含β缓发中子前驱体裂变系统中的含时Monte Carlo
Time-dependent Monte Carlo in fissile systems with beta-delayed neutron precursors
摘要：
在核反应堆物理领域，瞬态现象的研究通常采用确定性方法或混合方法。这些方法需要许多近似，例如：几何，时间和能量离散化，材料均匀化和扩散条件的假设，等等。在这方面，蒙特卡罗模拟特别适合研究这些问题。在裂变系统的时空动力学中使用蒙特卡罗模拟时所面临的挑战是瞬发和延迟中子发射所涉及的不同时间尺度，这意味着如果使用模拟蒙特卡罗模拟，则所获得的结果具有较大的方差。此外，在确定性和蒙特卡罗模拟中，缓发中子前体被分组在$6$ -或$8$ -群结构中，但是现在没有一个可靠的理由保持这种聚集。在这项工作中，第一次，个人的前兆数据实施的蒙特卡罗模拟，明确包括相关的$β$延迟中子发射的时间依赖性。这是通过修改开源蒙特卡罗代码OpenMC来实现的。在修改后的程序--时间相关OpenMC或OpenMC（TD）--中，讨论了与源自$β$衰变的缓发中子发射有关的时间相关性。所用的连续能量中子截面数据来自JEFF- $3$。$1$。$1$库。个体前体数据取自JEFF- $3$。$1$。$1$（累积产率）和ENDF-B/VIII。$0$（缓发中子发射概率和缓发中子能谱）。OpenMC（TD）在以下国家进行了测试：i）单能系统; ii）能量依赖的未慢化系统，其中前体被单独地或以组结构获取;和最后iii）轻水慢化的能量依赖系统，使用$6$ -组、$50$和$40$单独的前体。
英文摘要：
In the field of nuclear reactor physics, transient phenomena are usually studied using deterministic or hybrids methods. These methods require many approximations, such as: geometry, time and energy discretizations, material homogenization and assumption of diffusion conditions, among others. In this context, Monte Carlo simulations are specially adequate to study these problems. Challenges presented when using Monte Carlo simulations in space-time kinetics in fissile systems are the different time-scales involved in prompt and delayed neutron emission, which implies that results obtained have a large variance associated if an analog Monte Carlo simulation is utilized. Furthermore, in both deterministic and Monte Carlo simulations delayed neutron precursors are grouped in a $6$- or $8$- group structure, but nowadays there is not a solid reason to keep this aggregation. In this work, and for the first time, individual precursor data is implemented in a Monte Carlo simulation, explicitly including the time dependence related to the $β$-delayed neutron emission. This was accomplished by modifying the open source Monte Carlo code OpenMC. In the modified code -- Time Dependent OpenMC or OpenMC(TD) -- time dependency related to delayed neutron emission originated from $β$-decay was addressed. Continuous energy neutron cross-sections data used comes from JEFF-$3$.$1$.$1$ library. Individual precursor data was taken from JEFF-$3$.$1$.$1$ (cumulative yields) and ENDF-B/VIII.$0$ (delayed neutron emission probabilities and delayed neutron energy spectra). OpenMC(TD) was tested in: i) a monoenergetic system; ii) an energy dependent unmoderated system where the precursors were taken individually or in a group structure; and finally iii) a light-water moderated energy dependent system, using $6$-groups, $50$ and $40$ individual precursors.
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