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  2 neutrons + 92Kr + 142Ba + ENERGY
• 235U + 1 neutron  2 neutrons + 92Sr + 140Xe + ENERGY