Thermodynamics, Heat Transfer, And Fluid Flow. V.2. Heat Transfer (776131), страница 11
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Tocheck the current heat load on the primary system due to decay heat, cooling is securedto both heat exchangers. The primary system heats up at a rate of 0.8°F/hr. The primarysystem contains 24,000 lbm of coolant with a specific heat capacity of 0.8 Btu/lbm-°F.Will one heat exchanger be sufficient to remove the decay heat?Solution:Q̇m cp∆T∆tBtu 0.8°F (24,000 lbm) 0.8 lbm °F 1 hr 15,360BtuhrOne heat exchanger removes 12,000 Btu/hr.One heat exchanger will not be sufficient.Decay Heat LimitsReactor decay heat can be a major concern. In the worst case scenarios, it can cause melting ofand/or damage to the reactor core, as in the case of Three Mile Island.
The degree of concernwith decay heat will vary according to reactor type and design. There is little concern about coretemperature due to decay heat for low power, pool-type reactors.Rev. 0Page 55HT-02DECAY 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.Decay Heat RemovalMethods 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-02Page 56Rev.
0Heat TransferDECAY HEATSummaryThe important information in this chapter is summarized below.Decay Heat SummaryDecay heat is the amount of heat generated by decay of fissionproducts after shutdown of the facility.The amount of decay heat is dependent on the reactor’s powerhistory.Methods for removing decay heat usually fall into one of thefollowing categories.-Closed loop systems, where coolant is circulated between thereactor and a heat exchanger in a closed loop. The heatexchanger transfers the decay heat to the fluid in the secondaryside of the heat exchanger.-Once through systems, where coolant from a source is injectedinto the reactor core. The decay heat is transferred from the fuelassemblies into the coolant, then the coolant leaves the reactor andis either collected in a storage structure or released to theenvironment.The limits for decay heat are calculated to prevent damage to thereactor core.end of text.CONCLUDING MATERIALReview activities:Preparing activity:DOE - ANL-W, BNL, EG&G Idaho,EG&G Mound, EG&G Rocky Flats,LLNL, LANL, MMES, ORAU, REECo,WHC, WINCO, WEMCO, and WSRC.DOE - NE-73Project Number 6910-0018/2Rev.
0Page 57HT-02DECAY HEATHeat TransferIntentionally Left BlankHT-02Page 58Rev. 0.
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