Topic 10

 

What are nuclear fusion and nuclear fission?

 

In the exam you need to know how:

  • to sketch a labelled diagram to illustrate how a chain reaction may occur.

In the exam you need to know about:

  • There are two fissionable substances in common use in nuclear reactors, uranium 235 and plutonium 239.

  • Nuclear fission is the splitting of an atomic nucleus.

  • For fission to occur the uranium 235 or plutonium 239 nucleus must first absorb a neutron.

  • The nucleus undergoing fission splits into two smaller nuclei and 2 or 3 neutrons and energy is released.

  • The neutrons may go on to start a chain reaction.

    •

  • Nuclear fusion is the joining of two atomic nuclei to form a larger one.

  • Nuclear fusion is the process by which energy is released in stars.

 

Key Words

Nuclear Energy

Fission

Fusion

 

Nuclear Energy

Sixty years ago the power of nuclear energy was demonstrated to the world.  Little Boy, a free-fall atomic bomb was dropped on Hiroshima in August 1945.  A few days later an aerial mine, Fat Man, was detonated over Nagasaki.  The destruction and carnage caused by these bombs is well known.  The energy released was due to the conversion of 20 grams of nuclear material to heat.

 

The conversion of nuclear energy to heat is at the heart of nuclear energy.  Scientists have learned to control the process so that instead of an explosion, a steady heat source is achieved.  A nuclear reactor can boil water to steam to turn a steam turbine.  In the early days, nuclear energy was greeted with unbounded optimism.  All sorts of nuclear powered devices were conceived, including railway engines and aeroplanes.  It was hoped that nuclear power would be so cheap that it would not be necessary to meter electricity.  However this proved not to be the case.

 

We will look at the processes that release nuclear energy.

 

 

Fission

Very large nuclei tend to be rather unstable.  This means that they are radioactive.  Some nuclei, for example, Uranium-235 and Plutonium-239, can be made so unstable that they split into two or more nuclei of more stable elements.  This is called fission.  The nuclei are called fissile.

 

These fissile nuclei are isotopes of more stable elements (e.g. Uranium-238).  If left alone, they decay radioactively by emitting alpha particles.

 

Fission is not a spontaneous process.  It has to be started by injecting a neutron into the nucleus. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The neutron has to be injected at the right speed:

Many pictures show the neutron smashing the nucleus like a bullet.  This is wrong.  It's more like that the neutron "tickles" the nucleus.

 

The nucleus is not a neat array of protons and neutrons.  It is very active , changing shape all the time.  It's like a "wobbly drop".  When the extra neutron is taken into the nucleus, the wobbly drop goes dumbbell-shaped like this:

 

 

 

 

 

 

 

 

 

 

 

 

The weak spot at the neck makes the nucleus fly apart to form two or more new nuclei.  A lot of energy is released.  Nuclear energy gives off far more heat energy than chemical reactions. 

 

 

 

 

Also two or three (or more) neutrons are released.  These can go on to be absorbed by other nuclei to cause a chain reaction, which is shown in the picture below.

 

 

 

If the chain reaction is not controlled, a nuclear explosion will occur.  In a nuclear reactor, only one neutron is allowed to pass on to be incorporated into one nucleus.

 

Question 1  What sort of nuclei are needed for fission?  ANSWER

 

Reactors in nuclear power station do the same job as the boiler; they boil water to steam.  They also can be used to make radioactive isotopes for medical purposes.

 

 

  Fission has NOTHING whatever to do with radioactivity.  Alpha and beta particles are NOT emitted during fission.  However many of the new daughter nuclei are radioactive.

 

 

Fusion

This involves light nuclei, two isotopes of hydrogen, deuterium and tritium.

 

 

Question 2  What nucleons are found in the deuterium and tritium nuclei?  ANSWER

 

If two helium nuclei are forced together, they join together or fuse to form a helium nucleus, giving off lots of energy, more than in fission.  The process is shown in the picture below:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The process is easier to illustrate than to achieve.  The tritium and deuterium nuclei have to be slammed together by heating them to temperatures of millions of degrees Celsius before they fuse.

 

The vast amounts of energy can be released in a massive explosion.  The amount of hydrogen involved in a hydrogen bomb explosion is tiny; it would fill a party balloon.  Achieving controlled fusion has proved more challenging, and commercial fusion power stations remain a distant prospect.

 

Fusion is the process that fuels stars.  In the Sun, four million tonnes of hydrogen fuel is consumed every second.  This sounds a lot, but the Sun has enough fuel to keep burning for another 4500 million years, by which time we will all be long gone and forgotten.

 

  Do not write "fussion".  This indicates that you are not sure of the difference between fission and fusion.  You will not score any marks.

 

Now answer Question 3.

 

Now answer Question 4.

 

Have a go at the Crossword that gets you to think about nuclear physics.

 

Summary:

  • Fission has nothing to do with radioactivity.

  • Fission occurs with certain large fissile nuclei.

  • A neutron is absorbed by a fissile nucleus which splits into smaller nuclei, releasing energy and neutrons.

  • More neutrons are absorbed by other fissile nuclei to form a chain reaction.

  • Fusion is the joining of light nuclei to form a larger nucleus.

  • Fusion releases more energy than fission.

  • Fusion powers stars.

 

 

Now try the Topic Quiz

 

 

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