Heat TransferDECAY HEATwhere:= decay heat (Btu/hr)Qm = mass of coolant (lbm)cp= specific heat capacity of coolant (Btu/lbm-oF)DT = temperature change of coolant (oF)Dt = time over which heatup takes place (hr)Example:Three days after a planned reactor shutdown, it is desired to perform maintenance on oneof two primary heat exchangers. Each heat exchanger is rated at 12,000 Btu/hr. 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:Qm cpDTDt(24,000 lbm)0.8Btulbm °F0.8°F1 hr15,360BtuhrOne heat exchanger removes 12,000 Btu/hr.One heat exchanger will not be sufficient.DecayHeatLimitsReactor 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. 0 Page 55 HT-02
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