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While nuclear fission is splitting heavy nuclei, nuclear fusion is joining light nuclei. Both reactions provide energy but there are quite a few differences.
In a reactor
The following picture shows you the reaction that takes place in nuclear fusion reactors, Deuterium and Tritium are fused into Helium. This fusion releases an immense amount of energy.
Assignments
101. Write the nuclear reaction just like you did in chapter 4. Deuterium and tritium can be written as isotopes of hydrogen (what they are) or you can use the symbols D and T respectively. A neutron is written with a lower-case n.
102. Can you get a chain reaction this way? Why (or why not)?
Under normal circumstances, nuclei can’t get close to each other. The fact that the atoms in your feet aren’t fusing with those in the floor is proof of that. Even when the atoms are in a gaseous state, they don’t fuse. When two atoms (or ions) approach each other, the bounce off each other long before the nuclei get close.
Deuterium and Tritium are no different. This means that to fuse these two nuclei, either the pressure has to be so high, the nuclei don’t have anywhere else to go to avoid a collision, or the temperature has to be really high. When the temperature is high enough, the nuclei will be vibrating at such a high speed that they can’t avoid each other when they’re on a collision course.
103. There’s a fundamental difference in the type of work the fission- and fusion reactors have to do. Describe this difference.
The reactors
There are a couple of reactors that are built to enable nuclear fusion. As you’ve discovered in the previous paragraph, the reactor has to do quite a bit of work to get nuclei to fuse.
The tokamak
This device uses ring-shaped electromagnets to create a magnetic field in which the superheated gas is caught. These gasses are so hot that the atoms have lost their electrons. The material has passed the gaseous phase is now a plasma.
The electromagnets do require a large current to produce a magnetic field strong enough, about 15 Mega-ampere.
A more practical problem lies in the magnets. Magnetism doesn’t work at high temperatures and it works best at really low temperatures. This means that you need a really, really cold device in which you have to contain plasma as hot as the sun.
Another problem with this reaction is the fact that it emits neutrons. Neutrons can’t be caught by a magnetic field. After a while, the walls of the tokamak need to be replaced because they’ve become lightly radioactive.
104. What is the only (other) waste product of nuclear fusion?
Because of the high energy required and the fact that there’s no chain reaction, nuclear fusion is only achieved in short bursts called pulses. A reactor being built in France now should be able to fix that problem.
Stellarator
Several of this type of reactor have been built. The strange curves are designed by a computer to keep the plasma contained as efficiently as possible. This design does have its drawbacks. The magnetic field of one ring is affecting the next ring, trying to break the device. This means that the strength of the magnetic fields is limited.
Natural fusion
The only place apart from short pulses in reactors where nuclear fusion occurs, is in the heart of stars. Including our own closest star of course, the sun.
The process with which the sun produces the energy we feel every day is quite a bit more complicated than the nuclear fusion in a reactor. For one, neutrons are being created by smashing protons together, something that requires more energy than a nuclear reactor on earth could produce.
Nuclear fusion in the sun is made possible by gravity, the hydrogen gas is compressed by the enormous mass of the gasses piled on top of it.
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