12.179. The accident, which occurred in March 1979 near Harrisburg, Pennsylvania, to a PWR of Babcock and Wilcox design, was the result of an unusual combination of operator errors and equipment deficiencies. A lengthy chain of events began with an inability to pump feedwater into the secondary system, which resulted in a turbine trip shortly thereafter. The pressurizer relief valve (§13.16) then opened in response to the rise in primary system pressure but was stuck in the open position, which was unrecognized by the operators. Thus, we had what was comparable to a small-break loss-of-coolant accident.

12.180. As a result of operator confusion, the coolant inventory was further reduced by their throttling the high-pressure injection flow and draining water through the letdown line in the system used to regulate the coolant boron concentration. Boiling in the core as a result of the reduced pressure caused the operators to shut down the primary circulating pumps, leading to further steam buildup, core uncovering, and severe core damage. Earlier, overpressure on a tank receiving discharge from the pressurizer led to a minor radioactive release to an auxiliary building and then to the environment through a vent stack.

12.181. The loss of coolant was finally halted after several hours and the core was subsequently reflooded. However, it was still difficult to cool the core with the primary pumps because of the large quantities of steam and some hydrogen in the system as well as the degraded core geometry. A number of “feed and bleed” maneuvers in which water would first be injected and then the pressure reduced proved helpful. The operators were then able to activate a primary pump and achieve a relatively stable con­dition after about 16 hours.

12.182. Feed-and-bleed cooling as an operator option has since been modeled extensively with the aid of such system codes as MAAP [16]. These analyses provide a picture of the response of a given system should feedwater be unavailable and depressurization be attempted by alterna­tively injecting emergency coolant and bleeding off inventory through the pressurizer. Such a picture provides guidance for the emergency meas­ures required, particularly the time “window” that may be available for countermeasures.