Fast Spectrum Reactor vis-à-vis Pressurized Water Reactors
Heat is extracted by the coolant for power generation. Heat generation and the removal process occur in the reactor core. Under thermal equilibrium condition, heat generation is equal to heat removal. Based on the process requirements, the core can be designed to sustain any power level, even nearly zero power by appropriately manipulating the number of neutrons and thereby the number of fissions in the core consisting of fissile, fertile, and absorber materials. These materials are surrounded by another class of materials to reflect/shield the neutrons leaking from the inner core. All these materials are placed in some specific configuration in the reactor core. In an FSR, the core is composed of the active core, blankets, absorber rods, neutron reflectors, and shields. While the blankets positioned at the top and bottom regions of the active core are called axial blankets, those surrounding the active core are called radial blankets. With this configuration, the neutrons leaking out in all directions are effectively captured by the fertile materials housed in blankets for breeding as well as fission reactions. Figure 2.1 depicts the schematic sketch of an FSR core configuration. The power distributions in the active core and blanket regions along the axial direction and across the core are depicted in Figure 2.2 at the beginning of life as well as at the end of life. Power generation in the blanket increases gradually and can attain a level close to the power generated in the active core. However, the power generated in the blankets is restricted due to safety considerations. The power generated in the blanket at the end of life is compared with the power distribution at the beginning of life of the core in Figure 2.2. In the case of thermal reactors, neutron leak is insignificant and no significant number of neutrons would be available for breeding. Hence, blankets are not introduced. However, in the case of fast breeder reactor (FBR) there will be more neutron leakage and fertile blankets are provided surrounding the core to profitably capture neutrons. In view of low cross sections, unlike thermal reactors, there is no large reactivity change of the core due to the accumulation of fission products.