Summary of "CANDU 101 Part 1: Introduction- A Little Bit About the CANDU Core Lunch and Learn"
Overview
This document summarizes Part 1 of a multi-part introductory series on CANDU reactors presented by WIN (Women in Nuclear) — Durham chapter. The session combines CANDU history, basic reactor physics, core design features, and a survey of major Canadian reactors and design evolution. Presenters highlighted Canadian contributions to nuclear technology (research reactors, commercial CANDU development, medical isotopes) and explained why specific design choices were made.
Key concepts
- CANDU = CANada DEUterium Uranium: a heavy-water–moderated reactor design that can use natural uranium fuel.
- Heavy water (D2O): water in which hydrogen atoms are replaced by deuterium (one proton + one neutron). D2O is an efficient neutron moderator, slowing fast neutrons to thermal energies and increasing the probability of fission in fissile isotopes.
- Deuterium and tritium:
- Deuterium (D) — one proton + one neutron; used in heavy water.
- Tritium (T) — one proton + two neutrons; produced in reactors and used in niche applications (self-powered lighting, some signs).
- Natural uranium: contains about 0.7% U-235 by mass (the transcript contained an apparent error quoting “7%”). CANDU reactors can operate on natural uranium because the D2O moderator has low neutron absorption.
- Moderation/thermalization: slowing neutrons to thermal energies raises the fission probability for U-235; moderators (D2O in CANDU) accomplish this.
Historical timeline (major milestones)
- Late 1800s–1930s: foundational science — Rutherford, Chadwick (neutron), discovery/description of fission (Lise Meitner and others).
- WWII era: heavy water interest; allied research and the Montreal Project advanced Canadian heavy-water research.
- ZEEP (Zero Energy Experimental Pile), Chalk River (1945): first Canadian experimental heavy-water pile; reached a self-sustaining chain reaction in Sept 1945 and provided data (e.g., lattice spacing) used in later designs.
- NRX (National Research Experimental reactor) — first critical in 1947; experienced a major accident in the early 1950s; provided lessons for subsequent reactors.
- NRU (National Research Universal reactor) — first critical in 1957; high neutron flux, major isotope producer (Mo-99, Co-60); operated until 2018.
- NPD (Nuclear Power Demonstration) — first CANDU prototype for electricity (operational 1962–1987); introduced pressure-tube/fuel-channel layout and zirconium-clad UO2 fuel.
- Douglas Point (1966–1984): first full-scale commercial CANDU prototype.
- Major commercial scale-up: Pickering, Bruce, Darlington, Gentilly, Point Lepreau, Wolsong (Korea), Cernavoda (Romania), and exports to several countries.
- Evolution to CANDU 6, conceptual CANDU 9, and ongoing interest in SMRs and new projects.
Design features and technical innovations
Moderator and core geometry
- Heavy-water moderator housed in a calandria (a vessel containing D2O).
- Lattice pitch: early experiments (ZEEP) suggested an approximate optimum center-to-center spacing (~30 cm) for fuel rods.
- Calandria tubes separate fuel channels from the moderator; an annulus gas gap thermally isolates the moderator from hot fuel/channels.
Fuel and fuel-channel design
- Transition from uranium metal (research reactors) to uranium oxide (UO2) pellets for power reactors for improved high-temperature performance.
- Fuel pellets assembled into bundles (“pencils” into bundles); cladding is zirconium alloy (zircaloy) for low neutron absorption and good corrosion/high-temperature properties.
- Pressure-tube / fuel-channel architecture (vs. large pressure vessel) enabled large plants to be built in Canada and allowed on-power refuelling.
- On-power refuelling: fueling machines insert/remove bundles while the reactor is at power; shared fueling machines are used on larger units to reduce exposures.
Reactivity control and shutdown
- Shut-off rods (gravity drop) used as a primary shutdown mechanism.
- Moderator dump (draining heavy water) was an early shutdown method (later considered slow).
- Liquid zone control: zones of light water within the moderator used to adjust local flux (liquid acts as a neutron absorber).
- Gadolinium injection used in later designs as a rapid secondary shutdown (gadolinium is a strong neutron absorber).
- Boron has been used in some absorber rods in research cores.
Heat transport and layout
- Heat transport system: heavy water circulates through fuel channels, transfers heat to steam generators, and returns via inlet/outlet headers (figure-8 loop layout described).
- Steam generators move heat from the heavy-water loop to the light-water/steam cycle driving turbines.
