. ( ___ )
C. Choose the correct option
- Kudankulam is in (a) Tamil Nadu (b) Gujarat (c) Punjab
- The Fukushima accident happened in (a) 1986 (b) 2011 (c) 2021
- In India’s PHWR reactors, the moderator is (a) heavy water (b) sand (c) coal
D. Answer in detail
- Explain “defence in depth” with examples from Aditi’s story.
- Why was it important for scientists to listen to the villagers near Kudankulam?
- Write a balanced paragraph: should India build more nuclear power plants? Give points on both sides.
Brain Buster
Line up dominoes so that each one knocks over two others. Then try a line where each knocks over just one. Which is like a controlled reactor?
Check your understanding
Tap an answer to see if it is correct. This is just for practice — nothing is saved.
1. What happens in fission?
- Light atoms join
- A heavy nucleus splits
- Water turns into steam only
Show answer
B. A heavy nucleus splits
2. What do control rods do?
- Add fuel
- Absorb neutrons
- Make steam
Show answer
B. Absorb neutrons
3. Roughly what share of India’s electricity is nuclear?
- About 3%
- About 30%
- About 70%
Show answer
A. About 3%
4. Which statement is true?
- Nuclear plants release lots of CO₂
- Nuclear waste is dangerous for a very long time
- Nuclear plants need no safety systems
Show answer
B. Nuclear waste is dangerous for a very long time
5. What makes thorium special for India?
- India has large reserves of it
- It is a kind of coal
- It comes from the sea breeze
Show answer
A. India has large reserves of it