This study addresses the selection and optimization of Temporary Neutron Detectors (TNDs) for monitoring initial fuel loading in a pressurized water reactor nuclear power plant (Units H5 and H6) utilizing irradiated secondary neutron sources (SS). During the initial fuel loading of Unit H5, 3He proportional counters were employed as TNDs. However, measured count rates deviated significantly from expectations due to interference from the strong gamma radiation emitted by the SS affecting the 3He counters’ performance. In response, the TNDs for the Unit H6 fuel loading were replaced with boron-coated proportional counters. To ensure these new detectors met requirements for minimum count rate and effective signal filtering, neutron and signal response tests were conducted in an environment representative of the secondary source conditions. This allowed for the precise experimental determination of the boron-coated counters’ optimal working parameters. Finally, a comparison of TND count rates between Unit H6 and a reference unit validated the feasibility and effectiveness of the boron-coated counters, successfully addressing the challenge of subcritical monitoring during core fuel loading in strong gamma fields.