Thermal Criteria for Plant Startup

In the Super LWR, a design criterion is set on the maximum cladding surface temperature (MCST) in order to maintain fuel rod integrity and prevent cladding failures. MCST is limited to not greater than 620°C as an example for the following discussion [9]. 

Beyond the critical pressure and temperature, there is no actual distinction between liquid phase and vapor phase. The moisture content in the main steam is not a limiting factor during normal operation of the Super LWR (i.e., at 25 MPa pressure and 500°C temperature) because the turbine inlet steam is superheated steam. However, during Super LWR startup at subcritical pressure, the moisture content in the steam becomes a limiting factor as the turbine inlet steam is saturated steam. The moisture content in the steam is limited to not greater than 0.1% so that turbine blade damage can be prevented. This criterion is consistent with that of current BWRs. 

Since the Super LWR plant system does not have a superheater, the main steam conditions need to be adjusted during startup and low power operations. The enthalpy of the core outlet coolant must be high enough to provide the required turbine inlet steam enthalpy. At subcritical pressure operation in the sliding pressure startup scheme, boiling and dry out in the descending moderator water rods are undesirable and should be prevented because they affect the inlet subcooling. 

the following criteria are set to be satisfied at startup of the Super LWR. 

The moisture content in the turbine inlet steam must be less than 0.1%. 

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