📖 Chapter 7 of 12 · Green Energy - Advance 2 · Class 9–10
Chapter 7: What Is Fusion, and How Close Are We?
In this chapter
- Fusion versus fission: joining light nuclei instead of splitting heavy ones.
- Why fusion needs 100 million degrees, and how a tokamak holds plasma.
- ITER, India’s role, and the near-term option of small modular reactors.
- Meet Nithya from Kerala and Aryan from Tamil Nadu.
The Big Idea
Fusion joins light nuclei — usually deuterium and tritium, both forms of hydrogen — into helium, releasing energy. It is what powers the Sun. On Earth we must heat a plasma to over 100 million °C and hold it long enough and densely enough for fusion to release more energy than we put in. Fusion cannot melt down or run away, and its waste is far shorter-lived than fission waste — but no power plant runs on it yet.
Did You Know?
India is one of seven partners building ITER in France, the world’s largest fusion experiment. At home, the Institute for Plasma Research in Gandhinagar operates the ADITYA-U and SST-1 tokamaks, and commissioned a small spherical tokamak in 2026.
Story Time — Nithya and the Star in a Bottle
Nithya, age 16 · Thiruvananthapuram, Kerala
At a science centre in Thiruvananthapuram, Nithya attended a talk titled “Building a Star on Earth”. The speaker, a plasma physicist, began with a question: “Why is fusion hard, when the Sun does it effortlessly?”
The answer surprised her. “The Sun cheats. It has crushing gravity and enormous size, so its core can be a ‘cool’ 15 million degrees and still fuse slowly. We have no such gravity, so we must reach over 100 million degrees — and hold that plasma away from every wall, because nothing solid can touch it.”
He explained the triple product: to reach ignition, the plasma’s density, temperature and confinement time must all be high enough together. “Raise one and the others often fall. That is the entire history of fusion research.”
Nithya asked about the milestone she had read about — the 2022 experiment in the United States where a laser-driven capsule released more fusion energy than the laser light delivered to it. “A genuine scientific first,” he said, “though the lasers themselves consumed far more electricity than the experiment produced. Scientific gain and plant-level gain are different bars.”
Walking out past the ITER model, Nithya felt both the distance and the pull of the problem. In her notebook she wrote: “Fusion is not a machine waiting to be switched on. It is a physics problem being slowly surrounded.”
Story Time — Aryan and the Reactor That Fits on a Lorry
Aryan, age 15 · Kalpakkam, Tamil Nadu
Aryan’s school near Kalpakkam sits close to one of India’s nuclear research campuses. At an open day, a scientist showed his class a model of a small modular reactor — an SMR.
“A big reactor is built on site over ten years,” she said. “An SMR is smaller, and most of it is built in a factory and transported in modules. The aim is faster construction, lower upfront cost, and reactors that can sit where old coal plants already have grid connections.”
Aryan asked whether small meant less safe. “The opposite is intended,” she replied. “Smaller cores produce less heat to remove after shutdown, so many designs can cool themselves by natural circulation, without pumps.”
He also learned about India’s longer game: the fast breeder reactor at Kalpakkam that reached its first criticality in 2026, and the plan to use India’s large thorium reserves in a later stage.
“And fusion?” Aryan asked. The scientist smiled. “Fusion is the goal for the second half of this century. Fission, done carefully, is what we have now. Your generation will have to run both conversations at once — and be honest about the waste and the risks in each.”
Fusion Physics: Temperature, Density, Time
Nuclei repel each other because both are positively charged. Only at extreme temperatures do they move fast enough to overcome that repulsion and fuse. The most accessible reaction is D + T → He-4 + neutron, which releases about 17.6 MeV — several times more energy per kilogram of fuel than fission, and millions of times more than burning coal.
- Plasma: at these temperatures, matter becomes a gas of free nuclei and electrons.
- Magnetic confinement (tokamak): a doughnut of plasma held by magnetic fields so it never touches the walls.
- Inertial confinement: lasers compress a tiny fuel pellet for a fraction of a second.
- Q factor: fusion energy out divided by heating energy in. Q > 1 means scientific gain; a power plant needs Q of roughly 10 or more, plus efficient conversion.
- Fuel: deuterium comes from seawater; tritium must be bred from lithium inside the reactor.
Fusion, Fission and the Near Term
It is tempting to treat fusion as the answer to everything. A scientist compares them carefully instead.
