Mixing of helium with air in reactor cavities following a pipe break in HTGRs - High fidelity and engineering CFD model development and validation

高温气冷堆（HTGR）设计是第四代国际论坛（GIF）选定的六种先进反应堆类型之一，将在不久的将来进行开发。它是唯一一种允许在发电的同时产生氢气的设计。高温气冷堆是目前美国和其他一些国家支持的关键反应堆设计之一。在英国，商业、能源和工业战略部（BEIS）最近支持了八个先进模块化反应堆（AMR）的可行性研究，其中三个是高温气冷堆，以促进英国参与这些先进技术的开发。高温气冷堆的设计目的是通过使用非能动安全系统，避免在任何条件下，甚至在设计基准事故以外的条件下发生裂变产物释放。然而，高温气冷堆降压后的空气进入已被确定为对堆芯安全的重要风险。最近已经进行了大量的工作来研究这种现象，但大多数都集中在该过程的后期阶段，包括反应堆建筑的空气再填充和空气进入反应堆压力容器。相比之下，第一阶段，排污，迄今为止很少进行详细研究。这是一个非常复杂的瞬态过程，气流从高度欠膨胀的超音速射流过渡到弱欠膨胀的超音速射流，最后过渡到复杂的自然循环。这对建模提出了巨大的挑战。该提案和相关的美国提案旨在调查在排污阶段期间和之后每个反应堆建筑空腔中空气/氧气和氦气的空间分布。我们在美国的合作伙伴提出的研究的目的是获得实验验证数据，在高温气冷堆（如通用原子公司的350 MWt MHTGR）的排污期间和之后，反应堆建筑空腔中的氦气和空气混合。在英国进行拟议研究的目的是进行数值模拟，以补充美国合作伙伴进行的实验工作。联合王国的工作分为两个工作包。工作包1旨在开发和验证基于工程雷诺平均纳维尔-斯托克斯（RANS）的计算流体动力学模型，用于模拟排污期间和排污后的整个瞬态过程，从最初的管道破裂到达到平衡的时间，并继续到随后的空气再填充阶段。这个模型将是第一个看完整的瞬态过程，我们的目标是带来创新的数值方法来处理从可压缩到不可压缩流的过渡。工作包2的目的是开发高保真CFD模型的基础上，以及解决大涡模拟（LES）的基本流动物理研究的空气进入现象的高温气冷堆。这是为了促进对这种现象的理解，并提供详细的信息和数据，补充实验，以支持工程CFD模型和相关性的发展。此外，我们亦会比较英国和美国所采用的电脑编码，以评估两者的一致性和差异。

The High Temperature Gas-cooled Reactor (HTGR) design is one of the six advanced reactor types selected by the Generation IV International Forum (GIF) for development to be employed in the near future. It is the only design which allows the generation of hydrogen alongside the generation of electricity. HTGR is one of the key reactor designs that are currently supported in the US and a number of other countries. In the UK, the Department for Business, Energy and Industrial Strategy (BEIS) has recently supported feasibility studies for eight advanced modular reactors (AMRs), three of which are HTGRs, to facilitate the UK's engagement for the development of such advanced technologies. HTGRs are designed to avoid fission product release under any conditions, even beyond design basis accidents, by utilizing passive safety systems. However, the air ingress following the depressurization of an HTGR has been identified as an important risk to the core safety. Significant work has been carried out recently to investigate this phenomenon, but most have focused on the later stages of the process including the air-refill of the reactor building and air-ingress into the reactor pressure vessel. In contrast, the first stage, blowdown, has rarely been investigated in detail to date. This is a highly complex transient process, with the flow transitioning from a highly under-expanded supersonic jet, to a weakly under-expanded supersonic jet, and finally to complex natural circulation. This poses a huge challenge to modelling.This proposal and the associated US proposal are aimed at investigating the spatial distribution of air/oxygen and helium in each reactor building cavity during and after the blowdown phase. The objective of the proposed research by our partners in the US is to obtain experimental validation data on mixing of helium and air in reactor building cavities during and after blowdown in HTGRs such as a General Atomics 350 MWt MHTGR. The purpose of the proposed research in the UK is to carry out numerical simulations to complement the experimental endeavours carried out by the US partners. The UK work is organised into two work packages. Work Package 1 is aimed at developing and validating engineering Reynolds-Averaged Navier-Stokes (RANS)-based CFD models for the simulation of the full transient process during and after blowdown, from the initial pipe break to the time when equilibrium is reached and continuing to the following air-refill phase. This model will be one of the first to look at the complete transient process, and we aim to bring in innovative numerical methods to deal with the transition from compressible to incompressible flows. Work Package 2 is aimed at developing high fidelity CFD models based on well-resolved Large Eddy Simulation (LES) for the study of fundamental flow physics underpinning the air-ingress phenomena in a HTGR. This is to advance the understanding of such phenomena and provide detailed information and data, complementary to experiments, to support the development of engineering CFD models and correlations. In addition, effort will be made to compare computer codes used in the UK and the US to evaluate the consistency and discrepancies between them.