L.T. Business Consultants is participating with AEA Technology Engineering Services, Inc. and Equipos Nucleares, S.A. in an international Consortium established in the year 1996, to test and validate the Mechanical Stress  Improvement Process (MSIP) for preventing or mitigating stress corrosion cracking in operating nuclear power plant Reactor Vessel Head CRDM penetrations. 

MSIP is a patented process invented, developed and implemented to protect weldments against stress corrosion cracking in nuclear power plants.

The presence of high tensile stresses in the weld region is one of the most significant contributors to stress corrosion cracking. The mechanical process developed by AEA Technology Engineering Services, Inc. (AEAT ES) involves introducing small plastic strains to redistribute or remove the residual tensile stress from the critical weld regions.

MSIP's typical application involves a slight circumferential contraction of the pipe on one side of the weldment. Recently AEAT ES has developed a variant of MSIP involving axial contraction for application to the CRDM penetrations in a PWR RV Head, which would eliminate the potential stress corrosion cracking in the penetration weldments.

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.

IF YOU WANT TO VIEW SOME PHOTOGRAPHS SHOWING MSIP EQUIPMENT AND FIELD APPLICATIONS CLICK HERE. 

CRDM nozzles are usually shrunk fit in reactor head penetrations and then welded to the dome on the inner side by partial penetration J-welds. Since all the nozzles are vertical, the outer radial locations require welding of the nozzle to the dome at angles of up to 47º. Welding produces a complex pattern of residual stresses where the tensile stresses are enhanced by significant ovalization of the nozzle extending inside the dome. The pattern of stress becomes more complex after a hydrotest which introduces local plastic yielding at the hole boundary and weld discontinuity.

The mechanical means of applying loads must be adapted to the complex geometry of the CRDM penetration. Such loads can be imposed using a simple device with a central rod extending through the penetration having a head at one end and a hydraulic cylinder on the other (Figure 4). The imposed axial compressive stresses interact with the as-welded circumferential tensile stresses to  enhance plasticity. The resulting plastic flow redistributes the residual stresses to remove tension from the critical weldment region. After removing the imposed axial loads, both the axial and the circumferential residual stresses are reduced to an almost stress-free condition.

Two and three-dimensional inelastic finite element stress analyses were performed on the central and outermost CRDM penetration weldments of typical RPV head design to verify the concept.

Considering the CRDM penetration pattern in the Reactor Vessel Head (RVH), a three dimensional pie-shaped segment of the RVH dome is modeled. This model extends from the center of the head down to the flange connection. One side of the model bisects the center of the outermost CRDM. The area in the vicinity of this CRDM weldment to the RVH dome is modeled with a fine mesh to accurately determine the resulting stresses. Such a shaped model allows symmetry boundary conditions to be accurately applied to the model. The RVH flange is included in the model to account for the effect of the flange stiffness on the deformation and resulting stresses on the analyzed CRDM (Figure 5).

This model was used in comprehensive elastic-plastic analysis to demonstrate the benefits of the MSIP for reducing residual stresses in the weldment to mitigate stress corrosion cracking.

While analyses show that as-welded tensile stresses are reduced by axial contraction of the nozzle, our results show even better improvement of stress would be achieved if the penetrations are contracted when the RPV head is subject to internal pressure, or when a difference in average temperature exists between the RPV head shell and the Inconel penetration. This average temperature difference should be in the order of 85º to 140º C. A combination of heating and cooling would be applied locally to the RPV head penetration weldment being treated with MSIP. The average temperature difference would eliminate the shrink fit and, along with MSIP, improve stresses on the inner surface of the penetration. The cooling process should take a few minutes to achieve the desired cooldown and would be followed by an axial squeeze.

Initial hoop stresses within a range of 275-345N/mm² tension are reduced, such that after returning to operation, the tensile stresses remain low. Significant improvement is obtained at all locations around the nozzle, below and above the weld. Application of the process eliminates both the hoop and axial 'as-welded' stresses above the weld as well as in the weld region and beneath it.

Qualification tests have been run at  Equipos Nucleares, S.A (ENSA) factory in Santander (Spain) to verify the removal of the 'as-welded' residual stresses. Full scale mock-ups have been designed and fabricated using the same procedures as those used in an actual RV head. Residual stress measurements were made before and after application of MSIP. Geometry and locations of stress measurements are shown in Figure 6, Figure 7 and Figure 8 show the level and distribution of stresses before and after MSIP application. The attached photograph (Figure 9) shows the stress measuring equipment designed by ENSA for measuring residual stresses inside the penetration.

The following table summarizes the results:

The results show that the tensile stresses are either removed or reduced to such low levels  as to be below the threshold of corrosion cracking.

It should be noted that the Inconel 600 used in the mock-up was conservatively chosen with a high yield strength of 518 N/mm2. This is considerably higher than the typical yield strength values for Inconel 600 which are in the order of 300 N/mm2. Even with such high yield strength material, the process was able to eliminate or reduce the tensile 'as-welded'  residual stresses to begin levels along the inner surface of the penetration. For the normal yield strength values, it is expected that the tensile stresses will be completely removed.

The analyses and qualification tests performed by the AEAT-ENSA-LT Consortium have verified the applicability and effectiveness of MSIP to eliminate residual weld stresses present in PWR RVH CRDM Inconel 600 penetrations, thus providing protection against stress corrosion cracking.

For CRDM penetrations, MSIP represents a cost-effective means to extend the life of operating RV heads and additional protection for new or replacement Reactor Vessel Heads.

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