Through-Thickness Microstructure Characterization in a Centrifugally Cast Austenitic Stainless Steel Nuclear Reactor Primary Loop Pipe Using Time-of-Flight Neutron Diffraction

Matthew M Schmitt,James J Wall,Sven C Vogel,Chanho Lee,John D Yeager,Daniel J Savage

doi:10.3390/qubs5020012

Abstract

The US code of Federal Regulations mandates regular inspection of centrifugally cast austenitic stainless steel pipe, commonly used in primary cooling loops in light-water nuclear power plants. These pipes typically have a wall thickness of ~8 cm. Unfortunately, inspection using conventional ultrasonic techniques is not reliable as the microstructure strongly attenuates ultrasonic waves. Work is ongoing to simulate the behavior of acoustic waves in this microstructure and ultimately develop an acoustic inspection method for reactor inspections. In order to account for elastic anisotropy in the material, the texture in the steel was measured as a function of radial distance though the pipe wall. Experiments were conducted on two 10 × 12.7 × 80 mm radial sections of a cast pipe using neutron diffraction scans of 2 mm slices using the HIPPO time-of-flight neutron diffractometer at the Los Alamos Neutron Science Center (LANSCE, Los Alamos, NM, USA). Strong textures dominated by a small number of austenite grains with their (100) direction aligned in the radial direction of the pipe were observed. ODF analysis indicated that up to 70% of the probed volume was occupied by just three single-grain orientations, consistent with grain sizes of almost 1 cm. Texture and phase fraction of both ferrite and austenite phases were measured along the length of the samples. These results will inform the development of a more robust diagnostic tool for regular inspection of this material.

Highlights

Cast austenitic stainless steels (CASS) have historically been used in primary cooling loop piping and components in light-water reactor nuclear power plants
In cast austenitic stainless steels (CASS), the microstructure is inhospitable to ultrasonic waves and the macroscale elastic anisotropy produces a large magnitude of ultrasonic attenuation and scattering, effectively reducing the signal to noise ratio for conventional ultrasonic inspections [3]
Bulk textures were obtained from two samples cut along the radial direction from the walls of pipes of cast austenitic stainless steel historically used in the primary cooling loops of nuclear power reactors

Summary

Introduction

Cast austenitic stainless steels (CASS) have historically been used in primary cooling loop piping and components (e.g., pump housings) in light-water reactor nuclear power plants. The centrifugal and static casting methods cause directional solidification; as the metal solidifies, it does so outward from the mold surface, which is below the liquidus temperature, into the melt, with grains growing preferentially in low-energy crystallographic directions. This results in a highly oriented (textured), coarse-grained and often inhomogeneous microstructure [2]. This presents a problem for conventional UT inspection methods since CASS grains have anisotropic elastic moduli, anisotropic ultrasonic velocity, which results in attenuation and a low signal to noise ratio [3]. Alternative approaches to inspection of CASS, including methods to characterize microstructural changes associated with reduced resistance to stress corrosion cracking and thermal embrittlement, are being pursued

Methods

Results

Conclusion