CANDU Nuclear Station Reliability

by Morgan Brown, P. Eng.
Updated April 23, 2009

Table of Contents


  1. Introduction
  2. What is Reliability?
  3. Factors Affecting Reliability
  4. The Capacity Factor
  5. Notes on the Maximum Capacity Rating (MCR)
  6. Availability
  7. The CANDU Reactor - A Brief History
  8. Ontario's Power Reactors
  9. CANDU 6 Capacity Factors
  10. Canadian CANDU Capacity Factors
  11. Ontario CANDU Capacity Factors
  12. Ontario's 'B' CANDU Reactors
  13. Ontario's CANDU Reactor Availability
  14. Conclusion
  15. Addendum A - Update for 2005 to 2009
  16. Addendum B - Canadian CANDU Output History and Predictions
  17. Addendum C - Additional Resources


To Table of Contents

A common claim, from nuclear opponents and in the media, is that Canada's CANDU nuclear reactors are unreliable, particularly those in Ontario.   This document provides definitions and examples of terminology related to reliability, and also shows the calculations and assumptions used to quantify the reliability of Canada's nuclear generated electricity.

The Independent Electricity System Operator (IESO) is responsible for coordinating the Ontario electricity grid. It depends upon a variety of electricity sources to meet the grid demand.   Reliability of supply is key to the IESO, as it matches production with demand that fluctuates throughout the day.   In 2004, 2005, 2006, 2007 and 2008, nuclear power plants provided over half of the electricity produced in Ontario. The capacity factor of Ontario's entire fleet of fifteen operating reactors was 81.0% in 2004, according to the IESO's expectations.   This is equivalent to the reactors functioning at their maximum allowable power for 81% of the time.   In 2005, 16 reactors (Pickering-1 was restarted) had a net capacity factor (CF) of 81.6%, and the CF was 84.9% in 2006, 81.3% in 2007 and 85% in 2008.   In January 2009, the 12 reactors at Darlington, Bruce B and Pickering B attained their highest ever monthly gross output.

No other generation source in Ontario (coal, hydro, natural gas, wood waste, gas/oil) had a capacity factor as high as that of the nuclear generators.   Low capacity factors do not necessarily reflect the reliability of the above sources, because different sources are used for different purposes: base, intermediate, or peak load.   But it is clear, based on the high capacity factor of Ontario's reactor fleet, that CANDU reactors are reliable - providing much of the baseload generation required by the province.

  1. Introduction
  2. To Table of Contents

    Electricity grid managers, such as the Independent Electricity System Operator (IESO) of Ontario, must manage a complex interplay amongst generators, consumers and the grid itself.   They cannot control the demands of small consumers (households and small businesses), and have limited control over medium and large consumers (mainly via financial penalties or incentives).   Grid managers must obtain sufficient generation at all times, augmented with purchases when necessary, to meet the demand.   Hence grid managers want generators to provide electricity reliably, when it is needed and until the demand drops, for the lowest price.   Reliability is essential.

    Opponents of nuclear energy often assert that nuclear power reactors, in particular the CANDU design, are unreliable and cannot maintain a stable supply of electricity to consumers.   Here is one such assertion about the reliability of Ontario's reactors, in an open letter from Energy Probe discussing the "Environmental Benefits, Controllable Emissions, Reliability Advantages, and Cost of Ontario’s Installed Cleaner Coal".   It was written to Mr. Jack Gibbons, Chair of the Ontario Clean Air Alliance, on October 7, 2005:

    Nuclear generators are similar to intermittent renewable generation in that they produce only when technical conditions are suitable, not when consumers require electricity.   Planned and unplanned nuclear outages, some of which have extended for periods of up to seven years, have reduced Ontario’s overall nuclear reliability from planned levels of 85-90% to a level of about 57% in 2004.   While this average reliability number is still roughly twice as high as that of a typical wind generator, the average hides an enormous diversity of reliability data - including entire multi-reactor nuclear stations generating nothing at all for years in a row, a result that is itself unusually threatening of grid security.

    Intermittent renewable generation is suitable for replacing somewhat less intermittent nuclear generation but is not suitable for replacing Ontario’s coal generators, which are able to increase or decrease output on demand.   Ontario's grid can accommodate a significant capacity of intermittent wind or nuclear capacity, as long as it has enough reliable and dispatchable capacity like coal or peaking hydroelectric to keep the lights on when the wind dies, or the reactors do.   But we cannot replace reliable and dispatchable capacity with intermittent capacity, and still expect the lights to stay on.
    . . .
    But, the poor production record of Pickering 4 this spring and summer illustrates the risk to reliability of relying on refurbished Candus.

