Canadian Nuclear Society Response to
Paul McKay's article August 20, 2003

Updated May 10, 2006

Original Media Report
Canadian Nuclear Society Response
How our nuclear reactors failed us
by Paul McKay
The Ottawa Citizen
August 20, 2003
It took nine seconds to knock out Ontario's high-priced fleet of Candu nuclear reactors last Thursday, but it may take nine days to bring them all back to full power -- leaving the province acutely dependent on imported electricity and vulnerable to rolling blackouts. Much of the electrical grid in the northeastern United States of America and Ontario was blacked out on August 14, 2003, leaving approximately 50 million people without electricity. The Ontario grid blacked out between 16:10:50 and 16:11:57, as all types of electrical generation were disconnected. The generating stations automatically disconnected themselves from the grid to avoid damage to the generating equipment which could take weeks or months to repair. Eleven of Ontario's 12 operating nuclear reactors were running at the time of the blackout; four reduced power to 60% and were re-connected to the grid on the evening of August 14, 2003. Reconnection of the 11 CANDU reactors, that were on-line at the time of the blackout, occurred at:
  • Bruce 5, 7 and 8, 19:13 to 21:13 August 14
  • Darlington 3, 21:18 August 14
  • Darlington 2, 20:36 August 17
  • Darlington 1, 03:15 August 18
  • Darlington 4, 11:12 August 18
  • Pickering 5, August 22
  • Bruce 6, 01:03 August 23
  • Pickering 8, August 23
  • Pickering 6, August 25
Also, Pickering-4 was reconnected to the grid on August 22, 2003, for the first time since being closed for upgrades and refurbishment on April 2 1996.
The big test will likely come tomorrow afternoon. Sweltering summer weather and business power demands typically drive demand into the 23,000-25,000 megawatt range. Tomorrow, temperatures are expected to exceed 30 degrees. The peak OPG demand to date was 25,495 MWe on August 1 2002. The demand in Ontario peaked at 20726 MWe at 16:00 on August 21, 2003 [ IMO Weekly consumption report August 20 - 26, 2003].
Provincial utility managers will be in a race against time to bring the hobbled nuclear units back online, because Ontario can import a maximum of only 4,000 megawatts to meet any power deficit -- and power is in high demand in all areas recovering from the biggest blackout in North American history. Nuclear units are not brought on line as a race, but must be re-started in accordance with set procedures and physical limits, like all power generation sources. Nuclear reactors have more complex systems than other generation systems and cannot be hurried into production. The reactors are best operated as baseload power, with constant full-power output.
When the blackout hit last Thursday, most of Ontario's nuclear units tripped into full shutdown mode - despite technology designed to de-couple them from the grid, but leave them in standby mode at 60 per cent of power output. At the time of the blackout, four Bruce Power and seven OPG reactors were operating.

The Bruce and Darlington reactors can rapidly decrease their output to 60% of full power to avoid a xenon poison outage (explained later), and the steam produced can be diverted directly into the condenser (cooled by lake water) rather than going through the turbines. This is called "poison prevent mode". Thus the reactors can remain in operation indefinitely at 60% power, while not delivering electricity until the grid is ready. As described later, all eight Bruce B and Darlington reactors decreased power automatically upon the loss of the grid and four were successfully put into poison prevent mode. Two others were shut due to system problems, and two were shut manually because the required system reviews could not be completed and checked in time.

The Pickering reactors can also go into poison prevent mode, but do not have bypass lines to their condensers. Instead, they only have atmospheric steam discharge valves to dump the (non-radioactive) steam to the atmosphere if the steam is not needed in the turbines (because the grid cannot take the electricity). The supply of demineralized water, to replenish the steam lost to the atmosphere, is limited. With all three operating reactors disconnected from the grid, the water may have lasted only a couple of hours. As it was, the grid to Pickering was not restored for over 5 hours, so the reactors could not have remained in poison-prevent mode for that long.

Three Bruce reactors and one Darlington reactor were reconnected to the grid between 7:13 and 9:18 pm on the evening of the blackout. [ IMO Releases Details of Power Restoration Effort, August 29, 2003]

Instead, three of the four 500-megawatt units at the Pickering B nuclear station automatically slammed into total shutdown mode, along with one 750-megawatt unit at the Bruce B complex. The station operators have declined to disclose why this occurred. Pickering B unit 7 was just in the process of returning to power after a maintenance outage when the blackout occurred, and the reactor was manually tripped (shut down). Pickering 4 - the first of the four "Pickering A" reactors to be refurbished - was being prepared to be connected to the grid for the first time since refurbishment when the blackout occurred and the reactor automatically tripped. The three operating 516-megawatt (MWe) reactors at Pickering B were tripped by the grid failure, and all four Pickering B reactors were cooled, depressurized and placed into a guaranteed shutdown state [OPG Technical Briefing, August 21, 2003].

