Misleading behavior of a safety valve quickly compounds the problem.

1. Secondary loop boils down to a low level, superheating the primary loop.

2. Pressure rises above 2350 psi, opening safety valve to quench tank. Radioactive steam and water move to quench tank.

3. Valve tries to close when pressure drops below 2300 psi, but doesn't seat. However, it closes enough to trip the indicator in the control room, so operators think it is closed.

- With the cooling effect of the restored feedwater and the stuck relief valve, the reactor pressure drops quickly.

- At 1600 psi the Emergency Core Cooling System kicks in automatically, flooding the core with high pressure water.

- Even though the reactor has "scrammed" and the control rods have stopped the fission chain reaction, the residual reactions still produce 5-7% of full heating power. For an 800 MW electrical, 2400 MW thermal reactor, this amounts to 120-170 Megawatts of heating power.

- The operators were afraid that the water level would get too high, that the reactor would "go solid". If the water filled the reactor, it would act like a hydraulic press and force the reactor lid off.

- After over an hour of heating, the coolant pumps were shut off, worsening the already bad situation.

- Two-thirds or more of the 12 foot core was uncovered. At 150 MW of heating it would take only 30 seconds for fuel melting to start if all cooling was lost.

- A large hydrogen "bubble" was formed at the top of the reactor by reaction between the zirconium fuel rod cladding and the superheated water.