MSIP for BWR piping systems

The basic concept of MSIP for piping welds is briefly described below. A simple hydraulically operated clamp (Figure 1) is used to locally contract the pipe in the direct vicinity of the circumferential weld. The permanent contraction under the tool generates a concave contour at the weld location and results in a corresponding reduction of the pipe circumference (Figure 2). The amount of contraction needed to complete the stress redistribution depends on the geometry of the weld joint and materials. Once the tool has been removed, the weldment remains in axial compression through about half of the wall and is protected by a layer of compressive hoop stress which extends almost all through the wall.

The generation of residual compressive stresses has been verified and confirmed by independent tests. These include residual stress measurements on 12" and 28" weldments by Argonne National Laboratory (ANL) for US NRC, pre-packed 28" pipe-to-elbow weldment by EPRI for BWR Owners Group and several 12" nozzle-to-safe-end welds by EPRI.

At ANL, residual stresses on MSIP treated 12" and 28" weldments were measured on the inner surface as well as through the wall. The stresses on the inner surface were highly compressive in both the axial and hoop directions ranging from -207N/mm2 to -345N/mm2 in the Heat Affected Zone (HAZ) for the 12" weldment and from the -152N/mm2 to -345N/mm2 in the HAZ for the 28" weldment. Through wall axial residual stress distributions were almost linear across the thickness near the HAZs and the compressive stresses were found to extend for almost 50% of the wall thickness. Similar results were found for the 28" weldment (Figure 3).

Two basic types of tools are used for applying the process. The stud tensioner tool is typically used for standard weldments such as pipe-to-pipe and pipe-to-elbow joints for sizes up to 14" in diameter. In the second type, a specially designed hydraulic box press is used for bringing the clamp halves together This type is typically used to squeeze heavy wall nozzles and large diameter pipes. A portable hydraulic pump is used to actuate the stud tensioner or box presses. Plant compressed air is used for pump operation.

The process is applied using approved Engineering and Field Service Procedures. Weld Travelers with Performance and Verification Records are used to document application results and to record measurements and verification. The process is displacement controlled and verification is provided by measuring pipe contraction between circumference measurements before and after MSIP.

MSIP is accepted by the U.S. Nuclear Regulatory Commission in NUREG-0313 as a Stress Improvement (SI) method for mitigating stress corrosion cracking in BWR plants. Early application of MSIP in existing piping systems eliminates the otherwise inevitable need for piping replacement. Additionally, in the U.S. context, the use of an NRC-approved stress remedy reduces  the required inspection frequency, thus reducing both outage time and radiation exposure. EPRI information for the first application of MSIP at Commonwealth Edison shows a saving of about US$430 million achieved by avoiding pipe replacement in the utility's operating BWR plants. Use of MSIP makes all the weldments in the system immune to stress corrosion damage. By replacing 'as welded' residual tensile stresses with compressive residual stresses in the weld inner surface, pre-existing cracks are arrested and crack irritation prevented. The prevention application of MSIP is fully justified considering not only potential losses related to repairs and interrupted energy production, but also plant safety.

MSIP was first used to improve weldments in 1986. Since then over 1,300 welds including over 500 nozzles/safe-end weldments have been treated in over 30 BWR plants worldwide (See Table 1). Recently its use has been extended to PWR's in USA for mitigating stress corrosion cracking in some Inconel safe-end weldments.

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