Operational benefits of relaxed axial power distribution control limits

Constant axial offset control (CAOC) was developed in the early 1970s in response to lower loss-of-coolant accident-based peaking factor limits. Th CAOC requires control of the axial power distribution within a specified band, typically +/- 5% or +3, -12% axial flux difference (AFD), about a measured target value of AFD. Operational outside of the CAOC limits results in the accumulation of penalty time. One hour of penalty time in any 24-h period is permitted. Although CAOC is sufficient to ensure peaking factor limits are satisfied, operation outside of CAOC limits is beneficial under certain conditions. Allowing a relaxation in CAOC restrictions can be used both to enhance the load follow capability of the plant by allowing control strategies that minimize the boron system duty or increase the return to power capability and to greatly increase the ability to return to power after a plant trip or shutdown. To achieve these benefits, relaxed axial offset control (RAOC) was developed. Other benefits of RAOC include a simplified technical specification and the ability to perform in-core/ex-core calibrations at higher powers. Duke Power Company has benefited in many of these ways by changing from CAOC power distribution limits to RAOC power distribution limits at the McGuire Nuclear Station. One of the chief benefits has been the ability to achieve full power much more quickly following shutdowns of short duration and reactor trips during the last half of the cycle lifetime