An assembly of fissionable material (uranium-235 or plutonium-239) designed to produce a sustained and controllable chain reaction for the generation of electric power. The first reactor (then called a “pile”) was constructed at the University of Chicago in 1942 under the leadership of Enrico Fermi. The essential components of a modern nuclear reactor are (1) The core, composed of metal- or ceramic-clad rods containing enough fissionable material to maintain a chain reaction at the necessary power level; as much as 50 tons of uranium may be required (d 19). (2) A source of neutrons to initiate the reaction, such as a mixture of polonium and beryllium. (3) A moderator to reduce the energy of fast neutrons for more efficient fission (called slow or thermal neutrons); materials such as graphite, beryllium, heavy water, and light water are used. (4) A coolant to remove the fission-generated heat; water is generally used converted to steam in heat exchangers and used to drive turbines. Sodium, helium, and nitrogen may also be used. The heating of streams and estuaries by reactor effluent is a serious environmental problem, which can be surmounted by construction of special cooling towers. (5) A control system such as rods of boron or cadmium that have high capture cross sections, to absorb neutrons rapidly when their concentration becomes too high. (6) Adequate shielding, remote-control equipment, and appropriate instrumentation are essential for personnel safety and efficient operation. The primary use of nuclear reactors is for electric power generation. This has been a reasonably successful endeavor; nuclear energy has made a notable contribution to the overall energy supply situation in the U.S. But the efficiency of reactors is often reduced far below their potential by failures resulting from the highly corrosive operating conditions to which the materials of construction of the reactor and the associated hardware are exposed. Frequent and extended shutdowns are commonplace. This, together with the environmental radiation hazard and escalating construction costs, has restricted the development of nuclear energy as a power source. Plans for construction of many plants have been delayed or cancelled. A further negative fact is that the estimated safety factor has changed radically since 1975, at which time the risk of serious accident that might result in core meltdown was reported to be 1 in 20,000 years of operation. A study conducted by ORNL based on accident data in the decade 1969–1979 concluded that this risk has risen to 1 in 1000 operating years—a 20-fold decrease in the safety factor. See Fission; Breeder; Acceptable Risk.
