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Meltdown at Three Mile Island | Article

What Happened?

What exactly happened within Pennsylvania's Three Mile Island plant? 

The time is just before 4am. The reactor is currently running smoothly. 

Time elapsed...

0 sec
The pumps in the secondary loop shut down automatically, due to a slight malfunction.

1 sec
Alarms sound within the control room.

2 sec
Because heat is no longer being transferred to the secondary loop, water pressure and water temperature in the primary loop rise. This is normal and no cause for concern.

3 sec
The PORV (pressure relief valve) opens automatically, releasing steam into a holding tank.

4 sec
Backup pumps within the secondary loop automatically turn on. However, these pumps are disconnected from the system by cutoff valves. Operators assume that the pumps are doing what they're supposed to do.

9 sec
The control rods , made of boron and silver, are lowered, which slows down the nuclear chain reaction within the core. Heat is still generated, though. In this state, the core produces enough energy to light 18,000 homes.

The PORV light goes out, indicating that the valve is now closed. The valve is actually still open.

Steam and water continues to be released through the PORV, creating a LOCA (loss of coolant accident).

2 min
The EIW (emergency injection water) is activated, and water flows into the primary loop. This is a safety device, designed to keep the water at a safe level in the event of a LOCA. Operators are not too concerned when this happens, though — the EIW has turned itself on many times in the past when there has been no leak.

4 min

Operators observe that the water level in the primary system is rising and that the pressure is decreasing. They turn off the EIW . The water level still appears to be rising. The level is actually dropping. The water, along with the steam, is now released through the PORV.

8 min

An operator notices that the valves for the backup pumps in the secondary loop are off. He opens the valves. The secondary side now operating normally.

15 min

By this time approximately 3,000 gallons have escaped from the primary loop. The instrument that checks radioactivity levels does not trigger an alarm, so operators still have no reason to suspect a LOCA.

45 min

The water level in the primary loop continues to drop. Gauges in the control room erroneously indicate that the water level is up.

1 hr 20min
The pumps that push water through the primary loop begin to shake violently. This is caused by the steam passing through the pumps. Two of the four pumps are turned off.

1 hr 40min
The other two pumps turn off. Steam within the primary loop, now no longer circulating with the water, rises.
Also, because the water isn't circulating, the core heats up even more, converting more of the water into steam.

2 hr 15min
Water no longer covers the top of the core. The heat from the exposed core soon turns the steam into superheated steam. The control rods, in turn, react to the superheated steam and begin to release hydrogen and radioactive gases. These are also released through the PORV.

2 hr 20min
An operator from the next shift comes on duty and notices that the PORV discharge temperature is abnormally high. He stops the leak by shutting the PORV's backup valve. More than a quarter of a million gallons of radioactive cooling water has been discharged since the PORV first opened.

Operators still don't realize that the water level in the primary loop is low. The water within the loop continues to boil away, which causes more damage to the core, more heat, and more radioactivity.

2 hr 30min
The operators receive the first indication that radiation levels are going up.

Radiation alarms sound. A site emergency is declared.

Half of the core is now uncovered, and the radioactivity of the water in the primary loop is 350 times its normal level.

Even higher radiation levels prompt the declaration of a general emergency.

High temperatures in the core lead some to believe that the core is uncovered; others do not trust the temperature readings.

7 hr 30min
Even after operators pump water into the primary loop, pressure is still high. The PORV' backup valve is opened to lower the pressure.

9 hr
Hydrogen explodes within the containment structure, causing a pressure spike on the control room gauges and an audible thud. The spikes are believed to be caused by an electrical malfunction; the thud is thought, at least by some, to be just a ventilator damper.

15 hr 50min
The primary loop's pumps are turned on, which circulate water around the core. The core's temperature is finally under control, although half of it is melted and part of it has disintegrated. Also, there's still hydrogen in the primary loop.

The primary loop's pumps are turned on, which circulate water around the core. The core's temperature is finally under control, although half of it is melted and part of it has disintegrated. Also, there's still hydrogen in the primary loop.

Now that the path toward meltdown has been averted, the significant problem of leaking radiation must be dealt with.

NOVA. "Sixty Minutes to Meltdown." WGBH Educational Foundation, 1983.
Stephens, Mark. "Three Mile Island." New York: Random House, 1980.


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