Mechanical Stress Improvement Process (MSIP®)
Stress corrosion cracking is a recognized problem which can compromise nuclear plant safety and availability. Mechanical Stress Improvement Process (MSIP®) is a NuVision Engineering patented process for preventing or mitigating this problem in newly fabricated and operating nuclear plant pipe weldments. The process works by removing tensile residual stresses and generating a favorable compressive stress pattern in its place. The process:
- Extends plant life by eliminating Intergranular Stress Corrosion Cracking
- Has been approved by NRC (NUREG 0313) since 1986
- Has a 100% success record in more than 3000 weldments worldwide on which it has been used
- Saves money through elimination of pipe replacements
The MSIP® Process 
MSIP® works by using a hydraulically operated clamp to slightly contract the pipe on one side of the weldment. The permanent contraction generates a concave contour at the weld location and results in a corresponding reduction (typically <1%) in pipe circumference. Once the tool is removed, the weld area remains in axial compression through about half of the wall and is protected by a layer of compressive hoop stresses which extends through almost the whole wall. Finite element analyses of the axial (left) and hoop stresses (right) are shown below.
During application of MSIP®, the inside wall is subjected to monotonically increasing compressive strains. It does not extend pre-existing shallow cracks but rather prevents their further propagation and thus is also appropriate for use on older pipes which may contain undisclosed pre-existing minor cracks.
Application
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. Measuring pipe contraction between circumference measurements before and after MSIP® provides the necessary verification.
Experience
MSIP® has had a 100% successful track record since its introduction in 1986. Since then over 3000 welds, including those with pre-existing crack indications, have been treated in nuclear plants worldwide. To date, no cracks have initiated in any welds treated and cracks in weldments with pre-existing flaws have been arrested.
Approvals/Standards
MSIP® is approved and accepted by the United States Nuclear Regulatory Commission in NUREG-0313, Rev. 2, as a Stress Improvement (SI) process for the mitigation of stress corrosion cracking in BWR power plants. It has now been used in most of the BWR plants in the USA to provide safe life for existing piping systems and to protect replacement or new piping systems for the entire lifetime of the plant. MSIP® is now being successfully applied to PWRs in the USA and has been transferred to Europe to help ensure safe operation of nuclear plants.
Benefits of MSIP®
- Early application can eliminate the otherwise inevitable need for pipe replacement.
- Reduces the required inspection frequency, thus reducing both outage costs and radiation exposure to workers.
- Saves money for the utility. EPRI information for the application of MSIP® at a utility shows a savings of $430 million achieved by avoiding pipe replacement in the utility's operating BWR plants.
- Makes all welds in the system immune to corrosion damage. By replacing "as welded" residual tensile stresses with compressive stresses in the weld inner surface, pre-existing cracks are arrested and crack initiation prevented.
In summary, the preventative application of MSIP® is fully justified considering not only potential losses related to repairs and interrupted energy production but also plant safety. |
