Most commercial pressurized water reactors with alloy 600 steam generator tubes are susceptible to stress-induced corrosion at locations such as the tube sheet transition, the tube-to-tube support structure interface, U-bend regions with high localized stresses, and to a lesser extent, free-span locations between supports where deposits or manufacturing defects have caused accelerated local attack. Under postulated main steam-line break (MSLB) accident conditions (and in rare instances during normal operation), some leakage of reactor coolant inventory through these cracks occurs. The result is an iodine source term to the environment.

A simplified probabilistic iodine release model has been developed that is different from previous conservative deterministic models, which were developed for the routine steam generator tube rupture analysis, which is performed as part of a plant's safety analysis. The model described herein was developed to calculate the probability that the iodine release for MSLB-induced steam generator leakage will result in thyroid and whole body doses that do not exceed the criteria in 10CFR100 for the projected condition of the plant's steam generator tubes after a specified period of full-power operation.

This simplified probabilistic model treats the intrinsic statistical nature of the projected population of degraded tubes, the probability of leakage for multiple degradation mechanisms, and the probability distributions for iodine release for a preexisting spike and a coincident spike.

Results from applying this methodology to data from a plant with substantial steam generator degradation indicate that steam generators with multiple degradation mechanisms can be operated safely for normal operating cycles. Safely, in this case, means without a significant probability of exceeding thyroid and whole body dose criteria under normal operation and postulated accident conditions.