One 795 MWe reactor (unit 6) at Bruce B was shut following the blackout because a mechanical failure prevented the reactor from going into poison-prevent mode.

Nuclear station operators, erring on the side of safety, also put three 850-megawatt units at Darlington into full shutdown mode, as well as one 500-megawatt unit at Pickering B. That left only one 850-megawatt Darlington unit, and one 750-megawatt unit at Bruce B, operating at standby power levels. Both were feeding power back into the grid by Friday. The four Darlington reactors each produce 881 MWe (net), and all units automatically reduced power to attain poison-prevent mode (60% power) following the blackout. An inverter for class 2 power failed on unit 4, so it was decided to manually shut the reactor. The control room operators completed the required system safety reviews and determined that units 1 through 3 were safe to operate in poison prevent mode. The shift manager was unable to complete the required review in the time available, however, so units 1 and 2 were manually shut down. The managerial review of unit 3 was completed in time, so the reactor was placed in poison prevent mode and was reconnected to the grid on Thursday evening. As mentioned above, all four Pickering B reactors were shut down following the blackout, and put into a guaranteed shutdown state.

There were three 795 MWe (net) CANDUs in poison-prevent mode at Bruce B after the blackout on Thursday, and "within hours were reconnected to the provincial grid" (i.e. Thursday) [Bruce Power press release, August 15, 2003]

Another four 500-megawatt units at Pickering A, and four 750-megawatt units at Bruce A, have been out of commission since 1997 because of poor performance, costly repairs and safety upgrades. After repair and safely upgrade costs estimated at $2.5 billion, the Pickering A reactors are scheduled to return to service during the next year. For cost and safety reasons, the four Bruce A reactors will likely never be restored to service. One Bruce A reactor was shut in 1995, one in 1997 and two in 1998. One Pickering A reactor was shut in 1996 and another three at the end of 1997. Seven of the reactors were shut by Ontario Hydro (OPG's predecessor) following the August 13 1997 release of the Integrated Independent Performance Assessment report by Ontario Hydro. The report recommended that the A nuclear stations be shut for assessment, but "all of the Ontario Hydro Nuclear plants were being operated in a manner that meets defined regulations and accepted standards related to nuclear safety" (Ontario Hydro release, August 13, 1997).

As mentioned above, Pickering A unit 4 (one of the shut reactors) was about to be reconnected to the grid for the first time since it was shut for refurbishment, when the blackout occurred. It was reconnected on August 22 2003.

Bruce Power spent 610 million dollars (to the end of June 2003) to restore units 3 and 4 of Bruce A to service. On April 4, 2003 the regulator - the Canadian Nuclear Safety Commission (CNSC) - granted Bruce Power a licence amendment for Bruce 3 and 4 to be started pending completion of specific requirements. On August 19, 2003 (the day before the Ottawa Citizen article was published), Bruce Power was given permission by the CNSC to remove the guaranteed shutdown state from unit 4, in preparation for re-start and testing at low power (CNSC news release).   Bruce 4 was restarted and reconnected to the grid on October 7 2003, followed by Bruce 3 on January 8 2004.   Bruce Power is refurbishing and restarting Bruce 1 and 2, as described here.

