Stress corrosion cracking as a result of excessive tensile stress, is a recognized problem which can compromise nuclear plant safety and availability. The process works by removing tensile residual stress and generating a favorable compressive stress pattern in its place.
The permanence, ease of application, low radiation exposure, short schedule that is easily accommodated within the planned outage duration, and minimal interference with other outage activities are some of the advantages of MSIP®.
Removal of Tension Provides Permanent Protection of Weldments Against Stress Related Cracking
Basic Concept of MSIP®
Displacement Controlled – Uses a hydraulically operated mechanical clamp to contract pipe on one side of the weld.
- MSIP® Tool Placement - Application of Pressure to Contract Pipe
- Mechanism of Compression Generation in Axial and Hoop Directions
During the application of MSIP®, the inside wall is subjected to monotonically increasing compressive strains. It is important to note that MSIP® does not impose tensile or reversed plastic strains at the inner surface. Hence, it does not extend pre-existing shallow cracks. Application of MSIP® is also appropriate for older piping systems which may contain undetected indications or pre-existing cracks.
Most recently (2009) the NRC commissioned Battelle Labs to perform an evaluation of MSIP® as a mitigation technology for primary water stress corrosion cracking in PWRs. The evaluation covered the reactor vessel hot leg and pressurizer surge and safety nozzles. It also included an evaluation of the process in the presence of a pre-existing flaw in the weld. The study confirmed:
The earlier evaluations completed by NRC/ANL and EPRI that MSIP® is an effective method to reduce tensile weld residual stresses and generate compressive residual stresses in the inner weld region.
That MSIP® can be applied for cracks up to 30% through the pipe wall thickness