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 l/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 l-k_eff as soon as the core approaches to critical. (authors)