The design of fuel channels has taken into consideration the following factors related to pressure tube diametral expansion and wall thinning due to creep and growth:

• Stress

• Creep ductility

• Flow by-pass

• Spacer nip up (no gap between the pressure tube, calandria tube and spacer)

Diametral expansion occurs mainly by irradiation creep. For operating reactors, stress analyses to address strength requirements have been performed for operation of pressure tubes to 5% diameter increase and 0.368 mm wall thinning. Based on data from in-reactor experiments 5% is considered to be a very conservative limit with respect to creep rupture and creep ductility.

Results from diameter measurements from several plants suggest that the fastest creeping pressure tubes are experiencing an upperbound diametral expansion rate of about 0.2% per 7000 EFPH. Based on this upper bound rate, the following is predicted for the fastest creeping pressure tubes:

• Nip-up will occur before design life

• Diametral strain of 5% will be reached before design life.

However, several plants have lower diametral strain rates because of the lower operating temperature and flux. Data from these reactors currently suggest that the maximum diametral strain will not exceed 5% during the design life.

It is also recognized that the measured pressure tube diametral expansion rates will result in flow by-pass and a reduction in margins on cooling capability for the fuel. Because the operation of a unit depends upon the prediction of the maximum pressure tube diameter in the core, there may be a need to obtain additional data on high power channels to more precisely determine the distribution of diameters in each unit and to identify the fast creeping pressure tubes such that remedial action can be taken, if required.

Life management strategies therefore have been developed to evaluate the need for increased inspections. As the units age, these inspections enable the variability in diametral expansion to be more precisely quantified, in order to address the following:

• Strength and creep ductility requirements for operation with diametral strains greater 5%.

• Coolant flow bypass around the fuel bundles for diametral strains greater than 5%.

• Operating in a “nipped-up” condition (i. e. with no gap between the pressure tube, calandria tube, and spacer).