This is one of the future energy sources into which a huge amount of research effort has been poured. Attempts to make fusion power feasible have been ongoing since before world war II.
Fusion is the release of energy that occurs as smaller atoms join together
to form larger ones. There is a slight loss of mass from the smaller to
larger atoms. This mass is converted into energy and we can calculate
this using Einstein's formula.
E = mc2
- Fusion converts mass into energy.
- Fusion is the same process that powers the sun.
- The basic reaction in the sun is that 4H are converted into 1 He atom
Stars can also produce other larger atoms by fusion. All the matter that makes
us up, for atoms bigger than H, or He was formed either in a star, or in a
supernova explosion. This fusion process is important not only to generate
light and energy, but also to produce the atoms that make us up.
The chances of 4H colliding to create a He, is statistically almost impossible,
so this reaction actually proceeds in 3 steps that we refer to as the proton-proton
(P-P) chain.
In the first 2 normal H collide and produce 1 D(euterium), a neutrino and a positron (antimatter) The positron being antimatter will immediately annihilate with a normal electron and produce energy in the process. The neutrinos will escape out from the sun.
In the second step 1 D collides with 1 normal H to produce a He -3 (this is an isotope of He with only 1 neutron) and a gamma ray (light energy as a photon - this photon can take up to 1 million years to escape the center of the sun, and in the process it gets change to other wavelengths of light).
In the last step two He -3 collide to make He -4 (2 neutrons) as well as 2 normal H.
In order for fusion to proceed, first all the electrons on the atoms have to be split off, (all this material is completely ionized) The conditions also have to be extreme enough to overcome the repulsion between two positive nuclei, so that when they collide, they interact, instead of just repelling each other. The nuclei involved need to have sufficiently high kinetic energy (velocity) to overcome this repulsion, they also have to be packed tightly enough so that they will collide often enough to sustain the reaction. Inside of stars, the temperatures needed to ignite and sustain the fusion reactions are in the 5-10 million degree Kelvin range. In the star this material holds together under the force of gravity, since the star has such a large amount of mass.
Deuterium is easily available as a part of heavy water (although like the production of H, energy has to be expended to extract the deuterium from the heavy water. In this case however the energy expended is much less than you will get back from the fusion process)
Tritium does not occur naturally in any abundance, so it must be generated in breeder nuclear reactors using Li. Li(thium) is a very common element on Earth, so it is readily available for this process.
The problem with fusion as a source of energy is the extreme conditions needed inside of the reactors in order to sustain the fusion process.
Fusion reactors have been researched since the 1950's. Fusion energy is very environmentally friendly. It would produce some radioactive waste, but only a fraction of what fission reactors do, and that waste has much shorter half-lifes, so it needs only a few decades to a century of containment.
The chief problem is how to contain the fusion reactions given the extreme conditions needed. The very hot ionized gasses in which fusion can occur is called a plasma. The key to this process then is the generation and containment of the plasma in which the reaction can occur.
You also need to have some sort of mechanism to extract the energy you have generated and convert it from heat to electricity.
In designing a reactor we consider then two key factors:
1. Plasma temperature
2. Lawson criteria - this is how well the plasma is confined, and this
is the product of the plasma density and the confinement time. This must exceed
10 to the power 14 seconds/centimeter cubed.
There are two main methods for producing a fusion reaction, in both cases we are
more likely to see a process whereby the reaction happens in spurts or pulses,
rather than the kind of continuous process we see in a fission reactor.
The two methods of containment are magnetic or inertial. Inertial containment involves creating extremely high temperatures and pressures in a fuel pellet by using lasers to heat the fuel. This type of fusion is probably very far away, as we don't have the laser or other devices that can produce sufficient conditions of temperature and pressure.
Much more likely to produce within the next couple of decades a viable fusion reactor is magnetic containment. This uses extremely powerful magnetic fields to contain the plasma (similar technology is used in particle accelerators).
Most modern magnetic containment devices (this includes designs by Princeton, JET, and ITER) all use a donut shape containment field called the Tokamak. The Russian were the first to have this design, and Tokamak is from the Russian abbreviation for "toroidal magnetic containment".
The last part of the reactor is extracting the energy to turn it into electricity. This is done by placing layers of Li around the reactor that can absorb the heat and other particles. This heat can then be used to heat water and turn steam turbines. The heat extraction technology (though it isn't well discussed in the reactor designs,) has been done, this aspect of the technology exists and is effective.
Fusion reactors will play a significant role on our societies needs for
electricity as they do produce wastes but in significantly smaller quantities
and radioactivity lasts for only a hundred years or so. We are still a
ways from economical reactors (at least 20 years) we have 3 major milestones
to overcome.
1. Getting the fusion reaction to happen, this means you have to reach
sufficient temperatures for the fusion to occur. This stage has been reached in a
couple of different designs.
2. Breakeven. This is where you can get the same kind of energy
out of the fusion reaction as you need to initiate and contain it (some reactor
designs are near this stage).
3. Enough energy above the breakeven point is reached to make the reactor
economically feasible (could be a while yet).
Once these 3 stages are overcome large amounts of energy will likely be available, although we shouldn't be to dependant as there may be some significant flaws that have not been discovered. Either way we are getting closer to a fusion future.