Session: MF-06-01/11-01 Materials and Technologies for Nuclear Power Plants I and Small-scale and miniature mechanical testing (Joint with C&S)

Paper Number: 61467

Start Time: Tuesday, July 13, 2021, 05:00 PM

61467 - Study on Fracture Behavior and Assessment for Dissimilar Metal Weld of Low Alloy Steel and Ni-Base Alloy Weld Using a BWR Reactor Pressure Vessel Material 

Stress corrosion cracking (SCC) experience in Ni-base alloy weld metal welded to low alloy steel (LAS) has indicated the possibility of propagation of SCC into LAS of reactor pressure vessel (RPV) components. Fracture evaluation for a crack into RPV in structural integrity assessment is generally performed with the fracture evaluation methodology and K_Ic criterion based on the linear elastic fracture mechanics, provided in such as Section XI of ASME Code and “Rules on Fitness-for-Service for Nuclear Power Plants of the Japan Society of Mechanical Engineers Code (JSME FFS Code)” in Japan. The fracture evaluation methodology, however, has been originally developed by considering irradiation embrittlement of RPV material where neutron irradiation damage accumulates during operation, causing decreases in ductility and fracture toughness, and where pressure and thermal loadings are expected at the temperatures below the upper shelf temperature range. Fracture behavior and assessment methodology applicable to dissimilar metal weld (DMW) components have not been enough studied and established, despite the neutron irradiation damage is negligible and the operation temperature is above the upper shelf temperature range in general for such DMW components.

In this study, a fracture mode of DMW of LAS and Ni-base alloy weld has been investigated by using a large-scale, heavy forged steel part named “Bottom Head Ring” of RPV, manufactured for a recent BWR. In addition to fracture toughness tests with compact tension specimens, fracture tests using the plates cut from a DMW material with a semi-elliptical surface crack have been conducted. The crack tip locations have been designed and controlled to be at the heat affected zone with a variation of the distance from the weld fusion line. All the mechanical tests conducted showed highly ductile fracture behavior and properties; elastic-plastic fracture toughness evaluated was above as high as 400kJ/m² in the fracture toughness tests. The results of the fracture tests showed good agreement with fracture load evaluated with the elastic-plastic fracture mechanics methodology. Based on the results, it is considered that the fracture assessment methodology based on the elastic-plastic fracture mechanics is reasonably applicable for the RPV LAS material where embrittlement is not a concerned degradation mechanism.

Presenting Author: Takahiro Hayashi Toshiba Energy Systems & Solutions Corporation

Authors:

Takahiro Hayashi Toshiba Energy Systems & Solutions Corporation
Takuya Ogawa Toshiba Energy Systems & Solutions Corporation
Shuichi Yoshida Toshiba Energy Systems & Solutions Corporation
Masao Itatani Toshiba Energy Systems & Solutions Corporation
Yasuhiro Hattori Toshiba Energy Systems & Solutions Corporation
Tetsushi Yamaoka Toshiba Energy Systems & Solutions Corporation
Shigeaki Tanaka Toshiba Energy Systems & Solutions Corporation
Toshiyuki Saito Toshiba Energy Systems & Solutions Corporation