The UK site for KS3 and KS4 Physics

KS4 Atomic Structure:
There have been many occasions in the world of science when a chance discovery has opened up a whole new branch of research. Such a discovery occured in 1938 which led firstly to a devastating use of the power of the atom, then to a practical, beneficial use of its power.
Fission and fusion
In this fourth and final section we learn about nuclear fission and its use today, and about physicists' aim to power the world with a new type of energy from the atom using nuclear fusion.

Nuclear Fission - a chance discovery !

In 1938, shortly before the second world war, two Physicists were bombarding Uranium-235 with neutrons, trying to make new elements, heavier than Uranium.
But to their surprise, they found that rather than make a heavier nucleus, on absorbing the neutron, the Uranium nucleus split into two lighter nuclei and in the process released a significant amount of ENERGY!
They called the process, fission, since it means "to split".

At the time they found that the process, fission, only occured with Uranium-235. Uranium-235 was a very unusual and special isotope.

The fission of Uranium-235 is best described using a diagram:

All of this happens because of just one extra neutron! Amazing.

For a long time scientists thought that Uranium-235 was unique in behaving in this way, but eventually one other heavy isotope was discovered that will undergoe fission and that is Plutonium.

The Energy released

The energy released from the fission reaction is due to the kinetic energy of the fission products; their rapid movement generates heat such that the nuclear core, in a nuclear power station, becomes very hot which is then used to turn water into steam to drive turbines and electricity generators. Energy is also released in the form of gamma rays which will be absorbed by material surrounding the core, again turning to heat.

The neutrons

Fission starts when the Uranium-235 nucleus absorbs a slow moving neutron; just one, thats all it takes, but it has to be slow; if it isn't, it won't be absorbed.

Notice the 3 neutrons that are released whenever fission occurs. It is incredibly fortunate that this happens because in a nuclear reactor at the core of a nuclear power station, one of these neutrons is allowed to become the "slow moving neutron" which causes another Uranium-235 nucleus to undergo fission. One of its 3 neutrons is allowed to do the same, and so on. In this way, the fission process keeps going and a steady amount of energy is produced. We call this a controlled chain reaction.

What would happen if all 3 of the extra neutrons were allowed to go on to cause further fission? Well, we would start with 1 fission, then 3, then 9, then 27 etc. Each time more and more energy would be released and a vast amount would eventually be produced. We call this an uncontrolled chain reaction and it is what happens in the atom bomb.

Also, it shows how critical it is in a nuclear power reactor to control the reaction, so control rods are used to do this which will drop into the core, to shutdown the reaction by absorbing all the neutrons, preventing fission, in the event of any problems.

Advantages and Disadvantages of Nuclear Fission

Fission is a great way to produce energy compared to the burning of fossil fuels. As already mentioned above, it releases a million times more energy from an amount of uranium than from the same amount of coal or gas or oil. And it does not contribute to global warming. These are two major advantages of nuclear fission.

However, the fission products are themselves very radioactive, useless and have a very long half-life; so they have to be safely contained for a very long time. This is a major disadvantage of nuclear fission. Other disadvantages might include radiation leaks and other equipment "failures" but these are less likely to happen in our current era but still have to be considered. Oh, before we forget, uranium has to be mined from the ground, like coal; and like coal it is not renewable, so it will run out.

If Fission Means "to split" then Fusion Means...

Nuclear fusion is the joining together of two light nuclei to form a heavier nucleus. It is found that in doing so a significant amount of energy is released.

The following is one example of a fusion reaction:

3 2 H + 3 2 H → 4 2 He (+ 1 1 H + 1 1 H )+ ENERGY

On the left, in blue, we have two nuclei of tritium (this is an isotope of hydrogen, sometimes called "heavy water").
When the two tritium nuclei fuse togeth, they produce Helium (plus ordinary Hydrogen) and ENERGY.

What a beautifully simple and safe reaction? The "fuel" is readily available, being just a heavy version of water (unlike uranium which is not renewable) and the "products" are helium and hydrogen which can either be put to good use back in the fusion reactor or elsewhere or simply released into the atmosphere, safely.

So, why aren't we doing this already?

The problem is, the only places where we know that fusion takes place readily, are inside STARS!

And that is because - the fusion of the nuclei requires an environment with an enormously high temperature such as that found inside (not even at the surface of) a star. The temperature needs to be of the order of 10 million degrees.

Making Fusion Happen on Earth

We have explained why fusion needs a very high temperature such as found inside a star, so does that mean it can never be made to work on Earth?
Hopefully the answer to that is NO. There are hundreds of physicists across the world working on nuclear fusion as you read this. Perhaps the most significant two groups are a UK group (Tokamak Energy) in Oxfordshire, whose reactor reached a temperature of 15 million degrees C in early 2018 and an International effort (ITER) based in France.
The UK group are optimistically forecasting that they will be able to begin supplying electricity to our National Grid, generated by Fusion power, by 2030. If it happens that will be a wonderful day! It could be the end of our need for any fossil fuels, assuming all vehicles are electric powered, and as a nation we would no longer rely on imports of fuels from foreign countries.

Time for a few questions: