The Unique Maintenance Demands of CANDU Reactors

CANDU reactors, a Canadian-designed pressurized heavy water reactor, differ fundamentally from light water designs. Instead of a single large pressure vessel, they use hundreds of horizontal fuel channels within a calandria containing heavy water moderator. This distributed core, combined with on-power refueling, provides outstanding neutron economy and fuel flexibility but also creates maintenance challenges not found in other reactor types. Fuel channels must be inspected for creep, deformation, and garter spring displacement. Thousands of pressure tubes, feeder pipes, and steam generator tubes demand periodic assessment. The sheer number of welds, valves, and rotating equipment dwarfs that of a light water plant.

Adding to the complexity is the radiation environment. Tritium produced by neutron absorption in heavy water migrates into carbon steel systems, creating a contamination hazard that makes personnel entry into the reactor vault or primary coolant loop extremely controlled. Traditional manual inspections involved extensive planning, protective suits, dose tracking, and limited time windows. Even routine tasks like monitoring vibration on a primary heat transport pump required careful scheduling. The result was either conservative maintenance intervals that increased cost or, conversely, delayed detection of incipient failures that escalated into forced outages.

The industry has long sought to minimize human presence in high-radiation areas while maintaining the data richness needed for advanced condition assessment. Remote monitoring has emerged as the natural answer, leveraging materials science, telecommunications, and artificial intelligence to bring the diagnostic center to a desk rather than a worker to a danger zone. The specific maintenance demands can be categorized into several key areas:

  • Fuel channel integrity: Creep and sag of pressure tubes, garter spring migration, and hydride blister formation require periodic measurement. Underestimation of these phenomena can lead to pressure tube rupture, a beyond-design-basis event.
  • Heat transport system: Pumps, valves, and steam generators need vibration analysis, leak detection, and thermal performance monitoring. Bearing wear in primary pumps can propagate to entire system outages if not caught early.
  • Moderator and calandria: Heavy water chemistry, cover gas purity, and structural aging of the calandria shell must be tracked. Any leak in the calandria tubes can contaminate the moderator with fission products.
  • Cable and conduit systems: Aging insulation, connection resistance, and fire barrier integrity are safety-critical. Deteriorated cables can cause spurious safety system actuations or prevent essential signals from reaching the control room.
  • Ancillary systems: Cooling water, air handling, and fire suppression equipment all require condition monitoring to ensure overall plant reliability.