Refurbishing major components4 July 2017
A recent project at Surry shows that there is now a proven, systematic approach to refurbish and replace major reactor components like reactor coolant pumps. By Gerry Ottman
Today, more than 450 nuclear power plants in 30 countries provide more than 10 percent of the world’s electricity. More than half of these plants are nearly 30 years old. As the nuclear fleet ages, plant operators seek ways to extend their operation. In the USA, for example, 84 plants have been granted 20-year licence renewals, meaning these plants will remain operational into the 2050s. In order to keep plants operational for decades to come, the nuclear industry constantly explores innovative and efficient ways to maintain the facilities and ensure operational excellence.
Components operate for extended periods of time under harsh conditions. As a result, normal vibrations and wear take their toll and, eventually, these components need to be refurbished or replaced. To date, replacing components rather than refurbishing them has been a more common practice. But refurbishing major components is now a viable, cost-effective solution, as demonstrated by a project Areva NP completed at Dominion Virginia Power’s Surry power station in southeastern Virginia, USA.
The decision to refurbish at Surry was driven by the issue of intergranular stress corrosion cracking (IGSCC) in reactor coolant pump turning-vane bolts. Nuclear plant operators carefully monitor for IGSCC and have procedures in place to replace components to maintain the plant’s safe operation.
One of the most susceptible locations for IGSCC is the 1-inch diameter turning- vane bolts in the reactor’s coolant pumps. These bolts secure the turning-vane diffuser assembly, which is inside the reactor coolant pump, above the pump impeller. Failure of multiple turning vane bolts could cause the turning vane to drop. This would cause the turning vane to come in contact with the impeller and potentially cause a locked-rotor condition, reducing coast-down time, which could adversely impact core cooling during certain accident scenarios.
All Model 93A reactor coolant pumps are susceptible to IGSCC, but the early 20-bolt, 1-inch diameter configuration is more so. Only three US units use this configuration; the rest use a 24-bolt, 1.5-inch diameter configuration.
After evaluating this issue, Dominion Virginia Power decided to upgrade and replace all the reactor coolant pumps in its fleet that contained 1-inch diameter turning-vane bolts. Two reactor coolant pumps had to be replaced at Surry 1 and three at Surry 2. Due to the timing of the project and an aggressive outage schedule, Dominion Virginia Power decided that refurbishment rather than replacement was the optimal choice.
Dominion contracted Areva NP to refurbish all five of the reactor coolant pumps at Surry during the 2015 spring and fall outages. To date, this is the largest reactor coolant pump refurbishment project undertaken in the USA.
This project posed three challenges: obtaining spare pumps; manufacturing replacement components on an aggressive and accelerated timeline; and planning replacement of several reactor coolant pumps during a normal maintenance outage.
Historically, the lead-time to secure new reactor coolant pumps is two to three years. Dominion’s aggressive outage schedule required that spare pumps be obtained in less than 10 months. So Areva NP explored reclaiming two reactor coolant pumps from Dominion’s retired Kewaunee plant in Wisconsin and refurbishing them to like-new condition.
There were significant obstacles that needed to be overcome. They included:
- Ensuring that Kewaunee’s reactor coolant pumps, already used for more than 30 years, could support more than 20 years of additional service at Surry.
- Checking that the Kewaunee pumps would meet the American Society of Mechanical Engineers (ASME) code requirements governing the Surry plant. ASME codes and standards vary from plant to plant, based on the US state where they are located and the plant’s date of construction.
- Evaluating the physical interchangeability of the Kewaunee pumps and Surry’s pump casing, including pump flow rate and pressure, the location of the interface for the motor (coupling, motor stand bolts, etc.), and the location of the piping connections (seal water, cooling water).
After a detailed engineering study, Areva NP determined that the Kewaunee pumps were a safe and reliable solution for installation at the Surry station. Dominion decided to proceed with the project.
The refurbishment project also included replacing the shafts, impellers, couplings, bearings and all bolting on the reactor coolant pumps. Dominion’s accelerated delivery schedule required Areva NP to manufacture these replacement parts in less than six months – a task that normally takes one to two years.
