Neutron Diffraction Measurement of Residual Stress in an Unirradiated Boiling Water Reactor Core Shroud

	Micrograph of section through weld showing strain map locations.
	Measured axial residual stresses in core shroud sample.

HTML No. 95-031, Stan T. Rosinski

Through efforts coordinated at Sandia National Laboratory, the DOE Office of Nuclear Energy is presently supporting the Boiling Water Reactor Vessel and Internals Project (BWRVIP) to address integrity issues arising from service-related degradation of reactor pressure vessel (RPV) and RPV internal components. Experimental measurement of residual stresses is fundamental to the prediction of in-service stress corrosion crack growth in boiling water reactor components such as the core shroud, which is a welded stainless steel cylinder located inside the reactor pressure vessel that directs cooling water around the nuclear fuel. Welding fabrication residual stresses are the main loads on the material in this otherwise unloaded structure. Stress corrosion cracking occurs in BWR core shroud welds due to the combination of sensitized material, harsh environment, and residual stress. In some instances complete 360^o circumferential cracks to a depth of 50% of the wall thickness have been observed. Realistic predictions of crack growth are required in order to optimize maintenance schedules and safety assessments. In this regard, information as to the residual stress present after fabrication is particularly important.

Residual strains in a 2-inch thick 304L stainless steel plate were measured by neutron diffraction and interpreted in terms of residual stress. The curved plate was removed from a 20-foot diameter unirradiated boiling water reactor core shroud, and included a multiple-pass horizontal weld which joined two of the cylindrical shells which comprise the core shroud. Residual stress mapping was undertaken in the heat affected zone, concentrating on the outside half of the plate thickness. The variations in residual stresses were consistent with the case histories and in agreement with independently conducted finite element calculations and "blind hole drilling" measurements. The results will appear in a Sandia National Laboratories Report, and were presented at the ASME 1996 Pressure Vessels & Piping Meeting.