Cores are designed to assure the inherent negative reactivity feedback characteristics. The fuel temperature coefficient (Doppler temperature coefficient) is always negative and it is designed so that the moderator temperature coefficient is negative at hot power operation. Getting the negative power coefficient by the combination of fuel and moderator coefficients suppresses the power rise in abnormal transients.

Items Design principles

Reactor shutdown Designed to assure the complete core shutdown capability at hot temperature condition even with the most reactive rod cluster control assembly (RCCA) stuck in the fully with­drawn position. Designed to maintain the core shutdown capability even at cold tem­perature condition by boric acid injection of chemical and volume control system Reactivity Designed with appropriate limits of the maxi­

insertion limit mum rod cluster control assembly (RCCA) worth so that the core internal structures function for a core cooling without damage to integrity of coolant pressure boundary at the ejection of a rod cluster control assembly Designed with appropriate limits of the maxi­mum reactivity insertion rate so that the fuel integrity is secured at a withdrawal of two banks of RCCAs at the maximum speed Designed with appropriate limits of the maxi­mum RCCA worth so that the fuel integrity is secured at a sudden drop of a fully- withdrawn RCCA at hot full power condition Self controllability Designed to assure the inherent negative reac­tivity feedback characteristics where the Doppler coefficient is always negative and the moderator temperature coefficient is negative at hot power operation

Fuel integrity Designed to assure that the minimum DNBR is

larger than the allowable limit

Designed to assure that the maximum fuel cen­terline temperature is lower than the fuel melting point

Designed to assure that the maximum burnup is lower than the design limit

Power distribution Designed to assure that the nuclear enthalpy rise restriction hot channel factor (FNH) is lower than the

design limit at normal operation

Designed to assure that the heat flux hot channel factor (Fq) is lower than the design limit at normal operation

Designed to assure that the maximum linear power density does not exceed the design limit at abnormal transients

Stability Designed to assure no abnormal oscillation of

power distribution where the oscillation decay characteristics are sufficient or any oscillation is detected and easily suppressed