    Tom Adams, Executive Director, Energy Probe

    See also Energy Probe's contrasting 1997 opinion of coal generation, which stated:

    "Ontario Hydro is also threatening to replace the dying nuclear plants with dirty coal-fired power.   Energy Probe is advocating an environmentally and economically attractive alternative to the irresponsible coal option, based on the principles of customer choice, efficiency, and tight regulation of polluters.   We need your support to ensure that cogeneration and renewable energy, not coal, replace the dying nuclear plants."

    The sentiments expressed above are still held by Energy Probe, as seen in the following quote in the New Brunswick Times and Transit of February 5, 2008:

    "If you don't contract for all of the Point Lepreau II capacity, then you can save some of it to squeeze into the grid when available," he [Norm Rubin of Energy Probe] said, noting that the unreliable nature of nuclear power means it competes with renewable like wind energy rather than grid mainstays like coal and oil.

    So, are Canada's indigenous CANDU reactors reliable or not?

  3. What is Reliability?
  4. To Table of Contents

    The reliability of any mechanical system is its ability to

    1. provide the service for which it is designed,
    2. when it is expected to do so, and
    3. for as long as it is expected to do so.

    No system can operate all of the time, nor can it be available to start operation all of the time, nor can it operate indefinitely once started.   Instead, each system has a probability of being available for operation during specific time periods and, if operated, a probability of remaining operational for a given period.

  5. Factors Affecting Reliability
  6. To Table of Contents

    All physical systems age (or wear) from the time they are built, usually aging fastest when they are in operation.   Many systems also age when they are not in use, due to ongoing physical processes like corrosion.   Motor vehicles wear as they are driven - tires, valves, exhaust, pistons, brakes, batteries, etc. - but they also age simply sitting in the driveway - corrosion, leaks, tire creep (i.e. flat spots), batteries, etc.

    Since all physical systems age, maintenance must be done.   For the vehicle example, oil changes are typically scheduled every 5000 km or three months, whichever comes first (oil should be changed even if the vehicle sits idle).   Other subsystems and components are changed or checked on longer schedules - air and gas filters, brakes, tires, etc.   Some components are run until failure, such as the exhaust system.   Replacing a component only when it has failed is acceptable if the failure does not compromise safety; e.g., a failed exhaust system might be temporarily acceptable, until a replacement can be fitted, provided exhaust fumes do not enter the vehicle passenger compartment.

    Sometimes one can continue to operate a system with failures because the failures only affect how one operates the system, but not operational safety.   For example, unreliable electric door locks on a motor vehicle are not a safety problem, since the locks can be operated manually; the cost of replacing the locks does not warrant their replacement, though it modifies vehicle operation.

  7. The Capacity Factor
  8. To Table of Contents

    A common way of measuring the useful output of a given device is to take the total measured output, during a period of time, and divide by the theoretical maximum output during the same time period.   The resulting ratio is called the capacity factor.

    The theoretical maximum output is the product of the maximum continuous rating (MCR) and the length of time.   For electric power plants, the MCR usually refers to the useful output - electricity.

    Using the vehicle example again, its useful output is the number of tonne-kilometres or passenger-kilometres achieved (i.e., the distance travelled multiplied by the useful load).   To make it simpler, however, we'll consider the distance travelled as the sole useful output.   Note that a vehicle produces no useful output while it is idling in the driveway or at a stoplight.

    The MCR of a vehicle is its maximum continuous operating speed.   The MCR is only achievable if there is a sufficient inflow of fuel, air, oil and a qualified operator.   Also, the MCR can only be attained if the machine is operating as designed, and the road conditions and laws allow.

    Imagine a long-distance truck operating with a two-person crew on a 2000 km trip.   Being good truckers, the crew doesn't excessively exceed the speed limits, stopping only for fuel, food and bio-breaks.   The trip takes a total of 23 hours from door to door.   The capacity factor of the truck during the 23 hour period is therefore:

    2000 / (23 x 100) = 0.870 or 87.0%

    assuming the truck is designed for a maximum continuous speed of 100 km/hour.

    Note, however, that it is entirely conceivable the truck could exceed 2300 km in the time period; in such a case the capacity factor would be over 100%.   Power plants sometimes exceed 100% capacity factors as well.   This can occur if (a) there is no planned maintenance, (b) the plant has sufficient fuel, suffers no unexpected failures and does not need maintenance that affects safety or production, (c) the regulatory environment allows the power plant to exceed its MCR to a limited degree if safe to do so, and (d) the grid demand requires high production.

    How can one exceed the MCR for a power plant (item c above), without breaking the regulations governing the plant?   Unlike a truck, which could be driven a few km/hr faster than the speed limit without being stopped by the police (the traffic regulator), a power plant cannot exceed the regulated operational limits without special dispensation from the regulator.   The power plant MCR, however, is the electrical power and is not a regulated value.   For a nuclear power plant, factors like the reactor power (i.e., the fission power, not the electrical power) and operating temperatures and pressures are regulated and cannot be exceeded.