The combination of blackout-triggered shutdowns and out-of-commission nuclear units has put Ontario in a razor-thin power-supply deficit. It has prompted pleas from Premier Ernie Eves for large industrial users to suspend operations, a sharp cutback in civil services, and a request for citizens to minimize power consumption, especially air conditioning. The requests for cut backs in electricity consumption were prompted by the blackout and subsequent restoration of the grid.
The problem has been magnified by a unique feature on Candu reactors. One of their split-second shutdown systems sends high-pressure jets of an exotic element called gadolinium into the heavy water that both cools and controls the chain reactions inside the atomic plants. The gadolinium "poisons" the reaction process by absorbing the neutrons that split uranium atoms. As a safety feature, this automatic shutdown system appears to have performed well as the blackout hit last Thursday. But once the Candu reactor moderator is "poisoned," it takes up to three days for the element to naturally dissipate so that the reactor can be restarted. That means extra power has to be found -- and paid for -- to replace that lost during downtime. A short physics lesson: fast (high energy) neutrons are released when the nucleus of a uranium or plutonium atom is fissioned (split). These same neutrons are slowed (moderated) by striking the nuclei of heavy hydrogen (deuterium) atoms in heavy water. If a slowed (thermal) neutron enters another uranium or plutonium atom's nucleus, the atomic nucleus will probably split, releasing heat and more neutrons. Thus the fission reaction continues as a chain reaction. The released heat is used to produce steam to spin a turbine, which turns a generator to generate electricity.

There are two phenomena of interest regarding "neutronic poisons" - materials where neutrons are absorbed but do not split the atomic nuclei of those materials.

The first neutron poison phenomenon is the ability of a reactor to be controlled or shut down by systems that absorb neutrons. CANDU reactors have two fast shutdown systems, in addition to the reactor regulating system. All three systems use materials that absorb neutrons and each system can shut down the reactor. The first shutdown system is a set of cadmium and steel absorber rods that drop into the core. The second shutdown system injects a solution of gadolinium nitrate into the moderator water (which is separate from the coolant water). If the second shutdown system is used, it takes a couple of days to extract the gadolinium nitrate from the moderator, via ion-exchange columns, in order to re-start the reactor. US reactors use dissolved boron, control rods and shutdown rods to absorb neutrons and control or shutdown the reactor. Not better or worse, just different. Additional information on the CANDU shutdown systems is available in course notes (3.81 MB pdf) written by the CNSC and placed on the CANTEACH web site.

The second neutron poison phenomenon is due to the waste product xenon-135, a very effective neutron absorber produced in all nuclear reactors. Xenon is produced continuously by the decay of tellurium-135 (a fission product left when some uranium atoms are split) into iodine-135, which decays to xenon-135. Xenon-135 builds up in an operating reactor and decays away (or is changed by absorbing neutrons) at an equal rate (an equilibrium state). After a reactor decreases in power (or shuts down), there is a delayed effect when the xenon-135 builds up from previously-created tellurium-135 and iodine-135. If a reactor is not re-started within about half an hour after shutdown, the xenon concentration will get too high and will absorb too many neutrons to allow reactor restart. Thus the xenon-135 must decay sufficiently before the reactor can be re-started. The effect of xenon poison is dependent upon many things, including the pre-trip power level and duration, and the reactor design. American reactors require about 20 hours to re-start following a xenon "poison outage". CANDU reactors require about 36 hours to re-start after a xenon poison outage, due to design differences and natural uranium fuel.

When station managers at Darlington and Pickering B also chose to put units there into total shutdown mode for safety reasons, the gadolinium "poison" process also kicked in. Once a reactor goes into cold mode, it can take days to power the unit back up because thousands of complex performance and safety circuits have to be checked. During that downtime, replacement power has to be found and purchased. The three shutdown Darlington units were not put into guaranteed shutdown mode (i.e. with the gadolinium nitrate solution). The three units were reconnected to the grid by Monday August 18, 2003. The second shutdown system (gadolinium nitrate injection) operated on two Pickering B units (5 and 6), in conjunction with shutdown system 1 (shutdown rods).

Replacement electricity has to be purchased whenever any type of generator is shut down, if the grid requires it. The cost of the electricity is dependent upon the supply from other sources, both within the utility and from imports from other generating companies. Hydraulic power is typically the cheapest, from the perspective of operating cost, but a utility can only use this if there is an adequate supply without drawing down the water levels too much. Coal and nuclear electricity are next cheapest, with relatively similar costs. Natural gas is often the most expensive to operate because of the higher fuel costs, although it serves well for peaking loads.

As an example of the cost of replacement power, New Brunswick Power operates a single CANDU 6 reactor at Point Lepreau which can generate approximately 14,400 MWhe net per day. In 2002-03 this was done at about $29/MWhe net, based on a 71.9% capacity factor and $117 million operating, maintenance and administration cost reported in the 2002-03 Annual Report. Note that the 2002-03 year was expensive - the 2001-02 OM&A cost was about $23/ MWhe net.

