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Three Mile Island Nuclear Accident



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Year 1998
Publisher provided Keywords Assessment Assessment, Case Case, Cases Cases, Corporate Corporate, Discipline: ENGINEERING Engineering, ENVIRONMENTAL Historical issues Issues, Nuclear PUBLIC radioactive RISK risks risks, safety safety, Setting: Structural Structural, substances substances, Type: University University, worker
Publisher National Academy of Engineering
Language English

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Timeline of Events Occurring During the Three Mile Island Disaster
Elapsed Time Event
00:00:00 Pumps feeding water to the secondary loop shut down.

This was the first of two independent system failures that led to the near meltdown of the Three Mile Island Nuclear Reactor.

00:00:01 Alarm sounds within the TMI control room.

At the time this alarm is disregarded by the operators.

00:00:02 Water pressure and temperature in the reactor core rise.

The failure of the secondary loop pump has stopped the transfer of heat from the Primary Loop to the secondary loop. The rise in temperature and pressure is considered to be part of the normal plant operations, and hence ignored.

00:00:03 Pressure Relief valve (PORV) automatically opens

When the pressure of steam in the reactor core rises above safe limits, the pressure relief valve is designed to automatically open, releasing the excess steam to a containment tank.

00:00:04 Backup pumps for the secondary loop water system automatically turn on.

Four seconds into the accident the secondary loop water pumps are automatically turned on. This is indicated to the operators by the presence of lights on the control panel. The operators are not aware that the pumps have been disconnected and are not functioning.

00:00:09 Boron and Silver control rods are lowered into the reactor. PORV light goes out, indicating valve is closed.

Lowering of the control rods into the reactor core slows down the rate of the reaction. The effect of which is also a reduction in the heat produced by the reactor. When the PORV light goes out the operators incorrectly assume that the valve is closed. In reality the valve is not only open but is also releasing steam and water from the core. This is now a LOCA (Loss of Coolant Accident)

00:02:00 Emergency Injection Water (EIW) is automatically activated.

The EIW is a safety device that causes water to flow into the reactor core. It is designed to ensure that when there is a LOCA the water in the core remains at a safe level. In the past the EIW system has turned itself on when there has been no leak so the operators are not unduly concerned by this.

00:04:30 Operators observe that the water level in the Primary System is rising while the pressure is decreasing.

When they observe that the water level in the core is rising the operators shut off the EIW system.

00:04:30 Water level in the core still appears to be rising.

In actuality the water level in the core is dropping and turning off the EIW increases the amount of steam being produced by the reactor core. The combination of steam and water is still being released through the PORV.

00:08:00 Operator notices that the valves for the secondary loop backup pumps are off.

8 minutes into the accident the closed valve is noticed by an operator. Once he turns the valves back on the Secondary Water loop is functioning correctly.

00:45:00 Water level in primary loop continues to drop.

At this point in the accident the operators still do not suspect a LOCA. The instrument checking the radiation has not registered an alarm, and the gauges in the control room are wrongly indicating that the water level is up.

01:20:00 Primary loop pumps start to shake violently.

Steam produced by the lack of cooling water in the core passes through the primary loop pumps and causes them to shake. Assuming they are not functioning correctly the operators turn off two of the four pumps

01:20:00 Remaining two pumps in the primary loop turn off.

The automatic shut down of the two remaining pumps in the primary loop causes the water within the nuclear core to stop circulating. This in turn causes the heated core to convert more water into steam, further reducing the transfer of heat away from the core.

02:15:00 Water level drops below the top of the core.

Once the top of the core is exposed the steam is converted to super heated steam. This reacts with the control rods and produces hydrogen and other radioactive gases.

02:15:00 Hydrogen gas is released through PORV.

Since the Pilot Operated Relief Valve is still in the open position it allows the hydrogen gas produced to be released along with the steam.

02:20:00 Operator from next shift arrives and closes PORV backup valve.
02:20:30 Operators receive first indication that the radiation levels are up.
02:45:00 Radiation alarm sounds and a site emergency is declared.

At this point half the core is uncovered and the radiation level of the water in the primary loop is 350 times its normal level.

03:00:00 Due to higher radiation levels a General Emergency is declared.

There is still confusion as to whether the core is uncovered or not. There are some that feel the temperature readings may be erroneous.

07:30:00 Operators pump water into the primary loop and open the PORV backup valve to lower the pressure.
09:00:00 Hydrogen within the containment structure explodes.

The explosion is recorded by the instruments in the control room. It is dismissed as just being a spike caused by an electrical malfunction. The sound of the explosion heard is thought by some to be a ventilator damper.

15:00:00 Primary loop pumps are turned on.

By now a large portion of the core has melted and there is still hydrogen present in the primary loop. Water from the primary loop pumps is circulated and the core temperature is finally brought under control.

Return to the Three Mile Island Nuclear Accident.

Cite this page: "Timeline of Events Occurring During the Three Mile Island Disaster" Online Ethics Center for Engineering 2/16/2006 OEC Accessed: Saturday, August 17, 2019 <>