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Fusion Reactor Technology

In my previous post, I mentioned the various problems facing fusion scientists and engineers today. If you have not read it yet, I suggest doing so. It is an introductory to nuclear fusion, and will clarify some of the topics discussed in this post.

In this post, I will present to you, the various aspects of modern fusion reactors, as well as how these designs solve some of the aforementioned problems in my previous post. There are two common methods of creating controlled fusion reactions that I will discuss: Magnetic confinement fusion, and Inertial confinement fusion.

Magnetic Confinement

Magnetic confinement rests upon the property that charged particles, like those in a plasma, will travel along the lines of a magnetic field because objects with a charge are subject to electromagnetism. By arranging magnetic fields in the right way, scientists have been able to trap the plasma within the fields. While the plasma is held, it can be heated with a combination of multiple methods. The plasma can be heated by passing a current through the plasma. It is called ohmic or resistive heating; the heat generated depends on the resistance between the plasma and current. However, as temperature rises, resistance drops, making this form of heating less and less effective. Other methods are neccessary in addition in order to heat the plasma to required temperatures. Another method of heating is neutral-beam injection where high energy, neutral atoms are shot into the plasma, and are immediately ionized. These ions then get trapped by the magnetic fields, and transfer some of their energy to the surrounding plasma particles through collisions, thus raising the overall temperature. The plasma can also be heated through a rapid compression. This is called the magnetic compression method, which is possible by increasing the magnetic field. In the tokamak, this compression occurs by moving the plasma to an area of a higher magnetic field. Radiofrequency heating is another option, where high-frequency waves are launched into the plasma through the use of oscillators. If the waves have the right wavelength, their energy can be transferred into certain particles, which then transfer the energy through collisions with others.
Currently there are two types of magnetic confinement systems: the mirror (open) and the toroidal (closed). The mirror and the toroidal method are both combinations of what you see in the above picture. The mirror method, is where the superconducting field magnets are arranged in a line with both ends open, and the plasma can be reflected back and forth by magnets on both sides (above). Something similar to this, is shown in the picture below. It is a linear accelerator, and is more suited to future spacecraft where the plasma can be ejected out one end as propellant.

The other type of magnetic confinement device is called the tokamak, a word formed from the Russian words for Toroidal Chamber and Magnetic Coil. Tokamaks were originally designed and used in Russia. In this design, the chamber is toroidal, or doughnut-shaped, thus having no open ends. The magnetic field is generated through the current running in the solenoid coils that are wrapped around the reactor. The field is stronger towards the center, causing the plasma to tend towards the outer wall. However, another magnetic field generated by a current going through the plasma itself combines with the coils’ magnetic field to create magnetic lines that spiral around the torus. This spiralling counteracts the drifting effect on the plasma because of the strong inner field, and effectively traps the plasma.

In this method, increasing the magnetic feild also increases the density of the plasma, and thus increases the amount of collisions and fusions that occur. All fusion reactors attempt to meet the Lawson criterion, which varies for different types of fusion, and states the overall conditions which must be met for a yield of more energy than is required for the heating of the plasma. These conditions are usually stated in terms of the product of ion density and confinement time.

Inertial Confinement

Inertial confinement is another method of plasma confinement. This technique involves imploding a small fuel pellet. If it is compressed quickly and hard enough, temperature and density rise, allowing the reaction to reach or exceed the Lawson criterion. It is the inertia of the imploding pellet that keeps it confined momentarily, and because it is confined only by its own inertia, the plasma lasts for about one nanosecond. Therefore, to achieve breakeven point, a very large density is needed, usually around 1024 particles/cm3.
The fuel pellet, or target, is compressed and heated with what are called energy drivers. These high-powered sources of energy are usually either high-powered laser or ion beams, which bombard the target from all sides symmetrically. The outer layer of the pellet vaporizes and moves away from the pellet like a rocket. This projection creates shock waves which go on to compress and heat the core. The compressed fuel then burns, releasing much energy, and expands. This is partially offset by the shock waves, which tend to continue compressing the material. This behavior is known as inertia. The result is an inertal confinement fusion reaction.
There are two types of targets: a direct-drive inertial fusion energy target, and an indirect one. The direct-drive targets are just the spherical pellets containing the fuel which will be pounded directly by a laser or ion beam.

The indirect-drive targets have the fuel pellet placed inside a hohlraum, which is a small and thin cylindrical container composed of a high atomic number material, like gold or lead. The container will convert the driver beams into x-rays, which subsequently compresses the fuel.

