The minimum departure from nucleate boiling ratio (DNBR) and maxi­mum fuel centerline temperature are limited to secure fuel integrity at normal operation and abnormal transients. DNBR is defined as the ratio of predicted critical heat flux (heat flux at the time when a departure from nucleate boiling occurs in boiling heat transfer, referred to as DNB heat flux) to actual heat flux. Cores are designed to have a minimum DNBR larger than the allowable limit and a fuel centerline temperature lower than the fuel melting point.

In addition, the cores are designed to assure that maximum burnup is lower than the design limit confirmed for fuel integrity. There is also another maximum burnup limit obtained from fuel cycle considerations such as the acceptance limit set by reprocessing facilities.

(4) Power Distribution Restriction

To meet the minimum DNBR limit at normal operation and abnormal transients, cores are designed to assure that the nuclear enthalpy rise hot channel factor (FNH) is lower than the design limit at normal operation. The heat flux hot channel factor (FQ) must be lower than the design limit at normal operation to satisfy safety analysis (LOCA analysis) initial condi­tions, and moreover, maximum linear power density must not exceed the design limit at abnormal transients due to the limits of minimum DNBR and maximum fuel centerline temperature at abnormal transients.

(5) Stability

Cores are designed to assure no abnormal oscillation of the power distri­bution where oscillation decay characteristics are sufficient or if there are any oscillations, that they can be easily detected and suppressed.