On September 1, 1972, a routine start-up operation was proceeding on the Mill­stone 1 reactor in Connecticut. This reactor was a 660-MW(e) B^^. When the reactor had achieved less than 0.1% of full power, the operator noted that the water purification system was malfunctioning. He switched to a second water purification system and continued the start-up. About half an hour later the sec­ond system also failed and the operator began to shut down the reactor. When it became obvious that salt from seawater was penetrating the primary coolant circuit, the reactor was tripped rapidly. Upon investigation it was found that tubes in the condenser (which was cooled by seawater) had corroded, allowing a massive amount of seawater to enter the primary circuit. One consequence of the saltwater ingress was the failure of the instruments that measured power in the reactor; the failure was due to stress corrosion cracking of the stainless steel sheaths of the instruments, which are sensitive to chloride attack.

The reactor was successfully repaired and resumed operation. Although this

accident caused no injuries and no radioactivity was released, it demonstrates the relative vulnerability of direct-cycle systems such as the B^^ in comparison with indirect-cycle systems such as P^WR CANDU, or AGR. In a B^^ the pri­mary circuit coolant passes directly to the turbine and is condensed in the con­densers before returning to the reactor. If the condensers are cooled by seawater, ingress into the primary circuit is always a potential problem. One way to overcome this is to isolate the condensers in the event of leakage of sea­water, but this leads to loss of the main heat sink and a need to provide alter­native cooling or means of energy release.