A variation of the standard inertial confinement methods is the fast ignitor. The difference is that an extremely short and intense laser creates a hotspot in the center of the fuel which ignites the core. First, the pellet is compressed the standard way as explained above. Then, an ultra short and intense laser pulse punches a hole through the atmosphere left over from the compression after which an even smaller, intense laser pulse is shot down the newly formed channel, creating a hotspot on the dense fuel for ignition. The burn then spreads throughout the rest of the fuel, releasing large amounts of energy. The size and complexity of the primary compression laser system is reduced, and the amount of energy released to energy absorbed could also increase.

There are other, less conventional methods of achieving controlled plasma fusion, such as sonofusion, but so far, these two methods are the most widely used, as well as the most widely accepted possibilities for future plasma fusion reactors.

The science of Nuclear Fusion

          With the detonation of the very first hydrogen bomb in 1952, we tapped into the ultimate source of energy in our solar system- the power source of the sun. Since then, nuclear fusion has been percieved as the holy grail of clean energy and scientists worldwide have tried to harness it’s seemingly limitless power. In the face of all this hype, we must ask ourselves- what is fusion, and if were so good, why dont people use it? What about fission, and whats the difference? That seems awesome, but what’s the catch? These are some of the questions that I hope to answer in the following discussion, as well as dispell several myths about fusion, and nuclear power in general.

What is Fusion?

     fusion_1.jpg                        

          Fusion is the process of joining lighter elements together to form a heavier element. The opposite of nuclear fission where heavier elements are split to make lighter elements. The sun is a perfect example of an uncontrolled fusion reaction in nature, and we benefit from the sun’s heat and light energy every single day. Fusion works because of Einstein’s equation: E=MC². Einstein proposed that Mass and energy are variations of the same thing and one can be converted into another. In nuclear fusion, just like in nuclear fission, when the atomic nuclei are fused, or split, some of the mass is lost, and converted into great amounts of energy in the form of heat, electromagnetic radiation, or kinetic energy of the released nuclear products. This energy released is huge, far greater than the chemical energy we recieve from chemical reactions such as the burning of coal, or natural gas, and doesn’t leave any CO2, or nuclear waste products that have to be stored for tens of thousands of years for them to decay. Fusion fuel is also relatively plentiful, and can be achieved with isotopes found in seawater! This means that you’re getting energy, using relatively common fuel, without trashing the environment.

Whats the catch?

          What I’ve explained above seems like the perfect solution, right? Now, how come all this energy is taking so long to get into the energy grid, and into our homes? The sad truth is, that fusion takes far more energy to create, than what you get back. You are probably familiar with the fundamental interactions of nature (Strong nuclear force, weak nuclear force, electromagnetism, and gravity). The laws of electromagnetism dictate that objects with like charges repel each other, and objects with unlike charges attract each other, and the force gets stronger as distance between the objects decrease. Another fundamental interaction of nature that is, at the atomic scale, about 100 times more powerful than electromagnetism, is the strong interaction, or the strong nuclear force. This is the force that on the larger scale, holds protons and neutrons in the atomic nucleus together, and that on a smaller scale, holds quarks and gluons together to form particles such as protons and neutrons. In order for nuclear fusion to occur, you must apply enough force to overcome the forces of electromagenetism separating the like charges of the atomic nuclei (coulomb barrier), so that the more powerful strong nuclear force can take over and bind the nucleus together. There is a problem, there must be temperatures of over 10 million degrees to overcome the coulomb barrier and cause the nuclei to fuse.

          Another problem, is that when matter is present at the high temperatures necessary for fusion, it changes into another form. We’re all familiar with the 3 main states of matter: solid, liquid, and gas. Each state is more energetic than the last. The atoms of a substance in the gaseous state move about much more than those of a substance in the solid state. Plasma, what we called the “fourth state of matter,” consists of a cloud of charged particles (the intense energies carried by the electrons on the atoms have broken the electromagnetic force holding it to the atom, and are now flying around independantly, the result is a cloud of ions (nuclei without electrons), and electrons) and is an even more energetic state of matter. The problem with plasma, is that it is very difficult to contain, seeing as it moves around all the time. In order for fusion to be practical, it must have a good method of confinement. Aside from plasma, neutrons emitted from the reactor can also damage the reactor walls, by making them radioactive.

          Finally, another problem is to get the nuclei to collide at an efficient rate. The nucleus is about 10000 times smaller than the atom, and trying to get two nuclei to fuse at the exact same point in space is like trying to shoot two bullets and have them hit head on head. If the total energy does not balance out the amount you put in, then you are losing rather than gaining. Thus, great pressures are required to increase the denity of the plasma to a density at least one million times larger than that of our sun in order to make collisions much more likely to make up for all the energy you put in.