    If, however, the efficiency of energy conversion (thermodynamic efficiency) is improved, more of the heat generated by a nuclear reactor can be converted to electricity.   The MCR is an expected average value calculated for the plant based on tests performed during the commissioning period.   If the cooling water is cold (i.e., winter temperatures), the thermodynamic efficiency increases because the steam condensers (for the turbine exhaust) operate at a lower pressure than during the summer.   Thus more electricity is generated per unit of reactor heat; while the reactor output does not increase (i.e. beyond the regulatory limits), the useful electrical output does.   Since the useful output of a power plant is the electricity, it is sometimes possible to produce a little more than what is expected based on the plant's MCR.

  9. Notes on the Maximum Capacity Rating (MCR)
  10. To Table of Contents

    Determining the actual value of a reactor's MCR can be problematic.   All four Bruce A reactors were originally designed to produce industrial steam as well as electricity.   To track reactor performance, the steam production was converted to an "electricity equivalent", and the total gross power of these reactors was listed as 904 MWe.   With the shutdown of the large users at the Bruce energy park, industrial steam is no longer produced by the Bruce A reactors.   Their turbines were under-sized when built, however, and the turbine MCRs are only 825 MWe (gross).   Presently, component aging limits the reactor power in order to maintain safety margins, so the turbine size is adequate.

    Note, also, the difference between net and gross output; a nuclear plant consumes some of its own output in order to run, especially the large primary heat transport system pumps.   The Darlington reactors consume 54 MWe internally, or 5.8% of their gross MCR.

    The IESO lists the two operating Bruce A reactors at only 770 MWe (net) each - this is the MCR that the IESO presently expects from these reactors.   Likewise, the IESO originally listed the four Bruce B at 795 MWe (net) each, rather than the 860 MWe (net) as originally built.   Two of the reactors have since received replacement turbine rotors, as part of their ongoing maintenance and refurbishment, and the IESO now lists their net output as 855 MWe.

    For the most part, this discussion uses the maximum "as-built" MCR values for each reactor.   Thus the Bruce A and Bruce B reactors are considered to be 904 and 915 MWe gross, respectively, and the CANDU 6 reactors are rated at the maximum values used by Nuclear Engineering International or Nucleonics Week.   The Wolsong units 2 to 4 were re-assessed in 2006 by the Korean utility, and are now listed as 729 or 730 MWe gross each (and are assumed to have been these values since being declared commercial).   This is is higher than NEI and NW have used in the past; using the maximum MCR values reduces the capacity factors, but maintains consistency.

    The exceptions to this rule relate to the IESO calculations, because the IESO lists only the net output of the reactors (the IESO is only interested in the amount of electricity supplied to the grid).   The IESO lists the output expected from each reactor; thus the Bruce A and Bruce B reactors have lower MCR values than as-built, because of physical, operational and regulatory limits.   The discrepancies between the "as-built" and the IESO MCR values are not large, and the other CANDU reactors are operated with the same MCR as when they were built.

  11. Availability
  12. To Table of Contents

    Availability is similar to the capacity factor, but a little more complex.   It is the total possible output, during a period of time, divided by the theoretical maximum output during the same time period.   The total possible output is the total output the device could (probably) have produced if requested to do so during the period of time.   The theoretical maximum output is once again the product of the maximum continuous rating (MCR) and the length of time.

    Using the long-distance truck example again, the truck travelled 2000 km in a 23-hour trip.   The crew spent an hour replacing a flat tire, so the truck was unavailable for that period (an unexpected failure reducing the possible distance travelled by 100 km).   Also, they spent one hour travelling at 75 km/h due to the speed limits (a regulatory constraint reducing the possible distance travelled by 25 km), and a half hour refuelling the drivers and truck (a planned outage reducing the possible distance travelled by 50 km).   Thus the truck would have been available to travel a possible:

    2300 - 100 - 25 - 50 = 2125 km

    during the 23 hour trip period.   The resulting availability factor is

    2125 / (23 x 100) = 0.924 or 92.4%

    Note that the availability factor is greater than the capacity factor (87%), which is typical.

    Now suppose that during the trip the truckers were informed that the receiver at their destination would be delayed opening the warehouse, so they slowed down from 100 km/h for some of the trip, to avoid having to hang around at the warehouse.   This represents a reduction in demand - the truck could have been driven at the 100 km/h speed limit but there was no demand to do so.

