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. Read the rest of this entry »