Predicting Fuel Channel Deformation in Heavy Water Reactors
Atomic Energy Canada Ltd. (AECL) is hosting an IAEA ICSP (International Collaborative Standard Problem) on ‘Heavy Water Reactor Moderator Subcooling Requirements to Demonstrate Backup Heat Sink Capabilities of the Moderator during Accidents’.
"An important safety feature of pressurized heavy water reactor is the ability to use the moderator as a backup heat sink during emergencies", explained Jong Ho Choi from the IAEA's Nuclear Power Technology Development Section.
In a number of postulated loss-of-coolant accident scenarios, with or without coincident loss-of-emergency core cooling, the fuel may overheat and transfer heat to the pressure tube. As the pressure tube overheats, it loses strength, plastically deforms and balloons into contact with the surrounding calandria tube. At the time of contact, the calandria tube experiences a large increase in heat flux at the contact points as stored heat is rejected from the pressure tube to the cooler calandria tube.
If the heat flux on the outer surface of the calandria tube exceeds the critical heat flux, film boiling (dryout) may occur on the surface of the calandria tube. If the area of dryout is sufficiently large and the dryout is prolonged, the pressure-tube/calandria-tube combination can continue to strain radially and may challenge fuel-channel integrity. The moderator subcooling limits required to avoid such dryout conditions are defined by the contact boiling curve. The boundary between immediate quench and patchy film boiling defined the moderator subcooling limits for CANDU reactors.
The first workshop to share experiences and establish an international collaboration scheme for the ICSP was held on 19-21 November 2013 in Ottawa, Canada. Ten organizations from Canada, India, the Republic of Korea, Pakistan, and Romania including research institutes, regulatory bodies, utilities and engineering companies are participating in this ICSP with several computer codes.
More detailed information on this ICSP is available here.