Study on Characteristics of Ex-core Detector Response on Primary Neutron Source and Criticality Extrapolation Method during Start-up Experiment

MCNP5 was used to simulate the ex-core source range detector（SRD） response to the primary source（PNS） at the stage of reactor start-up experiment. The relationship among k_eff, k_s and the detector counts C has been analyzed. Through the prediction of SRD response factor a, traditional criticality extrapolation method 1/C is optimized by a’/C in order to obtain a more linear extrapolated curve in the boron dilution process. The result shows that a’/C gives a better match with 1-k_eff as soon as the core approaches to critical.