Investigate-Design-Build Approach is Successful for Grouting Repair

The Monticello Nuclear Generating Plant, located in Monticello, Minnesota alongside the Mississippi River, is a single boiling water reactor that has been in operation since 1971. To accommodate plant-specific service conditions, a predictive maintenance program was established.

The 40 year old High Pressure Coolant Injection (HPCI) pump base at the facility had experienced excessive vibrational issues in the 1980s, so additional anchor bolts had to be added to support the system. More recently, despite these corrections, vibrations were still approaching high levels which put the pump at risk for misalignment and uneven wear on bearings. The HPCI pump is the first line of defense for a reactor vessel in cases where there is a loss of normal core coolant inventory, so it must stay operational. Without repair, the pump could wear out prematurely and cause a failure of this critical safety equipment component.

Investigation and Repair Approach

STRUCTURAL was called to serve as the repair contractor and perform the maintenance due to their expertise working within nuclear facilities and repair capabilities. To understand the best strategy for the needed repairs, STRUCTURAL worked with STRUCTURAL TECHNOLOGIES to identify the problem, develop a plan for the materials needed and the method to carry out the repairs, along with the quality control and assurance practices that would be implemented.
The team performed acoustic emission testing to determine whether a void existed under the substrate and its extent and detected numerous voids between the skid plate and grout. These voids resulted in excess vibration and created additional levels of voids between the base plate and foundation. Investigation also helped to determine the level of corrosion activity within the foundation and what impact it would have on the durability and performance of the existing foundation.

Planned Shutdown Allows for Smooth Repairs

Repairs began during a planned shutdown of the plant and the pump equipment stayed in place during the repair, so the biggest challenge was to avoid lifting the steel plate while injecting the grout. STRUCTURAL’s crews injected epoxy into the voids between the grout and the skid to improve the bearing support.

Grouting under the large metal base plates required a systematic approach that minimized air pocket buildup under the metal supports. Using the largest sized aggregate grout blend and the selection of proper grout material was critical to achieving success. Material properties such as flowability, pot life, and thixotropic blends had to be considered. Small voids were filled with epoxy and large voids were first filled with silica sand and then epoxy impregnated.

Ensuring that the existing anchor bolts penetrating the plate exceeded design capacity was also very critical. If there was slippage or issues with strain creep, the grouting would not compensate. To perform the inspection and determine the repair needed, STRUCTURAL drilled diameter holes using a pattern grid that was based on the void locations. A borescope was placed down the drill holes to determine extent of voids and high durometer silica sand was vacuumed placed prior to resin injection in all large air pockets.

After the ports were installed and sealed, the epoxy was injected, and the process repeated as necessary to complete the penetration of the entire skid. When the epoxy had cured, STRUCTURAL performed acoustic emission testing on the steel skid base to make sure all of the voids had been filled.

Another Predictive Maintenance Program Success

STRUCTURAL crews were equipped to accommodate the varying grout conditions that were discovered. Because repairs were addressed early on, the entire grout fill did not require removal and replacement. This avoided the added costs of providing pH, carbonation, and petro-graphical analysis because the conditions of the pump base did not exhibit considerable micro-cracking and/or dusting at the exposed grout surfaces on the sides of the metal supporting plate. The entire process took less than a month, optimized resources, and improved overall effectiveness of the plant.