The accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant in Pennsylvania on March 28, 1979 was one of the most serious in the history of the U.S. nuclear industry. It not only brought to light the hazards associated with nuclear power, but also forced the industry to take a closer look at the operating procedures used at the time. What makes the TMI-2 accident such an interesting case study is the series of events which led up to the partial meltdown of the reactor core. It was a combination of human error, insufficient training, bad operating procedures and unforeseen equipment...
The accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant in Pennsylvania on March 28, 1979 was one of the most serious in the history of the U.S. nuclear industry. It not only brought to light the hazards associated with nuclear power, but also forced the industry to take a closer look at the operating procedures used at the time. What makes the TMI-2 accident such an interesting case study is the series of events which led up to the partial meltdown of the reactor core. It was a combination of human error, insufficient training, bad operating procedures and unforeseen equipment failure that culminated in a nuclear accident that could have easily been prevented.
This was the first of two independent system failures that led to the near meltdown of the Three Mile Island Nuclear Reactor.
At the time this alarm is disregarded by the operators.
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.
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.
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.
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)
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.
When they observe that the water level in the core is rising the operators shut off the EIW system.
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.
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.
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.
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
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.
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.
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.
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.
There is still confusion as to whether the core is uncovered or not. There are some that feel the temperature readings may be erroneous.
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.
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.
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