Quantum Physics

30.2 Nuclear Reactors 979events in the reactor core supply heat to the water contained in the primary(closed) system, which is maintained at high pressure to keep it from boiling. Thiswater also serves as the moderator. The hot water is pumped through a heatexchanger, and the heat is transferred to the water contained in the secondary system.There the hot water is converted to steam, which drives a turbine–generatorto create electric power. Note that the water in the secondary system is isolatedfrom the water in the primary system in order to prevent contamination of the secondarywater and steam by radioactive nuclei from the reactor core.Reactor Safety 1The safety aspects of nuclear power reactors are often sensationalized by themedia and misunderstood by the public. The 1979 near disaster of Three MileIsland in Pennsylvania and the accident at the Chernobyl reactor in the Ukrainerightfully focused attention on reactor safety. Yet the safety record in the UnitedStates is enviable. The records show no fatalities attributed to commercial nuclearpower generation in the history of the United States nuclear industry.Commercial reactors achieve safety through careful design and rigid operatingprocedures. Radiation exposure and the potential health risks associated with suchexposure are controlled by three layers of containment. The fuel and radioactivefission products are contained inside the reactor vessel. Should this vessel rupture,the reactor building acts as a second containment structure to prevent radioactivematerial from contaminating the environment. Finally, the reactor facilities mustbe in a remote location to protect the general public from exposure should radiationescape the reactor building.According to the Oak Ridge National Laboratory Review, “The health risk of livingwithin 8 km (5 miles) of a nuclear reactor for 50 years is no greater than the riskof smoking 1.4 cigarettes, drinking 0.5 liters of wine, traveling 240 km by car, flying9 600 km by jet, or having one chest x-ray in a hospital. Each of these activities isestimated to increase a person’s chances of dying in any given year by one in amillion.”Another potential danger in nuclear reactor operations is the possibility thatthe water flow could be interrupted. Even if the nuclear fission chain reactionwere stopped immediately, residual heat could build up in the reactor to the pointof melting the fuel elements. The molten reactor core would melt its way to thebottom of the reactor vessel and conceivably into the ground below—the socalledChina syndrome. Although it might appear that this deep undergroundburial site would be an ideal safe haven for a radioactive blob, there would be dangerof a steam explosion should the molten mass encounter water. This nonnuclearexplosion could spread radioactive material to the areas surrounding thepower plant. To prevent such an unlikely chain of events, nuclear reactors aredesigned with emergency core cooling systems, requiring no power, that automaticallyflood the reactor with water in the event of a loss of coolant. The emergencycooling water moderates heat build-up in the core, which in turn prevents themelting of the reactor vessel.A continuing concern in nuclear fission reactors is the safe disposal of radioactivematerial when the reactor core is replaced. This waste material contains longlived,highly radioactive isotopes and must be stored for long periods of time insuch a way that there is no chance of environmental contamination. At present,sealing radioactive wastes in waterproof containers and burying them in deep saltmines seems to be the most promising solution.Transportation of reactor fuel and reactor wastes poses additional safety risks.However, neither the waste nor the fuel of nuclear power reactors can be used toconstruct a nuclear bomb.Accidents during transportation of nuclear fuel could expose the public toharmful levels of radiation. The Department of Energy requires stringent crash1 The authors are grateful to Professor Gene Skluzacek of the University of Nebraska at Omaha for rewriting this section.