Analyses of Flow Decreasing Events

Since the transient subchannel analysis code does not have the functions prepared in typical system analysis codes, several parameters are taken from the calculation results by the single channel safety analyses performed in Sect. 6.7. These parameters are the flow rate, temperature and pressure at the inlet of the hot fuel assembly, and the relative power. The radial and axial power distributions are assumed not to change with time. This is reasonable because the reactivity is not locally changed at the flow decreasing events. 

The time profiles of the increase in the maximum cladding surface temperature (AMCST) are shown in Fig. 6.64. The peak values of Cases 1 and 2 are almost equal to that calculated by the single channel code while that of Case 3 is higher than the result by the single channel code by about 25°C. 

The distributions of the cladding surface temperature at the axial position of the highest temperature are compared between the steady-state and the moment of 

For Case 2, the temperature distribution is expanded at the transient condition. In the subchannels surrounded by the fuel rods with relatively high power, the coolant density decreases earlier and hence the pressure drop caused by acceleration, friction, and grid spacers increases earlier, then the coolant escapes to the surrounding subchannels as cross flow. This phenomenon is called flow-redistribution . For Case 2, the difference between the highest and lowest temperatures in the fuel 

Moment of highest AMCST at partial loss of reactor coolant flow 

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