Thus the cost of operating Point Lepreau is about $375,000 per day, based on the average 2001-03 OM&A cost of $26/MWhe. In the Sept 18 2003 New Brunswick Telegraph Journal, it was reported that "The nuclear plant is NB Power's cheapest producer of electricity. Each day it is offline, the utility has to spend between $500,000 and $750,000 to produce or purchase replacement electricity."

By contrast, the reactor design used in the U.S. does not use a Candu-type moderator/ coolant, so they can be shut down without "poisoning" the reaction process. Consequently, the U.S. reactors affected by the blackout have returned to service much more quickly than those in Ontario. CANDUs also can be shut without poisoning the moderator, either with the reactor regulating system or with shutdown system 1 which uses steel and cadmium rods. US reactors can be shut down by injecting boron into the reactor coolant to absorb neutrons. As mentioned above, all reactors can be forced to stay shut down by the xenon-135 poison phenomenon, until it decays away.

"Four of the largest CANDU units in the province were the first nuclear units in the affected area to reconnect to the grid, six hours after the blackout started and more than two days before the first U.S.-based plants were reconnected. ... Ontario's nuclear capacity returned at a rate comparable to the United States overall and, in some cases, almost 10 per cent faster than affected states in the U.S. "
['Blackout demonstrates need for nuclear revival', Letter to the Editor by Robert Van Adel and David Torgerson, Toronto Star, August 26, 2003, p. A21]

A final hurdle in bringing Ontario's nuclear fleet back into full service is that provincial utility managers must carefully match the power output to demand from Ontario industries, commercial businesses and 12 million consumers. A sudden excess of power sent into the grid, or a surge in demand that is not met instantaneously by power-plant output, could trigger new domino-effect blackouts. A balance between generation and load (consumption) is maintained in the grid. When a large load is added to the grid, it may cause the voltage to decrease slightly due to the imbalance. This is observed on a local scale in your home when lights dim slightly when a high-power appliance (e.g. a vacuum cleaner) is attached to the same house circuit and turned on. Hydro One is the company that maintains the grid and matches loads and generation, preventing large voltage swings. A large grid helps prevent large voltage swings because the individual loads and generators represent a smaller fraction of the total electricity supplied by the grid.
So literally, hour by hour, the nuclear units must be brought back online and adjusted as provincewide power demands fluctuate. It is a balancing act that has never been executed in Ontario because the massive 1965 blackout occurred before any commercial-scale Candus were operating in Ontario. The care with which generation and load must be matched applies equally to every generator, whether fossil-fired, renewable, nuclear or hydro. In 1965 there was approximately 10,000 MWe of generation capability in Ontario, whereas now there is over 25,000 MWe in operation. The complexity (total line length, number of customers, interconnects with other grids, transformers, etc.) of the grid has grown as well.

The nuclear reactors operate best at a constant power. Thus Ontario's reactors are typically run as baseload (full power). The smaller hydro dams are the preferred means of adjusting to grid demands, and the larger dams and fossil fuel-fired stations are the next best for load-following.

The task is complicated by the fact that each of the nuclear units represents a huge single block of power, so the reactors are not as agile to operate as a hydro station like those on the Ottawa River, or a coal plant. (For example, two Darlington units could supply all of Ottawa's power.) When Candu units come onto the grid, they provide huge jolts that must be absorbed within seconds. Also, because they are located a long way from Ontario's major load centres, fine-tuning output with demand can be a complex technical task. All electrical generators, including CANDU reactors, are "synchronized" with the grid at zero electrical power. Synchronization means that the electrical generator frequency and phase are aligned with those of the grid before the generator is physically connected to the grid. Thus all generators in the grid are generating the same 60 Hz output in step with each other.

For a CANDU reactor, synchronization usually occurs when the reactor power (i.e. its thermal, or heat, power as opposed to its electrical power) is at about 10 to 15% of full power. It can be synchronized at higher reactor power (but still at zero electrical power), however, and newer CANDUs can be run at 60% of reactor power indefinitely without being attached to the grid (i.e. poison-prevent mode).

Once the generator is synchronized to the grid, the steam turbine, and hence electrical generator, output is increased gradually, in step with the reactor output and grid demand. CANDU reactor electrical output to the grid is typically increased at approximately 1 MWe per minute, thus taking a Darlington-sized reactor about 16 hours to go from zero to 100% of its output. There are no jolts imposed by any generator being synchronized to the grid or run-up in power. The reactor systems are also designed to withstand a "load rejection" as happened when the grid failed.

