Saturday, 5 September 2009

Basic Nuclear Fission

Introduction:

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.

Choosing the Bullet:

When we spoke about the marble analogy earlier, we said that the marble being thrown is a like a "neutron bullet". But what does this mean, and why not use another type of particle to "throw" at a nucleus to fission it? First, what particles with distinct mass are available to launch at a nucleus? Think back to our lesson on radioactivity. Recall that two particles emitted by radioactive elements are the particle and the neutron. (There are other particles emitted too, but they are generally much smaller than the neutron and the particle.) Recall that the particle is essentially a 4He nucleus. Now, let's review the structure of an atom. Remember that an atomic nucleus is made up of positive protons and neutral neutrons? Because of this, the nucleus carries an overall positive charge. So, if we were to launch another particle with a positive charge at a nucleus, it wouldn't get there. Why wouldn't it get there? The answer lies in magnetism. Have you ever used magnets? If you have, you'd know that two like poles of a magnet repel each other. A positive particle and the positive nucleus would repel each other in the same way. The particle is positive. Why? Well, it's composed of two protons and two neutrons. Its positive protons give it a positive charge. Because it's positive, it would get repelled away from another positive nucleus. So, the only thing left is the neutron. The neutron is electrically neutral and thus would not get repelled from a positive nucleus.

Fissile Isotopes:

Fissile isotopes are isotopes of an element that can be split through fission. Only certain isotopes of certain elements are fissile. For example, one isotope of uranium, 235U, is fissile, while another isotope, 238U, is not. Other examples of fissile elements are 239Pu and 232Th. An important factor affecting whether or not an atom will fission is the speed at which the bombarding neutron is moving. If the neutron is highly energetic (and thus moving very quickly), it can cause fission in some elements that a slower neutron would not. For example, thorium 232 requires a very fast neutron to induce fission. However, uranium 235 needs slower neutrons. If a neutron is too fast, it will pass right through a 235U atom without affecting it at all.

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 " width="17" height="13"> 2 neutrons + 92Kr + 142Ba + ENERGY
  • 235U + 1 neutron " width="17" height="13"> 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 " width="17" height="13"> 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.

Runaway Reactions:

Earlier, we explained the concept of a chain reaction, in which neutrons are released and then produce more reactions that release more neutrons. The first neutron is a first generation neutron. This can release between 2-3 more neutrons, depending on the way in which the uranium nucleus splits. We will assume that about 2.5 neutrons are released during each fissioning. Those 2.5 neutrons are second generation neutrons. Then, those 2.5 second generation neutrons prompt the fissioning of more uranium, producing approximately 2.5 third generation neutrons, for each second generation neutron. So, the total number of neutrons in the third generation is now: 2.5 x 2.5, or 6.25 neutrons. In a nuclear reactor, there is approximately 1 millisecond between each generation. So, what does this mean? It means that every millisecond, the number of neutrons increases 2.5 times. So, if at time 0, there are 5 neutrons present, 10 milliseconds later, there are 5x 2.510 =47,683.7158, or about 47,700 neutrons. That's a lot of free neutrons to be created in .01 seconds!

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