By: Donald Jones, P.Eng., retired nuclear industry engineer, 2014 June
The Enhanced CANDU 6 (EC6) is just that, an enhanced CANDU 6 and is one of the contenders for new build at Darlington (references 1 and 2). Following on from some early conceptual work by Canadian General Electric (CGE), Atomic Energy of Canada Limited (AECL) based the CANDU 6 design on the four unit Pickering A station (that was brought into service 1971-1973) but as a single unit station with a significant power increase, major equipment simplifications, improvements in shutdown and emergency core cooling systems, extensive use of digital computers for control and safety systems etc. In fact the CANDU 6 is unrecognizable as being based on Pickering except maybe for the fuel channel sizing, even though fewer channels are in CANDU 6, and the two loop primary heat transport system that were retained. Since Ontario Hydro was enamored by multi-unit stations CANDU 6 was intended as a single unit for out of province build including off shore. The two lead CANDU 6 projects were Gentilly 2 in Quebec and Point Lepreau in New Brunswick and these were quickly followed by Embalse in Argentina and Wolsong, now Wolsong 1, in South Korea and all came into service in the early to mid 1980s. These can be regarded as the first tranche of CANDU 6 build.
The second tranche of CANDU 6 units came with Wolsong 2, 3 and 4 in South Korea, Cernavoda 1 and 2 in Romania, and Qinshan 4 and 5 in China (the other units at Qinshan site are not CANDU), all entering service between 1996 to 2007. Each of the second tranche CANDU 6 units incorporate lessons learned from operation of the earlier units with changes to meet latest regulatory codes and standards. All three Wolsong units came in on budget and on schedule and the two Qinshan units came in under budget and ahead of schedule. In fact the total project schedule for the CANDU 6 units at the Qinshan site in China was 81 months from contract effective date to in-service.
CANDU 6 Units
Point Lepreau, New Brunswick, Canada. Unit came into commercial operation in 1983 February. It was the first CANDU 6 to be licensed and go into commercial operation. The unit had excellent capacity factors for the first 12 years or so after start up but then developed design, maintenance and human factor issues that affected its future performance. These included a major outage, and subsequent outages, for fuel channel spacer relocation, failure of the pump shaft of a primary heat transport pump caused by failure to remove temporary plywood from inside a steam generator after maintenance outage, damage to the emergency water supply headers inside the steam generators due to flow-accelerated corrosion, forced outage due to a crack in an outlet feeder pipe bend caused by continual flexing from a fuel channel that had not been locked after a maintenance outage, another forced outage from a cracked feeder pipe likely from stress corrosion cracking and discovery of more cracks, more frequent outages and outage extensions to verify condition of feeder pipes (wall thinning from flow accelerated corrosion and cracks at bends) and fuel channels, spurious reactor trips due to electronic component failure etc. These are just some examples. All of this had an effect on the capacity factors of the unit.
Beginning in 1998 reactor power deratings were initiated to maintain adequate dry out margin on the fuel as a result of pressure tube diametral creep and this had reached a 12 percent derating by the time unit was refurbished. However despite the derating and maintenance outages the unit achieved an average annual capacity factor of 79.7 percent for the 5 years prior to the refurbishment outage, a commendable performance (see reference 3 for raw capacity factor data). All other CANDU 6 units, especially those in the second wave of build, benefitted from lessons learned at Point Lepreau.
The unit was taken out of service in 2008 March for refurbishment, the complete replacement of all fuel channel assemblies, inlet/outlet feeder pipes and significant other work. At the end of 2007, its last full year of operation and the end of its first life before the refurbishment outage, the unit had a respectable lifetime capacity factor of 82.1 percent and an annual capacity factor for 2007 of 74.9 percent. The unit was back in commercial operation in 2012 November after the 4.5 year outage for a refurbishment that was supposed to have taken 1.5 years. Wolsong 1 carried out a generally similar refurbishment in 2.3 years, it started later and finished earlier than Point Lepreau. Lessons learned in the Point Lepreau refurbishment proved useful for Wolsong 1 and will prove useful for future refurbishments of other CANDU 6 units. The unit achieved full power output 2013 November. At the end of 2013, its first full year of operation in its second life, its lifetime capacity factor was 70.9 percent with an annual capacity factor for 2013 of 72.6 percent.
Gentilly 2, Quebec, Canada. Unit came into commercial operation in 1983 October. The unit was permanently shutdown at the end of 2012 by Hydro-Quebec after election of the short lived Parti Quebecois government earlier in the year. At the end of 2012, when it was shutdown, it had a lifetime capacity factor of 76.9 percent with an annual capacity factor for 2012 of 71.9 percent. Operation at around 50 percent full power for a number of years in the 1980s when there was a surplus of generation no doubt contributed to the low lifetime capacity factor. The performance of Gentilly 2 was also likely affected by the large amounts of hydro-electric generation available in Quebec.