During the early 20th century the local electrical sources (typically hydro in Ontario) became fully utilized. As electrical demand increased, new sources were developed in remoter locations (particularly hydro dams). But about 60% of Ontario's generating capacity is located in the load centre of the Niagara - Hamilton - Toronto - Oshawa area, including eight of Ontario's 12 presently-operating reactors (64% of the operating nuclear generation capacity).

The main tactic will likely be to match the returning Candu units to the demands of Ontario's large industrial power users such as steel mills, automakers, pulp and paper plants, chemical producers and mines. Currently, most have suspended production precisely because of the downed nuclear units and fragility of power imports. Regardless of the generation source, load and generation must be balanced to avoid swings in grid voltage. As more generation capacity comes on line, additional industries and heavy users are able to restart production.
They use huge amounts of power around the clock, and often have designated transmission supply routes, so their return to production can be more easily matched to Candu units coming back onto the grid. It is possible, for example, to synchronize a common hour at which both the Stelco steel complex in Hamilton and specific Candu units can return to production -- but they must do so within seconds of each other. Despite lost production costs almost certainly exceeding billions of dollars, it will likely be the weekend before Ontario's largest industries and the Candu units operated by Ontario Power Generation are in balance again. While production and consumption are matched in the grid, no generation source is matched with a load within seconds. As mentioned earlier, the generators are slowly increased in output to match the grid loading. Similarly, large users do not/cannot suddenly turn on an entire production process, and must first get permission from the grid to draw a large load. The grid is a dynamic entity - the loads are continuously changing and the voltages change slightly all the time. Grid management ensures that generation closely matches the load and that the voltages and frequency stay very close to constant.
Meanwhile, Ontario's hydro and coal plants will be running flat-out, and the transmission lines bringing imported power into the province will likely be pushed to their limits. It will take another Sunday, when Ontario's power demand is lightest, for the province's largest industries and its largest power plants to complete the delicate electrical dance needed to keep the economy humming. Ontario resumed exporting electricity on Saturday August 23, 2003. See the figure adapted from Independent Electricity Market Operator data from the August 20 -26 2003 Weekly Market Report.
Even if no further blackouts occur because of Thursday's grid collapse, Ontario faces a daunting shortfall of power for years to come because of its problem-plagued nuclear fleet, and aging, smog-producing coal plants, which all three political parties have vowed to retire. They currently provide about 75 per cent of the power produced in Ontario. Nuclear power plants provided 43.1% of Ontario's electrical production, coal 25.2%, natural gas 5.9%, oil 2.4%, wood waste 0.7%, and hydro 22.8% [compiled from IMO generator disclosure monthly reports for the period May 2002 to April 2003]. Thus, together, nuclear and coal produce 68% of the electricity produced in Ontario. This is before the addition of the six Bruce and Pickering nuclear units being refurbished and returned to service.
Most ominously, Ontario's biggest, most costly power plant at Darlington is already overdue for a major overhaul. That has been repeatedly deferred because the province's acute power deficit has left no time or breathing space for any of the 850-megawatt units to be taken off-line. "The Darlington Nuclear Generating Station is not overdue for any such overhaul. The commission requires each operator of nuclear power reactors have ongoing maintenance and inspection programs that ensure that nuclear reactors in Canada remain fit for service at all times."
[Letter to the editor, by Jim Blyth, Acting vice-president, Operations branch, Canadian Nuclear Safety Commission, Ottawa Citizen, Aug 25, 2003]
If they go down due to mechanical failure or equipment stress related to running flat out, 24 hours per day for months on end, then only a miracle would avert future blackouts. The grid and generation sources are run with planned maintenance outages, with flexibility for possible problems requiring shutdown. Nuclear plants in particular are designed to be baseload - running at full power for many months on end. Pickering unit 7 has the world record for continuous operation: 894 days completed October 7 1994.
Paul McKay is a Citizen reporter, and author of 'Electric Empire: The Inside Story of Ontario Hydro.' He is involved in a proposed green energy project in Northern Ontario. Paul McKay is the author of other anti-nuclear articles such as:

Mr McKay was listed as a "Friend" of Toronto-based anti-nuclear organization Energy Probe [1984 Energy Probe Annual Report]. He also was the Peterborough board member of the "Ontario Public Interest Research Group" (OPIRG, an anti-nuclear organization), and launched what became the Nuclear Free Press.

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