Residents near the Three Mile Island power plant
were awaked at 4:00 AM on Wednesday, March 28, 1979, by a loud, airplane-like noise produced by a jet of steam released from the second power plant on the island, or TMI-2
. This tremendous noise might’ve given residents some foresight towards the impending disaster, but unfortunately, the screech from a loud jet of steam was not an uncommonly heard sound for those living near the island. Several times in the months since TMI-2 had started operating, residents had heard these loud releases of steam from the plant. As such, this instance gave them no real insight
into what was occurring at the plant.
One hour earlier, inside the plant, foreman Fred Scheimann
and two auxiliary operators were in the secondary cooling loop area, cleaning a condensate polisher, which is a special part used to filter the water heading into the cooling loop. The beads inside the polisher became stuck as the men attempted to pump them into a flushing tank. This was not an uncommon occurrence, but the air hose normally used to solve the problem was out of order at the time. Scheimann decided to innovate, and instead of waiting until the air hose could be fixed, he disturbed the water in the bottom of the polisher to create a bubbling mixture of air, water and resin. This worked, effectively clearing the beads from the pipe. However, as the men moved on to their next task, they didn’t realize that they had left a small leak in the system. A miniscule amount of water began to make its way through a series of pipes called the instrument air system
, interrupting the flow of air from the pipes. A built in safety procedure
went into effect, closing the valves to the feedwater pump and triggering other safety features
throughout the system
. One of these precautions was to release the steam normally used to turn the generator into the air outside, which worked as it should and generated the roaring blast that woke nearby residents. The system was working as it should except for one serious problem – the ERV
, a special valve that controls cooling system pressure and water levels, was supposed to open for 13 seconds and shut again, but it malfunctioned and stayed open. As a result, reactor coolant
continually poured from the core and into the ERV’s holding tank
. With the coolant slowly leaking out of the core, the temperature began to rise rapidly.
Inside TMI-2’s control room, it quickly became apparent that there was some sort of problem
within the reactor. The problem, however, was that nobody was quite sure what was wrong. Backup pumps
were engaged and were supposed to be delivering water to the steam generators to help control the temperature, but, unknown to operators, two valves that regulated the flow of this water were closed two days ago during a routine maintenance check and not opened again as they should’ve been. Operators might have noticed this problem, but a maintenance tag on the control board covered up at least one of the lights that indicated the position of the twelve valves. The problem could’ve been fixed almost immediately if operators were able to tell that the ERV was still open, but due to a flawed control board
design that showed what the valves were supposed to have done according to system commands, it appeared to operators that the ERV was actually closed.
As the steam generator
s began to boil dry, the emergency core cooling system
) pumps automatically started up and began sending water into the primary cooling system at a rate of 1,000 gallons
per minute. The reactor temperature quickly levelled off, but the core water level reading on the control board was rising rapidly. If too much water is pumped into the core, the nuclear reaction becomes hard to control and there is a danger that pipes connected to the core could rupture under too much water pressure. Fearing that too much water was being pumped into the core, operators shut off one of the ECCS pumps and decreased the water flow from the other. Just a few minutes later, they shut off the second pump. This, however, was a very bad move
– the core actually had very little water in it, but another flaw in the control board design told operators otherwise. If the ECCS pumps had been left on, the TMI-2 disaster still could’ve been averted completely, but unfortunately, the defects in the control system caused operators to act otherwise, sealing TMI-2’s fate.
of TMI-2 began to gradually worsen. Several operators were wracking their brains to figure out what was actually going on inside the core, but due once again to a number of flaws; the control panel was giving them conflicting information
. Finally, however, the situation took a turn for the better when Craig Faust
noticed the twelve valves were closed. He immediately opened the valves and restored water flow to the secondary cooling loop, but he was too late to fix anything. The real problem that was occurring within the reactor still had not been solved, and one of the cooling system’s steam generators was now damaged, leaving only one to remove heat from the plant.
TMI-2’s streak of bad luck was not over yet. The situation took another turn for the worst when the holding tank behind the ERV, now containing much more coolant than it was intended to hold, burst at the base and spilled 250,000 gallons of boiling hot, radioactive water
onto the floor of the containment building
. A sump pump
automatically engaged and began moving the water into the auxiliary building, which was another drastic mistake.
Twenty-nine minutes later, workers realized that the coolant being transferred to the auxiliary building was radioactive and immediately shut down the sump pump, but by this time, radioactive steam
from the water had already been vented into the atmosphere.
Another twenty minutes later, at 5:00 AM, the reactor core began to boil dry. Though the nuclear fission
process had been stopped earlier, “decay heat
” produced as a result of the fission, continued to increase the temperature inside the reactor. The cooling pumps, now pumping more steam than water, began to shake forcefully. Operators, still thinking the core was covered in water, didn’t understand this, but nevertheless, they shut down one set of cooling pumps at 5:20 to prevent the pipes from rupturing under the violent shuddering, and 20 minutes later, the second set shut down automatically.
Soon the fuel rods within the reactor, now very exposed to air and the volcano-like temperature caused by decay heat, began to melt, spilling radioactive pellets onto the floor of the reactor. The radiation and heat soon caused the remaining water molecule
s to begin splitting into their separate elements, and before long, hydrogen
and radioactive gases
began escaping through the ERV, which was still open.
