Editor Maureen McQueen has over 40 years ' professional radiation protection experience in the UK, USA and Canada.
CANDU reactors are Pressurised Heavy Water Reactors( PHWR) invented in Canada, which are fuelled by natural uranium and cooled and moderated by deuterium, hence the name CANDU( CANada Deuterium Uranium). The lack of dependence on the enrichment of uranium to create nuclear power has resulted in the sale of the CANDU reactor to many countries around the world. There are 34 CANDU reactors across the world, with the first one( Bruce Unit 2, Canada) licensed to operate in 1976.
In the design of the CANDU reactor, the fuel sits horizontally in 380 or 480 fuel channels which are cooled by heavy water fed to the fuel channels by“ feeder” pipes which transport the water to and from the steam generators forming the primary circuit of the reactor. The fuel channels sit in an array inside a large tank of heavy water( called the calandria). The heavy water moderates the neutrons and the very specific spacing of the fuel channels allows criticality. It has always amazed me that this specific design allows criticality of natural uranium!
Schematic of a CANDU reactor To date in the world, no CANDU reactor has been fully decommissioned. As the reactor fleet has aged, the practice of“ retubing” and refurbishment of the entire reactor has been practised. Retubing changes out all of the major components of the reactor( all of the fuel channels and feeders) for brand new components, allowing the reactor life to be extended for the same or greater duration as the original reactor design. Refurbishment of the reactor can also include changing major
Key and Characteristics 1. Fuel 2. Calandria 3. Adjuster rods( control nuclear reaction) 4. Heavy water( tritiated) 5. Steam Generator 6. Light water 7. Pump 8. Fuelling Machine 9. Moderator – heavy water 10. Fuel channel 11. To Turbine( not significant) 12. From Turbine 13. Containment
components, such as reactor coolant pumps, steam generators and turbine parts.
In Canada, the Point Lepreau and Bruce Units 1 and 2 were the first reactors to be retubed and refurbished, returning to service in 2012. This was followed by several other Canadian based reactors and also CANDU reactors in Argentina and Korea. In total to date ten CANDU reactors worldwide have been retubed leading to ongoing use of these original nuclear power plants for another 25-30 years. An additional 12 refurbishments are planned worldwide and yet to occur.
From a radiation protection perspective, the work of retubing a reactor is significant. Prior to the retube, all of the tritiated water is drained from the primary circuit and from the calandria using specially designed de-watering rigs, with potential exposure to tritium in significant concentrations( the tritium concentration in the CANDU moderator is around 400 GBq / kg). The irradiated core reactor components, with dose rates up to hundreds of Sv / h, have to be
34 Radiation Protection Today www. srp-rpt. uk