Moving to heavier atoms, less energy is released in each fusion event; until, at iron 26 protons and 30 neutrons , no more energy is released by fusion. Any bigger, it takes energy to make fusion happen. Atoms with really huge nuclei, such as uranium and plutonium do the opposite of fusion: they release energy when they break apart. The result of the instability is the nucleus breaking up, in any one of many different ways, and producing more neutrons, which in turn hit more uranium atoms and make them unstable and so on.
This chain reaction is the key to fission reactions, but it can lead to a runaway process resulting in nuclear accidents. In conventional nuclear power stations today, there are systems in place to moderate the chain reactions to prevent accident scenarios and stringent security measures to deal with proliferation issues.
Unlike nuclear fission, the nuclear fusion reaction in a tokamak is an inherently safe reaction. The reasons that have made fusion so difficult to achieve to date are the same ones that make it safe: it is a finely balanced reaction which is very sensitive to the conditions — the reaction will die if the plasma is too cold or too hot, or if there is too much fuel or not enough, or too many contaminants, or if the magnetic fields are not set up just right to control the turbulence of the hot plasma.
This is the same process that powers the sun and creates huge amounts of energy—several times greater than fission.
Fusion reactions are being studied by scientists, but are difficult to sustain for long periods of time because of the tremendous amount of pressure and temperature needed to join the nuclei together. All of the energy we produce comes from basic chemical and physical processes. To produce a flow of output energy, or power, a chain reaction must occur. A slow neutron n is captured by a fissionable heavy nucleus denoted by X. X fissions into two lighter, but more tightly bound, nuclei Y and Z, and fast neutrons are emitted.
A reactor contains an ample source of fissionable material in order for the chain reactions to continue. Too little fissionable material would be characterized as subcritical The reactor also contains a moderator, typically water, which is used to slow down the fast neutrons and make them easy to absorb Fast neutrons penetrate the heavy nucleus X Control rods are used to absorb neutrons to prevent the chain reaction to excalate too fast causing a supercritical or runaway situation.
A reactor that operates normally is said to sustain a critical chain reaction. The typical nuclear fission reactor uses the kinetic energy of the neutrons that that result from the chain reaction to heat water, make steam and drive a generator that produces electricity.
Nuclear fusion reactors don't exist because we don't know how to harness fusion to make a practical reactor However, there is great interest using fusion as an energy source, because it is a clean and abundant form of energy.
But the challenge remains to create a practical fusion reactor, one that puts out more energy than it takes in.
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