    Thus availability represents the output the device was available to do, but was not required to do.   Note, however, that availability would not necessarily equate to capacity even if the demand was there.   Increased demand would mean more refuelling, more maintenance and potentially more unexpected failures.   With the trucking example, the truckers could have driven a little faster if the warehouse had been open sooner (i.e., if the demand was there) but their increased fuel consumption might have required another fuelling stop. Therefore, while availability is a better measure of maximum possible production than the theoretical maximum output, the only true measure of the capability of a device is its actual output measured over a long period.

  13. The CANDU Reactor - A Brief History
  14. To Table of Contents

    The CANDU is a Canadian pressurized heavy water reactor (PHWR) design, spearheaded by Atomic Energy of Canada Limited (AECL) but incorporating a great deal of design input from Ontario Hydro (now Ontario Power Generation or OPG) and, in the early years, Canadian General Electric (now General Electric Canada).   The CANDU 6 design is solely AECL's.

    A total of 22 nuclear power reactors have been built in Ontario, supplying electricity to the Ontario grid since June 4, 1962.   The two oldest reactors, a 22 MWe proof-of-concept (Nuclear Power Demonstration or NPD) and a 220 MWe large prototype (Douglas Point), were decommissioned in the 1980s after many years of service.   They provided a great deal of field testing for various CANDU concepts, such as horizontal fuel channels, heavy water moderation, computerized control, and on-power refuelling.

    The other 20 Ontario power reactors are "full-sized" CANDUs, ranging from the four 515 MWe (each, net) Pickering A reactors to the four 881 MWe (each, net) Darlington reactors.   These can be divided into two groups - the older eight units at Bruce A and Pickering A, and the 12 newer units at Pickering B, Bruce B and Darlington.

    In addition, there are eleven CANDU 6 units in operation (or refurbishment) around the world, two AECL-built copies of Douglas Point in India, and a larger version of NPD in Pakistan (CGE-built).   India has built another ten copies (later ones significantly modified) of Douglas Point, and have built two 515 MWe (net) indigenous PHWRs.

  15. Ontario's Power Reactors
  16. To Table of Contents

    After over a decade of Ontario's power reactors ranking amongst the best in the world, their performance declined in the 1980s and 1990s.

    The eight A reactors in Ontario were shut down in the period from October 8, 1995 to April 9, 1998.   The shutdowns occurred due to poor management, backlogged maintenance, reduced staffing, maintenance errors, and component aging.   The eight A reactors were earlier versions of the B reactors, and many features were improved in the B reactors.   For example, the Pickering A reactors had two safety shutdown systems, in addition to the control system, but due to changing nuclear safety philosophies only one shutdown system was permitted to fully operate.   Ontario Hydro (as it was then) promised the regulator (the Atomic Energy Control Board, now the Canadian Nuclear Safety Commission) that the Pickering A reactors would be fitted with an enhanced shutdown system by the end of 1997, or it would shut the Pickering A reactors until the systems were installed.   By December 1997, only Pickering-4 was nearing completion of its shutdown system refit, so all four Pickering A reactors were shut.

    All the reactors were safe; the regulator had permitted continued operation, although it had reduced some licence periods and expressed concerns over diminishing safety margins (i.e., the differences between operating conditions and the upper and lower limits on those conditions), especially in the A reactors.   The decision to shut the A reactors was made by Ontario Hydro to enable the company to concentrate its effort and expertise on the 12 newer reactors.   The goal was to restart the A reactors, after improving safety and operating systems and after performance had been improved in the 12 newer reactors.

    Since 1997 much work has been done and reactor performance has improved.   As of September 26, 2005, four of the eight A reactors are again supplying electricity to the Ontario grid.   Two more (Bruce 1 and 2) are being completely refurbished, and the final two (Pickering 2 and 3) are not being restarted.   2004 marked a major improvement in Ontario's nuclear output; in that year, Ontario's 15 operating reactors generated 81,131,506 MWh gross, more than any year since and including 1997 when 18 reactors were operating.   Sixteen reactors are now operating in Ontario.

  17. CANDU 6 Capacity Factors
  18. To Table of Contents

    The 10 CANDU 6 reactors in operation around the world, in 2004, produced 54,015,811 MWh gross.   They could have theoretically produced 61,777,872 MWh if they had been able to operate at their full MCRs for the entire year.   The resulting capacity factor was:

    54,015,811 / 61,777,872 = 87.44%

    An annual capacity factor of 87.44% is exceptionally good for any reactor type, but not unusual for CANDU 6 reactors. The following table shows the gross fleet capacity factor for all the CANDU 6 reactors in operation around the world:

    YearNumber of CANDU 6
    reactors in operation
    Overall CANDU 6 Capacity Factor
    20081181.42% *

    * Point Lepreau was removed from service on March 28 2008, for refurbishment. If this time is taken into account, the 2008 CANDU 6 capacity factor is 87.2%.

