Abstract

The operation of many nuclear pressurized water reactors is being extended beyond their design lifetime threshold. From the perspective of possible further lifetime extension, satisfying safety requirements is a priority. Characterization of the structural integrity of the reactor pressure vessel (RPV) is an important issue as it is a guiding parameter that influences the reactor lifetime. Embrittlement of RPV material is primarily induced by the bombardment of fast neutrons (with energies greater than 1 MeV). Consequently, fast neutron fluence is one of the quantities used by safety authorities to characterize the structural integrity of RPV. However, future RPV aging assessments might lean on new variables with respect to current laws, such as neutron fluence considering the whole neutron spectrum or displacements per atom (dpa) since the latter is more representative of overall damage generated in the RPV. In order to meet these challenges, a versatile calculation scheme for RPV aging assessments is proposed in this paper. The developed methodology allows one to compute (fast and non-fast) neutron fluence as well as dpa rate, using the Norgett-Robinson-Torrens dpa model and the Athermal Recombination Corrected dpa model, for a wide azimuthal and axial range on the RPV and in the capsules of the aging monitoring program (which contain dosimeters and vessel material samples). This methodology is based on a coupling between deterministic (CASMO5 and SIMULATE5) and Monte Carlo (MCNP6) numerical approaches. First, the deterministic approach is used to evaluate the full-core fission neutron source term. Second, Monte Carlo modeling is used to perform the neutron attenuation from the core to sites of interest, such as the RPV. The computational efficiency, accuracy, and potential benefits of the methodology are presented. Moreover, the frequency at which neutron transport calculations should be performed in order to obtain sufficiently accurate time-integrated data over a reactor cycle is discussed. Finally, the validity of the fast neutron fluence as an indicator of RPV aging is compared against the use of dpa.

摘要

许多核压水反应堆的运行正在延长到超出其设计寿命阈值。从可能进一步延长使用寿命的角度来看，满足安全要求是当务之急。反应堆压力容器 (RPV) 结构完整性的表征是一个重要问题，因为它是影响反应堆寿命的指导参数。RPV 材料的脆化主要是由快中子（能量大于 1 MeV）的轰击引起的。因此，快中子注量是安全当局用来表征 RPV 结构完整性的量之一。然而，未来的 RPV 老化评估可能会依赖于与现行法律相关的新变量，例如考虑整个中子谱的中子注量或每个原子的位移 (dpa)，因为后者更能代表 RPV 中产生的整体损伤。为了应对这些挑战，本文提出了一种通用的 RPV 老化评估计算方案。所开发的方法允许使用 Norgett-Robinson-Torrens dpa 模型和无热复合校正 dpa 模型计算（快和非快）中子注量以及 dpa 速率，用于 RPV 上的宽方位角和轴向范围和在老化监测程序的胶囊中（其中包含剂量计和容器材料样本）。该方法基于确定性（CASMO5 和 SIMULATE5）与蒙特卡罗 (MCNP6) 数值方法之间的耦合。第一的，确定性方法用于评估全核裂变中子源项。其次，蒙特卡洛模型用于执行从核心到感兴趣位点（例如 RPV）的中子衰减。介绍了该方法的计算效率、准确性和潜在优势。此外，讨论了应该执行中子传输计算的频率，以便在反应堆周期内获得足够准确的时间积分数据。最后，将快中子注量作为 RPV 老化指标的有效性与 dpa 的使用进行了比较。讨论了应该执行中子传输计算的频率，以便在一个反应​​堆周期内获得足够准确的时间积分数据。最后，将快中子注量作为 RPV 老化指标的有效性与 dpa 的使用进行了比较。讨论了应该执行中子传输计算的频率，以便在一个反应​​堆周期内获得足够准确的时间积分数据。最后，将快中子注量作为 RPV 老化指标的有效性与 dpa 的使用进行了比较。