Axial power monitor rod issues and resolution for K-14.1

A recent concern arose over the treatment of uncertainty associated with the K-Reactor axial power monitors (APMs). There are nine axial power monitor rods located at various positions in the K-Reactor core. By comparing the output of one sensor near the top of the rod to the output of another sensor near the bottom of the rod, the relative ratio of the neutron flux from the top to the bottom of the core can be determined. This ratio is called the roof-top-ratio (RTR) and is the output of a top sensor (Sensor 2) divided by the output of a bottom sensor (Sensor 6). The RTR is important to the safety analyses because when the RTR is maintained within certain ranges, the severity of reactivity transients is limited. There are uncertainties associated with the equipment`s ability to measure the true roof top ratio. It was determined recently that sufficient uncertainty was not accounted for either in reactor operation or in the safety analyses. The concern about uncertainty was addressed for three separate issues. One issue dear with the linear response of the sensors for power ranges planned for K-Reactor operation. The second issue dear with overall uncertainty in the RTR channel. The third issue dear with apparent large ranges in confidence bands for the RTR at low reactor powers as represented by original vendor data. Plots of sparse vendor data indicated unacceptably large uncertainties in RTR would have to be accounted for at the power ranges planned for K-Reactor operation. These concerns were brought to management`s attention through the existing procedures for notification, irrespective of their potential impact on the restart schedule. Analyses have been completed to resolve the APM issues described above, and work is progressing to take the needed steps to change operational procedures.