The understanding of the Natural Circulation (NC) mechanism and its flow regimes can be considered as a help to achieve a higher safety level and lower costs by reliance on this mechanism. NC Flow Map (NCFM) envelope diagram as an engineering tool for evaluating the NC Performance (NCP) has been attained from different test facilities database. Some Small Modular Reactors (SMRs) with their interesting safety and economic features have used the NC mechanism as their main primary flow drive mechanism. Uncertainty analysis of NCP of a SMR with NC flow drive mechanism, according to the NCFM envelope diagram, is the main objective of the current study. For this purpose, the NuScale reactor with helical coil steam generator at the normal operation NC mode has been modelled using RELAP5/Mod3.3 and the differences of the results in comparison to the designer’s data are in the acceptable range. Then an uncertainty quantification for several sensitive parameters of NuScale has been performed according to the Code Scaling, Applicability and Uncertainty (CSAU) method and NCFM uncertainties for this reactor have been derived. The NuScale NC uncertainties during the normal operation have been mapped to the NCFM envelope to evaluate the NCP of this reactor in high powers. The mass flow rate to power as an index parameter has been used in NCFM and the value of this parameter is evaluated in the range of 3–4 ($\frac{\mathit{Kg}}{\mathit{MW}.s}$). This range shows the better performance and good design of NuScale reactor for normal operation at the NC condition in comparison to the typical PWRs with high power NC.

对自然循环（NC）机制及其流动方式的理解可以帮助通过依赖该机制来实现更高的安全级别和更低的成本。已经从不同的测试设施数据库中获得了NC流量图（NCFM）包络图，作为评估NC性能（NCP）的工程工具。一些具有有趣的安全性和经济性的小型模块化反应堆（SMR）已将NC机构用作其主要的主要流体驱动机构。根据NCFM包络图，具有NC流体驱动机构的SMR的NCP的不确定性分析是当前研究的主要目标。为此，已使用RELAP5 / Mod3对正常运行NC模式下带有螺旋线圈蒸汽发生器的NuScale反应器进行了建模。与图3相比，结果与设计者数据的差异在可接受的范围内。然后根据“代码缩放，适用性和不确定性”（CSAU）方法对NuScale的几个敏感参数进行了不确定性量化，并得出了该反应堆的NCFM不确定性。已将正常运行过程中NuScale NC的不确定性映射到NCFM包络，以评估该反应堆在大功率下的NCP。功率的质量流量作为指标参数已在NCFM中使用，并且该参数的值在3-4（已经推导了该反应堆的适用性和不确定性（CSAU）方法以及NCFM不确定性。已将正常运行过程中NuScale NC的不确定性映射到NCFM包络，以评估该反应堆在大功率下的NCP。功率的质量流量作为指标参数已在NCFM中使用，并且该参数的值在3-4（已经推导了该反应堆的适用性和不确定性（CSAU）方法以及NCFM不确定性。已将正常运行过程中NuScale NC的不确定性映射到NCFM包络，以评估该反应堆在大功率下的NCP。功率的质量流量作为指标参数已在NCFM中使用，并且该参数的值在3-4（$\frac{\mathit{公斤}}{\mathit{兆瓦}。s}$）。与具有高功率NC的典型PWR相比，此范围显示了NuScale电抗器在NC条件下正常运行时具有更好的性能和良好的设计。