Today, it is the fear that terrorists might try to blow up a nuclear plant that has Americans jittery.

The Three Mile Island incident began on March 28 1979 when in the middle of the night an alarm sounded in the control room of reactor Number Two.

Two days later, the incident reached its peak when a radioactive gas bubble threatened the environment.

On April 1, the bubble began to shrink under the 900 megawatt reactor's dome, and President Jimmy Carter visited Harrisburg, Pennsylvania, and the plant to reassure the public.

No one died in the incident, which started with a failure in a non-nuclear section of the plant, according to the US Nuclear Regulatory Commission (NRC).

The main feedwater pumps stopped running after a mechanical or an electrical failure, preventing steam generators from removing heat, according at an NRC report.

The turbine and reactor Number Two shut down automatically.

Pressure in the nuclear part of the plant immediately began to increase.

A valve opened to reduce pressure, but it failed to close as it was supposed to when the pressure subsided. The control station never received a signal indicating that the valve remained open.

Coolant escaped from the valve through the pressurizer, causing the reactor's core to overheat.

The control station's indicators did not show that the core's coolant level was too low.

Unaware of this problem, technicians worsened the condition by taking steps that reduced the core's coolant level.

Since there was no adequate cooling, the nuclear fuel overheated to the point of causing the rupture of metal tubes that held the fuel.

Fuel pellets began to melt before technicians were able to reduce the temperature.

Luckily, although the plant suffered a "severe core meltdown, the most dangerous kind of nuclear power accident, it did not produce the worst-case consequences that reactor experts had long feared," according to the NRC report.

A Chernobyl-type disaster -- in which massive amounts of radiation were spread into the atmosphere in the Ukraine in 1986 -- was averted.

Today, one-fifth of US energy derives from nuclear power, 25 years after the country avoided a catastrophe.

No new plants have been ordered and Congress has only timidly proposed nuclear power as an alternative source of energy to modernize a fragile sector.

Last year, one-fourth of the US northeast lost electricity after a massive power failure.

Since the September 11, 2001, attacks in New York and Washington, nuclear plants are considered among the most likely targets of terrorists who want to cause mass casualties.

Every alert leads the security services of the country's 103 nuclear plants to prepare for the worst, while fighter jets are mobilized to stop hijacked planes from crashing into a reactor.

WAR.WIRE