Nuclear power plant in Pennsylvania which experienced a partial (52%) meltdown of its reactor core and minor radiation leakage (about 0.005 rems) on March 28, 1979 just after 4:00am. A water pump in the secondary cooling system of the Unit 2 pressurized water reactor failed, and a relief valve jammed open, flooding the containment vessel with radioactive water. A backup pumping system was down for maintenance, so the temperature inside the reactor core rose quickly, rupturing fuel rods after the core ran without coolant for 40 minutes.

If the coolant were not replaced in time, the molten fuel in the reactor could have escaped its containment vessel and come into contact with the nearby radioactive water, causing a steam explosion and rupturing the reactor's containment dome. The radioactive steam could have escaped and caused a disaster like Chernobyl.

Many people already know that Three Mile Island was a near catastrophic meltdown incident. However, many people are not totally aware of the events and consequences of the situation. I will try to lend a bit of information gathered from various online sources.

Background Plant Information

In order to understand what happened at TMI, it is necessary to understand the system that was in operation. The reactor at TMI was a PWR, or pressurized water reactor. The components of this reactor consist of a nuclear core enclosed in a steel pressure vessel. There are different water loops that are used to transfer the heat and cool the core. The first loop is the primary loop, which is entirely isolated from other components, except for the transfer of heat. The secondary loop flow through a heat exchanger where the heat from the primary loop is dumped to create steam which is used to power a steam turbine and generate power. It should be noted that the primary loop is maintained at extremely high pressure to ensure that the water remains in the liquid state and does not evaporate to steam.

The reactor had a number of different failsafes.

  1. Power Operated Relief Valve. This valve served the purpose of relieving pressure pressure in case there was an extreme buildup in the primary core. The particular model at TMI, a Dresser electromatic, was designed to never fail to open, but had a problem with closing.
  2. A manual block valve. This valve was added to be used in the event that the electromatic valve wouldn't close. If no extra valve was present and the electromatic valve failed, too much pressure would be released and the water in the primary loop would turn to steam, which would not be able to adequately cool the core.
  3. High pressure and Low pressure injection systems. These systems were capable of dumping unbelievable amounts of water on the core in the event that a loss of coolant, ie primary loop water, was to occur.
  4. Containment Dome All of the primary loop components were housed in this structure, which had 12 foot thick concrete walls.
  5. Emergency pumps. Basically everything had a redundant system in case one should fail.

The Chain Reaction

The entire system was sent on a course of near catastrophe based on a single event that started a chain reaction of coincidental mishaps that could almost never be duplicated. It was as if some hollywood screen writer was behind the scenes, composing each and every thing that could go wrong.

Here is the order and actions of events that nearly caused the total meltdown of Three Mile Island Reactor 2.

  1. The single event that started it all was the mistaken connection of a rubber hose between an air line and a water line. No one know why this was done for sure, but it probably happened because the person thought they were connecting two air lines. All the fittings for water and air lines were the same. The particular air line that was connected to the water line was a control line for many pneumatic controls of the feedwater.
  2. The water line was at a higher pressure than the air, so eventually the water crept into the pneumatic valves controlling the feedwater. These valves slammed shut, cutting of the cooling flow of water in the secondary loop. Ironically, this scenario had been designed for and a system was created to make the valves fail in the "as is" position, but this system was never hooked up.
  3. The closing of the valves immediately caused immense pressure due to the stopped flow. This ruptured the secondary pipes and damaged the flow pump.
  4. The loss of water through the secondary loop eliminated the cooling for the primary loop, and the temperature and pressure began to rise in the primary loop.
  5. Emergency feed water pumps were triggered to restore flow to the secondary loop. However, during maintenance, the valves to these pumps were closed and never reopened. Yellow maintenance tags were hanging over the indicator lights, so the operator did not see that they were closed. Although procedure called for immediate verification that they were open, the operator assumed they were open because they were supposed to be open.
  6. The pressure continued to rise in the primary loop because the there was no feedwater to cool the system. This caused a relief valve to open and steam was expelled into a quench tank in the containment building.
  7. The valve didn't close, but an indicator light said it had because the light was poorly configured to indicate only that the valve had been commanded to close, not that it actually was closed.
  8. Pumps were started to replace the water that had been lost in steam due to the relief valve opening. However, the indicators began to show that the water level was rising way too fast. This was, in fact, not the case. The stuck relief valve was causing erroneous water level readings. The emergency injectors were shut down and the water level in the primary loop plummeted.
  9. Pumps were then started to remove water from the loop. Now water was being pumped out and exiting through the relief valve. It continued to drop.
  10. Seeing that the feedwater pipe temperature was still rising, it was realized that the emergency feedwater pump's valves were closed. The valves were opened and flow was restored.
  11. The continuous loss of pressure in the primary loop made the primary water begin to boil. This created problems with the coolant pumps. The pumps had to be shut down so that they wouldn't be destroyed by trying to pump steam. The result was that the core was partially exposed and sitting in stagnant coolant.
  12. The suspicion that the relief valve was open now arose and a measurement was taken for the temperature at the valve outlet. A high temperature would indicate that the valve was still open, but the technician read the wrong indicator, so the valve remained open.
  13. The flow from the relief valve eventually overran the quench tank and the containment room became flooded with radioactive water.
  14. Eventually, the block valve was closed to remedy the relief valve problem. It was too late though, because water from the containment room reached the rest of the building through floor drains.
  15. A hydrogen bubble formed in the containment room and was sparked by an electrical relay. The room easily withstood the explosion, but it led to the fear of further explosions due to the formation of pure hydrogen in the system.
  16. A judgement was made that determined venting the hydrogen, along with some radioactive steam, was more favorable than a complete explosion.
  17. The coolant flow was eventually restored and the drama had ended.

Legal Battles

Over 2,000 personal injury claims were filed by people who felt they became ill because of the exposure to radiation from the accident. Many state reports that the average person was subjected to 1.4 mrem of radioactive material, which is not a very significant amount. The plaintiffs claimed that a highly concentrated plume of radioactive material had left the plant and had landed upon a smaller area of people leading to exposure of over 10 rems, which would be capable of causing cancer.

All suits were dismissed due to the plaintiff's inability to prove that any such concentrated plumes existed or that anyone was subjected to extreme amounts of radiation. In all reasonableness, however, it is unlikely that the radiation lost was neatly distributed among the 2 million people in nice little 1.4 mrem portions.

In the End

Three Mile Island was a terrible accident, but not nearly as terrible as it could have been. It was started by a seemingly insignificant action that lead to a string of mechanical and human errors. Looking back at this situation, it's pretty unbelievable that everything could have gone that wrong. Ultimately, it provides a valuable lesson for the future of nuclear power.

Information was taken from many sources with factual information. For the best TMI breakdown I've seen, go to The sight follows every event.
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 generators began to boil dry, the emergency core cooling system (ECCS) 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.

The plight 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 molecules 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. --
thus, when he said he looked at the plant, he was looking at it as someone who knew what he was looking at.

Log in or registerto write something here or to contact authors.