P2bL13 Nuclear Fission
Key Words
Chain reaction - an event that sets off one or more events as a result.
Critical mass - mass of uranium (or other fissile material) that is sufficient to trigger a chain reaction.
E = mc2 - Einstein's famous equation that links material to energy.
Nuclear fission - some unstable nuclei fall apart, releasing energy.
Grade E
It is strange to think that matter (material) can be turned into energy and back again, but in nuclear physics, this happens. The amount of energy that can be released from materials is huge. It is described by Einstein's simple equation:
E = mc2
The Physics code are:
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E - energy in joules;
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m - the mass in kg
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c - speed of light (300 million m/s)
So the term c2 gives us 9 × 1016, a huge number (almost the annual turn-over of a Premiership football club).
The mass of material converted into energy in the detonation of an atomic bomb is about 20 grams.
Grade C
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:
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too fast, the neutron will pass right through, or knock out another neutron.
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too slow, the neutron will bounce off the nucleus.
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 "laughs" itself to bits.
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. If the chain reaction can be controlled, the energy can be used to boil water, to turn a turbine, to make electricity.
For the reaction to occur, there has to be a critical mass. For uranium, this is about the size of a tennis ball. The critical mass has a mass of about 5 kg (uranium has a very high density, 19 g/cm3). Anything less, the neutrons escape without setting off a chain reaction.
Nuclear fission has nothing whatever to do with
radioactive decay. Although the uranium and the products are
radioactive, the events are entirely unlinked. Be a good chap and don't
call it "fussion".
Grade A
This is a typical nuclear fission equation:
Note that:
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The mass numbers balance (235 + 1 = 90 + 143 + 3).
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The atomic (proton numbers) balance (92 = 36 + 56).
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Three neutrons on average are released, which go on to be captured by other nuclei.