In all these complications of using the right source of energy, this is where the purpose of nuclear fusion research comes into picture. The smart features of this type of fusion power includes a no harmful gas emitted characteristic. This is because the only waste product produced by this reaction is helium (He) which is a chemically harmless type of gas found abundantly in the earth’s atmosphere. To conclude, this makes fusion energy an environmentally friendly type of energy resource.
Also, from calculations, a 1000MW fusion reaction would produce about 250 kilograms of helium as a by product which is in huge contrast compared to fossil fueled energy power plants. Below is a table showing the waste products from different types of 1000MW energy resources per year:
In conclusion of the previous table above, Fusion power would be the best source of energy to use in terms of harmful greenhouse gas emissions.
Another attractive feature of this type of fusion power is of it’s safety handling as there wouldn’t be any chance of tragic events happening such as nuclear meltdowns. This is because only the collision of atomic nuclei would provide energy and not the breakdown of it’s nuclei to release energy.
In the area of economics, fusion power is certainly predicted to cost much cheaper than that of the current price of the today’s leading energy source. This is because of the reactants used in the fusion process which are easily accessible. In great contrast to fossil fuels, huge amounts of money would have to be spent on large equipments in digging deeply into the earth to acquire the energy.
The purpose why the remote handling systems are used in the Jet tokamak is because of the tokamak’s harmful radioactivity after when tests are performed on the nuclear reactor. This makes it very difficult and poses a dangerous threat to people working on the nuclear experiment. In the olden days, scientists were no more allowed than 4 hours to perform system works on the tokamak after experiments were performed on JET which made the torus radioactive. They wore white suites and helmets which reflected radiation and thus protecting themselves from the harmful radioaction inside nuclear reactor.
Thanks to the advances of modern technology these days, the remote handling system offers less man labour on Jet experiments and also prevents the chance of human exposure to radioactivity. Their sterile and clean structure prevents contamination of any sort inside the fusion reactor and ensures work done to the highest efficiency.
Account Of Physics Principles
Fusion is a process whereby nuclei collide into each other so fast that they overcome their repulsive forces and are joint together forming a heavier nuclei. This nuclear colliding process only occurs at high temperatures and releases vast amounts of energy when collision occurs. The fusion reaction process practiced in the JET tokamak reactor is illustrated in the diagram below which uses a Deuterium and Tritium isotope of hydrogen as reactants:
The equation for the above fusion reaction is:
In a fusion reaction, a tiny amount of mass is converted into energy. Once we know the amount of energy released in the fusion process (17.59 Mev), The amount of mass converted into energy can be calculated by the famous Einstein equation E=mc2 .
1eV = 1.6 × 10-19 Joules
1 MeV = 1.6 × 10-19 (× 106 ) = 1.6 × 10-13 Joules
Therefore 17.59 MeV = (17.59 × 106 ) × ( 1.6 × 10-13 )
= 2.8144 × 10-6 Joules
E=mc2
Whereby E = 2.8144 × 106 Joules
c = 3×108. Therefore c2 = (3× 108)2 = 9×1016 ms-1
m =???? kg
E
To find “m”, you manipulate the equation E=mc2 to give m =
c2
2.8144 × 10-6 joules
9×1016 ms-1
The above calculation concludes that the amount of mass destroyed and converted into energy is = 3.13× 10-23.
Deuterium and tritium are both isotopes of hydrogen. Isotopes are basically atoms of the same element which have the same number of protons but with a different number of neutrons in the nuclei.
Deuterium occurs naturally in nature (from ordinary water) and there is about 35 grams of deuterium in every cubic metre of water. Tritium is quite hard to acquire as it is not found in nature naturally. Therefore tritium is artificially extracted from lithium (a metal which is abundantly found in the earth’s crust). This process is done by lithium being bombarded by a neutron which leads to a reaction in producing tritium. The equation for this process is:
To conclude, the primary reactants needed for fusion energy practiced at JETis infinite in use.
Plasma Explained
A plasma is an ionized form of extremely hot gas matter which occurs at temperatures of above 10,000ºC. It is usually considered as the fourth form of distinct matter besides solid, liquid and gas and it is made of free ions and electrons. The free ions are composed of atomic nuclei being stripped of electrons when exposed at hot temperatures. This therefore makes a collection of electrically positive charged ions and negatively charged electrons. Examples of plasmas are the sun, lightening and fluorescent light tubes. Different types of plasmas are mixtures of negatively charged electrons and positive charged ions (and sometimes negative ions as well as neutral atoms and molecules). Particles in a plasma move freely in the molecule. An example of a plasma is shown below inside a nuclear reactor:
In order to achieve the incredible temperatures of a plasma state on earth to make fusion reactions happen inside the JET nuclear reactor, special heating techniques are used
There are four main types of heating techniques in achieving a plasma temperature. These are:
Ohmic heating: This is the type of technique whereby the use of strong electric currents (of up to 7MA) is put into the reactor which heats up and generates a plasma (just like how a current heats up a wire). The large current is produced by the use of an enormous “eight limbed transformer”. A transformer is a device used to increase or decrease electric voltages and currents.
A set of wires wrapped around a non conductive material (called coils) is put around the “center limbs of the transformer core” which forms a primary winding and also another around the plasma and thus forming the secondary winding. Both windings of the transformer is what the current is induced from to heat up the plasma.
Although this type of Ohmic heating technique can heat plasmas into temperatures of around 20-30 million Kelvin, it is not quite effective because the amount of heat generated depends on the resistance between the plasma and the current (the Joule Effect which states that the amount of electrical energy produced is dependent upon the resistance of the substance through which electricity is passing). The reason for the ineffectiveness of Ohmic heating from the Joule effect principle is because at temperatures of over 1 million degrees centigrade, the electrical conductivity of the plasma is very high which means that there is very low resistance.
