| After nuclear fission, more than 99% of neutrons are prompt neutrons which are emitted in the 10-17s～10-14s, and less than 1% of neutrons are delayed neutrons which are emitted between a few seconds to a few minutes. Delayed neutrons emission time is much longer than that of prompt neutrons, and the control and operation of the reactor can be realized only by the delayed effect. Effective delayed neutron fraction βeff is one of the basic parameters of the reactor, which not only is the basic data of reactivity measurement, but also can be used as an integral parameter for macro nuclear data inspection work.The experiments of measuring effective delayed neutron fraction βeff are very few in home and abroad. At present, there is no successful experiment of measuring βeff in a fast critical assembly at home and it is badly in need of carry out experitment research. This experiment adopts the Nelson number method. This method is accurate and has a low dependence on theory, which has more advantages than other methods. In this paper, three key problems of measuring effective delayed neutron fraction βeff in Nelson number method are studied:the key parameters, the key experiment conditions, n-γ discrimination performance.For determining the key parameters, the spatial correction factor g and effective coefficient of external neutron source g* are calculated by DOT and MCNP programs. Besides, combined with the reactor fuel and neutron energy spectrum, number of prompt neutrons released per fission vp and neutron dispersion factor Dv are selected. For optimizing the key experiment conditions, by simulation and setting up an evaluation parameter, the optimal value of external neutron source strength S and the optimal range of reactivity ρ$ are determined. For improving n-y discrimination performance, the effects of the y ray to the Rossi-α measurement results is simulated. Only the lithium glass scintillator is available and is satisfied the requirement of response time, but the n-γ discrimination performance of the lithium glass scintillator is poor. In order to reduce the influence of γ rays, the n-γ discrimination performance of the detection system is improved by using lead for shielding, using the thinner lithium glass scintillator and pulse amplitude discrimination.Finally, the experiment of measuring βeff is carried out in the delayed critical state and the six subcritical states by Rossi-α measurement. Through the analysis of the measurement results with the variation of reactivity, it is found that the y ray has a little impact on the measurement, and the uncertainty of measurement results is determined. After effective delayed neutron fraction βeff of five measurement points are obtained, effective delayed neutron fraction of the fast critical assembly is obtained:βeff=0.00712, with 7.55% uncertainty.Through the research and experiment work of this paper, the key technology of Nelson method for measuring effective delayed neutron fraction βeff is mastered, and effective delayed neutron fractionβeff’s value of the fast critical assembly is successfully measured. The measuring method can be applied to other similar critical assembly. |
