U.S. pilot-operated relief valve (PORV) on the
U.S. History II
19 march 2004
Three Mile Island is a nuclear power plant located in Harrisburg, PA. It had two pressurized water reactors. One in which began its service in 1974 and is the best performing reactor in the US. However its other reactor is almost brand new and suffered a server nuke clear meltdown. March 28th 1979 at approximately 4:00 a.m. a minor malfunction created a rise in temperature to the primary coolant.
The reactor shut down as a safety result. In no time a pilot-operated relief valve (PORV) on the reactors cooling system opened but did not close. This caused reactor coolant water to leak out and soon drained the tank of its coolant (Wikipeia). As a effect of the lost coolant, high pressure pumps pushed replacement water into the reactor system. Water and steam then escaped through its relief valve as cooling water surged to the reactor.
In this type of situation, the operators were trained to reduce the flow of the replacement water. Their training told them that the pressurizer water level was the only dependable indication of the amount of cooling water in the system. Because the pressuriser level was increasing, they thought the reactor system was too full of water They were told to do all they could to keep the pressuriser from filling with water. If it filled, they could not control pressure in the cooling system and it might rupture.
Operators responded by reducing the flow of replacement water. Steam then formed in the reactor cooling system. Pumping a mixture of steam and water caused the reactor cooling pumps to vibrate. If the severe vibrations could have damaged the pumps they would made them unusable, so the operators shut down the pumps. This ended the forced cooling of the reactor.
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 cooling water. At 6:22 am operators closed a block valve between the relief valve and the pressuriser. This action stopped the loss of coolant water through the relief valve. However, superheated steam and gases blocked the flow of water through the core cooling system (Wikipeia). 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 pm, 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. From March 29 and 30, operators used a system of pipes and compressors to move the gas to waste gas decay tanks(Wikipeia). The compressors leaked, and some radioactive gas was released to the environment
After an anxious month, on 27 April operators established natural convection circulation of coolant. The reactor core was being cooled by the natural movement of water rather than by mechanical pumping. The plant was in “cold shutdown”.
The cleanup of the damaged nuclear reactor system at TMI-2 took nearly 12 years and cost approximately $973 million. The Plant surfaces had to be decontaminated. Any water used and stored during the cleanup had to be processed. And about 100 tones of damaged uranium fuel had to be removed from the reactor vessel — all without hazard to cleanup workers or the public. (Wikipeia)
I see Three Mile Island as history repeating itself; It reminded me a lot of the Titanic. The crew on titanic and in the operators room were told that an accident was nearly impossible so that when something happened they didnt know how to react properly or knew entirely what was going on. However, they responded with there instincts which only made the problem worse. Unlike the Titanic though, no one died in Three Mile Island.
The Three Mile Island incident was in a way a good lesion to the US in working with nuclear generated power. We saw that it is a force of nature that is very powerful. Its dangers are very real, anything could happen, and if something did happen when using the nuclear power many could die. We saw that we should not assume anything in a time