Fuel Channel Design Improvements for Large CANDU Reactors

Abstract

From the initial designs used in NPD and Douglas point reactors, the CANDU fuel channel and its components have undergone considerable development. Two major designs have evolved: the Pickering/CANDU 6 design which has 12 fuel bundles in the core and where the new fuel is inserted into the inlet end, and the Bruce/Darlington design which has 13 bundles in the channel and where new fuel is inserted into the outlet end. In the development of a single unit CANDU reactor of the size of a Bruce or Darlington unit which would use a Darlington design calandria, the decision has been made to use the CANDU 6 fuel channel rather than the Darlington design. The CANDU 6 channel has provided excellent performance and will not encounter the degree of maintenance required for the Bruce/Darlington design. The channel design in turn influences the fuelling machine/fuel handling concepts required. The changes to the CANDU 6 fuel channel design to incorporate it in the large unit are small. In fact, the changes that are proposed relate to the desire to increase margins between pressure tube properties and design conditions or ameliorate the consequences of postulated accident conditions, rather than necessary adaptation to the larger unit. Better properties have been achieved in the pressure tube material resulting from alloy development program over the past 10 years. Pressure tubes can now be made with very low hydrogen concentrations so that the hydrogen picked up as deuterium will not exceed the terminal solid solubility for the in-core region in 30 years. The improvements in metal chemistry allow the production of high toughness tubes that retain a high level of toughness during service. Life limits due to material inadequacy from service degradation of properties are unlikely, and the life limits will be set by dimensional changes. A small increase in wall thickness will reduce the dimensional changes without significantly affecting burnup. Changes to increase safety margins from postulated accidents are concentrated on containing the consequences of pressure tube damage. The changes are concentrated on the calandria tube and the annulus gas bellows. The calandria tubes have been modified to fail at a higher pressure to contain the heat transport system pressure until the reactor cools down. The inside surface is conditioned to increase heat absorptivity so that the pressure tube cannot overheat as much subsequent to a loss of coolant accident. The calandria tube inside surface is roughened or textured to ensure that pressure tube contact from a LOCA/LOECC will result in optimum heat transfer to the moderator and prevent pressure tube rupture. The annulus bellows will be strengthened to limit the consequences of a postulated pressure tube break by preventing a loss of heavy water into the reactor vault and also prevent a loss of moderator if the calandria tube were to fail.