One of the design basis fault conditions for a PWR listed in Chapter 4 is rupture of a steam generator tube. Such an event occurred at the R. E. Ginna PWR sta­tion in New York State on January 25, 1982. The Ginna station is based on a two-loop Westinghouse PWR. At 9:25 a. m. the plant was operating at 100% power [490 MW(e)]. Shortly thereafter the primary reactor coolant system pres­sure dropped significantly, followed by nearly simultaneous activation of the HPIS, reactor trip-turbine trip, and containment isolation. The pressurizer went almost empty. This is the behavior expected when a steam generator tube bursts (ruptures), allowing primary circuit water to pass into the (lower-pres­sure) secondary side of the steam generators. Following standard procedures for responding when it is suspected that a steam generator tube has ruptured, the operators tripped the main coolant pumps and closed the main steam isola­tion valves on the suspect steam generator.

The operators opened the PORV connected to the pressurizer in order to equalize the primary and secondary circuit pressures quickly and stem the leak.

This action allowed reactor coolant to drain into the pressurizer relief tank. However, when this operation was completed and an operator tried to close the PORV again, it failed to close (as at TMI-2), requiring the operator to shut the block valve and thus isolate the flow, which he did promptly. The depres­surization resulting from the opening of the PORV caused flashing in the pri­mary circuit, pushing water into the pressurizer and causing a void to form in the top part (upper head) of the reactor vessel. This situation was recognized and rectified by starting a main circulating pump some 2 h after the start of the incident. No excessive fuel temperatures were noted.

The operation of the PORV caused the pressurizer relief rupture disk to blow and some 5,000-10,000 gallons of water drained into the sump of the contain­ment building, which had been isolated by this stage. During this time, the damaged steam generator was isolated on the secondary side and the pressure in this steam generator went up to the point where the secondary relief valve lifted. This resulted in a minor radioactive release to atmosphere, mainly of krypton and xenon.

The plant was subsequently cooled down, first by using the undamaged steam generator to remove the residual heat and then, after about 24 h, by the low-pressure residual heat removal system.

Subsequent inspection of the damaged steam generator showed that a loose pie-shaped metal object, weighing about 2 lb, was present in the steam gener­ator. This object had vibrated and severely damaged a number of steam gener­ator tubes, causing one of them to rupture and leading to the events described above. The object appears to have been present in the steam generator for a number of years, having been introduced inadvertently during earlier mainte­nance work. Flow through the damaged tubes was blocked by plugging them, and the unit has been returned to power.

Unlike the accident at Three Mile Island, the operator response at Ginna was good, although somewhat delayed compared with the operating guidelines for this type of incident. Although the Ginna incident has received the most pub­licity, it is noteworthy that steam generator tube ruptures had occurred previ­ously, one example being an incident in P’^T. Unit No. 2 at Prairie Island, Oregon, on October 2, 1979.