Bruce A Proves There Are Second Acts in Nuclear Power

By James M. Hylko and Dr. Robert Peltier, PE

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The refurbishment and restart of all four CANDU reactors at Bruce A may be Ontario’s most significant and complex power generation project since the first phase of the Bruce Nuclear Generating Station was built more than 30 years ago. Units 1 and 2 are expected to be synchronized in 2011 and return to commercial service by early 2012, joining Units 3 and 4, which restarted in 2004 and 2003 respectively. POWER visited Bruce A in April to witness the project’s progress.

In its first act, Bruce Nuclear Generating Station—whose Bruce A and Bruce B stations sit on the eastern shore of Lake Huron, about 155 miles northwest of Toronto—was the king of nuclear capacity factors. Thanks to the refurbishment of the long-idle Bruce A, it may become the king of capacity factors in its second act as well.

Bruce A and Bruce B each house four CANDU pressurized heavy water nuclear reactors (see sidebar “Inside the CANDU Reactor”). Bruce A reactors (Units 1 through 4) entered commercial service during 1977–79 and were followed by Bruce B reactors (Units 5 through 8) between 1984 and 1987 (Figure 1). All eight units featured a stellar operating record through the 1980s. In 1981, Unit 1 operated with a 97% capacity factor, garnering accolades as the number one reactor in the world. In 1982, Unit 3 set a new world record of 494 days of continuous operation. In addition, Bruce A was the most reliable multi-unit station in the world in 1984, with all four units finishing the year with capacity factors greater than 90%.

1. Eight is enough. A 2,300-acre site accommodates the Bruce A (foreground) and B (background) generating stations. Each station houses four CANDU reactors. All four units of Bruce B (Units 5 through 8) and Units 3 and 4 of Bruce A are in commercial service. When the Bruce A Unit 1 and 2 restart is completed in 2012, the entire site will produce 6,200 MW. Courtesy: Bruce Power

These performance figures may seem average by today’s standards, but they were stellar 25 years ago. In contrast, the U.S. nuclear fleet’s average capacity factor was an embarrassing 56.3% in 1984. By 1987, Bruce Units 3, 4, 6, and 7 were on the global top 10 list of best-performing reactors. By the end of that decade, Ontario Hydro was recognized as owning one of the world’s largest and most reliable fleets of nuclear power plants. (For a more complete picture of Canada’s energy portfolio, see “Canada Moves to Rebalance Its Energy Portfolio” in our June 2009 issue.)

In the mid-1990s, Ontario Hydro, then-owner of the Bruce stations, was grappling with an unexpected situation: Overbuilding had produced the largest capacity surplus in its history. The business decision was made to temporarily lay up the Bruce A units in order to “concentrate resources on other initiatives.” Bruce A Unit 2 was prematurely mothballed in October 1995, in part because of damage to its steam generators from a lead blanket left inside during a 1986 outage. Unit 1 was subsequently shuttered in October 1997, Unit 4 in March 1998, and Unit 3 in April 1998.

Inside the CANDU Reactor

The heart of each Bruce unit is the CANada Deuterium Uranium (CANDU) pressurized heavy water reactor. Developed in Canada, the first CANDU reactor entered service in 1962. The last CANDU reactors to enter service were RPAS-3 and -4 and Kaiga-1 and -2 in India in 2000, followed by China’s Qinshan Unit 3-1 in 2002 and 3-2 in 2003. According to the CANDU Owners Group, there are 34 operating CANDU reactors in Canada and abroad, not including the two Bruce units currently being refurbished.

The CANDU design is quite different from U.S. light-water designs. Instead of enriching U-235 to 5% and using regular or “light water” as a moderator to slow down the neutrons for fission to occur and as a coolant, CANDU reactors are fueled with natural uranium (uranium with an isotopic composition found in nature, containing 99.3% U-238, 0.7% U-235, and a trace of U-234) and use deuterium (D₂O) as a moderator and coolant. Deuterium (heavy water) is an isotope of hydrogen that contains an extra neutron not found in hydrogen. Deuterium occurs naturally in all bodies of water (1 part per 7,000 in the Great Lakes).

CANDU Reactor Core. Within a CANDU reactor core, the calandria (a large metal drum with flat end shields) is filled with the heavy water moderator and pierced by an array of horizontal tubes approximately 6.3 meters in length called “calandria tubes” (Figure 2). Each calandria tube, in turn, contains an inner “pressure tube,” in which natural uranium fuel bundles—the source of the heat that creates steam—are laid end-to-end. The pressure tubes and calandria tubes are made of a zirconium alloy that absorbs fewer neutrons than other metals. Collectively, the pressure and calandria tube systems are referred to as “fuel channels.” The calandria contains 480 fuel channels.

2. Down to the core. This photo shows the core of the CANDU reactor, including the calandria and shield tank assembly, during a fuel channel replacement. Courtesy: Bruce Power

CANDU Fuel. Fuel for Bruce Nuclear Generating Station is in the form of compressed and sintered natural uranium dioxide pellets that are sheathed and sealed in Zircaloy-4 tubes. Thirty-seven tubes (elements) are assembled between two end plates to form one fuel bundle. Each of the 480 fuel channels contains 12 bundles, and each bundle weighs 22.5 kg. A small number of depleted uranium fuel bundles are inserted at specific locations to fine-tune reactivity in the core. Fresh fuel bundles are received at a fuel-receiving area, enclosed in protective palletized crates containing 36 bundles. Fuel storage areas have the capacity to store approximately 23,000 bundles.

CANDU Refueling. The CANDU system employs a unique, online, full-power refueling system (Figure 3). Two identical fueling machines rise from a fueling duct under the reactor and latch onto opposite ends of a designated fuel channel. Each machine is remotely operated from the control room. With both machines latched on and brought up to system pressure, the ends of the fuel channel are opened and new fuel is simultaneously exchanged for used fuel—one machine discharging and the other accepting. Each bundle stays in the reactor from 12 to 20 months, depending on its location in the core.

3. Automated refueling. Specialty robotics automate the task of feeding fuel assemblies into one of the Bruce A reactors. Courtesy: Bruce Power

Spent Fuel Processing. Once fuel is removed from a reactor, it is transferred by remote control to a water-filled storage bay or spent fuel pool. The bay is about 6 m deep and roughly the size of an Olympic swimming pool. Fuel bundles are stored in the cooling bays at the stations for at least 10 years to dissipate decay heat. Afterward, the bundles are transported to a dry fuel storage facility owned and operated by Ontario Power Generation (OPG) on the Bruce Power site. Used fuel from Bruce Power’s operations is owned by OPG, and as part of the lease agreement with OPG, Bruce Power pays a per-megawatt fee to OPG for storage of the used fuel.

Special Safety Systems. The CANDU reactor design has two independent and physically separated shutdown systems. Shutdown System No. 1 is the primary means of quickly shutting down the reactor. Neutron-absorbing rods made of stainless steel–sheathed cadmium are suspended vertically above the reactor and held in place by an electromagnetic clutch. The rods drop into the core within 2 to 3 seconds of a reactor trip signal by cutting off electricity to the clutch (Figure 4).

4. Double your safety. The CANDU reactor design has two independent fast shutdown systems that are physically separate and have their own power supplies and monitoring equipment. Courtesy: Bruce Power

Shutdown System No. 2 is a fully independent system consisting of tanks containing gadolinium nitrate in heavy water connected to perforated tubes that run horizontally through the reactor. Opening the isolation valves at the top of the tanks activates the shutdown system. High-pressure helium injects the gadolinium nitrate into the moderator that absorbs neutrons and shuts down the reactor in about 2 seconds.

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