Mechanical methods of improving resistance to stress corrosion cracking in BWR piping systems

Abstract

Pipelocks and the mechanical stress improvement process (MSIP) have been applied in BWR plants. Pipelocks restore the integrity of the weldments with identified cracks. MSIP removes residual tensile stresses from weldments, thus preventing initiation of cracks or retarding growth of pre-existing flaws in piping systems. The first 12-in pipelock installed at Commonwealth Edison's Quad Cities plant was inspected after operating for 18 months. Pipelocks for 10-in, 12-in and 28-in reactor safe-ends were fabricated for Carolina Power & Light's Brunswick plant. MSIP was applied at Commonwealth Edison's Dresden and LaSalle BWR plants. Extensive qualification has been completed for MSIP under US Nuclear Regulatory Commission and Electric Power Research Institute sponsorship. Sectioning of the pipe wall by Argonne National Laboratories provided stress distribution before and after MSIP for the 12-in and 28-in pipes. Measured ‘as welded’ tensile stresses before MSIP were within the range 30–50 000 psi. Compressive stresses after MSIP at the inside surface of the weldment reached more than 30 000 psi in both hoop and axial directions. The axial compressive stresses extended to the middle plane of the wall. Hoop stresses remained compressive through the wall. The stresses were uniformly distributed around the circumference of the pipe. J.A. Jones Applied Research Company completed an evaluation of MSIP applied to a precracked weldment between 28-in pipe and elbow. The pipe was squeezed to about 1·7% in the presence of cracks 25%, 50% and 90% through-wall. High compressive stresses were measured after MSIP. The cracks did not extend and could be identified after completion of the process by the usual UT technique. The use of mechanical methods becomes especially adequate for reactor safe-ends including bi- or tri-metallic joints. The use of overlay technique or induction heat stress improvement is more difficult due to high thermally induced strains at the strong discontinuity interface between materials of different thermal expansion. Basic concepts and practical application of mechanical methods to inhibit stress corrosion attack are described.