Around 6:35 AM, a water sample was taken from the drain line between the containment and auxiliary buildings. The sample was 350 times more radioactive than normal. A worker was also dispatched with a Geiger gauge
into the auxiliary building, and returned with ominous results – the radiation level was thousands of times higher than the naturally occurring level. At 6:56, Bill Zewe
declared a site emergency
Twenty-eight minutes later, at 7:24 AM, Gary Miller
checked a monitor on the containment building and was shocked by the results. The monitor showed that radiation was penetrating the steel walls of the reactor at a rate of about 800 millirems (the default unit for radiation measurement) per hour. With this latest development in mind, Miller declared a general emergency
, which signified an accident that could potentially put the public in danger of severe exposure to radiation
. It was about an hour later that WKBO radio station
broadcast the first report of the crisis.
Around 10:30 AM, three hours since the first estimate of the radiation in the containment dome was made, another measurement showed that the radiation level had increased substantially, shooting up to 40,000 or more rems per hour. Miller figured out that there must be steam pockets in the pipes preventing the primary coolant from circulating. Still believing that there was water in the core, Miller, rather than reactivating the cooling pumps, opened the motorized blocking valve on the pressurizer. By doing this, the pressure inside the core would drop to 500 pounds per square inch, which in turn would activate an automatic core flooding process that would send 500,000 gallons of coolant into the core. Unfortunately, Miller’s plan was flawed in that the system pressure did not drop far enough – it only reached 600 PSI
, not nearly low enough to activate the flooding process. This had also made the situation worse – the reactor core started losing even more coolant through the now open blocking valve. The fuel rods continued to decay, and through a reaction with the radioactive water, hydrogen was created. This hydrogen floated to the top of the reactor and built up, forming a huge hydrogen bubble.
Nearly four hours later, at 2 PM, Miller heard a loud slamming noise from within the reactor. He didn’t know it at the time, but the sound was actually caused by the hydrogen bubble exploding. The explosion was nearly powerful enough to destroy the whole containment building, which would’ve released enough radiation to poison a large part of North America, but luckily, the thick concrete walls of the containment building held together.
News from inside the plant was scarce until two days later, on Friday the 29th of March. Jim Floyd
discovered a pocket of radioactive gas inside a makeup tank
, which held the water for the ECCS system. Floyd, knowing this could be dangerous, faced the dilemma of either venting the gas into a waste-gas decay tank or into the outside air. It would seem that venting the gas into the waste decay tank would be the obvious choice, but Floyd feared that the gas could escape through leaks in the pipe, or that the ECCS makeup tank valve might malfunction and stay open. If he vented the gas outside, though, he was releasing radiation into the environment, and there was the inherent
risk that the valve to the outside would stick, letting a considerable amount of radiation into the outside air. Floyd eventually decided to vent the gas outside. This information was quickly communicated, and soon Governor Dick Thornburgh
issued an announcement suggesting that pre-school age children and pregnant women evacuate a 5-mile radius around TMI. Though the Governor worded his speech carefully, it set off a mass panic, and by the end of the day, 40,000 people had evacuated the area.
Plant officials were still busy at work Friday morning, unaware of the panic outside. Another problem quickly became apparent, though – the hydrogen bubble that had exploded on Wednesday was building up again, and measurements showed that it was already 100 cubic feet in size. Roger Mattson
, director of the US Nuclear Regulatory Commission
, soon heard about the reforming bubble. Mattson believed the bubble was an immediate threat – he thought it could prevent proper cooling of the reactor and lead to a catastrophic meltdown. Mattson also believed that the hydrogen and oxygen might blend with radioactive materials in the core and produce a flammable mixture, causing an enormous explosion that would undoubtedly rip apart the containment building.
Tension was still high Saturday morning, but relief came in the form of an announcement by President Jimmy Carter
to the public. President Carter told citizens that he had toured the plant, seen the facts, and saw no impending danger to citizens within the area. He also said that plant officials were working on a change in the cooling system that would put a permanent end to the situation. This was confirmed on Sunday when NRC officials said there was no danger of the hydrogen bubble in TMI-2’s core bursting – Mattson had based his figures on the bubble in an uncompressed environment, while pressure inside the core was 1,000 pounds per square inch. By 7:00 PM on Sunday, the bubble had shrunk to a size of 350 cubic feet.
By Friday, April 6, the cold shutdown process
(in which water is naturally circulated to cool the reactor) was well on its way, and with the evacuation advisory lifted, about 90 percent of evacuated residents had returned to their homes. The crisis was finally over, and as of now, no deaths have been linked to the radiation released from the TMI-2 plant.
Source: "Three Mile Island" by Therese De Angelis and Gina De Angelis; NRC fact sheet "Accident At Three Mile Island"
2004.01.03 at 20:37: m_turner says minor point to make that might be useful to note - Jimmy Carter was a Nuclear Engineer. -- Carter began his career as a naval engineer after receiving a B.S. degree from the U.S. Naval Academy in 1946. -- http://tinyurl.com/yto9h
thus, when he said he looked at the plant, he was looking at it as someone who knew what he was looking at.