Nuclear Context & The Station

Darlington houses four CANDU 850-class pressurized heavy water reactors (PHWRs), the largest CANDU units ever built. Each unit produces 878 MWe net from a thermal output of 2,776 MWt, giving the station a combined net capacity of 3,512 MWe. That is enough to power approximately two million Ontario homes and represents roughly 20% of the province's electricity, or about one-third of Ontario's total nuclear output.

The station sits on the north shore of Lake Ontario in Clarington, Ontario, about 70 km east of Toronto. Construction began with a first concrete pour in June 1981. The units came online in an order that does not match their numbering: Unit 2 achieved commercial operation on October 9, 1990, followed by Unit 1 on November 14, 1992, Unit 3 on February 14, 1993, and Unit 4 on June 14, 1993. The original projected cost was $7.4 billion CAD; the final cost escalated to $14.4 billion due to post-Three Mile Island and post-Chernobyl safety upgrades, design changes, and funding constraints.

The physical scale of Darlington is striking. Four individual reactor buildings with 1.8-metre-thick reinforced concrete walls are connected to a single turbine hall spanning roughly 580 m long, 137 m wide, and 45 m high (approximately six football fields long and twelve storeys high). A 71-metre-high vacuum building, a cylindrical concrete structure unique to multi-unit CANDU stations, serves as a passive containment system. It is maintained at sub-atmospheric pressure and connected to all four reactor buildings by pressure relief ducts.

Supporting facilities include the Tritium Removal Facility (TRF), used fuel wet and dry storage facilities, heavy water management systems, and the Engineering Support & Services Building (ESSB), where I worked on-site during the first five months of my internship.

CANDU stands for CANada Deuterium Uranium. It is a reactor design that uses heavy water (deuterium oxide, D₂O) as both moderator and coolant, and burns natural (unenriched) uranium fuel. Developed from the late 1950s by Atomic Energy of Canada Limited (AECL), Ontario Hydro, and Canadian General Electric, the CANDU design has several distinguishing features that create fundamental differences from the pressurized water reactors (PWRs) and boiling water reactors (BWRs) used elsewhere in the world. Understanding how CANDU works was essential context for every modification I tracked.

Where PWRs and BWRs enclose the entire reactor core in a single massive steel pressure vessel, CANDU reactors distribute the pressure boundary across 480 individual horizontal pressure tubes per unit (at Darlington). Each tube is made of cold-worked Zr-2.5%Nb alloy with an inside diameter of approximately 104 mm and a wall thickness of approximately 4.2 mm. These tubes sit inside a low-pressure stainless steel calandria vessel filled with heavy water moderator at only about 70°C and near-atmospheric pressure. The annular gap between each pressure tube and its surrounding calandria tube is filled with CO₂ gas for thermal insulation, with four garter spring spacers maintaining separation.

The Primary Heat Transport (PHT) system circulates pressurized D₂O coolant at approximately 310°C and 10 MPa through the fuel channels in a "figure-of-eight" two-loop configuration. Four steam generators per unit transfer heat from this primary D₂O circuit to a secondary light water (H₂O) circuit, producing steam at approximately 260–268°C and 4.7 MPa.

Meanwhile, the moderator system independently maintains the calandria D₂O at approximately 70°C using dedicated pumps and heat exchangers. This separation means the moderator acts as an enormous passive heat sink in accident scenarios, a safety advantage unique to the CANDU design.

Because heavy water absorbs far fewer neutrons than light water, CANDU reactors sustain a chain reaction with natural uranium dioxide (UO₂, only 0.7% U-235), eliminating the need for costly enrichment. The fuel comes in 37-element bundles weighing 23.5 kg each, approximately 495 mm long and 102 mm in diameter. Darlington loads 6,240 fuel bundles per unit across its 480 channels.

Two fueling machines, one at each end of the reactor, perform on-power refueling by attaching to opposite ends of a fuel channel, pressurizing to match the PHT system, removing closure plugs, pushing fresh fuel in from one end while spent fuel exits the other. This eliminates the lengthy refueling outages required by PWRs and BWRs.

Darlington employs two fully independent shutdown systems. Shutdown System 1 (SDS1) uses mechanical shutoff rods held above the reactor by electromagnets. On a trip signal or power loss, the magnets de-energize and gravity drops the rods into the low-pressure calandria. It is fail-safe and fully passive.

Shutdown System 2 (SDS2) injects gadolinium nitrate solution, a powerful neutron absorber, into the moderator, driven by high-pressure helium tanks through independent valves, sensors, and logic. SDS2 is completely independent from SDS1 in design, instrumentation, and actuation.

The Emergency Core Cooling (ECC) system provides multi-phase cooling water injection in a loss-of-coolant accident: high-pressure nitrogen-driven tanks followed by sustained pumped injection.

The containment system comprises multiple barriers: ceramic fuel pellets, Zircaloy cladding, pressure tubes, calandria vessel, reactor building concrete, and the vacuum building. The vacuum building automatically draws radioactive steam inward and condenses it through a gravity-fed dousing system, providing at least seven days of containment hold-up time without any external power.

