Overview of the International Comparative Assessment Study of Pressurized Thermal-Shock in Reactor Pressure Vessels (RPV PTS ICAS)

摘要

This paper summarized the recently completed International Comparative Assessment Study of Pressurized Thermal-Shock in Reactor Pressure Vessels (RPV PTS ICAS). The ICAS project brought together international experts to perform comparative evaluations of methodologies employed in the assessment of RPV integrity under prototypical PTS conditions. ICAS grew out of a strong interest expressed by the participants to proceed with further evaluations after completion of the earlier FALSIRE II project. FALSIRE focused on evaluation of structural analysis and fracture assessment methods on the basis of experimental and analytical results. The ICAS problem statement defined transient thermal-mechanical conditions postulated to result from loss-of-coolant accidents in a Western type four-loop RPV with cladding on the inner surface. The primary focus was on the behavior of relatively shallow cracks located under and through the cladding. The assessment activities were divided into three tasks: Deterministic Fracture Mechanics (DFM), Probabilistic Fracture Mechanics (PFM) and Thermal-Hydraulic Mixing (THM). The results showed that a best-estimate methodology for RPV integrity assessment could benefit from a reduction of the uncertainties in each phase of the process. Within the DFM task, where account was taken of material properties and boundary conditions, reasonable agreement was obtained in linear-elastic and elastic-plastic analyses. Results from linear-elastic and J-estimation analyses were shown to provide conservative estimates of peak crack driving force when compared with those from complex 3D finite element analyses. For the PFM task, linear-elastic solutions were again shown to be conservative with respect to elastic-plastic solutions (by a factor of 2 to 4). Scatter in solutions obtained using the same computer code was generally attributable to differences in input parameters, e.g. standard deviations for the initial value of RT_(NDT), as well as for nickel and copper content. In the THM task, while there was a high degree of scatter during the early part of the transient, reasonable agreement was obtained during the latter part of the transient. Generally, the scatter was due to differences in analytical approaches used by the participants, which included correlation-based engineering methods, system codes and three-dimensional computational fluids dynamics codes. Based on concluding discussions from ICAS participants, a project is being organized to develop a computer software tool named QUAMET (acronym for QUAlification METhodology) for future use in qualifying codes and analysts engaged in the structural integrity assessment of RPVs.