Nuclear fission is the process of splitting atoms, or fissioning them. This page will explain to you the basics of nuclear fission. Before we talk about that, however, I would like to discuss marbles. Everyone's played with marbles at one time or another, right? Well, imagine about 200 marbles lying on a flat surface, all jumbled together, and roughly forming a circle. What would happen if someone took another marble and threw it at them? They would fly all around in different directions and groups, right? That is exactly what happens in nuclear fission. The filled circle is like an atom's nucleus. The marble being thrown is like a "neutron bullet". The only differences are that the marbles are protons and neutrons and the protons and neutrons aren't in a filled circle, but in the actual atom are in the shape of a sphere. Of course, an atom is also a bit more complicated than a pack of marbles.
Splitting the Uranium Atom:
Uranium is the principle element used in nuclear reactors and in certain types of atomic bombs. The specific isotope used is 235U. When a stray neutron strikes a 235U nucleus, it is at first absorbed into it. This creates 236U. 236U is unstable and this causes the atom to fission. The fissioning of 236U can produce over twenty different products. However, the products' masses always add up to 236. The following two equations are examples of the different products that can be produced when 235U fissions:
* 235U + 1 neutron -> 2 neutrons + 92Kr + 142Ba + ENERGY
* 235U + 1 neutron -> 2 neutrons + 92Sr + 140Xe + ENERGY
Animation of Fissioning of 235U
Let's discuss those reactions. In each of the above reactions, 1 neutron splits the atom. When the atom is split, 1 additional neutron is released. This is how a chain reaction works. If more 235U is present, those 2 neutrons can cause 2 more atoms to split. Each of those atoms releases 1 more neutron bringing the total neutrons to 4. Those 4 neutrons can strike 4 more 235U atoms, releasing even more neutrons. The chain reaction will continue until all the 235U fuel is spent. This is roughly what happens in an atomic bomb. It is called a runaway nuclear reaction.
Animation of a chain reaction fissioning many 235U atoms
In this animation, one can see how the fissioning of each 235U atom (red) releases more neutrons (green) that go on to fission more 235U atoms, thus producing a chain reaction.
Where Does the Energy Come From?:
In the section above we described what happens when an 235U atom fissions. We gave the following equation as an example:
235U + 1 neutron -> 2 neutrons + 92Kr + 142Ba + ENERGY
You might have been wondering, "Where does the energy come from?". The mass seems to be the same on both sides of the reaction:
235 + 1 = 2 + 92 + 142 = 236
Thus, it seems that no mass is converted into energy. However, this is not entirely correct. The mass of an atom is more than the sum of the individual masses of its protons and neutrons, which is what those numbers represent. Extra mass is a result of the binding energy that holds the protons and neutrons of the nucleus together. Thus, when the uranium atom is split, some of the energy that held it together is released as radiation in the form of heat. Because energy and mass are one and the same, the energy released is also mass released. Therefore, the total mass does decrease a tiny bit during the reaction.
Nuclear energy can also be released by fusion of two light elements (elements with low atomic numbers). The power that fuels the sun and the stars is nuclear fusion. In a hydrogen bomb, two isotopes of hydrogen, deuterium and tritium are fused to form a nucleus of helium and a neutron. This fusion releases 17.6 MeV of energy. Unlike nuclear fission, there is no limit on the amount of the fusion that can occur.
Nuclear fusion is the energy source of the future. It is what provides the sun and the stars with the energy to shine
continuously for billions of years. Fusion has been used here on earth to produce nuclear bombs, but has not yet
been controlled so that we can obtain useful energy.
We will try to show how fusion works, and describe current efforts to tame this limitless energy source.
Fusion is what happens when two atomic nuclei are forced together by high pressure ... high enough to overcome the strong repulsive forces of the respective protons in the nuclei. When the nuclei fuse, they form a new element, and release excess energy in the form of a fast-moving neutron. The energy is 'extra' because the mass of the newly formed nucleus is less than the sum of the masses of the original two nuclei; the extra mass is converted to energy according to Einstein's equation E=mc2 This energy can be used to do useful work!
