By: Donald Jones, P.Eng., retired nuclear industry engineer.
According to Ontario’s 2013 Long-Term Energy Plan the last nuclear unit to be refurbished will be Bruce unit 8 with its refurb scheduled to start in 2028. Since this unit was brought into service in 1987 it means it would have operated for 41 years before its mid-life refurbishment. Bruce unit 5 would have operated for 37 years before its refurb and units 6 and 7 for 40 years. Conventional wisdom calls for refurbishment at 25-30 years.
The operating life of a CANDU depends on the pressure tubes and calandria tubes maintaining the mechanical integrity and the physical geometry assumed in the original design. Designers knew that the fuel channel components would set the limit on the operating life of the reactor. They would have been well aware of the effect on these components of the pressure, temperature and neutron flux conditions in the reactor core when arriving at their original prediction of operating life. The aging mechanisms restricting operating life of the pressure and calandria tubes, as well as the feeder pipes, are well known and are summarized in reference 1.
As operating experience accumulated and components were examined in situ using specially designed equipment a better picture of age related degradation enabled designers and the operating utilities to keep adjusting their original prediction of operating life of the limiting components. Pressure tubes are also removed for detailed examination. Once the limit on the most life limiting aging mechanism was reached the pressure tubes and calandria tubes would be replaced, the so called CANDU mid-life refurbishment. In other words the decision to refurbish is not based on the original prediction but on the results of on going examinations of the components as mandated in the Canadian Standards Association inspection requirements, CAN/CSA N285.4 Periodic Inspection of CANDU Nuclear Power Plant Components. This can be likened to the civil aviation industry practices. Some of the the Douglas DC-3 aircraft that were introduced in 1936 are still flying today 78 years later. After many hundred of reactor years of CANDU operation Atomic Energy of Canada Limited and Ontario Power Generation have likely become the world’s experts in predicting operating life of CANDU fuel channels, although India will also be very knowledgeable as well from their experience with heavy water pressure tube reactors. Reactors in the USA were licensed for 40 years not due to technical limitations but for financial reasons. The owners of most of these 100 or so reactors have submitted applications to the US nuclear regulating authority to have their operating licences extended for another 20 years. This requires on going plant life cycle management plans to satisfy the regulator that the plant systems, structures and components related to safety are being managed, and will be managed, in accordance with all applicable rules, regulations and standards. Properly managed units could likely see further life extensions beyond 60 years.
The pressure tube life for a CANDU 6 unit was originally predicted to be 210,000 Equivalent Full Power Hours (EFPH) or 30 years at an 80 percent capacity factor. Point Lepreau had operated for 25 years with a life time capacity factor of 79.5 percent (based on CANDU Owners Group data in the 2009 Nuclear Canada Yearbook produced by the Canadian Nuclear Association) when the unit went into its refurb outage in 2008 accumulating 174,105 EFPH. For Wolsong unit 1 the most life limiting aging mechanism based on regular inspections was found to be pressure tube axial elongation, expected at around 190,000 EFPH. With a lifetime capacity factor of 84.1 percent Wolsong 1 reached this limit in 2009 and went into its refurb outage. Gentilly 2 had run for 29 years up to 2012 December, when it was permanently shutdown, at a lifetime capacity factor of 76.9 percent so it had accumulated 195,356 EFPH by that time.
Now getting back to the Bruce B units. Bruce B would have had the same 210,000 EFPH prediction as CANDU 6 units since they went into service around the same time in the early 1980s. Up to 2012 December Bruce B had a four unit average lifetime capacity factor of 83.3 percent and Bruce unit 8 had a lifetime capacity factor of 82.6 percent meaning it had accumulated 180,894 EFPH by the end of 2012 after 25 years of operation. Extrapolating this to 2028, the start of refurbishment, at an assumed capacity factor of 80 percent, means it would accumulate an additional 112,128 EFPH for a grand total of 293,022 EFPH, substantially more than 210,000 EFPH. These are not the only units exceeding their predicted life since Pickering will have completed 247,000 or more EFPH by the time it is permanently shutdown in 2020. For how Pickering is going about achieving this increase in EFPH see reference 2. No doubt Bruce Power would also be involved in this major program.
A major risk in all this of course is if the rate of change of the life limiting aging mechanisms change significantly from that expected based on many years of tests, analysis, and periodic examinations of operating components. If this happened units would have to be shutdown earlier than anticipated which would result in a serious loss of generation during the refurb program. With Darlington and two Bruce A units being refurbished the Ontario grid will need all the nuclear electricity it can get so derating Bruce B units to lower their EFPH accumulation may not be an option.
A pressure tube failure in an operating unit is a small loss of coolant design basis event that is well understood and controlled and in fact has occurred in the past. It is more an economic problem than a real safety problem since the unit has to be shutdown to replace the failed fuel channel but more than one failure would likely not be tolerated by the nuclear regulator. US light water reactor plants use one large reactor pressure vessel to hold the nuclear fuel undergoing fission instead of the many fuel channels of CANDU. This vessel is under intense neutron bombardment and operates at higher pressures and temperatures than those in CANDUs. These vessels are subject to rigorous inspections as part of plant life management programs since pressure vessel catastrophic failure is inconceivable.
If mid-life refurbishments of CANDUs start occurring at 40 years instead of at 25-30 years then as units approach 80 years of operation other life limiting aging mechanisms will come to the fore, obvious ones are the degradation of the insulation properties of electrical cables and safety related concrete. All this is considered as part of present Plant Life Management Plans.