球床结构对PB-FHR中子物理特性的影响研究

氟盐冷却高温球床堆（PB-FHR）中燃料球的装卸依靠浮力完成。球床结构受堆芯几何、装卸料速度、熔盐密度、熔盐流动等诸多因素的影响，其不确定性是反应堆物理设计和安全分析中重点考虑的内容。参考装卸料实验台架（PRED）的实验结果，采用蒙特卡罗程序（MCNP）完成了固态燃料钍基熔盐实验堆（TMSR-SF1）球床堆积密度、球床底部形状、冷却剂泄漏导致的液位下降等因素对中子物理关键参数的影响分析。结果表明，堆积密度的增加（50%~64%）导致燃料球装载量的增加、有效增殖因数的增加、温度系数的增加和控制棒价值的减小；相对于平坦型球床底部结构，外锥型结构会随着锥角的增加导致反应性先增加后减小，内锥型和斜面型结构则会引入负反应性；冷却剂泄漏事故引起的堆芯冷却剂液位大幅降低会导致堆积密实并引入负反应性。 

Effect of Pebble Bed Structure on PB-FHR Neutron Physics Characteristics

In the Pebble Bed Fluoride-salt-cooled High-temperature Reactor(PB-FHR), the pebbles are loaded by buoyancy?in the coolant. Hence,?the packing structures of the pebbles in the core are greatly influenced by?various factors such as?the core structure,?the loading rate,?the coolant density, and the coolant flow. The uncertainty of the pebble bed structures is the key issue in the reactor design?and safety analysis. To provide a mock up for the Solid Fueled Thorium Molten Salt Reactor (TMSR-SF1), the Pebble Recirculation Experiment Device(PRED) was developed. Packing experiments has been performed under different conditions.?For the purpose of verifying the design,?the?study on pebble structures has been performed based on the TMSR-SF1 design using Monte-Carlo code MCNP. The results quantitatively evaluate the effect of various packing fractions, bottom shapes of the pebble bed and the loss of coolant?on the key neutron physics parameters. It shows that,?when?the packing fraction is increased from 50% to 64%, the effective multiplication factor, the pebbles loaded and the temperature coefficients are?increased, but the control rod worth?is decreased; the bottom shape of the pebble bed has different effects depending on?the structures. The positive cone can cause?both increasing and decreasing of the reactivity, and the inclined plane and the negative cone cause a decreasing of the?reactivity; the loss of coolant causes?a denser packing which results in a decreasing of the?reactivity.