Canada’s fleet of nuclear power plants have realized impacts worth hundreds of millions from research supported by a decade of stress measurements using neutron beams to ensure their safe, reliable and economic operations.
Image: Mock-up of a CANDU reactor face. (NA Engineering)
This series of articles reviews the impacts of a line of research using neutron beams that was especially valuable for the Canadian nuclear power industry, which include informing safety evaluations, providing confidence for a multi-billion export project, minimizing down time, informing lifetime management, and ultimately solving the underlying issue for the long term. Paul Spekkens who was Vice-president – Science and Technology Development for Ontario Power Generation from 2004 to 2016, now retired, recently reflected on this research:
“Research using neutron beams provided critical knowledge needed by operators of power plants based on the CANDUTM design in Canada and abroad. Cracking detected in the feeder tubes at some of Canada’s stations led to the need to inspect feeders across the CANDUTM fleet, resulting in additional downtime and dose in order to perform on-going inspections of the feeder tubes. The neutron beam analyses of residual stress were used to identify which manufacturing practices resulted in tubes that were susceptible to cracking, and where on the tubes the cracks would be located. This enabled inspection staff to focus their efforts and in some cases, eliminated the need for inspection or reduced their frequency.
“This result led to public benefits in several forms: the radiation dose to inspection workers was significantly reduced by minimizing their time spent near the reactor face. Plant downtime was decreased while maintaining safety margins, resulting in financial savings. Power utilities in other countries using Canadian technology were provided knowledge to support the safe, reliable and economic operations of their plants as well, thereby enhancing the cooperative relationships across the CANDU industry.”
Two unexpected leaks in feeder bends at the Point Lepreau Generation Station (PLGS) resulted in costly outages and had repercussions throughout the Canadian nuclear power industry. New Brunswick Power needed to understand the underlying causes to assure safety of continuing operation.
The Canadian Neutron Beam Centre (CNBC) played a critical role in the failure analyses, demonstrating that stress left by the manufacturing procedure was a major factor in producing the cracks. Subsequently, the industry pooled money through the CANDU Owners Group (COG) to study the cracking mechanism, to assure safety for relicensing, and to determine how to manage the scope of on-going feeder inspection programs. The COG program supported substantial research efforts to further understand the role of stress and to help determine which feeder bends were most likely to be affected. These studies used the CNBC’s neutron stress-scanning capabilities frequently from 2001 to 2008, and most recently in 2011, to determine the distribution and magnitude of residual stresses in various feeder bends.
The initial failure analyses and the following decade of research and operating experience led to many positive impacts on the industry, informing safety evaluations, providing confidence for a multi-billion export project, minimizing down time, informing lifetime management, and ultimately solving the underlying issue for the long term. These impacts are summarized as follows:
- New Brunswick Power was able to assure the regulator during the two forced outages of PLGS in 1997 and 2001, which together cost over $50M, that PLGS could be restarted safely thus avoiding further unplanned down time costs in these incidents.
- AECL was able to assure the China National Nuclear Corporation that it could continue with the $4B construction of two reactors at Qinshan by demonstrating that the problems at PLGS could not occur in its reactors, thereby avoiding the prospect of costly mid-construction design changes.
- All Canadian nuclear utilities were able to assure the Canadian Nuclear Safety Commission (CNSC) that the feeder cracking issues were manageable. Had the refurbishments of OPG’s fleet – planned to begin in 2016 – begun a decade early in order to replace the feeders, as proposed by intervenors in relicensing hearings, the lost value of these reactors’ design lives would be worth billions of dollars.
- NB Power was able to confidently make key decisions regarding the timing of a $1B project to refurbish PLGS and regarding $5M/yr activities to manage the cracking. The latter decisions included (1) not to invest $10M on a R&D program to try to prevent the cracking, (2) to pursue an aggressive inspection program, (3) where to focus limited inspection resources, having determined the locations most at risk of cracking, and (4) to secure substantial resources to be ready to repair cracks discovered during inspections. NB Power’s strategy was effective in preventing further unplanned down time.
- OPG was able to successfully propose an innovative approach to meet regulatory requirements to manage the risk of cracking. Its approach based on enhanced leak detection capability has become a CNSC licensing requirement for new power reactors.
- A welding services firm developed new technology to perform repairs in tight spaces, such as around feeder pipes, and is now competing to use it in commercial applications.
- Better materials and maintenance methods have been qualified with neutron stress measurements, providing high confidence that feeder cracks of the types seen at PLGS are unlikely to appear in new reactors or in the existing reactors after the feeders are replaced during the planned refurbishments.
- In the meantime, safety is assured through the use of standard fitness-for-service guidelines for feeders that are now in place across the industry.
- These guidelines have enabled the stations to keep the scope of inspection programs down to manageable levels, thereby saving time and resources during outages, minimizing down time and radiation dose for inspection staff.
The availability of neutron beams at the CNBC to easily obtain stress data non-destructively has been of immense benefit to the entire industry, helping it to demonstrate safety, assure export clients, and operate reliably with reduced dose to staff. While many other research studies and methods contributed to these impacts, stress data from the CNBC played a critical role. The value to Canada of the impacts of this one line of research can be estimated to be on the order of hundreds of million dollars, exceeding all of Canada’s direct investments in the neutron beam laboratory at Chalk River since neutron scattering was pioneered there in the 1950’s.
 This material is adapted from: D. Banks, R. Donaberger, B. Leitch, R.B. Rogge. Stress Analysis of Feeder Bends Using Neutrons: New Results and Cumulative Impacts. 2014. Pacific Basin Nuclear Conference. Vancouver, BC, Canada. PBNC2014-186.