- Pressurizer manages system pressure and accommodates density changes (shrink/swell) in the heat transport system.
- Feed-and-bleed and heavy-water recovery systems minimize D2O losses.
Control, instrumentation, and safety
- Digital control/computer control: Douglas Point used a stored-program digital computer to position a reactivity control element; Pickering was the first station with full computer control.
- Separation of safety systems from normal-operation systems, with testability at full power.
- Thermal siphoning/natural convection paths for decay heat removal if pumps fail.
- Emergency core injection systems and containment evolved (vacuum buildings to single large containments in later designs such as CANDU 6+).
Operational and program features
- On-load refuelling (world-first demonstrated in NRU and later used in power CANDUs).
- Use of enriched “booster” rods to manage reactivity transients and xenon poisoning.
- Modular scaling: multi-unit sites (Pickering, Bruce) and single-unit designs (CANDU 6) to match customer needs.
Products and outputs
- Significant producer of medical isotopes (Mo-99, Co-60).
- Early Canadian projects provided uranium and participated in research relevant to plutonium production (military-related exports ceased by the 1960s).
Concrete data and claims from the talk
- ZEEP: self-sustaining chain reaction in Sept 1945; lattice pitch found around ~30 cm.
- NRX: first critical 22 July 1947; initial design 10 MW(t) later increased (transcript cites up to 42 MW); serious 1950s accident; in service until 1993.
- NRU: critical 22 July 1957; thermal output up to ~200 MW (varied in later operation); ran until March 31, 2018; major isotope producer.
- NPD: began operation in 1962.
- Douglas Point: commercial prototype operating 1966–1984.
- Large multi-unit stations (Pickering, Bruce, Darlington) now supply a large fraction of Ontario’s electricity (presenter cited “over 50–60%”).
Note: some numeric items in the transcript appear mis-transcribed (for example, the presenter was quoted as saying “7% U-235 in natural uranium,” which conflicts with the well-known ~0.7% value). Where exact numeric precision matters, consult the original slides or recording.
Lessons and broad takeaways
- CANDU design choices (heavy-water moderator, pressure tubes, on-power refuelling) reflect resource availability (Canada’s natural uranium), manufacturing constraints, and a desire for operational flexibility.
- Experimental reactors (ZEEP, NRX, NRU) were essential stepping stones that validated core physics, lattice spacing, and moderator performance for commercial designs.
- Iterative evolution: each station (NPD → Douglas Point → Pickering → Bruce → Darlington) increased scale and introduced improved safety, control, and heavy-water management features.
- Canadian nuclear technology had significant global impact through reactor exports and isotope supply and is considered a major national engineering achievement.
- The CANDU family continued evolving (CANDU 6, CANDU 9 concepts) and Canada is exploring SMRs and future isotope production priorities.
Suggested future topics (from the session)
- Reactor auxiliaries and detailed control systems
- Fuel design and behavior
- Detailed CANDU power control methods
- Waste management and small modular reactors (SMRs)
- Medical isotope production and its supply chain
Notes on transcript quality and likely transcription errors
- Auto-generated subtitles contained many name and number errors (examples: Otto Hahn, Fritz Strassmann, Lise Meitner, Enrico Fermi were distorted in the transcript; other names/dates/numbers may also be wrong).
- When precise figures (enrichment percentages, power ratings, dates) are important, consult the original slides or recording for authoritative values.
Speakers and referenced historical figures
Live presenters / session participants
- Lisa Heath — Vice-chair, WIN Durham; long career at Ontario Power Generation (session lead/presenter).
- Britney Morazz — WIN Durham communications officer (session moderator / QR-code contact).
- Amarie (transcribed as “Amry” / “Amarie”) — former Pickering operator, licensed and later ATS; spoke briefly.
- Christa Huzeric (transcribed Huseric / Huzeric) — OPG engineer, reactor safety, control room supervisor, ATS; spoke briefly.
- Other WIN panel members were present but not introduced in detail.
Historical scientists / project figures referenced
- Ernest Rutherford
- James Chadwick
- Otto Hahn and Fritz Strassmann
- Lise Meitner
- Hans von Halban
- Lew Kowarski
- Frédéric and Irène Joliot-Curie (and references to Marie & Pierre Curie)
- George Laurence (Canadian researcher)
- Enrico Fermi
- Institutions/projects referenced: ZEEP, NRX, NRU, Montreal Project, Manhattan Project
End of summary.
Category
Educational
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