Sag occurs by irradiation creep from the weight of the fuel and heavy water in the pressure tube. Gross sag deformation of the fuel channel is primarily controlled by the relatively cool calandria tube. There are several limits to pressure tube/fuel channel sag that must be monitored:

• Calandria tube contact with horizontal structures that are perpendicular to the fuel channels (such as the liquid injection shutdown nozzles and horizontal flux detector guide tubes);

• Pressure tube to calandria tube contact leading to blister formation;

• Pressure tube sag leading to fuel bundle passage problems.

To maximize the life of the channels with respect to potential contact with horizontal mechanisms, the current strategy is to perform in reactor gap measurements so as to determine when contact could occur and to identify which channels would be affected. To address channels predicted to be in contact prior to the design life, the following remedial actions could be implemented depending on when contact is predicted to occur relative to the design life:

Perform testing to demonstrate that fretting between the components would be acceptable,

Defuel the channel in contact with the liquid injection nozzle or the horizontal flux detector to remove contact,

Adjust the tension on the liquid injection nozzle to increase the sag rate of the nozzle, Replace the liquid injection nozzle with an offset design, and Replace the calandria tube

Potential pressure tube to calandria contact resulting from movement of the loose fitting spacers is addressed by inspections to detect the spacers and reposition them, if required. To ensure that pressure tube to calandria tube contact does not occur, the repositioned spacers must be both adequately loaded so that they do not move after being repositioned and appropriately located so that the pressure tube will not sag onto the calandria tube.

Fuel bundle passage, as proven by tests using predicted end of life curvature is not impaired during the fuel channel design life.