| To ensure the safety of nuclear reactor start-up and avoid“blind zone”for detection,the counting rate of at least two detecting channels shall guarantee no less than 0.5 cps in the course of reactor start-up after refueling,which is required by relevant nuclear safety regulations.While 252Cf was used as start-up neutron source for the initial start-up of China Experimental Fast Reactor?CEFR?,the out-of-core detectors may not meet the counting rate requirement in the future reactor start-up after refueling because of 252Cf's shorter decay period?2.64 years of half-life?.Considering the higher price and longer delivery time of252Cf neutron source,and for meeting the requirement of CEFR's long-time operation,the feasibility of using secondary neutron source to replace 252Cf source is studied in this paper.Several kinds of Sb-Be secondary neutron source assemblies and their irradiation schemes are designed in this paper.123Sb?n,??reaction rate and 9Be single nucleon??,n?reaction rate in the secondary source assembly were calculated using MCNP code,and the strengths of each secondary source assembly at specified location inside the reflector assembly under different exposure conditions were obtained.Comparing with the measured date of 252Cf source at specific location inside the reflector and taking into account of the count difference between 252Cf source and Sb-Be secondary neutron source caused by their different energy spectrum,the minimum source strength of Sb-Be source was calculated in this paper which will meet the requirement of CEFR start-up,and the feasibilities of the designed Sb-Be sources and irradiation schemes using in CEFR are analyzed.The calculations show that better secondary neutron source assemblies are obtained when adding no moderator.Those designed secondary neuron source assemblies and their irradiation schemes can meet the minimum counting rate requirements for out-of-core source range detectors under most operating conditions of CEFR start-up. |
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