- Safety: fusion plasma is fragile — any disturbance stops the reaction, so a runaway chain reaction is impossible. Fission needs active control and cooling.
- Waste: fusion produces no long-lived fission products, though neutrons make reactor components radioactive for about a century — far shorter than fission waste.
- Status: no fusion plant supplies electricity anywhere; ITER’s research campaign is expected in the mid-2030s, with demonstration plants later.
- Meanwhile: fission — including SMRs — solar, wind, hydro and storage are the tools available this decade.
🔢 Numbers Corner
Q = fusion power out ÷ heating power in. Q = 1 is scientific break-even; ITER targets Q ≈ 10 in its plasma. A commercial plant must also overcome the electricity used by the whole facility, which is a much harder test.
Myth or Fact?
Myth: Fusion is just around the corner and will make all other clean energy unnecessary.
Fact: Fusion is real physics with real progress, but no plant generates electricity from it yet. Climate targets for the 2030s and 2040s must be met with today’s technologies; fusion is a prize for later in the century.
🔬 Try it in the Green Energy Lab
Experiment with these ideas in Lab 5: Fusion Q Explorer.
Solved Examples
Work it out first, then check the answer.
1. State the difference between fission and fusion.
Show answer
Answer: Fission splits a heavy nucleus such as uranium-235 into smaller ones. Fusion joins light nuclei such as deuterium and tritium into a heavier one, releasing energy in both cases.
2. Why must fusion plasma be so hot?
Show answer
Answer: Nuclei are positively charged and repel each other. Only at over 100 million degrees do they move fast enough to overcome this repulsion and fuse.
3. What does Q = 5 mean?
Show answer
Answer: The fusion reactions released five times as much energy as was supplied to heat the plasma. It is scientific gain, but not yet a power plant.
4. Where do the two fusion fuels come from?
Show answer
Answer: Deuterium is extracted from water; tritium is bred from lithium, usually inside the reactor blanket.
5. Why can a fusion reactor not melt down?
Show answer
Answer: The plasma holds only a tiny amount of fuel and needs perfect conditions. Any disturbance cools it and the reaction simply stops.
Check Your Understanding
A. Fill in the blanks
- Fusion joins nuclei; fission splits heavy ones.
- A doughnut-shaped magnetic confinement device is a .
- The ratio of fusion energy out to heating energy in is called .
- Tritium is bred from inside the reactor.
- India’s fusion research institute at Gandhinagar is the Institute for Research.
B. True or False
- Fusion can cause a runaway chain reaction. ( ___ )
- The Sun fuses hydrogen at lower temperatures than Earth laboratories need. ( ___ )
- A fusion power plant is already supplying electricity to a grid. ( ___ )
- SMR modules are largely factory-built. ( ___ )
C. Choose the correct option
- The main fusion fuels are (a) deuterium and tritium (b) uranium and thorium (c) carbon and oxygen
- ITER is being built in (a) France (b) India (c) Japan
- The D–T reaction produces helium and a (a) proton (b) neutron (c) photon only
D. Answer in detail
- Explain the triple product and why it makes fusion difficult.
- Compare the waste from fusion and fission honestly.
- Should India invest in fusion research while also building solar, wind and fission? Argue with reasons.
Brain Buster
Try pushing the north poles of two magnets together. That resistance is a rough analogy for the repulsion between nuclei. What does it take to overcome it?
Check your understanding
Tap an answer to see if it is correct. This is just for practice — nothing is saved.
1. Fusion releases energy by…
- Splitting heavy nuclei
- Joining light nuclei
- Burning hydrogen chemically
Show answer
B. Joining light nuclei
2. A tokamak confines plasma using…
- Magnetic fields
- Thick concrete only
- Cold water
Show answer
A. Magnetic fields
3. Q = 1 means…
- Fusion energy equals heating energy in
- The plant is commercial
- No fusion occurred
Show answer
A. Fusion energy equals heating energy in
4. Which statement is accurate today?
- Fusion supplies part of India’s grid
- No fusion plant supplies electricity anywhere
- Fusion has replaced coal
Show answer
B. No fusion plant supplies electricity anywhere
5. An advantage of SMRs is…
- Factory building and simpler passive cooling
- They need no fuel
- They produce no waste
Show answer
A. Factory building and simpler passive cooling