The Chernobyl Accident-Can it Happen Here?

Abstract

It is very difficult to predict the future of scientific developments, and few would even dare to make predictions extending beyond the next 50 years. However, based on everything we know now, one can make a strong case for the thesis that nuclear fission reactors will be providing a large fraction of our energy needs for the next million years. If that should come to pass, a history of energy production written at that remote date may well record that the worst reactor accident of all time occurred at Chernobyl, USSR, in April of 1986. In that accident, a substantial fraction of all of the radioactivity in the reactor was dispersed into the environment as airborne dust-its most dangerous form. It is difficult to imagine how anything worse could happen to a reactor from the standpoint of harming the public outside. In the wake of the Chernobyl accident, the primary question on American minds was-can it happen here? Let us try to answer that question. We have just seen how extremely improbable an accident of that magnitude should be. But if it is so extremely improbable, how could it have happened so early in the history of nuclear power? The response to that question is that there are very major differences between the Chernobyl reactor and the American reactors on which our previous discussion was based. In order to understand these differences, we must delve much deeper into the details of how reactors work. This discussion may also be useful to those with an interest in the basic science behind nuclear power. HOW NUCLEAR REACTORS WORK In an ordinary furnace, energy is produced in the form of heat by chemical reactions between the fuel and oxygen in the air. A chemical reaction is actually a collision between atoms in which their orbiting electrons interact. The other constituent of an atom is the nucleus. If two nuclei collide and interact we have a nuclear reaction. However, unlike atoms, which are electrically neutral, nuclei have a positive electric charge and therefore strongly repel one another. Hence nuclear reactions do not normally occur in our familiar world. An exception to this situation is the neutron, one of the two constituents of nuclei (the other is the proton), which does not have an electric charge. It can therefore approach a nucleus without being repelled and induce a nuclear reaction. Because this happens so easily, a neutron can move about freely for only about 0.0001 seconds before it collides with a nucleus and becomes involved in a nuclear reaction. Since free neutrons last for such a short time, they must be produced as they are used. Neutrons can only be produced in nuclear reactions, so what is needed is a nuclear reaction induced by a neutron which releases more than one neutron. These can then induce further reactions which produce more neutrons, and so forth, in a self-sustaining chain reaction. Such a reaction is available in the interaction of a neutron with a uranium-235 (U-235) nucleus. This is the basis for a nuclear reactor. When a U-235 nucleus is struck by a neutron, it often splits into two nuclei of roughly half the size and mass in a process called "fission." Since all nuclei have a positive electrical charge, these two newly formed nuclei repel one another very strongly. As a result they end up traveling in opposite directions at