  19. Canadian CANDU Capacity Factors
  20. To Table of Contents

    In 2004, Canadian reactors (in Canada) produced 90,962,729 MWh gross.   Seventeen reactors were operating or operable, and could have theoretically produced 116,367,888 MWh if they had been able to operate at their original MCRs for the entire year.   The resulting capacity factor was thus:

    90,962,729 / 116,367,888 = 78.17%

  21. Ontario CANDU Capacity Factors
  22. To Table of Contents

    Ontario's 15 (now 16) operating reactors produced 81,131,506 MWh gross in 2004, and could have theoretically produced 104,465,568 if they had been able to operate at their original MCRs for the entire year.   The resulting capacity factor was thus:

    81,131,506 / 104,465,568 = 77.66%

    So how did Energy Probe calculate a capacity factor of "about 57%" for Ontario's reactors?   One can approach this value by including all the reactors that were shut and (at that time) had indefinite futures.   It was impossible to restart the five shut reactors in 2004 (only Pickering-1 was being refurbished at the time, and is now in service).   Including the "theoretical" (actually, impossible) output from these five reactors, all 20 "could" have produced 134,781,696 MWh in 2004 if they had been able to operate at their original MCRs for the entire year.   The resulting capacity factor was thus:

    81,131,506 / 134,781,696 = 60.19%

    Using a simple arithmetic average of all 20 Ontario reactor capacity factors (based on the original as-built MCRs, and despite the unavailability of five reactors), Energy Probe calculated a capacity factor of 57.36%.   This is the lowest possible capacity factor calculable from the Ontario reactors, and it depends upon including the five shut reactors.   By including the shut reactors in their October 7 2005 letter, Energy Probe presumably expected all five to be restarted in the future, despite the August 12 2005 announcement by OPG that units 2 and 3 at Pickering A will not be restarted.

    Another calculation of the capacity factor considers reactor production from the perspective of the IESO.   This organization, charged with overseeing the Ontario electrical grid supply and demand, established net MCRs for each electricity producer.   Power plants that are under construction, being refurbished, or are mothballed are not expected to produce any electricity.   Only those that can produce electricity (or are undergoing short-term maintenance) are relied upon to supply the grid.

    In 2004, the total net Ontario nuclear output was 77,024,317 MWh, according to the IESO (50.2% of Ontario's electricity).   Based on the IESO MCR values, the theoretical maximum output of Ontario's reactors was 95,069,232 MWh (net).   The resulting net capacity factor was thus:

    77,024,317 / 95,069,232 = 81.02%

    IESO data is also useful for calculating the capacity factors of all Ontario's major electricity sources:

    Generation Type2004 Capacity Factor in Ontario*Portion of Ontario Electricity in 2004**
    Natural Gas42.82%7.0%
    Oil/Gas3.78%0.8% combined with wood waste
    and all other sources
    Wood Waste54.85% 

    * Calculated from the 2004 IESO monthly generation reports
    ** IESO January 25 2005 Press Release

    Low capacity factors do not necessarily reflect the reliability of the above sources, because different sources are used for different purposes - base load (nuclear and large hydro), intermediate load (coal), and peak load (small hydro, oil/gas, and natural gas).   Low capacity factors may be caused by lower grid demand and higher operating costs, but are only caused by low reliability if there is a fuel shortage (which includes low water periods) or technical reasons.   Hydro dams, even the large ones, typically have capacity factors of no more than 60%, due to seasonal variation in water flow; Niagara Falls is an exception, due to the huge size of the reservoirs (four Great Lakes).

    On the other hand, high capacity factors do imply a higher reliability - a capacity factor of 100% means the power plant was entirely reliable to operate at its MCR for the entire period.

    Energy Probe claimed that relying on refurbished CANDUs was risky, due to a 16 week shut down of Pickering-4 in the Spring and early Summer of 2005.   This outage was used to examine the feeder pipes, and determine their fitness for duty.   Despite this outage, and a requisite 18 week shutdown in 2007 to add a back-up electrical connection, Pickering-4 has operated with a capacity factor of 62.8% for the period of commercial operation (i.e., Sept 25 2003 to April 22 2009).

  23. Ontario's 'B' CANDU Reactors
  24. To Table of Contents

    The 12 more modern reactors at Pickering B, Bruce B and Darlington have operated together as a 'cohort' since the last one (Darlington 4) was declared in commercial operation in June 1993.   None were shut for any reason, other than periodic maintenance or an occasional trip, although the Bruce B reactors were derated from 860 to 795 MWe net, with more recent upratings (see Section 5).   As of March 31, 2009, these 12 reactors range from 15.8 to 25.9 years in commercial service, with an average age of 21.6 years.   Despite the aging processes, and requisite maintenance, the overall performance of these reactors has remained approximately constant during the 15.8 years they have operated together, as shown in Figure 1.   In fact, the graph shows a generally improving performance of these 12 reactors, especially since 1997 - 1998.

