The Three Mile Island nuclear accident was probably the most serious nuclear accident ever in the United States of America. The power station had two pressureised water reactors. At 4 am on 28 March 1979 reactor number 2 was operating at 97% power. A sequence of events started with a minor malfunction in the secondary cooling circuit. The temperature in the primary circuit started to rise which caused the reactor to shut down automatically.
The pilot-operated relief valve (PORV) on the reactor cooling system opened. About 10 seconds later it should have closed, and in fact the instruments implied that a siganal had been sent to close it - but it remained open, leaking coolant water to the reactor coolant drain tank.
Responding to the loss of cooling water, high-pressure injection pumps automatically pushed replacement water into the reactor system. As water and steam escaped through the relief valve, a surge of cooling water rtaised the water level.
Operators responded by reducing the flow of replacement water. Because the water level was increasing, they thought the reactor system was too full of water. If it filled, they could not control pressure in the cooling system and it might rupture.
Steam then formed in the reactor primary cooling system. Pumping a mixture of steam and water caused the reactor cooling pumps to vibrate. Severe vibrations could have damaged the pumps and made them unusable, so operators stopped the pumps. This ended forced cooling of the reactor core. As coolant water boiled away, the reactors fuel core was uncovered and became even hotter, damaging the fuel rods and leaking radioactive material into the cooling water.
At 6:22 am operators closed a block valve. This action stopped the loss of coolant water through the relief valve but superheated steam still blocked the flow of water through the cooling system. Operators at first tried to force more water into the system to condense the steam. By 7:50 pm on 28 March, they restored forced cooling of the reactor core when they were able to restart one reactor coolant pump. They had condensed steam so that the pump could run without severe vibrations.
Radioactive gases from the reactor cooling system built up in the makeup tank in the auxiliary building. During March 29 and 30, operators used a system of pipes and compressors to move the gas to waste gas decay tanks. The compressors leaked, and some radioactive gas was released to the environment.
When the reactor's core was uncovered, on the morning of 28 March, a high-temperature chemical reaction between water and the zircaloy metal tubes holding the nuclear fuel pellets had created hydrogen gas. In the afternoon of 28 March, a sudden rise in reactor building pressure shown by the control room instruments indicated a hydrogen burn had occurred. Hydrogen gas also gathered at the top of the reactor vessel.
It was feared the hydrogen could explode so between 30 March and 1 April operators removed this hydrogen gas "bubble" by periodically opening the vent valve on the reactor cooling system pressuriser.
After an anxious month, on 27 April operators established natural convection circulation of coolant. The reactor core was cooled by the natural movement of water rather than by mechanical pumping. The plant was in "cold shutdown", ie with the water at less than 100°C at atmospheric pressure.