Energy of Fission

The world’s natural resources are estimated to last about 30 – 70 years, but it could be much more or much less. Energy demand is increasing due to population growth, more energy use per capita and as more countries develop industry. This currently relies on fossil fuels; 80% of the world energy demand. There is a great demand for a safe renewable energy source to help control climate change and have the security of supply. Renewable energy technologies such as solar, wind, tidal, wave, biomass, geothermal and hydro are not very reliable due to their dependence on nature; which varies significantly and is subject to sudden change.

They suffer from isolated availability; certain environmental conditions are more suited to wind and solar energy for example, and hydro/wave energy can only be exploited if a water supply is nearby. The energy produced is also very small compared to Oil or Coal for example thus preventing the possibility of relying total on renewable energy. Nuclear Fission is the brother to Fusion, though it is more developed and is currently used to generate energy. However it causes great safety concern; in some severe circumstances radiation can be released from the reactor core putting the surrounding area at risk.

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There is also the concern of nuclear waste created from nuclear plants as to where to put it. Furthermore there is a security concern when hostile nations use fission power this is because though the fuel uranium-235 cannot be used for a bomb (enriched to only 3%-5%) the spent fuel elements contain plutonium-239 which could be separated chemically and diverted to nuclear weapons usage. These reasons alone are why the need for fusion is so great. Fusion is the answer to all these problems; it yield approximately four times the energy of fission, has near-abundant fuel, and has none of the concerns of fission.

There is no chance of meltdown or a fusion explosion being caused, there’s no highly radioactive waste and the fuel cannot be used to create fusion bombs. Nuclear Fusion comes with all the advantages of fission and little of the disadvantages. It’s safer, cleaner, cheaper and more efficient. Nuclear Fission involves a massive nucleus, usually Uranium-235 absorbing a low-energy, or slow neutron causing the nucleus to ‘break apart’ or fission causing more neutrons to be released thus creating a chain reaction.

This is the main reason why Fission is so dangerous; once it is started it is difficult to stop it. Only a small amount of energy is required to start a fission reaction which is where it has the advantage; fusion requires huge magnitudes of energy. There is also only about two grams of fuel present at any one time, only enough for a few seconds of usage; the reaction cannot become unstable. The pure fact the Fusion is so hard to achieve makes it inherently safe; entirely specific condition are required so any equipment failure would cause the plasma to extinguish.

Just like nuclear fission, fusion produces no greenhouse gases such as carbon dioxide, or any other hazardous gases. Nuclear Fusion produces four times the energy of fission, which could be enough to power two million households, thus providing for the increasing energy needs of the future due to growing cities; more power means less power stations and less impact on the environment. Less radioactive waste is produced in fusion; only metal parts close to the plasma may become radioactive so there is no burden for future generations.

The waste will have short half-lives and has the possibility of reuse after 100 years. Deuterium is in abundant supply in water; 1 in 5000 of the hydrogen in water is Deuterium, amounting to about 1015 tons worth. This means that a gallon of seawater is equivalent to 500 gallons of gasoline. All this can be extracted by electrolysis and would provide an almost limitless fuel supply to last thousands of years. Collecting Tritium however is a more complex process as it does not occur naturally and must be ‘bred’ from Lithium. This will come from the earth’s crust which has it in great supply.

When Lithium ‘absorbs’ a free neutron it produces Helium and Tritium, when Lithium-7 is used a further neutron is produced allowing more Tritium to be ‘bred’; a neutron is also produced in the D-T cycle. These two ‘breeding’ reactions are expressed as: The Tritium is radioactive but as it is produced in the reactor there is no problem with the transportation of radioactive materials. This also solves the problem of the spare neutron; this is because neutrons can join themselves with other nuclei, sometimes making them radioactive or starting a new reaction.

Lithium is used as a cushion on the inside of the reactor so the free neutron creates a tritium and some more energy. However the neutron will not always fuse with the Lithium, it may join with other parts of the reactor creating radioactivity and the need for another neutron for more tritium. This can be overcome with neutron multipliers, or the use of the second reaction shown; Lithium-7 which produces an extra neutron. To limit the radioactivity however, careful selection of materials can be applied such as advanced low-activation materials or the use of neutron-free reactions in future reactors.

An example of such a reaction is Deuterium and Helium-3 but this is considered more advanced. In order to control fusion and use it in power stations we must first overcome some formidable scientific and engineering challenges. Although nuclear fusion has been achieved on Earth it has yet to be maintained. Energy will be produced from Fusion the same way it is in fission; the heat produced creating steam which turns a turbine connected to a generator. Firstly the atoms must be in the form of plasma; often referred to as the fourth state of matter.

To create plasma a gas is heated until the electrons separate from the nucleus creating a ‘cloud’ of charged particles or ions, this process is referred to as ionisation. This cloud of equal amounts of positively charged nuclei and negatively charged electrons is called plasma. Suns, stars, auroras and lightning are all examples of naturally occurring plasmas. The plasma itself needs to be heated further to overcome the repulsive forces of the nuclei; the hotter it is the more energy they have. The primary conditions required for fusion are temperature, density, containment and confinement time.

The plasma needs to be generally as hot as possible, the hotter it is the more energy the nuclei have to overcome the repulsive forces and be attracted by nuclear forces. However if it gets too hot then the nuclei have too much energy and they zoom straight past each other. Around 100 million degrees Kelvin is the most optimal temperature. Density is important, the denser it is the more fusion reaction will occur; there is more to react with. Containing the plasma is also an important task; there is no material which can withstand such temperatures.

There are two solutions to this problem, magnetic confinement and inertial confinement. Magnetic confinement stops the hot plasma from touching the container by magnetic fields which keep it moving in circular or helical paths that prevent it from coming in contact with the outside wall. The tokomak is the most promising design for a commercial Fusion power station and comes from the Russian word for a torus shaped chamber with magnetic coils. Magnetic confinement works in the way because the particles in the plasma are charged and charged particles are deflected by a magnetic field.

Magnetic confinement has been under work since the 1950s, the first investigations of which were on linear machines, but particles were easily lost at either end so the idea evolved into the circular torus. As I mentioned fusion has been achieved but in order for it to be used in power stations the fusion reaction needs to produce more energy than is given out, this means the reactor needs to achieve ignition where it can sustain itself. This is done when it exceeds the Lawson criterion at which This is basically the plasma density n times by the confinement time t for a given temperature (in this case 100 million Kelvin).

This tells us that the plasma can contain a lot of particles for a short amount of time, or a small number of particles for a long amount of time. This is achieved by inertial confinement and magnetic confinement respectively. As for the energy outputs of fusion in a power station I shall compare it to fission. Figure 2 shows the energy yields of both thermonuclear reactions for 1kg or fuel as you can see although Fission yields more on average (215 MeV compared to 17. 6) the fractional yield, as the percentage of the original energy, Fusion is just less than 4 times more the energy of Fission.