The worst ever US nuclear disaster explained

MIDDLETOWN, PENNSYLVANIA — Forty years ago, a calm early morning in Middletown, Pennsylvania turned chaotic when a nuclear plant began leaking toxic fumes into the atmosphere. According to the US Nuclear Regulatory Commission, roughly 140,000 people were evacuated.

Today we know this as Three Mile Island Accident and here is how it all went down.

On March 28, 1979, The Three Mile Island Unit 2 reactor, near Middletown, Pennsylvania partially melted down, releasing radioactive fumes into the atmosphere.

According to the US Nuclear Regulatory Commission, the plant experienced its first malfunction at 4 a.m. that same day on one of the two reactors on site.

A mechanical break down was causing the reactor's cooling system to fail, which in turn caused pressure in its primary system to rise. In order to fight the increasing pressure, a valve located on top of the reactor's pressurizer opened.

The valve was supposed to close once the pressure levels were stabilized, however, it jammed and stayed open. Staff did not know the valve had failed to close because instruments in the control room did not indicate otherwise.

It wasn't until alarms began to ring that the staff realized the reactor was releasing coolants due to the open valve.

The US Nuclear Regulatory Commission reports that roughly 2 million people were exposed to a small amount of radiation, which they claim cannot cause health issues.

The reactor is now permanently shut down and 99 percent of its fuel has been removed.