Wolsong 1, South Korea. Unit came into commercial operation in 1983 April. Unit was taken out of service for refurbishment in 2009 April. At the end of 2008, the last full year of operation before the shutdown for refurbishment, the annual capacity factor was 93 percent and the lifetime capacity factor was a very respectable 85.7 percent. For the 5 years prior to the refurbishment outage the average annual capacity factor was 88.4 percent.
The unit returned to service in 2011 July after the 839 day (2.3 years) refurbishment outage making it the first CANDU 6 unit (ahead of Point Lepreau) to be refurbished for life extension, and lessons learned will undoubtedly reduce the refurbishment outage time on future projects. However the operating licence expired 2012 November at the end of the unit’s 30 year design life so it had to be taken offline. A ” Peer Review Service on Safe Long Term Operation (SALTO Peer Review Service)” was conducted on Wolsong 1 by the International Atomic Energy Agency in 2012 May/June so the safety related ageing and plant life management issues raised by this comprehensive review would have to be resolved and recommendations acted upon and this takes time. An application for continued operation had been submitted to the Korean nuclear regulator in 2009 December and it is still being reviewed. The Fukushima Daiichi event in 2011 has resulted in safety assessments by the Korean nuclear regulator of all reactors in South Korea including Wolsong 1 and has identified short and long-term measures for improvement.
The capacity factor for 2012 (less than a full year of operation after refurbishment) was 80.1 percent. The unit will start (recommence?) its second life when the Korean nuclear regulator approves the application for continued operation for another 10 years, to the time of its next Periodic Safety Review in 2022.
Wolsong 2, South Korea. Unit came into commercial operation in 1997 July. This unit and its two sister units, units 3 and 4, came in on budget and on schedule. Basically same as Wolsong 1 but with significant design updates and advancements and incorporates operating experience from earlier CANDU 6 units. At the end of 2013 it had an excellent lifetime capacity factor of 93.5 percent and an annual capacity factor for 2013 of 83.7 percent.
Wolsong 3, South Korea. Unit came into commercial operation in 1998 July. Basically same as Wolsong 2. At the end of 2013 it had an excellent lifetime capacity factor of 94.8 percent and an annual capacity factor for 2013 of 92.6 percent (based on figures in reference 3). According to Nuclear Engineering International magazine up to end of 2012 this unit had the world’s second best lifetime capacity/load factor, 93.4 percent. However it lost this position in 2013, according to Nuclear Engineering International magazine.
Wolsong 4, South Korea. Unit came into commercial operation in 1999 October. Basically same as Wolsong 2. At the end of 2013 it had an excellent lifetime capacity factor of 95.7 percent and an annual capacity factor for 2013 of 90.2 percent (based on figures in reference 3). According to Nuclear Engineering International magazine up to end of 2012 this unit had the world’s best lifetime capacity/load factor, 93.7 percent. At the end of 2013 Wolsong 4 dropped to second place in world’s best lifetime capacity factor ranking with 93.3 percent with Rajasthan 5 in India now becoming the world leader at 94.4 percent. Note that Rajasthan 5 is not a CANDU 6 but a 220 MWe pressurized heavy water pressure tube reactor similar to CANDU that came into service in 2009.
Embalse, Argentina. Unit came into commercial operation in 1984 January. It is expected to be taken out of service in 2014 for refurbishment for life extension. At the end of 2013, its last year of operation before start of refurbishment, it had a respectable lifetime capacity factor of 83.1 percent despite operating for significant periods at reduced power because of low demand and some ageing issues, and an annual capacity factor for 2013 of 63.3 percent. For the 5 years prior to the expected start of the refurbishment outage the average annual capacity factor was 74.2 percent. The life extension work will last 5 years for replacement of fuel channel assemblies, inlet/outlet feeders and the four steam generators as well as replacing obsolete control system equipment together with other changes in the balance of plant to improve thermal efficiency and increase power output. When all this is done the unit will then start its second life.
Cernavoda 1, Romania. Unit came into commercial operation in 1996 December. At the end of 2013 it had an excellent lifetime capacity factor of 90.3 percent and an annual capacity factor for 2013 of 99.4 percent. Unit 1 will have its planned bi-yearly maintenance outage in 2014.
