Managing the risk of feeder cracking at PLGS from 2001 until its refurbishment in 2008 proved to be challenging for NB Power. On one hand, the cost of many activities to manage the threat was high. On the other hand, the cost of more unexpected outages or extensions of planned outages were also high, especially because of the high cost of replacement electricity. The scale of these challenges is illustrated by the following:
- Feeder degradation was one of two main drivers for the decision to refurbish PLGS three years early. The second life-limiting factor was degradation of the fuel channels. Reducing the 28-year design life of a facility, a $1.4B capital investment, by three years represents a cost impact of at least $6M/yr over 25 years.
- Feeder cracking caused two forced outages (90 days total in 1997 and 2001) and extended three planned outages (13 days total, after 2001) resulting in direct and indirect costs of approximately $60M.
- Before the decision in 2005 to refurbish PLGS, the station’s activities to manage the risk of cracking and maintain adequate safety margins required as much as 5% of the total operations and maintenance budget as of 2005, or roughly $5M/yr.,
- Radiation exposure to perform inspection and maintenance on the feeders was about 30% of total outage radiation exposure. Key personnel qualified for these inspections would reach regulatory limits, and thus were not available to perform inspections of other components.
- Feeder cracking made a disproportionate contribution to the total safety risk of PLGS and increased regulatory concern. NB Power reported in 2005 that low margins in the feeders at highest risk and the frequency of cracking had “the potential to increase the probability of consequential feeder rupture and multiple feeder ruptures in the absence of remedial measures. The PLGS Safety Report does not permit design basis events to propagate into further failures.”
- NB Power was challenged by a loss of credibility with stakeholders, including the public, as a result of the recurring cracking.
NB Power developed an economic model to compare strategies to manage the cracking: (1) reliance on leak detection, (2) inspection and repair, and (3) prevention of cracking by performing stress relief or replacing sections of feeder with the highest probability of cracking. As inputs to the model, NB Power needed reliable predictions of how frequently inspections were needed to eliminate surprises and the number of cracked feeders that would be found during the outage inspections.
Because high stresses accelerate crack growth, stress measurements were one factor needed to predict these key parameters. Thus, AECL accessed the CNBC numerous times between 2001 and 2005 to examine stresses thoroughly in various feeder bends of the types found at PLGS, and it used theoretical modelling to corroborate the results. Key outcomes based on the model included:
- A decision to inspect extensively during planned outages rather than to pursue a $10M R&D program to develop prevention methods or to build a safety case based on leak-detection and low safety risk.
- A decision to secure substantial resources to be ready to repair two cracked feeder bends during any yearly outage, thus preventing any unplanned maintenance or outage extension when two cracked feeders were detected in 2006.
- The ability to focus valuable inspection resources at the locations most at risk of cracking.
In retrospect, NB Power’s strategy was effective, since all 10 partial cracks after 2001 were found during planned inspections, and several other pipes with potential for cracks were replaced. All of the cracked bends were consistently found in specific locations of high residual tensile hoop stress that was introduced during bend manufacture, further confirming the importance of stress as a major factor in these material failures. Researchers have concluded that cracking was of the two main modes of feeder degradation at PLGS. The primary drivers of feeder cracking included residual tensile stress, cold work during materials preparation and maintenance, and the in-service temperature.
During the refurbishment of PLGS, which was completed in November 2012, all the feeders were replaced. The materials, preparation methods and maintenance for new feeders were chosen to reduce stress and be less susceptible to feeder wall thinning, and thus feeder cracking is no longer expected to be a life-limiting factor.
Without stress data from the CNBC, NB Power may have had to compensate for a lack of scientific understanding of the cracking by making even more conservative decisions, such as starting the refurbishment prior to 2008, or greatly reducing the operating cycle to facilitate exhaustive inspections, either of which would have come at significantly higher cost.
The next article reviews the impacts on the rest of the industry, and how the CNBC contributed to their responses the concerns of cracking.
This research story was republished with the permission of the Canadian Institute for Neutron Scattering.