1,000 day breaker-to-breaker run is possible with CANDU

By: Donald Jones, P.Eng., retired nuclear industry engineer – 2015 June 22

How far can we extend the continuous on-line power operation (breaker-to-breaker runs) of the world’s commercial Generation II and Generation III nuclear power plants. Is 1,000 days possible?

To date the world record for PWR (Pressurized Water Reactor) continuous on-line operation is the 705 day run by Three Mile Island unit 1 an 819 MWe (net) unit in the U.S. that went into commercial operation in 1974 September. The run ended in 2009 October when the unit went into a planned refuelling outage. This run broke the previous world record of 692 days of another PWR, Calvert Cliffs unit 2, an 850 MWe (net) unit in the U.S. that was put into commercial operation in 1977 April. This run ended in 2009 February with a refuelling outage.

LaSalle unit 1, a 1137 MWe (net) unit in the U.S., that was put into commercial operation 1984 January, holds the world record for a BWR (Boiling Water Reactor) with 739 days when it came off-line in 2006 February. As it happens its twin, LaSalle unit 2, became the second place world record holder when it completed a run of 711 days on 2007 February. LaSalle unit 2 went into commercial operation in 1984 October. LaSalle units now hold first and second places in the world for a continuous run of any LWR (Light Water Reactor).

The world record for any type of reactor is held by a CANDU. This is Pickering unit 7, a 516 MWe (net) unit in Ontario, Canada, with a continuous run of 894 days when it came off-line for maintenance in 1994 October. This unit was put into commercial operation in 1985 January. CANDU is a PHWR (Pressurized Heavy Water Reactor). Rajasthan unit 5, a 202 MWe (net) PHWR in India, put into commercial operation in 2010 February, holds second place to Pickering unit 7 in world ranking after completing a 765 day continuous run and going into its planned biennial maintenance outage in 2014 September. Besides these record breaking runs there have been many runs of over 400 days by the different types of reactors.

These long runs are terminated when it is time for the planned maintenance outage and are not extended until safety targets can no longer be met, which would mean shutting down the unit at an inopportune time. The practical limit of continuous operation of PWRs and BWRs is set by the need to replace about a third of the nuclear fuel and do maintenance after about two years (720 days) or less. In the U.S. most light water reactors units operate on a 18 month fuel cycle and have maintenance outages scheduled for the spring and autumn months when electricity demand is low. Since a pressure tube PHWR like CANDU can refuel on-line at power the length of continuous operation is indeterminate but in practice there is a need to come off-line for certain tests, maintenance and inspections and upgrades that cannot be done at power. For a PHWR the run could be terminated by initiating one of the two reactor safety shutdown systems with the other reactor safety shutdown system being tested during the maintenance outage. The Enhanced CANDU 6 (EC6) is designed to operate for about three years (1080 days) before coming off line for a month for maintenance and inspections. Having some testing and maintenance done on-line reduces the inspection load during unit maintenance outage.

To run continuously at high power for these lengths of time means human performance and preventive and predictive maintenance have to be of a high order. This is helped by the design of the plant being tolerant of single failures with extensive redundancy enabling on-line testing of systems and components to be carried out. Since the unit safety systems are dormant during normal operation on-line testing of these systems is carried out at the frequencies necessary to demonstrate to the nuclear regulator (in Canada this is the Canadian Nuclear Safety Commission) that the targets for safety system unavailability are being met. This is part of the normal operation of a nuclear power plant.

At all times the critical parameters have to be within the Safe Operating Envelope (or Technical Specification) for the station. Sophisticated software monitors equipment in critical systems to warn of deviations from normal. There are redundant instrument channels for monitoring the same important process variable or safety parameter which means one channel can be taken out of service for testing without affecting safety system availability, and channels can be cross compared for anomalies. Having redundant safety system equipment trains means one train can be tested/maintained at power with only a slight temporary reduction in availability. Similarly for process systems critical for production having redundant equipment means testing/maintenance can be carried out on the stand-by equipment. The chemistry of the water in the primary and secondary heat transport systems and sub-systems has to be strictly controlled to meet the chemistry specification. All this means operators, maintainers, system engineers and technical support staff are kept very busy.

The record for a PWR by Three Mile Island unit 1 in 2009 October was set 35 years after the unit went into commercial operation. The BWR record by LaSalle unit 1 in 2006 February was set 22 years after going into commercial operation. Good operating performance does not depend on age of the unit. By comparison Pickering unit 7 achieved its record in 1994 October only 10 years after going into commercial operation.

If 1,000 days of continuous operation on-line is to be achieved by a nuclear unit it will likely be by a CANDU unit working on a three year outage cycle. Darlington units are operating on a three year outage cycle so have the potential for very long continuous operation and this is also the way EC6 will operate. Darlington units bring in consistently high annual capacity factors but no noteworthy long runs. Bruce units have had some long runs. Unit 4 completed 569 continuous days on-line in 2012 August 33 years after going into commercial operation and Bruce unit 8 achieved 580 days in 2003 16 years after going into commercial operation, a site record. Other Bruce units have achieved in excess of 500 days. Pickering unit 5 ended a 523 day continuous run on 2015 January, 32 years after going into commercial operation.

CANDU has all the necessary attributes for achieving a 1,000 day run. CANDU units have extensive computerized on-line testing and monitoring capability for their safety systems and provision for testing safety related equipment and process equipment critical to production. Also any failed fuel can be detected, located and, unlike light water reactors, replaced while operating at high power. CANDU (reference 1), CANDU 6 (reference 2) and other PHWRs (reference 3) have demonstrated excellent lifetime and annual capacity factors. The EC6 (reference 4) units proposed for Darlington have the operational flexibility to adjust electricity output to meet any demand during long periods of operation.

It should be noted that these world record runs were done at high continuous power. For example Calvert Cliffs unit 1 had a capacity factor of 99.3 percent and Rajasthan unit 5 had a capacity factor of 105 percent after their record breaking runs. Such performance, long continuous runs at high power with practically zero greenhouse-gas emissions, would be impossible from wind and solar energy producers and undesirable, due to greenhouse-gas emissions, from coal or gas-fired units even if it were possible to run them baseload for a year and more without a
maintenance outage.

1. Performance of Ontario’s CANDU nuclear generating stations in 2014, Don Jones, 2015 March 23, https://thedonjonesarticles.wordpress.com/2015/03/24/performance-of-ontarios-candu-nuclear-generating-stations-in-2014/

2. CANDU 6 performance in 2014, Don Jones, 2015 March 24, https://thedonjonesarticles.wordpress.com/2015/03/25/candu-6-performance-in-2014/

3. CANDU cousins in India – Performance in 2014, Don Jones, 2015 March 28, https://thedonjonesarticles.wordpress.com/2015/03/29/candu-cousins-in-india-performance-in-2014/

4. 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/


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