DECAY HEATHeat TransferEach reactor will have some limits during shutdown that are based upon decay heatconsiderations. These limits may vary because of steam generator pressure, core temperature,or any other parameter that may be related to decay heat generation. Even during refuelingprocesses, heat removal from expended fuel rods is a controlling factor. For each limitdeveloped, there is usually some safety device or protective feature established.DecayHeatRemovalMethods for removing decay heat from a reactor core can be grouped into two general categories.One category includes methods which circulate fluid through the reactor core in a closed loop,using some type of heat exchanger to transfer heat out of the system. The other categoryincludes methods which operate in an open system, drawing in cool fluid from some source anddischarging warmer fluid to some storage area or the environment.In most reactors, decay heat is normally removed by the same methods used to remove heatgenerated by fission during reactor operation. Additionally, many reactors are designed such thatnatural circulation between the core and either its normal heat exchanger or an emergency heatexchanger can remove decay heat. These are examples of the first category of methods for decayheat removal.If a reactor design is such that decay heat removal is required for core safety, but accidents arepossible that will make the closed loop heat transfer methods described above unavailable, thenan emergency cooling system of some sort will be included in the reactor design. Generally,emergency cooling systems consist of some reliable source of water that is injected into the coreat a relatively low temperature. This water will be heated by the decay heat of the core and exitthe reactor via some path where it will either be stored in some structure or released to theenvironment. Use of this type of system is almost always less desirable than the use of theclosed loop systems described above.Students should research systems, limits, and protective features applicable to their own specificfacilities.HT-02 Page 56 Rev. 0
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