Stochastic fluctuations of the neutron population within a nuclear reactor are typically prevented by operating the core at a sufficient power, since a deterministic (i.e., exactly predictable) behavior of the neutron population is required by automatic safety systems to detect unwanted power excursions. In order to characterize the reactor operating conditions at which the fluctuations vanish, an experiment was designed and took place in 2017 at the Rensselaer Polytechnic Institute Reactor Critical Facility. This experiment however revealed persisting fluctuations and striking patchy spatial patterns in neutron spatial distributions. Here we report these experimental findings, interpret them by a stochastic modeling based on branching random walks, and extend them using a “numerical twin” of the reactor core. Consequences on nuclear safety will be discussed.

通常通过以足够的功率操作堆芯来防止核反应堆内中子种群的随机波动，因为自动安全系统需要中子种群的确定性（即，准确可预测）行为来检测不需要的功率偏移。为了表征波动消失的反应堆运行条件，2017 年在伦斯勒理工学院反应堆关键设施设计并进行了一项实验。然而，该实验揭示了中子空间分布的持续波动和惊人的不规则空间模式。在这里，我们报告了这些实验结果，通过基于分支随机游走的随机建模来解释它们，并使用反应堆堆芯的“数字孪生”扩展它们。