Nuclear Fission and Fusion

The energy locked inside the nucleus is millions of times greater, gram for gram, than the energy released by burning fuels. There are two ways to release it: by splitting a very large nucleus apart (fission), or by joining two very small nuclei together (fusion). Fission powers nuclear power stations and nuclear weapons here on Earth; fusion powers the Sun and every star, and scientists are working hard to harness it. Although fission and fusion sound like opposites, both release energy and both are nuclear, not chemical, processes. This lesson, the last of the content lessons in Topic P6 (Radioactivity) of OCR Gateway Science A, explains how each process works, how a reactor controls fission, and how fission and fusion compare.

By the end of this lesson you should be able to describe nuclear fission and the chain reaction, explain how a reactor controls fission with a moderator and control rods, describe nuclear fusion and where it occurs, and compare fission and fusion.

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Nuclear Fission

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei, releasing energy. The fuels used are very large nuclei such as uranium-235 ($^{235}_{\ 92}\text{U}$ 92235​U) or plutonium-239 ($^{239}_{\ 94}\text{Pu}$ 94239​Pu).

Fission is usually induced (made to happen): a large nucleus absorbs a slow-moving neutron, which makes it even more unstable, so it splits. When it splits, it produces:

• two smaller "daughter" nuclei (the fission fragments), of roughly medium size;
• two or three more neutrons; and
• a large amount of energy, carried away mostly as the kinetic energy of the moving fragments and neutrons (which becomes heat).

We can show a typical fission of uranium-235 in words: a uranium-235 nucleus absorbs a neutron, becomes briefly unstable, and splits into two daughter nuclei (such as barium and krypton) plus, say, three neutrons and energy.

The key point is that fission of one nucleus releases two or three neutrons — and those neutrons can go on to cause further fissions. The daughter nuclei produced are themselves often radioactive, which is why nuclear power produces radioactive waste.

Exam Tip: Nuclear fission = a large unstable nucleus (uranium-235 or plutonium-239) absorbs a neutron and splits into two smaller nuclei, releasing 2 or 3 neutrons and energy. Remember: fission needs a starting neutron and gives out more neutrons.

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The Chain Reaction

Because each fission releases two or three neutrons, and each of those neutrons can trigger another fission, the number of fissions can grow rapidly. This self-sustaining sequence is called a chain reaction.

graph TD
  N0["neutron"] --> F1["fission of U-235"]
  F1 --> N1["neutron"]
  F1 --> N2["neutron"]
  F1 --> N3["neutron"]
  N1 --> F2["fission"]
  N2 --> F3["fission"]
  N3 --> F4["fission"]

If, on average, one neutron from each fission goes on to cause exactly one further fission, the reaction proceeds at a steady rate — this is what happens in a nuclear reactor, where the energy is released steadily and used to generate electricity. If, instead, the reaction is allowed to grow uncontrolled, with more and more neutrons causing more and more fissions each generation, the energy is released almost instantaneously in a huge explosion — this is what happens in a nuclear (fission) bomb.

Exam Tip: A chain reaction happens because each fission releases neutrons that cause further fissions. A controlled chain reaction (one new fission per fission) releases energy steadily in a reactor; an uncontrolled one releases it explosively in a bomb.

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Controlling Fission in a Reactor

A nuclear reactor's job is to keep the chain reaction going at a steady, controlled rate, so that energy is released gradually and used to heat water, raise steam and drive turbines. Two components do the controlling, plus one that makes fission possible in the first place:

• The moderator. The neutrons released by fission move too fast to be readily absorbed by uranium-235. The moderator (often water or graphite) slows the neutrons down by collisions, making them more likely to be absorbed and cause the next fission. Without a moderator the chain reaction would not be sustained.
• Control rods. Control rods (made of a neutron-absorbing material such as boron or cadmium) are lowered into or raised out of the reactor core to absorb neutrons and so control how many are left to cause further fissions. Lowering the rods absorbs more neutrons and slows the reaction; raising them absorbs fewer and speeds it up. To shut the reactor down, the rods are pushed fully in.

By adjusting the control rods, operators keep exactly one neutron per fission causing a further fission, holding the reaction steady.

Component	Material (example)	Job
Fuel	Uranium-235 / plutonium-239	The nuclei that undergo fission
Moderator	Water or graphite	Slows neutrons so they cause further fission
Control rods	Boron or cadmium	Absorb neutrons to control the reaction rate

Exam Tip: Don't mix up the two: the moderator slows neutrons down (so fission continues), while control rods absorb neutrons (to control the rate). Lowering the control rods absorbs more neutrons and slows the reaction.

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Nuclear Fusion

Nuclear fusion is the joining together of two light (small) nuclei to form a heavier nucleus, releasing energy. It is, in a sense, the opposite of fission. A typical fusion reaction joins two hydrogen isotopes — for example, deuterium and tritium — to make a helium nucleus plus a neutron and energy.

Fusion releases even more energy per kilogram than fission, and it is the process that powers the Sun and all the stars: in the Sun's core, hydrogen nuclei fuse to form helium, releasing the vast energy that reaches us as light and heat.

The great difficulty is that nuclei are positively charged and therefore repel one another strongly (like charges repel). To force two nuclei close enough to fuse, you need extremely high temperatures (millions of degrees) and extremely high pressures, so that the nuclei move fast enough and are squeezed close enough to overcome this repulsion. These conditions exist naturally in the cores of stars, but they are very hard to achieve and contain on Earth — which is why, despite decades of research, fusion is not yet a practical power source here, even though it would produce far less radioactive waste than fission.

Exam Tip: Nuclear fusion = two light nuclei join to form a heavier nucleus, releasing energy; it powers the Sun and stars. It needs very high temperature and pressure to overcome the electrostatic repulsion between the positive nuclei, which makes it hard to achieve on Earth.

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Comparing Fission and Fusion

Fission and fusion are easy to confuse because both are nuclear processes that release energy. The table sets out the differences clearly.

Feature	Nuclear fission	Nuclear fusion
What happens	A large nucleus splits into two smaller ones	Two small nuclei join into a larger one
Fuel	Uranium-235, plutonium-239	Light nuclei (hydrogen isotopes)
Started by	Absorbing a neutron	Very high temperature and pressure
Energy released	Large (per kg)	Even larger (per kg)
Where used / found	Nuclear power stations; nuclear bombs	The Sun and stars; experimental reactors
Conditions needed	Moderator and control rods to manage neutrons	Extreme heat and pressure to overcome repulsion
Main difficulty	Radioactive waste; safety	Achieving the conditions on Earth