An Investigation of Cladding Effects on Shallow-Flaw Fracture Toughness of Reactor Pressure Vessel Steel Under Prototypic Biaxial Loading

1999 ◽  
Vol 121 (3) ◽  
pp. 257-268 ◽  
Author(s):  
B. R. Bass ◽  
W. J. McAfee ◽  
J. W. Bryson ◽  
W. E. Pennell
Keyword(s):  
Fracture Toughness ◽  
Pressure Vessel ◽  
Biaxial Loading ◽  
Reactor Pressure Vessel ◽  
Biaxial Stress ◽  
Pressure Vessel Steel ◽  
Test Results ◽  
Vessel Steel ◽  
Nonlinear Stress ◽  
Reactor Pressure

Potential structural-integrity benefits or liabilities of the stainless steel cladding on the inner surface of a reactor pressure vessel (RPV) are important considerations in the effort to refine or improve safety assessment procedures applied to RPVs. Clad-beam tests were carried out to investigate and quantify effects of the clad structure on fracture initiation toughness of through-clad shallow surface flaws in RPV material. A cruciform beam specimen was developed at ORNL to introduce a prototypic, far-field, out-of-plane biaxial stress component that provides a linear approximation of the nonlinear stress distribution generated by thermo-mechanical loading transients in an RPV. The cruciform specimens (102-mm-thick test section) were fabricated from RPV shell segments available from a canceled pressurized-water reactor plant. The specimens were tested under biaxial load ratios ranging from 0.0 (uniaxial) to 1.0 (full biaxial), the ratio being defined as the total load applied to the transverse beam arms divided by that applied to the longitudinal arms. The test results imply that biaxial loading is effective in reducing the shallow-flaw fracture toughness of the clad/heat-affected zone/structural-weld region of the RPV shell below that determined from uniaxial loading conditions. The lowest toughness value from the clad cruciform specimens tested under biaxial loading is only slightly above the ASME Section XI KIc curve. For all biaxiality ratios, the test results imply that shallow-flaw fracture toughness data from the RPV structural weld material are significantly lower than that obtained from a high-yield strength plate material.

Microstructural Banding and Biaxial Fracture Toughness Tests in a Specially Heat-Treated Reactor Pressure Vessel Steel

2004 ◽  
Author(s):  
Randy K. Nanstad ◽  
Mikhail A. Sokolov ◽  
Philip J. Maziasz
Keyword(s):  
Fracture Toughness ◽  
Pressure Vessel ◽  
Biaxial Loading ◽  
Reactor Pressure Vessel ◽  
Biaxial Test ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Oak Ridge ◽  
Heat Treated ◽  
Reactor Pressure

The Heavy-Section Steel Technology (HSST) Program at Oak Ridge National Laboratory (ORNL) includes a task to investigate the effects of constraint on the cleavage initiation fracture toughness of reactor pressure vessel (RPV) steels in the lower transition temperature region using relatively large cruciform fracture toughness specimens under varying degrees of biaxial loading. One of the materials used for the project was a plate of A533 grade B steel (HSST Plate 14A) which was specially heat treated to result in a yield strength comparable to that of a radiation-sensitive RPV steel near the end of design life. During the testing phase to characterize the fracture toughness behavior of the plate with uniaxial three-point bend specimens, some relatively low fracture toughness values were observed. Subsequent metallography revealed the presence of varying degrees of dark bands in the microstructure. These observations prompted an investigation of the relationship between the experimentally determined fracture toughness results and the microstructure of the plate steel used for the biaxial-loading effects project, especially with regard to the results obtained from the biaxial test specimens. The primary issue in the investigation is whether the fracture toughness results obtained from the biaxially loaded specimens were influenced by the steel microstructure in a biased manner, i.e., were the observation regarding effects of biaxial loading on fracture toughness significantly affected by the microstructural segregation in heat treated HSST Plate 14A. A secondary issue is whether segregated microstructures are common in steels used for RPV construction and if the current procedures for evaluating fracture toughness of RPV steels adequately account for such microstructures. Various metallurgical tools, including metallography, microhardness testing, scanning electron fractography, electron microprobe analysis, and analytical electron microscopy were used to characterize the nature of the bands and evaluate the potential effects on the fracture toughness results.

Fracture Toughness Characterization of Midland Beltline Weld After High Dose of Irradiation

2006 ◽  
Author(s):  
Mikhail A. Sokolov ◽  
Randy K. Nanstad
Keyword(s):  
Fracture Toughness ◽  
Pressure Vessel ◽  
Neutron Fluence ◽  
National Laboratory ◽  
Reactor Pressure Vessel ◽  
High Dose ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Oak Ridge ◽  
Reactor Pressure

The Heavy-Section Steel Irradiation (HSSI) Program at Oak Ridge National Laboratory includes a task to investigate the shape of the fracture toughness master curve for reactor pressure vessel steel highly embrittled as a consequence of irradiation exposure, and to examine the ability of the Charpy 41-J shift to predict the fracture toughness shift. As part of this task, a low upper-shelf WF-70 weld obtained from the beltline region of the Midland Unit 1 reactor pressure vessel was characterized in terms of static initiation and Charpy impact toughness in the unirradiated and irradiated conditions. Irradiation of this weld was performed at the University of Michigan Ford Reactor at 288°C to neutron fluence of 3.4×1019 neutron/cm2 in the HSSI irradiation-anneal-reirradiation facility. This reusable facility allowed the irradiation of either virgin or previously irradiated material in a well-controlled temperature regime, including the ability to perform in-situ annealing. This was the last capsule irradiated in this facility before reactor shut down. Thus, the Midland beltline weld was irradiated within the HSSI Program to three fluences — 0.5×1019; 1.0×1019; and 3.4×1019 neutron/cm2. It was anticipated that it would provide an opportunity to address fracture toughness curve shape and Charpy 41-J shift compatibility issues at different levels of embrittlement, including the highest dose considered to be in the range of the current end of life fluence. It was found that the Charpy 41-J shift practically saturated after neutron fluence of 1.0×1019 neutron/cm2. The transition fracture toughness shift after 3.4×1019 neutron/cm2 was only slightly higher than that after 1.0×1019 neutron/cm2. In all cases, transition fracture toughness shifts were lower than predicted by the Regulatory Guide 1.99, Rev. 2 equation.

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