    Figure 1:Darlington plus Bruce B plus Pickering B averaged daily gross output since the last of these 12 CANDU reactors became operational in June 1993.   The dashed lines represent 80% and 100% capacity factors, based upon the total as-built gross output of the 12 reactors.

    The fluctuations in output are mainly due to scheduled maintenance outages.   The deepest dips in output are attributable to periodic maintenance outages to test the containment structures and vacuum building; Ontario's reactors are linked, in groups of 4 or 8, to common containment systems.   Thus when Bruce B is due for a containment outage, all four Bruce B units must be shut down, as occurred in the Fall of 2004.   Darlington's four reactor had a similar shared outage the year before, plus one in the Spring of 2009.

    One can also see the troublesome period in 1997-1998, when the 12 reactors produced significantly less electricity.   Part of this was caused by temporary deratings of some reactors, to address safety margins.   Since then, the reactors have increased output due to improved maintenance (decrease of maintenance backlogs) and the replacement of systems (e.g., new turbine rotors at some Bruce B units).

  25. Ontario's CANDU Reactor Availability
  26. To Table of Contents

    In 2004, Ontario's 15 operating CANDU reactors were unavailable for approximately 22% of the reactor-hours (e.g., 1 reactor out for 40 hours plus another out for 47 hours equals a total of 87 reactor-hours).   Thus the fleet availability was 100% - 22% = 78%, based on reactor-hours rather than actual production.   It would have been slightly better to calculate the availability based on production, but the necessary information was not available.

    The fleet availability is close to the fleet capacity factor of 77.66%, calculated in Section 11 based on actual production.   Despite the differences in the calculations (i.e., reactor-hours vs. actual production), the two numbers are very close, indicating that the grid demand was high enough to take all the electricity that the reactors produced.   This is what is expected of baseload generation - it is there as an essentially constant source of energy.

    The 22% unavailability is divided into four main categories.   Planned maintenance outages (i.e. planned many months ahead of time) accounted for 12.7%, outage extensions 2.6%, unplanned (human and technical) outages 6.7%, and unplanned natural event outages 0.2%.   Most planned outages are scheduled for low demand periods (Spring and Fall), so even with the outage extensions they have a small effect upon the capability of generators to meet grid demand.

    Unplanned outages (human, technical or natural causes) happen throughout the year.   Some of the technical causes reflect a declining performance in a subsystem, and maintenance is deemed necessary prior to the next scheduled maintenance outage.   In 2004, one natural cause of an outage was a large mat of algae in Lake Ontario, that was blown to the reactor site and partially blocked the cooling water intakes.   In that case, a reactor was shut to reduce the required inflow, while the algae were cleared from the intake screens.   Other natural causes in the past have included lake ice blockages and lightning strikes, which can trip protection systems and shut the reactor down.

  27. Conclusion
  28. To Table of Contents

    Ontario's nuclear reactors are not "similar to intermittent renewable generation in that they produce only when technical conditions are suitable, not when consumers require electricity", as claimed by Energy Probe.   Ontario's Independent Electricity System Operator relies upon the CANDU reactor fleet to provide the majority of its baseload requirements throughout the year, something intermittent sources could not do.   While the eight oldest reactors were prematurely shut down for a variety of reasons (technical, managerial, regulatory), four have since been restarted and two more are being refurbished.

    Only by including the non-existent output from five shut down reactors (including 2 which OPG declared will not be restarted) could Energy Probe claim a CANDU capacity factor of 57% in 2004 (60% if based on fleet capability rather than a simple arithmetic average).   Even this number is greater than the capacity factors of all other sources of electricity in Ontario: hydro, gas, oil, coal, wind and wood waste.

    The Independent Electricity System Operator controls the Ontario grid system, and it depends upon a variety of electricity sources to meet the grid demand.   Reliability of supply is key to the IESO, as it matches production with demand that fluctuates throughout the day.   In 2004, nuclear power plants provided 50.2% of the electricity produced in Ontario (50.7% in 2005, 54.1% in 2006, 51.5% in 2007, 53.0% in 2008).   According to the IESO's expectations, the fleet capacity factor of Ontario's operating reactors was 81% in 2004 (82% in 2005, 85% in 2006, 81% in 2007, 85% in 2008).

    Based on the high capacity and availability factors of Ontario's 16 operating reactors, and the 11 CANDU 6 reactors operating around the world, CANDU reactors are reliable - providing ovr half of the electricity required by Ontario.

