The TMI-2 Accident – In Brief
The TMI-2 accident involved a small leak of water from the reactor system that wasn’t correctly diagnosed until after the reactor’s nuclear fuel core was severely damaged. Inadequate control room instrumentation and emergency response training proved to be root causes of the operators’ inability to respond properly to an unplanned automatic shutdown of the reactor at 4 A.M. on Wednesday, March 28, 1979.
- Within seconds of the shutdown, the power-operated relief valve on the reactor cooling system opened, as it was supposed to. About 10 seconds later it should have closed, but it remained open, leaking vital reactor coolant water to the reactor coolant water drain tank. The operators believed the relief valve had shut because instruments showed them that a “close” signal was sent to the valve. However, they did not have an instrument indicating the valve’s actual position.
Pumps Come On, Are Shut Off
- Responding to the loss of water, high pressure injection pumps automatically pushed replacement water into the reactor system. As water and steam escaped through the relief valve, coolant water surged into the pressurizer, raising the water level in the pressurizer. (The pressurizer is part of the reactor system. It is a tank that maintains proper reactor system pressure. The relief valve is located on the pressurizer. In a pressurized water reactor like TMI-2, water in the reactor coolant system is kept under high pressure to keep it from boiling.)
- Operators responded to the high pressurizer water level by reducing the flow of replacement water. Their training told them that the pressurizer water level was the only dependable indication of the amount of water in the system. Because the pressurizer level was increasing, they thought the reactor system was too full of water. Their training told them to do all they could to keep the pressurizer from filling with water. If it filled when the rest of the reactor system was full of water, they could not control pressure in the reactor system and it might rupture.
Radioactive Releases Begin
- Steam formed in the reactor cooling system because of the loss of water through the open relief valve and the lack of replacement water. Pumping a mixture of steam and water caused the reactor coolant pumps to vibrate. Because the severe vibrations could have damaged the pumps and made them unusable, operators shut down the pumps. This ended forced cooling of the reactor. (The operators still believed the system was nearly full of water because the pressurizer level remained high.) However, as reactor coolant water boiled away, the reactor’s fuel core was uncovered and became even hotter. The fuel rods were damaged and released radioactive material into the reactor coolant water.
- At 6:22 A.M., operators closed a block valve between the relief valve and the pressurizer. This action stopped the loss of water through the relief valve. However, superheated steam and gases blocked the flow of water within the reactor coolant system. Throughout the morning, operators attempted to force more water into the reactor system to condense steam bubbles that they believed were blocking the flow of cooling water. During the afternoon, operators attempted to decrease the pressure in the reactor coolant system to allow a low pressure cooling water system to be used and emergency water supplies to be put into the reactor system.
- By late afternoon, operators began high pressure injection of water into the reactor cooling system to increase pressure and to collapse steam bubbles. By 7:50 P.M., they restored forced cooling of the reactor when they were able to restart one reactor coolant pump. They had condensed steam so that the pump could run without severe vibrations. Radioactive gases from the reactor system built up in the makeup tank in the auxiliary building. During March 29 and 30, operators used a system of pipes and compressors to move the gas to waste gas decay tanks. The compressors leaked. Some radioactive gas was released to the environment.
Hydrogen Bubble Restored
- When the reactor’s core was uncovered on the morning of March 28, a high-temperature chemical reaction between water and the metal tubes holding the nuclear fuel pellets had created hydrogen gas, some of which entered the containment. In the afternoon of March 28, a sudden rise in containment pressure shown by the control room instruments indicated a hydrogen burn had occurred. Hydrogen gas also gathered at the top of the reactor vessel. From March 30 through April 1, operators removed this hydrogen gas “bubble” by periodically opening the vent valve on the reactor cooling system pressurizer. For a time, some U.S. Nuclear Regulatory Commission officials believed the hydrogen bubble could explode. However, such an explosion was never possible. There was not enough gaseous oxygen in the system for an explosive mixture to exist.
- On April 27, operators established “natural” (convection) circulation of coolant water. The reactor core was being cooled by the natural movement of water rather than by mechanical pumping. The plant was in “cold shutdown.”