Within the reactor vessel, these pressure waves were approaching the frequency at which a piece of equipment resonated. Roughly halfway down the interior surface of the vessel was a titanium fitting, part of the backup cooling system. In the event of a coolant loss, and after a successful SCRAM, valves inside and outside the vessel would open, cooling the reactor either with a mixture of water and barium or, as a last measure, with seawater which could be vented in and out of the vessel—at the cost of ruining the entire reactor. This had been done once, and though it had been costly, the action of a junior engineer had prevented the loss of a Victor-class attack sub by catastrophic meltdown.

Today the inside valve was closed, along with the corresponding through-hull fitting. The valves were made of titanium because they had to function reliably after prolonged exposure to high temperature, and also because titanium was very corrosion-resistant—high-temperature water was murderously corrosive. What had not been fully considered was that the metal was also exposed to intense nuclear radiation, and this particular titanium alloy was not completely stable under extended neutron bombardment. The metal had become brittle over the years. The minute waves of hydraulic pressure were beating against the clapper in the valve. As the pump's frequency of vibration changed it began to approach the frequency at which the clapper vibrated. This caused the clapper to snap harder and harder against its retaining ring. The metal at its edges began to crack.

A michman at the forward end of the compartment heard it first, a low buzz coming through the bulkhead. At first he thought it was feedback noise from the PA speaker, and he waited too long to check it. The clapper broke free and dropped out of the valve nozzle. It was not very large, only ten centimeters in diameter and five millimeters thick. This type of fitting is called a butterfly valve, and the clapper looked just like a butterfly, suspended and twirling in the water flow. If it had been made of stainless steel it would have been heavy enough to fall to the bottom of the vessel. But it was made of titanium, which was both stronger than steel and very much lighter. The coolant flow moved it up, towards the exhaust pipe.