基于Feynman-α方法的小型铅铋快堆中子动力学参数测量

Measurement of Neutron Kinetic Parameter of Small Lead-bismuth Fast Reactor Based on Feynman-α Method

  • 摘要: 中子动力学参数对于评估反应堆的动态特性、确保安全运行以及优化物理启动过程至关重要,对其精确测量是反应堆物理实验的重要任务。铅铋快堆是新型核能系统发展的重要方向,为了解铅铋快堆的动力学特性,在中国原子能科学研究院铀棒栅临界实验装置铅堆堆芯上,基于Feynman-α方法开展了缓发中子有效份额βeff和中子代时间Λ的实验测量和数据分析。首先通过活化法标定BF3探测器探测效率,通过全堆输运模拟得到通量分布与裂变率比值关系,最终确定堆芯总裂变率;基于3He探测器与NI数据采集卡,搭建了反应堆噪声方法测量系统;在不同次临界度下进行多次的测量与拟合得到的缓发中子有效份额βeff和中子代时间Λ的实验测量结果与理论计算值较为接近,相对偏差不超过15%,初步验证了本文方法的可行性。

     

    Abstract: Neutron kinetic parameters are crucial for evaluating the dynamic characteristics of nuclear reactors, ensuring safe operation, and optimizing physical startup processes. Accurate measurement of these parameters remains a critical task in reactor physics experiments. As a key direction in advanced nuclear energy systems, lead-bismuth fast reactor (LFR) requires comprehensive studies on their kinetic properties. The effective fraction of delayed neutron (βeff) and neutron generation time (Λ) have been measured by Feynman-α method for the first time at the lead-bismuth zero power of uranium rod gate critical assembly at the China Institute of Atomic Energy, in order to provide basic data for the engineering design of lead-bismuth fast reactors. The experimental methodology integrated neutron activation, full-core transport simulation, and noise analysis techniques. Initially, the detection efficiency of BF3 proportional counters was calibrated via a gold foil activation method. Absolute neutron flux density was determined by irradiating gold foils at high-power critical conditions and measuring their activity using high-purity germanium (HPGe) detector. MCNP code was employed to establish the spatial distribution of neutron flux and fission rates, enabling the derivation of total core fission rate (F) from localized neutron flux measurements. A reactor noise measurement system, comprising 3He detectors, national instruments (NI) data acquisition cards, and pulse signal processing modules, was developed to collect neutron count time-series under subcritical conditions. Four subcritical configurations with effective multiplication factor (keff) in the vicinity of 0.98 to 0.99 were established by adjusting fuel loading. For each configuration, Feynman-α method was applied to the variance-to-mean ratio of neutron counts to extract the prompt neutron decay constant (α), βeff, and Λ. The critical prompt neutron decay constant (αc=(141.22±19.05) s−1) was deduced from the relationship between the measured α value and the detector count rate. Combining experimental α and F with theoretical models, βeff and Λ were calculated. The results show that βeff=0.006 203±0.000 276 and Λ=(43.926±2.325) μs, demonstrating relative deviations of −14.14% and −13.68%, respectively, from MCNP-predicted values (0.007 225 for βeff and 50.89 μs for Λ). These discrepancies highlight uncertainties of theoretical calculations but confirm the feasibility of the Feynman-α method for LFR kinetic parameter measurement. Key challenges include the indirect determination of total fission rates in subcritical states and statistical fluctuations in neutron pulse timing. Increasing the detector count rate and refined detector calibration are proposed to reduce uncertainties. The study successfully establishes a technical framework for neutron noise analysis in LFR, providing critical experimental data to support the design and safety evaluation of generation Ⅳ nuclear systems. Future work will focus on enhancing measurement precision and extending the method to other advanced reactor configurations.

     

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