Abstract: The output current of intermediate range channel (IRC) of nuclear instrumentation system (RPN) reached the saturation value prematurely when the nuclear power (Pn) was less than 30%FP during the commissioning startup of a pressurized water reactor nuclear power plant. The main influence factor of IRC premature saturation was its neutron detector manufactured sensitivity (SF) , after the first cycle refuel design scheme (FCRS) determined. It can be concluded that IRC premature saturation is the reason that the actual threshold of ATWT permit signal is lower than its design expected threshold, based on the ATWT design function and nuclear instrumentation system (RPN) safety criteria. In order to ensure the correct trigger of ATWT permit signal, the threshold Pnof ATWT permit signal has to be selected as Pn  under the saturation power when IRC premature saturation occurred. In order to decrease the probability of the IRC premature saturation, the IRC detector with smaller SF is sugguested for the nuclear power plants with FCRS of 18 months period (FCRS18) .