Help with Nuclear Fission and Fusion

Any essential things I need to know about these two processes? Thanks x

Fission reactions are nuclear reactions which produce energy, fission in particular takes place in a nuclear reactor to generate electricity on a larger scale. In fission, the nucleus 'splits'. A slow-moving neutron is absorbed by the nucleus of a Uranium atom, which causes it to be unstable (because it now has an extra neutron in it's nucleus.) The new nucleus splits into two smaller nuclei called 'daughter nuclei', in attempts to make it more stable and a few fast-moving neutrons are released also. Energy is released as kinetic energy through the daughter nuclei and the neutrons. Sometimes scientists trigger fission to happen by bombarding the Uranium atoms with neutrons. In a large sample of uranium, the fast-moving neutrons from the fission can go on the split other uranium nuclei - which is called a chain reaction. These chain reactions are controlled in nuclear power stations to make sure there is a constant and steady output of power - as opposed to different amounts of energy being released. They do this in four ways: fuel rods contain pellets of nuclear fuel in the form of uranium dioxide. A 'coolant' removes the thermal energy produced in the fission reactions in the reactor core, so it can be used to heat water to create steam to power generator turbines. A moderator surround the nuclear fuel rods and slows down fast moving neutrons, as slow moving neutrons have a greater chance of being absorbed by the nuclei of a Uranium atom than fast moving ones. Finally, control rods can be used to absorb neutrons which slows down fission reactions and controls the chain reaction.

Fusion reactions cause the energy generated by the Sun and stars. In fusion, two smaller, lighter nuclei join or fuse together to produce one larger nucleus - they produce a vast amount of energy. Extremely high temperatures are needed in order for fusion to take place, and fusion of hydrogen and other lighter nuclei are the energy sources in which keep our Sun and other stars burning.

Reply 2

sahil19

OP

18

Original post by caitlinford3

Fission reactions are nuclear reactions which produce energy, fission in particular takes place in a nuclear reactor to generate electricity on a larger scale. In fission, the nucleus 'splits'. A slow-moving neutron is absorbed by the nucleus of a Uranium atom, which causes it to be unstable (because it now has an extra neutron in it's nucleus.) The new nucleus splits into two smaller nuclei called 'daughter nuclei', in attempts to make it more stable and a few fast-moving neutrons are released also. Energy is released as kinetic energy through the daughter nuclei and the neutrons. Sometimes scientists trigger fission to happen by bombarding the Uranium atoms with neutrons. In a large sample of uranium, the fast-moving neutrons from the fission can go on the split other uranium nuclei - which is called a chain reaction. These chain reactions are controlled in nuclear power stations to make sure there is a constant and steady output of power - as opposed to different amounts of energy being released. They do this in four ways: fuel rods contain pellets of nuclear fuel in the form of uranium dioxide. A 'coolant' removes the thermal energy produced in the fission reactions in the reactor core, so it can be used to heat water to create steam to power generator turbines. A moderator surround the nuclear fuel rods and slows down fast moving neutrons, as slow moving neutrons have a greater chance of being absorbed by the nuclei of a Uranium atom than fast moving ones. Finally, control rods can be used to absorb neutrons which slows down fission reactions and controls the chain reaction.

Fusion reactions cause the energy generated by the Sun and stars. In fusion, two smaller, lighter nuclei join or fuse together to produce one larger nucleus - they produce a vast amount of energy. Extremely high temperatures are needed in order for fusion to take place, and fusion of hydrogen and other lighter nuclei are the energy sources in which keep our Sun and other stars burning.

Thank you! X

Reply 3

glowanti

14

Original post by sahil19

Thank you! X

No problem x

Reply 4

Luke Kostanjsek

11

Original post by sahil19

Any essential things I need to know about these two processes? Thanks x

The other answer was pretty comprehensive. The only thing I'd mention that was missed out is the iron peak. You only need to understand this pretty superficially at GCSE.

As stated before, fission is the act of splitting a nucleus and fusion the act of combining nuclei. But the processes don't always release energy. Put simply, there are two forces at play. You have the nuclear binding forces, which hold the nucleus together and is very very short range, and the electrostatic force, which pushes them apart but has an affect over a much larger distance. You can think of it like magnetism, it's the force that causes protons to repel each other, cause they're both positive. Now as the nuclear binding force holds the nucleus together, this force helps fusion. On the other hand, the electrostatic force hinders fusion (you don't need to be able to explain this bit, it's just to help you understand the next part).

In small nuclei, the nuclear binding force is much more powerful than the electrostatic force (because there aren't many protons to repel each other). This means that fusion is 'easy', and so releases energy. We call this exothermic, and it means that fusion of lighter nuclei will release energy. This is what generates energy in a star.

But as the nucleus gets bigger, we have more protons. These protons repel each other, which means that the electrostatic force gets bigger, making fusion harder and harder (less exothermic). Eventually, we reach a point where the electrostatic force is great enough that fusion now requires us to put more energy in to make it happen, than we get out of it. At this point, the fusion becomes endothermic. The point where this happens is when we try to fuse iron-56. This is why stars cannot create elements heavier than iron during their natural lifespan - they form only during the death of the star. This is also the point where fission becomes exothermic, for the opposite reasons to those given above.

I'm not sure how much of that you'll be required to know, but for my syllabus we had to know that:

a) Fusion up to iron-56 is exothermic and fission endothermic; fusion beyond iron-56 is endothermic and fission exothermic
b) The reason for this is because of the balance of the nuclear binding force and electrostatic force
c) This dictates what elements stars can fuse, and why even the biggest stars eventually die (because it's impossible to sustain fusion beyond iron-56, no matter how much energy you have)

Help with Nuclear Fission and Fusion

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