The nuclei used by the sun, and in experiments on earth, that undergo fusion, are two isotopes of hydrogen called deuterium and tritium.
The simple hydrogen atom, which has one proton in its nucleus, has two isotopes ... similar forms of hydrogen, but with extra neutrons in their nuclei. One is called deuterium, the other tritium. You can see the fusion process happening with these two nuclei, in the diagram at the top of the page.
The first generation fusion reactors will use deuterium and tritium for fuel because they will fuse at a lower temperature. Deuterium can be easily extracted from seawater, where 1 in 6500 hydrogen atoms is deuterium. Tritium can be bred from lithium, which is abundant in the earth's crust. In the fusion reaction a deuterium and tritium atom combine together, or fuse, to form an atom of helium and an energetic neutron.It only takes a small amount of these isotopes to produce a lot of energy! The deuterium-tritium fusion reaction results in an energy gain of about 450:1!! No other energy source we can tap releases so much energy for the amount that is input.
In fact, both the extra neutron and the new helium nucleus (called an alpha particle) carry off excess energy which can be used (to heat water, for example). Fusion is like lighting a match to a bucket of gasoline. You need that input energy (the match), but what you get as a result is far more powerful. Fusion fuel is very energy dense. A thimbleful of liquid heavy-hydrogen fuel could produce as much energy as 20 tons of coal. Or, more realistically, one pick-up truck full of deuterium would release the energy equivalent of approximately 2 million tons of coal (21,000 rail car loads), or 1.3 million tons of oil (10 million barrels), or 30 tons of Uranium Oxide (1 rail car load). Clearly, with seawater as our energy source, our energy problems would be over forever!
But there's a catch! In the sun, the energy to force nuclei together comes from the sun's immense internal temperatures, approaching 40,000,000 or more degrees at the center! In order to cause nuclei to fuse here on earth (and release their stored energy), they must either be heated to that temperature, or caused to move fast enough to simulate a correspondingly high temperature.
That has been done already, more than 50 years ago. The energy to set off the fusion reaction was supplied by an atomic bomb, and the fusion reaction that resulted was called a 'hydrogen bomb'! But the energy release was all at once, and uncontrollable. While scientists were easily able to control atomic explosions, to create reasonably safe nuclear energy in atomic power plants, no such controlled reaction has yet been achieved for fusion.
The reason lies in where the energy comes from.
Nuclear fission of a plutonium nucleus already happens naturally ... we just help it along by allowing the reaction to proceed faster. Nuclear fusion, on the other hand, requires that the fuel nuclei be moving very fast, or be heated to very high temperatures. Scientists for the last 50 years have been trying to figure out how to do this, but so far the technology at our disposal is not equal to the task!
Here are two different ways that we might achieve 'controlled' fusion, that are currently being explored in laboratories around the world.
In order for fusion reactions to occur, the particles must be hot enough (temperature), in sufficient number (density) and well contained (confinement time). These simultaneous conditions are represented by a fourth state of matter known as plasma. In a plasma, electrons are stripped from their nuclei. A plasma, therefore, consists of charged particles, ions and electrons. There are two ways that are being explored for confining these hot plasmas - magnetic and inertial.
Efforts to control fusion first relied on the principle of magnetic confinement, in which a powerful magnetic field traps a hot deuterium-tritium plasma long enough for fusion to begin.
In November 1997, researchers exploiting the magnetic confinement approach created a fusion reaction that produced 65 percent as much energy as was fed into it to initiate the reaction. This milestone was achieved in England at the Joint European Torus, a tokamak facility--a doughnut-shaped vessel in which the plasma is magnetically confined. A commercial fusion reactor would have to produce far more energy than went into it to start or maintain the reaction.
A 'Tokamak' reactor. Powerful magnets keep the charged nuclei moving in a circle, at high speeds.
