The outward-moving water carried the clapper into the pipe, which had a fifteen-centimeter inside diameter. The pipe was made of stainless steel, two-meter sections welded together for easy replacement in the cramped quarters. The clapper was borne along rapidly towards the heat exchanger. Here the pipe took a forty-five-degree downward turn and the clapper jammed momentarily. This blocked half of the pipe's channel, and before the surge of pressure could dislodge the clapper too many things happened. The moving water had its own momentum. On being blocked, it generated a back-pressure wave within the pipe. Total pressure jumped momentarily to thirty-four hundred pounds. This caused the pipe to flex a few millimeters. The increased pressure, lateral displacement of a weld joint, and cumulative effect of years of high-temperature erosion of the steel damaged the joint. A hole the size of a pencil point opened. The escaping water flashed instantly into steam, setting off alarms in the reactor compartment and neighboring spaces. It ate at the remainder of the weld, rapidly expanding the failure until reactor coolant was erupting as though from a horizontal fountain. One jet of steam demolished the adjacent reactor-control wiring conduits.

What had just begun was a catastrophic loss-of-coolant accident.

The reactor was fully depressurized within three seconds. Its many gallons of coolant exploded into steam, seeking release into the surrounding compartment. A dozen alarms sounded at once on the master control board, and in the blink of an eye Vladimir Petchukocov faced his ultimate nightmare. The engineer's automatic trained reaction was to jam his finger on the SCRAM switch, but the steam in the reactor vessel had disabled the rod control system, and there wasn't time to solve the problem. In an instant, Petchukocov knew that his ship was doomed. Next he opened the emergency coolant controls, admitting seawater into the reactor vessel. This automatically set off alarms throughout the hull.

In the control room forward, the captain grasped the nature of the emergency at once. The Politovskiy was running at one hundred fifty meters. He had to get her to the surface immediately, and he shouted orders to blow all ballast and make full rise on the diving planes.

The reactor emergency was regulated by physical laws. With no reactor coolant to absorb the heat of the uranium rods, the nuclear reaction actually stopped—there was no water to attenuate the neutron flux. This was no solution, however, since the residual decay heat was sufficient to melt everything in the compartment. The cold water admitted into the vessel drew off the heat but also slowed down too many neutrons, keeping them in the reactor core. This caused a runaway reaction that generated even more heat, more than any amount of coolant could control. What had started as a loss-of-coolant accident became something worse: a cold-water accident. It was now only a matter of minutes before the entire core melted, and the Politovskiy had that long to get to the surface.