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Three Mile Island

Next month sees the 20th anniversary of the accident at the Three Mile Island Number 2 nuclear power reactor in Pennsylvania. The accident on 28 March 1979 was nowhere near as severe as the explosion and meltdown at Chernobyl in 1986, but it came close. It also had a major impact on the public perception of the safety of nuclear power.

While civil engineering was not at the heart of the accident, the incident raised issues which have become increasingly relevant to civil engineers today.

The official accident report shows that those responsible for operating TMI2 had failed to train key staff to correctly interpret what was happening inside the reactor as it came dangerously close to meltdown. As a result they made a series of mistakes which worsened a situation they could have controlled.

The mistakes resulted in cooling water in the reactor overheating, boiling and and surging out through an emergency valve which subsequently jammed open. This left the reactor core exposed to the air at high temperature, increasing the risk of a nuclear explosion.

The core initially started to overheat after a simple pump malfunction resulted in the shutdown of one of the reactor's steam generators. Pressurised water pumped through a closed pipe circuit into the steam generator then increased its temperature as it was no longer losing heat to the steam making process.

Pressures and temperatures then increased inside the reactor core until an emergency valve released pent up water and steam. Meanwhile, engineers began to dump hundreds of thousands of gallons of water into the reactor to cool it down.

Temperature and pressure dropped. But the engineers failed to realise that the emergency valve had jammed open. As a result, much of the water from the emergency pumps was turning into steam immediately and rushed out through the jammed open valve.

Reactor pressure and temperature only fell because the volume of emergency water coming in was more than the amount leaving through the jammed valve.

Operators failed to realise this and, thinking they had the reactor temperature under control, shut down the emergency water pumps. This resulted in the evaporation of large volumes of water intended to keep the reactor core covered, and exposed the high temperature core causing a hydrogen explosion as the nuclear fuel reacted with steam.

In the end, disaster was avoided after more water was pumped into the core, although radioactive material was still released into the atmosphere as overflow water and radioactive gases from the reactor were released.

TMI2 has been in mothballs ever since the incident with a multi-billion dollar decommissioning price tag hanging over it. Operators of the plant are waiting for TMI1, the other reactor on the site, to reach the end of its natural life before decommissioning them at the same time.

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