'Tokamak' is a Russian acronym for 'toroidal magnetic chamber. This device was first developed by Russian scientists. A tokamak is a toroidal plasma confinement device, resembling a doughnut in shape. The plasma is confined not by the material walls but by magnetic fields. The reason for using magnetic confinement is twofold. First, no known material can withstand the hundred-million degree temperatures required for fusion. Second, keeping the plasma in a magnetic bottle insulates it well, making it easier to heat up.
(Such reactors are inherently safe. If the plasma escapes, it immediately cools down, and the reaction stops!)
Escaping neutrons and energy would heat a body of water; a steam turbine and generator would produce electricity.
This magnetic confinement method for producing fusion is regarded by some scientists as the most promising one for future commercial energy sites. This stems from the way Magnetic Confinement fusion works, which allows for a sustained reaction and thus continuous energy production. Many 'tokamaks' are in operation currently, around the world, and more are planned for the future. But so far, none have been able to sustain the reaction for more than a few seconds ... the plasma leaks out. Improved magnet design and higher input power will perhaps allow these reactors in the future to maintain a fusion reaction indefinitely, producing copious amounts of power ... from seawater!
Inertial confinement makes use of intense laser or electron beams to implode a fuel pellet. The pellet of deuterium/tritium fuel - a peppercorn-size fuel pellet - must be bombarded by two million joules, delivered in 4 nanoseconds. This is a power demand of 500 terawatts, and the equivalent of condensing up to ten hours' worth of electricity used by half a dozen homes into a fraction of a second!
Lasers can do this. After many years of research, scientists have plans to build a very powerful laser that will produce at least as much energy from fusion as the laser delivers to the pellet, ... but that will still not come close to producing the several 100-fold greater energy required to power the laser itself. That goal requires a fusion energy output much greater than the energy put into the laser. Present laser technology is too expensive to go further, for now.
A laser bombardment device.
Here's how it's supposed to work. Many pulsed laser beams hit the fuel pellet simultaneously, causing the surface of the pellet to become a very hot plasma.
This plasma expands inward, compressing the remaining deuterium and tritium so much that its temperature rises to the required 100,000,000 degrees. For about one tenth of a billionth of a second, there are the same conditions inside the pellet as those inside a star, ... and fusion takes place.
To generate 1000 MW of electricity using such a reactor would require microexplosions of about six pellets in one second. This takes into consideration the inefficiency of the conversion from heat to electrical energy.
In order to achieve these microexplosions, a chamber created to carry away the heat generated by the fusion would be built. A pellet would be shot into the center of the chamber and then the laser or particle accelerator would fire onto it, causing implosion and fusion. This would need to be repeated about six times a second.
This method would probably work, but because it is not self-sustaining, (you have to keep feeding in the pellets), it is not very efficient. Most researchers now believe that magnetic containment devices will be the first ones to actually sustain a fusion reaction.
Why Will Fusion Power Be Important?
By the middle of the next century, the world's population will double, and energy demand will triple. This will be due in large part to the industrialization and economic growth of developing nations. Continued use of fossil fuels (coal, oil and natural gas) will rapidly deplete these limited and localized natural resources.
There is, perhaps, another 50-100 years supply of oil and natural gas, and enough coal for several hundred years. Burning these fossil fuels threatens to irreparably harm our environment.
On the other hand, the deuterium in the earth's oceans is sufficient to fuel advanced fusion reactors for millions of years. The waste product from a deuterium-tritium fusion reactor is ordinary harmless helium.
Solar and renewable energy technologies will play a role in our energy future. Although they are inherently safe and feature an unlimited fuel supply, they are geographically limited, climate dependent and unable to meet the energy demands of a populous and industrialized world.
Another option, nuclear fission, suffers from a negative public perception. High-level radioactive waste disposal, and the proliferation threat of weapons-grade nuclear materials, are major concerns. The fuel supply in this case, uranium, is large, but ultimately limited to several hundred years.
The prospect of successful nuclear fusion technology, on the other hand, promises virtually unlimited energy, with very little danger. The radiation from a magnetic containment device is easily shielded, and (unlike uranium-fuelled fission power plants), if there is an accident and the magnetic containment is breached, the reaction immediately stops!
Nuclear fusion indeed looks like it may be the power source of the future!