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Accident at Three Mile Island

Updated on March 21, 2011

The accident occurred at Three Mile Island nuclear power station at Three Mile Island in 1979 and was the worst nuclear accident occurred in the United States, although not proven to have caused deaths.

The incident took place exactly two unit at 4:00 Wednesday, March 28, 1979, when the reactor was under a power of 97%. The incident began in the secondary cooling circuit with the block flow of food to the steam generators. This block flow in the primary cooling core to a substantial increase in pressure of the refrigerant, causing the first opening of a valve located on the pressurizer PORV release and then the "SCRAM" (emergency stop of the reactor by inserting the control rods). At this point, the release valve is not closed without that operators were aware of the problem, because there was no indication in the instrument the actual position of the valve. Thus the primary cooling circuit is partially emptied and the waste heat from the reactor core could not be discarded. Because of this, the radioactive core was severely damaged. Traders could not properly diagnose what was happening and react appropriately. The lack of control room equipment and inadequate training were found to be the main causes of the accident.

During the incident there was a dangerous partial melting of the core and result in serious damage reported that Unit 2 was closed and is still under monitoring, in anticipation of future actions for decommissioning.

The Chain Of Events


Within 10 seconds after SCRAM, the release valve should have been close. This did not happen and there was a loss of coolant accident classified as a small break loss of coolant (Small Break LOCA). The operators believed, mistakenly, that the PORV had closed, as had been sent the command closing the valve. However, there was a real response to the closure of the valve.


To respond to the loss of coolant pumps high pressure emergency cooling system automatically injects water in the primary circuit, which simultaneously kept losing coolant from PORV open. In this way, however, the primary circuit pressure remained high, with the level in the pressurizer, while the level in the reactor core continued to decline. In a PWR reactor, the water pressure is very important, since it is maintained at high levels to prevent boiling, on the other hand, the water level in the pressurizer is also essential to control the reactor level is too not allow the high pressurizer pressure control system which would tend to increase, causing the rupture of the cooling system.


The operators responded by reducing the water flow of the emergency system at high pressure. Their training provided that the water level in the pressurizer was the only reliable indication of the amount of cooling water in the system. Since the level of the pressurizer was increasing, they thought the primary was too full of water. Their training provided that the primary was not 100% full, or risk losing control of pressure in the system.


The water vapor before and after, were collected in a tank drainage. When it reached a pressure of 13 bar, the rupture disk broke. At this point, the integrity of the primary circuit was lost. Part of the water began to flow into the containment system and was automatically removed from the pumps, sending the liquid to the auxiliary buildings are not designed to accept contaminated fluids.


Following the decrease in pressure of the steam formed in the reactor's primary cooling system. The presence of steam caused strong vibrations in the pumps (cavitation). Since the vibration could damage the pumps and make them unusable, operators decided to stop them. This resulted in overheating of the reactor core (the operators believed the cooling system full of water because of the indication of high pressure in the circuit). However, as the reactor coolant water evaporated due to the reduced pressure, and also was lost from PORV open the reactor core was found, with the result that they heated up further. The fuel rods are damaged and the radioactive material they contain contaminated water from the primary circuit.


At 6:22 operators closed the block valve between the relief valve and the booster. This action stopped the coolant leak. However, the presence of steam and hot gas prevented the regular eflusso of water through the reactor's cooling system.


 During the morning, traders tried to increase the flow of water into the reactor cooling system to condense the vapor bubbles that impede the smooth flow of cooling water. In the afternoon, traders tried to lower the pressure in the cooling system for groped to stabilize the situation.


In the evening, operators began to inject water under high pressure in the reactor cooling system to increase pressure and reduce the bubbles of steam and hydrogen. At 19:50 on March 28, the forced cooling of the reactor was re-established. They condensed the steam so that the pumps could operate without excessive vibration.


The radioactive gas from the reactor cooling system had accumulated at the top of the Vessel (the first container of the reactor core).


 The hydrogen accumulated, being extremely light, it was collected at the top of reactor. From March 30 to April 1 operators periodically removed by opening the hydrogen vent valve on the pressurized reactor cooling system. For a time, officials of the inspection (NRC) thought that the bubble of hydrogen could be a danger of explosion, with obvious disastrous effects on the integrity of the containment system of the system.

Stopping Cold


After months of anxiety and expectation, 27 April traders settled the refrigerant circulation by natural convection: the reactor core was thus cooled by the movement of water by natural convection rather than by mechanical pumping. The plant came into being "cold shutdown."


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