In needing higher temperatures for larger fusion reactions to occur, other methods of plasma heating are introduced to enhance the Ohmic heating technique (in order to reach higher temperatures for bigger scaled fusion reactions to occur).
Neutral Beam Heating: This is an enhancing plasma heating technique of ohmic heating whereby accelerated neutral beams of deuterinium or tritium ions are injected into the already heated plasma.
The beams of ions carry large kinetic energies and are neutralized in order for easier access through the magnetic fields confining the plasma. When these beams are injected into the plasma, electrons are lost and the nuclei become ionized due to high temperature particle collisions (making them positively charged ions). “In the series of subsequent ion-ion, ion-electron and electron-electron collisions, the group velocity of beam ions is transferred into an increased mean velocity of chaotic motion of all plasma particles”. This results in the neutral beam of ions heating up the pre heated plasma and increasing it’s temperature by a maximum of 21MW.
Lower Hybrid Current Drive: This additional pre-heating technique is a process whereby micro electromagnetic waves of up to 10MW propel charged particles in a plasma at 3.7GHz of cycles per second to produce currents of up to 3MA which increases plasma temperature.
Radiofrequency Heating: This type of plasma heating technique is also known as Ion Cyclotron Resonant Heating (in a more scientifical understanding). It is a process whereby the antennae in the vacuum vessel of the tokamak “propagate waves in the frequency range of 25-55 MHz into the core of the plasma to increase the energy of the ions” thus increasing the temperature for more fusion reactions to occur. This heating method provides additional energy of up 20MW of power
Magnetic Confinement Of Plasma
A magnetic field is defined as a force produced by moving electric charges or currents that exerts a power on other moving charges.
Because the incredible temperature of plasma (which is over 100 million °C) inside the tokamak cannot be contained inside the inner vessel walls of the nuclear reactor alone, the use of magnetic fields gives an alternative technique in actually restraining the high temperature plasma particles from touching the walls of the reactor. This is called confinement.
The reason why plasma is restrained from touching the walls of the reacting vessel is that it cools down very rapidly and terminates itself at once when contact is made.
A plasma is made up of ionized particles. In the absence of magnetic fields, the ionized particles move in straight lines and in random directions. This eventually leads to contact with the inner chamber walls of the reacting vessel and results in the plasma quickly cooling down and terminating itself. But when a constant presence of a magnetic field is introduced, the ionized particles in the plasma flow through these fields in a spiral path on the magnetic lines. The movement of the particles across the magnetic field lines are restricted and this also prevents contact to the walls of the containment vessel.
Below is a picture illustrating the movement of charged particles with and without the presence of magnetic fields:
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On the torus, there two main components of magnetic fields used. The main type of magnetic field used in the plasma confining process is the toroidal field. This is a field whereby 32 large (D-shaped) coils surround the vacuum vessel of the nuclear reactor and when current flows around these coils, a toroidal magnetic field is generated which controls the position of the plasma inside the torus and also modifies the poloidal field (the second magnetic field).
The other magnetic field (the poloidal field) is produced by current being flown through the plasma which is initially induced from transformers. The created poloidal field with the combination of the toroidal field confines the plasma well and stabilizes it which allows confinement times of plasma temperatures to be longer.
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Limitations Of The Jet Fusion Tokamak
The foremost limitation on the Jet fusion tokamak is the small amount of time the plasma lasts for inside the reactor. This is because of the plasma being continuously cooled down by tiny impurities inside the chamber walls.
Also, the efficiency of the fusion reaction inside the nuclear reactor is quite an important aspect of limitations that has a need to be solved. This is because large amounts energy are put in initially just to start the fusion reaction at a plasma state and only half the energy is acquired when fusion is completed. On a typical Jet experiment, 30MW of energy is the power input used and only 16MW of energy is successfully extracted out from the reactor as power output . From an efficiency equation (Pout / Pin), this gives an efficiency of 53.3% .
Another important limitation of fusion reactions inside the tokamak reactor is the instability of the plasma that has a need to be controlled. This is because of excited and charged particles inside the plasma wanting to escape as they are being compressed into higher temperatures. Although this is controlled by the use of magnetic fields, the slightest error in confining these charged particles could result in the plasma escaping and quickly cooling down and thus all the total energy put initially put into the reaction being wasted. Also if the plasma escapes, this results in it touching the inner walls of the tokamak and eroding or damaging the components surrounding the plasma and by therefore reducing the lifetime of these components.
Although these problems or limitations cause less energy efficiencies inside the tokamak, modern day advances of computer aided technology is gradually providing remedies in sorting these limitations out.
Future Developments
From my knowledge of future developments in nuclear reactors, experimental studies which is currently being carried on the JET experiment is believed to provide some detailed background information in acquiring similar basic properties in the building process of ITER..
Currently, The JET experiment holds the world record for released fusion power at 16 million Watts (16MW). This is “a value comparable to the power needed for heating one thousand households in a cold winter.”
In future developments, it is predicted that the ITER experiment which is estimated to finish construction at the end of year 2006 in France would release an amazing fusion power of 500 Million Watts. This, by my accurate calculations, is enough to power up to 31,250 homes on a cold winter night.
The future properties of ITER when construction is finished is going to be entirely based around a hydrogen plasma torus which would operate at temperatures of above 200 Million °C and also by the use of super conducting coils to generate high temperature plasmas which would yield better energy efficiencies give more energy outputs.
In the size of ITER’s components compared to the current JET nuclear reactor, ITER is estimated to be 10 times bigger than JET in size.
Bibliography
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