In order for it to be self-sustaining, at least one of them must strike another U-235 nucleus and cause a fission reaction. It is these neutrons that sustain the chain reaction. The two original fragments from the fission process also have a substantial excess of internal energy which they largely dissipate by shooting off neutrons, typically two or three neutrons from each fission reaction. The purpose of a nuclear power plant is to convert this heat into electricity, as we described in Chapter 6. Thus, the fission reaction releases heat, 50 million times as much heat as is released in the chemical reaction between a carbon atom from coal and oxygen atoms from the air in the coal-burning process. It increases the speed of their normal random motion and our senses interpret this as increased temperature. By these processes, the energy released in the fission process is eventually shared by all of the atoms in the vicinity. The atoms they strike or their orbiting electrons are given additional motion and have collisions with other atoms, sharing their energy with them. As they travel through the surrounding material, whatever it may be, they strike other atoms, giving them some of their energy, until, after about a million such collisions over a few thousandths of an inch of travel, all of their energy is dissipated, and they come to rest. As a result they end up traveling in opposite directions at very high speeds, which means that their motion contains lots of energy. 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. Such a reaction is available in the interaction of a neutron with a uranium-235 (U-235) nucleus. These can then induce further reactions which produce more neutrons, and so forth, in a self-sustaining chain reaction. 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. Since free neutrons last for such a short time, they must be produced as they are used. 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. It can therefore approach a nucleus without being repelled and induce a nuclear reaction. Hence nuclear reactions do not normally occur in our familiar world.Īn 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. However, unlike atoms, which are electrically neutral, nuclei have a positive electric charge and therefore strongly repel one another. If two nuclei collide and interact we have a nuclear reaction. The other constituent of an atom is the nucleus. A chemical reaction is actually a collision between atoms in which their orbiting electrons interact. In an ordinary furnace, energy is produced in the form of heat by chemical reactions between the fuel and oxygen in the air. This discussion may also be useful to those with an interest in the basic science behind nuclear power. In order to understand these differences, we must delve much deeper into the details of how reactors work. 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. We have just seen how extremely improbable an accident of that magnitude should be. 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. It is difficult to imagine how anything worse could happen to a reactor from the standpoint of harming the public outside. 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. 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. 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. 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. Next=> THE CHERNOBYL ACCIDENT CAN IT HAPPEN HERE?
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