Improvement of Thermal Design Criterion

To evaluate the cladding temperatures directly during abnormal transients, it was necessary to develop a database of heat transfer coefficients at various conditions of heat flux, flow rate, and coolant enthalpy. The database of heat transfer coefficients was prepared by numerical simulations that successfully analyzed the deterioration phenomenon itself. The database, Oka-Koshizuka correlation, has been used for safety analysis. 

The concept for refining the transient criteria, without using the MDHFR criterion, was reported in 1997 [11]. Higher temperature cores for thermal reactors and the fast reactor SCTTl-H were designed using the new transient criterion of the 

MCST [12,13]. For high temperature reactors, the coolant enthalpy rise in the core is high and coolant flow rate is inevitably low. The gap between fuel rods is kept small to increase the coolant velocity in the core. 

Removing the critical heat flux criterion (i.e., the MDHFR) from the core design and taking the MCST criterion makes it possible to raise the outlet coolant temperature of the Super LWR and Super FR to that of the supercritical FPP. The high enthalpy rise and low coolant flow rate are advantages of the once-through coolant cycle. 

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