Quantum Physics

30.2 Nuclear Reactors 977Courtesy of Chicago Historical SocietyPainting of the world’s first nuclearreactor. Because of wartime secrecy,there are no photographs of the completedreactor, which was composedof layers of graphite interspersed withuranium. A self-sustained chain reactionwas first achieved on December 2,1942. Word of the success was telephonedimmediately to Washingtonwith this message: “The Italian navigatorhas landed in the New Worldand found the natives very friendly.”The historic event took place in animprovised laboratory in the racquetcourt under the west stands of theUniversity of Chicago’s Stagg Field.The Italian navigator was Fermi.of about 20 000 tons of TNT! An uncontrolled fission reaction, of course, is theprinciple behind the first nuclear bomb.A nuclear reactor is a system designed to maintain what is called a selfsustainedchain reaction. This important process was first achieved in 1942 by agroup led by Fermi at the University of Chicago, with natural uranium as the fuel.Most reactors in operation today also use uranium as fuel. Natural uranium containsonly about 0.7% of the 235 U isotope, with the remaining 99.3% being the238 U isotope. This is important to the operation of a reactor because 238 U almostnever undergoes fission. Instead, it tends to absorb neutrons, producing neptuniumand plutonium. For this reason, reactor fuels must be artificially enriched sothat they contain several percent of the 235 U isotope.Earlier we mentioned that an average of about 2.5 neutrons are emitted in eachfission event of 235 U. In order to achieve a self-sustained chain reaction, one ofthese neutrons must be captured by another 235 U nucleus and cause it to undergofission. A useful parameter for describing the level of reactor operation is thereproduction constant K, defined as the average number of neutrons from eachfission event that will cause another event. As we have seen, K can have a maximumvalue of 2.5 in the fission of uranium. In practice, however, K is less than thisbecause of several factors, which we soon discuss.A self-sustained chain reaction is achieved when K 1. Under this condition,the reactor is said to be critical. When K is less than one, the reactor is subcriticaland the reaction dies out. When K is greater than one the reactor is said to be supercritical,and a runaway reaction occurs. In a nuclear reactor used to furnishpower to a utility company, it is necessary to maintain a K value close to one.The basic design of a nuclear reactor is shown in Figure 30.3. The fuel elementsconsist of enriched uranium. The functions of the remaining parts of the reactorand some aspects of its design are described next.ControlrodsFuel elementsRadiationshieldModeratormaterialFigure 30.3 Cross section of areactor core showing the controlrods, fuel elements containingenriched fuel, and moderatingmaterial, all surrounded by aradiation shield.Neutron LeakageIn any reactor, a fraction of the neutrons produced in fission will leak out of thecore before inducing other fission events. If the fraction leaking out is too large,the reactor will not operate. The percentage lost is large if the reactor is very smallbecause leakage is a function of the ratio of surface area to volume. Therefore, acritical requirement of reactor design is choosing the correct surface-area-tovolumeratio so that a sustained reaction can be achieved.Regulating Neutron EnergiesThe neutrons released in fission events are highly energetic, with kinetic energiesof about 2 MeV. It is found that slow neutrons are far more likely than fastneutrons to produce fission events in 235 U. Further, 238 U doesn’t absorb slow