Nuclear Myths, the difference between fusion and fission

        In light of the disasters at Chernobyl, Three-mile Island, and most recently, Fukushima Daiichi, the general population have developed a great fear of nuclear power, no matter if it is fission or fusion. Common uneducated responses to the question “Why not nuclear fusion?” are:

  • It leaves nuclear waste
  • It’s hard to control and it explodes
  • Nuclear Energy is dangerous

          First off, I would like to dispell the myth that nuclear fusion leaves nuclear waste. A great number of people in the general population know very little, to nothing about nuclear fusion and nuclear fission. What they do know, is everything bad about nuclear fission, and because nuclear fusion is a nuclear process, they automatically associate it with the nuclear bomb, nuclear radiation, and nuclear waste. Currently, the most widely studied form of nuclear fusion is fusion of deuterium and tritium. The only products that can be released from D-T fusion is harmless helium-4, and neutrons. In D-D fusion, the only products are helium4, helium3, neutrons and tritium. Tritium can be called nuclear waste, but has a half life of only 12.3 years, after which it decays into harmless helium-3. Besides, Tritium is used as fusion
fuel, thus we dont have to worry about long term storage since it can be pumped back into the reactor, where it will be fused into Helium-4. This in comparison with the tens of thousands of years it takes for fission waste such as thorium-230 is a very short time.
         
Next, I will talk about the response “It’s hard to control and it explodes”. When people say fusion explodes, they are most likely thinking of the hydrogen bomb. The hydrogen bomb is a three step weapon that requires a lot of initial energy from the small plutonium bomb at the tip to start a fusion process which then heats up and causes another fission process. In a hydrogen bomb, the fusion reaction can never sustain itself in a runaway reaction. If the plutonium bomb were not there to provide a supply of power, the fusion reaction would stop. In nuclear fission, runaway reations happen because all you need is one neutron bullet to initiate an entire chain reaction with one reaction becoming two, becoming four, becoming eight, etc. The most that anyone can do is constantly pump in water to extract all that heat, and make it into energy, and hope that the pumps will keep running, because if it stops, the nuclear reaction will keep on running and cause a reactor meltdown. In nuclear fusion however, energy is constantly needed to provide the heat and pressure needed to sustain the reaction. Once that energy source is cut, the reaction stops.

          Finally, I will address the stigma around nuclear energy. Whenever people think of nuclear energy, people will think about atomic bombs, nuclear accidents, and all the people who died due to causes linked to nuclear weapons and pollution. Little do they know however, that a nuclear core melt-down is expected only once every 20,000 years of the reactor’s operation. In 2 out of 3 melt-downs there would be no deaths, in 1 out of 5 there would be over 1000 deaths, and in 1 out of 100,000 there would be 50,000 deaths. The average for all meltdowns would be 400 deaths. Since air pollution from coal burning is estimated to be causing 10,000 deaths per year, there would have to be 25 melt-downs each year for nuclear power to be as dangerous as coal burning. In China, about 6.6 people die DAILY from coal mining. Little do they know that a typical person is, and always has been struck by 15,000 particles of radiation every second from natural sources, and an average medical X-ray involves being struck by 100 billion. While this may seem to be very dangerous, it is not, because the probability for a particle of radiation entering a human body to cause a cancer or a genetic disease is only one chance in 30 quadrillion (30,000,000,000,000,000). Despite this, people still fear nuclear energy far more than fossil fuels. Most of this stems from the fact that coal, is something that people are familiar with. Most people know how coal burning works, barbecued food with coal, despite the fact that cooking with coal can cause a series of health problems, and have actually held a lump of coal in their hands. The fact that coal accidents and one unfortunate person here and there and everywhere dying of coal related health problem is usually not big enough to make it on the news also contributes to this. Nuclear power however, is something that people dont know about.  No one can see radiation, many dont know what radiation is and how it affects people, and fewer still have actually seen uranium. The only thing they do know, is all the terrible accidents appearing on the media. Nuclear fusion poses even less risks, seeing as the fuel is more safely obtainable, if expensive to produce (deuterium and tritium appear rarely in nature, and must be enriched. The oceans of the world however, contain enough of this fuel to last millenia, not to mention the fact that it can also be produced), and that there is no danger of a runaway reaction. 

Hopefully, this post answered many of your questions and shed light on many of the myths about nuclear fusion. In my next post, I will discuss about how many of the problems facing the developement of nuclear fusion are solved, and how these solutions work.

Welcome to Anatomy of the Universe

Dear Reader,    

          Hello, and welcome to Anatomy of the Universe where the world is laid open before your eyes. You may know me as the Quark of Nature.

The major topic discussed here is centered around future energy production methods. After reading some of my posts, perhaps there maybe a part that is difficult to understand, or in your view, wrong altogether, I will gladly accept any form of constructive criticism you have to offer since only through trial and error does one finally learn.

Happy reading!
-Quark of nature

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