In addition, Areva NP’s field implementation teams were faced with the complex logistics of replacing multiple reactor coolant pumps during a normal maintenance outage. To prepare, the Pump and Motor Field Service team and Surry’s maintenance technicians completed extensive mock-up training exercises. These were vital to the planning and preparation, and led to the development of new and improved tooling and equipment.
Before work on the project began, Areva NP and Dominion created full-scale mock-ups of the reactor coolant pumps so the project teams could rehearse their tasks. All team members participated in this training, from machinists, station management and engineers, to radiation protection and safety personnel. Practising the critical, high-dose tasks using the mock-ups allowed the team to save time and integrate dose savings into the plan. Additionally, a detailed machinist’s pre- job brief form was developed to decrease the time spent on turnover and briefing.
First, Areva NP removed the two reactor coolant pumps from Kewaunee, where they were still installed in the reactor coolant system. The pumps were shipped to Lynchburg, Virginia, where they were decontaminated, refurbished, modified and upgraded at Areva NP’s Pump and Motor Service Center to match the requirements at Surry. These pumps required different piping arrangements than those at Surry 1, so plant modifications were needed in conjunction with the reactor coolant pump exchange. New computer numeric controlled (CNC)-machined impellers were installed to change the pump flow rates to match that required at Surry. The pump bearing temperature thermal wells also had to be relocated. All this was completed in time for installation at Surry 1 during the spring 2015 outage.
In addition to refurbishing the reactor coolant pumps from Kewanee, Areva NP repurposed the two original reactor coolant pumps from Surry 1. After being removed during the spring 2015 outage, these two reactor coolant pumps were sent to Areva NP for a complete refurbishment. New shafts, bearings and bolting were installed. The impellers were reused by performing a specialised match machining operation between the old impeller and the new shaft to position the impeller for optimal pump performance. The reactor coolant pumps were installed at Surry 2, along with a previously refurbished reactor coolant pump, during the fall 2015 outage.
A story of success
The refurbishment project was a success. A key factor was close coordination among the teams from Surry, Dominion Engineering in Innsbrook, Virginia, the Areva Pump and Motor Service Center in Lynchburg and the Areva manufacturing facility in Jeumont, France.
The project was completed without a single safety issue, personal contamination event or human performance error. Safety was the number one priority during all stages of the project. The team made it a priority to minimise personnel radiological exposure and incorporate industrial safety into the tooling systems. Special tungsten-infused rubber shielding for machining activities was developed, which reduced the need for lead-shielding blankets. This helped reduce trip hazards and created a more orderly work area. As a result, measurable reductions in the levels of radiation were realised after each refurbishment: from the first pump to the last, 1.521Rem was saved, equivalent to a dose savings of approximately 76mRem per individual, per outage.
With simultaneous reactor coolant pump replacements during the normal outage schedule, Dominion realised significant savings on resources and equipment costs. By replacing all the reactor coolant pumps in each unit during one outage, instead of replacing one pump per outage over the course of three outages, Dominion saved approximately $5.6 million.
Improved communication between the field and project teams was also critical. Organising the entire project team in the same working area created a unified base location, which eliminated communication delays and improved hand-offs between teams. Remote video monitoring and communication equipment eliminated the need for multiple oversight personnel. As a result the team was able to provide status updates and troubleshoot issues in real time.
Use of an improved, shielded shipping cask reduced polar crane time and critical path time by eight hours per pump. The improved design incorporated trunnions and a storage cradle in the shipping container, so the rigging arrangement was much simpler. Its smaller footprint took up less laydown space and reduced the risk of a rigging issue or human error associated with multiple hooks or cranes.
Process and equipment improvements also helped ensure the outage schedule was maintained and budget was met. The team used Areva NP’s FARO Arm metrology technique to measure case diameters and the different dimensions of the pump components. This allowed three-dimensional overlays of new and old pumps, which quickly and accurately revealed possible deviations. As a result, Dominion was able to work with Areva NP to fabricate new shafts and impellers for the pumps to achieve flow requirements and extend plant life with new pump components and upgraded hardware.
Gerry Ottman is Sr. Manager, Innovation & Technology, Component Repair & Replacement, Areva NP. He has more than 35 years of experience in the nuclear industry, including 17 years as a plant and system engineer at operating pressurised and boiling water reactor plants.