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:
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.
- 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
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.
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)
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
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
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
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
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
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
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
[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.