Design Considerations with Heat Transfer Deterioration

3 Design Considerations with Heat Transfer Deterioration 

In conventional subcritical pressure LWRs, such as BWRs or PWRs, the core is effectively cooled by the boiling heat transfer. Therefore, the coolant inlet temperature is set below its saturation temperature and the saturated steam is sent to the turbine. (In BWRs, the core inlet and outlet temperatures are 216 and 286°C, respectively. In PWRs, the inlet and outlet temperatures are 289 and 325°C, respectively.) The boiling phenomenon starts as the coolant becomes heated close to its saturated temperature. The coolant starts its phase change from liquid to gas with large discontinuous property changes. The coolant flow becomes a two-phase flow and the bulk coolant temperature is kept below its saturation temperature. There have been very few reactors that could produce superheated steam one example was the American Boiling Nuclear Super heater Power Station (BONUS) an integral boiler-super heater, which was shut down permanently in 1968 and decommissioned by 1970. 

Considering the above points and by referring to experiences with supercritical FPPs, researchers are developing the concepts of the Super LWR with a system pressure of 25 MPa, core coolant inlet temperature of 280°C, and outlet temperature of about or higher than 500°C. 

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