![]() Another concern over fission reactors is the storage of hazardous radioactive waste. ![]() However, nuclear accidents in 1979 at Three Mile Island and in 1986 at Chernobyl raised concern over the safety of reactors, and these concerns were revived somewhat in 2011 after an earthquake and tsunami resulted in a nuclear disaster in Fukushima, Japan. Submarines and surface ships propelled by nuclear reactors and nuclear-powered electric generating stations are in operation. This heat energy can be dissipated or it can be used to drive conventional heat engines that generate usable power. This second medium, nonradioactive since it has not circulated in the reactor core, carries the heat away from the reactor. Outside the core, the coolant circulates through a heat exchanger where the heat is transferred to another medium. The heat energy released by fission in a reactor heats a liquid or gas coolant that circulates in and out of the reactor core, usually becoming radioactive. The excess nuclear fuel produced can be extracted and used in other reactors or in nuclear weapons. The transmutation of nonfissionable materials to fissionable materials in nuclear reactors has made possible the large-scale production of atomic energy. Similarly, U-238, the most common isotope of uranium, can be converted by neutron bombardment into fissionable plutonium-239. Thorium (Th-232) can be converted by neutron bombardment into fissionable U-233. In a breeder reactor, however, these surplus neutrons are used to transmute certain nonfissionable atoms into fissionable atoms. In an ordinary reactor, any surplus neutrons are absorbed in nonfissionable control rods made of a substance, such as boron or cadmium, that readily absorbs neutrons. It must first be primed with certain isotopes of uranium or plutonium that release more neutrons than are needed to continue the chain reaction at a constant rate. The breeder reactor is a special type used to produce more fissionable atoms than it consumes. ![]() If the fissionable fuel elements are intimately mixed with a moderator, the system is called a homogeneous reactor (e.g., a reactor having a core of a liquid uranium compound dissolved in heavy water). Reactors of these types, in which discrete fuel elements are surrounded by a moderator, are called heterogeneous reactors. Reactors in which the uranium rods are immersed in a bath of heavy water are often referred to as swimming-pool reactors. The moderator surrounds or is mixed with the fissionable fuel elements in the core of the reactor.Ī nuclear reactor is sometimes called an atomic pile because a reactor using graphite as a moderator consists of a pile of graphite blocks with rods of uranium fuel inserted into it. Substances that can be used as moderators include graphite, beryllium, and heavy water ( deuterium oxide). The neutrons carrying on the chain reaction in this type of reactor are slow (or thermal) neutrons. In this type of reactor the neutrons carrying on the chain reaction are fast neutrons.Ī chain reaction can also be accomplished in a reactor by employing a substance called a moderator to retard the neutrons so that they may be more easily captured by the fissionable atoms. Even though the neutrons move at high velocities, the enriched fissionable isotope captures enough neutrons to make possible a self-sustaining chain reaction. A chain reaction can be produced in a reactor by using uranium or plutonium in which the concentration of fissionable isotopes has been artificially increased. A reactor is so constructed that fission of atomic nuclei produces a self-sustaining nuclear chain reaction, in which the neutrons produced are able to split other nuclei. A fission reactor consists basically of a mass of fissionable material usually encased in shielding and provided with devices to regulate the rate of fission and an exchange system to extract the heat energy produced.
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