Nuclear fission—also known as atomic fission—is a process in nuclear physics and nuclear chemistry in which the nucleus of an atom splits into two or more smaller nuclei as fission products, and usually some by-product particles, Hence, fission is a form of elemental transmutation. The by-products include free neutrons, photons usually in the form gamma rays, and other nuclear fragments such as beta particles and alpha particles. Fission of heavy elements is an exothermic reaction and can release substantial amounts of useful energy both as gamma rays and as kinetic energy of the fragments (heating the bulk material where fission takes place).
Nuclear fission produces energy for nuclear power and to drive explosion of nuclear weapons. Fission is useful as a power source because some materials, called nuclear fuels, generate neutrons as part of the fission process and undergo triggered fission when impacted by a free neutron. Nuclear fuels can be part of a self-sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon.
The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very tempting source of energy; however, the byproducts of nuclear fission are highly radioactive and remain so for millennia, giving rise to a nuclear waste problem.
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 + ENERGY235U + 1 neutron
2 neutrons + 92Sr + 140Xe + ENERGY
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.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 + ENERGYYou 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.
Fission in Nuclear Reactors
To make large-scale use of the energy released in fission, one fission event must trigger another, so that the process spreads thoughout the nuclear fuel as in a set of dominos. The fact that more neutrons are produced in fission than are consumed raises the possibility of a chain reaction. Such a reaction can be either rapid (as in an atomic bomb) or controlled (as in a reactor).
In a nuclear reactor, control rods made of cadmium or graphite or some other neutron-absorbing material are used to regulate the number of neutrons. The more exposed control rods, the less neutrons and vice versa. This also controls the multiplication factor k which is the ratio of the number of neutrons present at the beginning of a particular generation to the number present at the beginning of the next generation. For k=1, the operation of the reactor is said to be exactly critical, which is what we wish it to be for steady-power operation. Reactors are designed so that they are inherently supercritical (k>1); the multiplication factor is then adjusted to the critical operation by inserting the control rods.
An unavoidable feature of reactor operation is the accumulation of radioactive wastes, including both fission products and heavy "transuranic" nuclides such as plutonium and americium.