The Darlington Refurbishment Project is a mid-life overhaul of all four reactors to extend station operation to at least 2055. Formally approved by the Ontario government on January 11, 2016 at a release quality estimate of $12.8 billion CAD, the project involves replacing 1,920 fuel channels (480 per unit), 3,840 feeder pipes, all calandria tubes and end fittings, rehabilitating steam generators, and overhauling turbine-generators.

The primary execution contractor is CanAtom Power Group, a 50:50 joint venture between SNC-Lavalin Nuclear (now AtkinsRéalis) and Aecon Construction Group, under a $2.75 billion retube-and-feeder-replacement contract. Other major contractors include GE Power (turbine work), BWXT Canada (feeder and fuel manufacturing), ES Fox, and Fluor.

Each unit's refurbishment follows a rigorous sequence: breaker open (grid disconnect), defueling of 6,240 fuel bundles over approximately 90 days, islanding (steel bulkhead installation separating the shutdown unit from operating units), containment pressure test, removal series (feeders, end fittings, pressure tubes, calandria tubes), calandria inspection, reassembly with new components, moderator fill, fuel load, and phased commissioning through four CNSC regulatory hold points before return to full power.

During my internship, all four reactor units were in different operational states simultaneously. This is the operational complexity that made my coordination role as SPOC so demanding; modifications had to be scheduled around all of it.

Unit 2 had just returned to service on June 4, 2020, weeks before I started. It was the first unit back from refurbishment.

Unit 3 was the active refurbishment unit during my entire internship. Its start was delayed four months (May to September 2020) to implement COVID-19 safety protocols, a deliberate decision that also kept Unit 3 generating clean electricity during the pandemic's early months. The 12-month window captured the full early refurbishment sequence:

• September 3, 2020: Breaker open. Reactor disconnected from the grid.
• September to November 2020: Defueling completed ahead of schedule. 6,240 fuel bundles removed remotely.
• Late 2020 to January 6, 2021: Islanding completed. 16 steel bulkhead panels (each over 5,000 kg) welded into place, finished 10 days ahead of the 44-day schedule.
• January 14, 2021: Containment pressure test passed, marking the start of the 311-day removal series.
• January to May 2021: All 960 feeder tubes removed from Unit 3.
• May to July 2021: End-fitting removal and preparations for pressure tube/calandria tube extraction advancing.

Despite the pandemic delay, no COVID-19 outbreaks occurred on the project, and once execution began, there was no further schedule impact.

Unit 1 set the world record for continuous nuclear power generation. Online since January 26, 2018, it surpassed the previous record of 962 days (held by India's Kaiga station) on September 15, 2020 and was not taken offline until February 4 to 5, 2021, having run for 1,106 consecutive days.

Unit 4 continued in normal operation.

Medical Isotope Production: OPG, Laurentis Energy Partners, and BWXT announced progress toward producing Molybdenum-99 (Mo-99) at Darlington, positioning it to become the first commercial operating reactor in the world to produce this diagnostic isotope, which is used in over 40 million medical procedures annually. Laurentis also began harvesting Helium-3 from stored tritium, becoming the world's first non-military source of He-3, a strategically important isotope used in neutron detection and quantum computing.

Small Modular Reactors (SMRs): In November 2020, OPG formally announced the resumption of planning for new nuclear at Darlington, the only site in Canada with a completed environmental assessment and site preparation licence for new reactors. OPG conducted deep-dive assessments of shortlisted SMR technologies during this period. The formal selection of GE Hitachi's BWRX-300 came in December 2021, shortly after my internship. By April 2025, the CNSC issued Canada's first-ever licence to construct a grid-scale SMR at Darlington.

Ontario hosts 16 of Canada's 17 operating commercial reactors, spread across three stations.

Darlington (OPG-owned and operated) is the province's newest and most powerful CANDU station with four 878 MWe units totaling 3,512 MWe.

Pickering (also OPG), located 35 km east of downtown Toronto, originally had eight smaller CANDU reactors. Units 5 through 8 continue operating, and in 2025, Ontario approved a $26.8 billion refurbishment of Pickering B with work scheduled to begin in early 2027.

Bruce, on Lake Huron near Kincardine, is the world's largest operating nuclear facility with eight CANDU reactors producing approximately 6,400 MWe. OPG owns the physical assets, but Bruce Power, a private partnership, leases and operates the station. Bruce Power is funding its own $13 billion life-extension program.

OPG's nuclear operations have never harmed a member of the public in over 50 years and have never exceeded regulatory limits on radioactive releases. Living near Darlington adds less than 0.001 mSv per year to natural background radiation exposure, compared to the 1.8 mSv Canadian average. Darlington is the first nuclear station in North America certified under ISO 14001 for environmental management, and in June 2016, WANO named Darlington "one of the safest and top-performing nuclear stations in the world" for the third consecutive time.