Three Mile Island, March 28, 1979

4:00am	The water pumps providing water to the generator stopped and three emergency pumps automatically started.

	Within minutes, 100 alarms were sounded and the plant's paging system was spouting instructions to the controllers.

	The operators did not notice that the valves on two of the emergency lines that were normally open were closed, blocking the flow of water to the core.  One light was covered by a yellow maintenance tag, and no one knows why the other was missed.

	A relief valve in the core should have been shut to contain the raising steam, but it remained open.  However, a light on the control panel indicated that it was closed.

4:01.45	The steam generators boil dry.

4:05.30	Steam bubbles began forming in the cooling system.

	Now more water was leaving the system through the relief valve then was entering.  Consequently, the core was being uncovered.	

4:08	The operators realize that no water is reaching the core.

	An operator scanned the panel for the status of the valves on the emergency water system, notices that they were closed and opened them.

4:11	An alarm sounds warning of high water in the reactor building sump.

	This would indicate a leak or break in the system.

4:15	A disc on the drain tank burst sending more radioactive water to the sump.

4:20	Indicators show a higher than normal radioactivity level inside the reactor room.

5:00	The four reactor cooling pumps start to vibrate.

5:27	The pumps are shut off, stopping the forced flow of water through the core.

6:22	An operator observed that the block valve that was normally closed, was open and shut the valve.

	The block valve backs up the relief valve, which was still open, but by closing the block valve the flow of water escaping the reactor was stopped.

The problem of the open relief valve was solved in two hours and twenty-two minutes.

Approximate cost to the parties involved: $2 Billion.

Source: Bailey Human Performance Engineering: A guide for System Designers, Prentice-Hall