Cernavoda 2, Romania. Unit came into commercial operation in 2007 October. Basically same as Cernavoda 1 but with updates based on experience with Cernavoda 1 and other CANDU 6 units. At the end of 2013 it had an excellent lifetime capacity factor of 93.9 percent and an annual capacity factor for 2013 of 89.1 percent. Unit 2 had its planned bi-yearly maintenance outage in 2013 reducing its capacity factor.
Qinshan 4, China. Unit came into commercial operation in 2003 January. This unit and its sister unit came in under budget and ahead of schedule. The design was based on that of Wolsong 3/4 and used advanced electronic engineering design tools including 3-D visualization for interferences and a data base for tracking drawings and documents and material usage. Open top construction and very heavy lift cranes reduced the construction schedule together with modularization and prefabrication. At the end of 2013 it had an excellent lifetime capacity factor of 91.2 percent and an annual capacity factor for 2013 of 92.2 percent.
Qinshan 5, China. Unit came into commercial operation in 2003 July. The design is based on that of Wolsong 3/4 and used advanced electronic engineering design tools including 3-D visualization for interferences and a data base for tracking drawings and documents and material usage. Open top construction and very heavy lift cranes reduced the construction schedule. At the end of 2013 it had an excellent lifetime capacity factor of 92.3 percent and an annual capacity factor for 2013 of 102.2 percent. Note that even though reactor thermal power is limited by the operating license the electrical output can increase above the nominal design rating if the cooling water (heat sink) temperature is below the nominal design temperature. Provided there are few, if any, outages during the year the annual capacity factor could exceed 100 percent with lower than nominal cooling water temperature and this is not unusual.
CANDU 6 summary
The average lifetime capacity factor for the four CANDU 6 units in the first tranche of build before they were shutdown for refurbishment, or in the case of Gentilly 2 shut down permanently, was 82 percent. The seven CANDU 6 units in the second tranche of build are amongst the world’s best performers. The average lifetime capacity factor for the seven CANDU 6 units in the second tranche of build was 93.1 percent up to end of 2013 and their average annual capacity factor for 2013 was 92.8 percent. Wolsong 4 had the world’s best lifetime capacity factor at the end of 2012 but dropped to second place in 2013 and Wolsong 3 had the world’s second best lifetime capacity factor up to end of 2012. A world record was set in 1994 by Pickering 7 in Ontario, Canada, another CANDU but not a CANDU 6, when it operated for 894 days (2.45 years) in continuous power production showing what can be achieved with on-power refueling, an inherent characteristic of all CANDU reactors. EC6 does indeed have a fine pedigree.
Enhanced CANDU 6 (EC6)
The CANDU 6 had a targeted operating life of 30 years at an 80 percent capacity factor. Refurbishment will extend its life for another 30 years assuming an 80 percent capacity factor. The EC6 has a targeted 60 year life with a lifetime capacity factor of 92 percent or more. This would include the refurbishment outage at half-life, planned average one month maintenance outage every 36 months, an allowance for forced outages, and a year to year capacity factor of 94 percent or more. The half-life refurbishment, as well as replacing life limiting components, gives the opportunity to replace items that have become obsolete, especially in the control and instrumentation area, and make changes to meet latest regulatory requirements and generally to improve safety and reliability. The Generation III EC6 has evolutionary improvements over the reference design Qinshan CANDU 6 units and will meet the latest codes and standards from nuclear regulators and incorporates all the lessons learned from CANDU 6 operation. Severe accident management has been upgraded as a result of the 2011 Fukushima event in Japan. Other enhancements are state of art instrumentation and control systems, an increased gross electrical power output to 740 MW, improvements in plant maintainability to shorten planned outages to improve capacity factor, improvements to life limiting components like fuel channel assemblies and feeders, and many other major improvements. For Darlington new build a two unit station was proposed. Canada is indeed fortunate, it mines its own uranium, manufactures its own nuclear fuel and designs and builds its own high performing CANDU reactors. All this with hardly any lifecycle greenhouse gas emissions.
1. Contenders for nuclear flexibility at Ontario’s Darlington B, AP1000 and EC6, and the winner is ….., Don Jones, 2013 January, https://thedonjonesarticles.wordpress.com/2013/01/10/contenders-for-nuclear-flexibility-at-ontarios-darlington-b-ap1000-and-ec6-and-the-winner-is/
2. Ontario’s IESO prefers Enhanced CANDU 6 over AP1000 for new build at Darlington, Don Jones, 2013 August, https://thedonjonesarticles.wordpress.com/2013/08/25/ontarios-ieso-prefers-enhanced-candu-6-over-ap1000-for-new-build-at-darlington/
3. Nuclear Canada Yearbook 2014, published by the Canadian Nuclear Society (CNS), http://media.cns-snc.ca/yearbook/CNS_Yearbook_2014_front.pdf