  29. Addendum A - Updates for 2005 to 2008
  30. To Table of Contents

    This document was originally written based on the January 1 to December 31 2004 year, since that was the time period covered in the Energy Probe letter to the Ontario Clean Air Alliance.   Since then, data for 2005 to 2009 has come available.


    In 2005, Ontario's reactors generated 82,970,000 MWh gross, an increase of 2.3% over 2004.   This was partly due to the restart of Pickering-1 on September 26 2005, plus improved performance by other reactors.   The gross fleet capacity factor, based only on operating reactors (and a partial year for Pickering 1), was 78.59% compared with 77.66% for 2004.   Including all four shut reactors (which includes the two permanently shut Pickering units), the capacity factor was 61.73%, up from 60.19% in 2004.

    The Canadian CANDU performance was 78.76% for 2005, up from 78.17% in 2004.   The worldwide CANDU 6 fleet capacity factor was 86.03% for 2005, down from 87.44% in 2004.

    In 2005, the total net Ontario nuclear output was 78,948,633 MWh, according to the IESO (50.3% of Ontario's production).   Total Ontario consumption increased to 157 TWh, up 2% from 2004.   Based on the IESO MCR values, and the restart of Pickering-1 on September 26 2005, the theoretical maximum output of Ontario's reactors was 96,744,240 MWh (net).   The resulting net capacity factor was thus:

    78,948,633 / 96,744,240 = 81.6%


    In 2006, Ontario's operating reactors had a fleet capacity factor of 81.4% (based on as-built MCRs).   Including the 4 non-operating reactors, the capacity factor was 66% (71% if the permanently shut Pickering 2 and 3 are excluded).   The output of the 12 reactors at Pickering B, Bruce B and Darlington was higher than in any other year in their collective history (i.e., since and including 1994).   CANDU 6 reactors also performed very well in 2006, with a fleet capacity factor of 90.8%.   The total output of CANDU reactors around the world exceeded that of 2005 by 6%; 2005 was the previous record year.


    In 2007, Ontario's operating reactors had a fleet capacity factor of 77.8% (based on as-built MCRs).   Including the 4 non-operating reactors, the capacity factor was 63.2% (68.0% if the permanently shut Pickering 2 and 3 are excluded).   The output of the 12 reactors at Pickering B, Bruce B and Darlington tied with 1995 for the third highest amount in a calendar year, for the 14 years in their collective history (i.e., since and including 1994).   The output was 0.6% less than the record 2006 year.

    CANDU 6 reactors also performed very well in 2007, with a fleet capacity factor of 88.6%.   The total 2007 output of CANDU reactors around the world was 140.57 million MWh gross, down by 3.7% from the record year of 2006, but 2.1% above that of 2005, the previous record year.   Much of the decrease in output, compared to 2006, was due to Pickering units 1 and 4, which lost a total of over 11 reactor-months between them (>4 million MWh gross).   Most of the loss was due to the need to upgrade a shared backup electrical system; these units were restarted in October 2007 and January 2008.


    In 2008, Ontario's operating reactors had a fleet capacity factor of 81.2% (based on as-built MCRs).   Including the 4 non-operating reactors, the capacity factor was 65.9% (70.9% if the permanently shut Pickering 2 and 3 are excluded).  In January 2008, the 12 reactors at Pickering B, Bruce B and Darlington attained their highest ever total monthly gross output of 6,734,000 MWh (94.7% monthly capacity factor).

    New October, November and December production records for these 12 reactors were set in 2008: 11.6%, 8.2% and 2.1% higher than the previous records for these months, respectively. The total production for the year, from these 12 reactors, was 0.2% higher than the previous annual record set in 2006.

    CANDU 6 reactors also performed well in 2008, with a fleet capacity factor of 81.4%.   This was down due to Point Lepreau being shut at the end of March 2008 for refurbishment; the fleet capacity factor was 87.2% if the shutdown time is taken into account.

    The total 2008 output of CANDU reactors around the world was 144.59 million MWh gross, down by 1.0% from the record year of 2006, but 2.9% above that of 2007.


    In January 2009, the 12 reactors at Pickering B, Bruce B and Darlington attained their highest ever total monthly gross output of 6,852,000 MWh (96.3% monthly capacity factor).   The total production for January, from these 12 reactors, was 1.7% higher than the previous monthly record set in January 2008.

    Wind Power

    Preliminary and some final data is now available from seven Ontario wind farms:

    From preliminary IESO data to April 22, 2009, a total of 3,602,000 MWh has been generated and the overall capacity factor for the wind fleet is estimated to be 30.3%.   This is for the period since the wind stations were declared commercial (i.e., fully operational), and accounts for the stations being declared commercial on different dates.   It takes a full year of operation to give a more accurate picture of the overall annual wind capacity factor, due to seasonal variations in wind; to date, December 2008 has the monthly maximum with a 47.1% capacity factor.

