Publisher Summary This chapter describes the dependence of numbers of neutrons and reactor power on the multiplication factor, which is in turn affected by temperature and control rod absorbers. It examines the precautions taken to prevent release of radioactive materials to the surroundings and discusses the philosophy of safety. Prevention of release of radioactive fission products and fuel isotopes is the ultimate purpose of safety features. Delayed neutrons and temperature effects provide inherent reactor safety. Control rods permit rapid shutdown, and reactor components are designed and constructed to minimize the chance of failure. Emergency core cooling equipment is installed to reduce the hazard in the event of an accident. Licensing is administered by the Nuclear Regulatory Commission, which expects plants to use probabilistic safety risk analysis (PRA).