摘要
先进反应堆对于缓解世界能源危机具有重要战略意义,其极强的探索性和复杂性决定了以蒙特卡罗(蒙卡)方法为核心的核过程模拟具有显著优势并适应未来发展需求。然而对于反应堆模拟而言,临界计算裂变源收敛问题、计算速度慢成为蒙卡发展的瓶颈,同时也是国际研究热点。现有蒙卡程序对于反应堆应用而言在程序功能及结构扩展性方面存在不足,特别对于先进反应堆而言不易实际应用及快速发展以适应新的需求。
本文针对上述问题展开研究,主要工作与创新之处包括:
(1)基于指数迭代中子输运方程开展有效增殖因子的三个估计因子组合估计方法研究。提出自适应叠加网格香农熵源收敛诊断方法并通过1/4堆芯基准源收敛例题Inp24正确性校验。相比于传统香农熵诊断方法,该方法将网格划分与物理关联起来,解决传统方法难以评估裂变源抽样是否充分的问题,网格的自动划分提高了复杂几何和材料问题计算时程序的易用性。在分析现有源收敛加速方法思路的基础上,转变源收敛加速的角度,提出根据物理特性预先进行裂变区域抽样的源收敛加速方法,使Inp24例题收敛速度提升约31.8%。
(2)将云计算引入反应堆蒙卡模拟中,设计了基于C/S框架和Socket通信技术的云计算网络框架及基于任务及资源监控的并行任务协同调度方法,使得用户可通过网络智能地进行反应堆模拟的分布式任务协同计算;提出了固定源问题基于粒子并行计算中负载动态调整方法,可根据资源与任务实际情况进行弹性云计算。以国际热核聚变实验堆ITER基准模型设计案例进行测试,该负载调整方法使任务计算效率提高42.99%,并保障结果的一致性。
(3)结合先进反应堆的需求与发展趋势,设计了耦合多物理并基于先进计算机技术的蒙卡程序功能架构。对粒子径迹与计数结构、中/光子输运流程、几何描述方式与结构进行了定义和设计,使得程序具有较好的扩展性。定义统一的XML标准数据交换格式以实现集几何与物理自动建模、中/光子输运计算、过程与结果可视化于一体,从而克服了难以描述及分析复杂先进反应堆的困难。采用基于上述方法与设计开发的超级蒙特卡罗计算软件SuperMC,从原始CAD模型开始开展ITER基准模型的第一壁、偏滤器、TF线圈、赤道口处分析,结果表明SuperMC在复杂几何描述与处理方面相比于MCNP具有优势。
The development of advanced nuclear reactors has strategic significance for alleviating the energy crisis. The simulation located with Monte Carlo method as the core is obviously advantageous and better accommodating to future needs because of the extreme exploration and complexity of advanced nuclear reactors. However, for reactors simulation, fission source convergence and calculation speed are bottleneck problems of Monte Carlo development and also the international hot research topics. Expansibility of functions and structure of present Monte Carlo codes is insufficient for reactors application. Especially for advanced nuclear reactors, these codes are not easy to apply and fast develop to accommodate to new needs.
Focusing on these aspects, the main research and innovation work of this thesis are as following:
(1) Based on exponential iteration neutron transport equation, research on the k-effective calculation method using combined estimator of three estimators was performed. A new adaptive overlapping mesh method for Shannon Entropy diagnostic was proposed and varified through one quarter reactor core benchmark Inp24. Compared to traditional method, this method makes the mesh division relate to physics to eliminate difficulties in estimation of whether fission source is sampled efficiently. This method also can automatically locate the meshes in fissile regions to make the code more easy to use for problems of complex geometry and materials distribution. After the analysis of present acceleration methods of source convergence, a method of pre-sampling fissile regions according to the physic characteristics and then taken as the initial fission source was proposed from different perspective. This method was tested with the Inp24benchmark case and the convergence speed was enhanced by almost31.8%.
(2) Cloud Computing was introduced to reactor Monte Carlo calculation. A Cloud Computing framework based on C/S framework and Socket network communication technology and collaborative parallel task schedule method based on monitor were designed. This design makes users can intelligently perform collaborative calculation of distributed task via network. A dynamic loading balance method for parallel calculation on particles was proposed. Calculations are performed in elastic Cloud Computing way according to the real situation of resources and tasks. The testing with ITER benchmarking model demonstrated that such loading balance method can enhance the calculation efficiency by42.99%and guarantee the consistency of calculation results.
(3) Considering the needs and development trends of advanced nuclear reactors, a functional architecture of coupling with multi-physics and basing on advanced computer technology was designed. The structure of particles trajectory and tallies, simulation flow path of neutron and photon transport, geometry description method and structure were defined and designed to gain better expandability of code. A uniform XML standard data exchange format was defined to integrate geometry and physics automatic modeling, neutron and photon transport calculation, process and results visualization. Such integrated approach can eliminate some difficulties in describing and results analysis of complex advanced nuclear reactors. SuperMC was implemented based on such code design scheme. With SuperMC, starting from original CAD models, the first wall, divertor cassettes, inboard toroidal field coils and equatorial port of fusion reactor ITER benchmark model were analysed. The advantage of SuperMC over MCNP on complex geometry description and dealing method were demonstrated.
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