    Five stations have attained at least their 1st anniversary of commercial operation, with annual capacity factors (bold = final data, otherwise preliminary data) of:

    The highest daily output of wind generated electricity to date was 16,585 MWh on March 11, 2009, corresponding to a daily wind fleet capacity factor of 82% (a peak hourly capacity factor of 93%).   The next day the total wind fleet hourly capacity factor was 17%, with the hourly CF dipping to 2%. The minimum daily nuclear output in Ontario (since IESO hourly output has been available) was 96,196 MWh (39.9% capacity factor) on Oct 10 2003.

  31. Addendum B - Canadian CANDU Output History and Predictions
  32. To Table of Contents

    In July 2003 the Campaign for Nuclear Phaseout (CNP) released a report Phasing out Nuclear Power in Canada - Toward Sustainable Electricity Futures.   The Executive Summary (page i) began:

    "The output of Ontario’s nuclear power plants has dropped by a third since it peaked in 1994.   It will soon begin a further steep decline.   By 2010 it will have dropped to 50% of its peak levels.   Sometime in the next 10-15 years, electricity production from nuclear power in Canada will drop to zero.   This projection assumes that the reactors that are still operating will continue producing until they are 27 years old, more than five years longer than any CANDU has ever operated without having to be shut down.   It also assumes that the current reconstruction of one unit at the Pickering A Station and two units at the Bruce A Station are successful and the rebuilt units operate like new for another 13 years or more. "
    Following the above paragraph was a graph (Figure ES-1) showing "Annual Energy from Canadian Nuclear Program (Historical to 2001, Projected from 2002-2020)", using data from the Power Reactor Information System (PRIS) of the International Atomic Energy Agency.   The graph is reproduced in Figure 2 below, with the addition of historical PRIS data 2002 to 2007.   The CNP predictions were interpolated from the original CNP graph, as no table of data was published in the CNP report.

    Figure 2:Reproduction and update of Figure ES-1 from CNP's July 2003 report "Phasing out Nuclear Power in Canada - Toward Sustainable Electricity Futures".   Updated with corrected historical PRIS data for 2001 and more recent historical data from 2002 to 2008.

    Inexplicably, the 2001 data plotted by CNP (supposedly historical data) was about 10% lower than the 2001 data from the IAEA PRIS database.   The 2001 historical data is corrected in Figure 2, and the 2001 CNP value is considered a prediction.

    As noted in the quote above, CNP considered two cases for future CANDU-generated electricity:

    CNP labelled the solid pink line "With current retubings", meaning that it included the restarts underway at Bruce 3, Bruce 4 and Pickering 4, but not the later restart of Pickering 1.   CNP labelled the pink dashed line as "Without retubings", assuming the refurbishments underway were never completed.   Note that CNP confused "retube" with "restart".   "Retube" means a complete replacement of all the pressure and calandria tubes in a reactor, whereas a "restart" is a refurbishment without retubing.   Retubes are presently underway at Bruce 1 and 2 and at Point Lepreau; Bruce 3, Bruce 4, Pickering 1 and Pickering 4 were all restarts without retubing.

    The thick solid green line "PRIS data with B3, B4, P4 restarts" is directly comparable to the CNP's prediction (thick solid pink line).   The CNP significantly underestimated the expected output from Canada's reactors - those already operating plus those restarted. The difference between these two lines, for the period 2001 to 2008, is equal to 168,000 GWh, the equivalent of three Darlington reactors operating at >90% capacity factor for that period.

    The restarted units at Bruce 3, Bruce 4 and Pickering 4 came on line through the latter half of 2003 and early 2004, and added significantly to the total output.   However, the output of the reactors excluding the restarted units (thick dashed green line) was significantly above the CNP predictions that included the restarted units. The already operating reactors were improving in performance, not declining dramatically as CNP predicted with their dashed pink line.

    In 2005, Pickering 1 was restarted, adding further to the output from Canada's reactors (thin solid green line).   In 2006, the output from Canada's 18 operating reactors was greater than any year except 1994 and 1995, when 22 reactors were operating.

    As of Dec 31, 2008, the longest serving power reactor in Canada, that has never been shut down, is Pickering 5; it has been in commercial service for 25.6 years (26.0 since first electricity).   Point Lepreau ran until March 28 2008 (25.1 years in commercial service, 25.5 years since first electricity) before being shut for a complete refurbishment including retubing.   Pickering 4 is 35.5 years old (commercial service), but has operated only 28.1 of those years; it was shut once for retubing and once for refurbishment (restart).

    CNP's predictions for Canadian nuclear generation appear unduly pessimistic.

  33. Addendum C - Additional Resources on CANDU Output
  34. To Table of Contents

    Canadian Nuclear Society home page