Three Mile Island accident

The location
The Three Mile Island accident was a partial nuclear meltdown that occurred on March 28, 1979, in 1 of the 2 Three Mile Island power plant's nuclear reactors, in Dauphin County, Pennsylvania. It was the worst accident in U.S. commercial nuclear power plant history to date and the 3rd worst case in the world, with Chernobyl and Fukajima being worse.

The atomic power station was built in 1974 on a sandbar on Pennsylvania’s Susquehanna River, just 10 miles downstream from the state capitol of Harrisburg. The state-of-the-art Reactor 2 was made in 1978, as a hoped for way of generating affordable and reliable energy in the 1970s energy crises caused by trouble in the Middle East.

Light bulbs
The technical design of the PORV indicator light and/or it's instillation were fundamentally flawed. The bulb was simply connected in parallel with the relief valve solenoid, thus implying that the PORV was shut when it went dark, without actually verifying the real position of the valve.

When everything was operating correctly, the indication was true and the operators became habituated to rely on it with out considering it may have been lying to them!

Unfortunately, when things went wrong and the main relief valve stuck open, the unlit lamp was actually misleading the operators by implying that the valve was shut, thus making the Three Mile Island accident a lot worse than it could have been if they had acted sooner.

The operators were not appropriately trained and did understand the ambiguous nature of the PORV indicator. Because of this, they did not look for alternative confirmation that the main relief valve was in fact closed.

An episode of the TV show Mayday, also known as Air Crash Investigation(s) also featured an aircraft, whose accident scenario became a lot worse after the crew became obsessed with a defective light bulb on the control panel.

Pre-SCRAM events
It was a civil partial nuclear meltdown that poisoned a large area of Pennsylvania, including a major city. The cause was human errors and mechanical problems with a stuck valve that controlled steam flows.

That night TMI-2 reactor was running at 97% of full power, while the companion TMI-1 reactor was shut down for refueling. It all began at 4:37 am EST, on March 28, 1979, in TMI-2's secondary loop, one of the three main water/steam loops in a pressurized water reactor (PWR).

An attempt by operators to fix a blockage in one of the 8 condensate polishers, the high tech filters cleaning the secondary loop water, 11 hours earlier jammed the valve. Blockages are common with these sophisticated resin filters and are usually fixed easily, but in this case the usual method of forcing the stuck resin out with compressed air did not succeed.

With the steam generators no longer receiving feedwater, the reactor became very hot and hyperactive. As heat and pressure increased in the reactor coolant system the reactor automatically perform a 'SCRAM' emergency shutdown and within 8 seconds, the control rods were inserted into the core to halt the nuclear chain reaction.

Post-SCRAM events
The hyper-hot/active reactor overwhelmed the (most likely boron or graphite) control rods and continued to generate decay heat. Since steam was no longer being used by the generating turbine, heat was no longer being removed from the reactor's primary water loop. Once the secondary feedwater pumps stopped, 3 auxiliary pumps activated automatically, but because the valves had been closed for routine maintenance in violation of a key NRC rule, so the system was unable to pump any water. The NRC rules stated the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This was later singled out by angry NRC officials as a key failure.

This triggered the pilot-operated relief valve (PORV) at the top of the pressuriser – a pressure active-regulator tank – to open automatically. It should have closed when the excess pressure had been released, and electric power to the solenoid of the pilot was automatically cut, but the open valve permitted coolant water to escape from the primary system and was the principal mechanical cause of the partial meltdown that followed.

Human factors: Confusion over valve status and the status of the solenoid by the operators meant did not correctly diagnose the problem for several hours.

The PORV indicator light's unlit lamp mislead the operators in to behaving the valve was shut. This caused the operators considerable confusion, because the pressure, temperature and coolant levels in the primary circuit, were apparently not behaving as they would have if the PORV were shut.

This confusion meant the staff were unable to break out of a cycle of assumptions that conflicted with what their instruments were telling them. The operators were not appropriately trained and did understand the ambiguous nature of the PORV indicator. Because of this, they did not look for alternative confirmation that the main relief valve was in fact closed.

Events after the new shift arrived
It was not until a fresh shift came in, who did not have the mind-set of the first shift of operators, that the problem was correctly diagnosed. By this time major damage had occurred thanks to the earlier shift's poor training, dithering, bad habits, inflexibility, confusion and incompetence.

There was a temperature indicator downstream of the PORV in the tail pipe between the PORV and the pressurizer that could have told them the valve was stuck open, by showing that the temperature in the tail pipe remained higher than it should have been had the PORV been shut. This temperature indicator, however, was not part of the "safety grade" suite of indicators designed to be used after an incident, and the operators had not been trained to use it. Its location on the back of the desk also meant that it was effectively out of sight of the operators.

The consequences of stuck valve
As the pressure in the primary system continued to decrease, reactor coolant continued to flow, but it was boiling inside the core. First, small bubbles of steam formed and immediately collapsed, known as nucleate boiling. As the system pressure decreased further, steam pockets began to form in the reactor coolant. This departure from nucleate boiling (DNB) into the regime of "film boiling" caused steam voids in coolant channels, blocking the flow of liquid coolant and greatly increasing the fuel cladding temperature. The overall water level inside the pressurizer was rising despite the loss of coolant through the open PORV, as the volume of these steam voids increased much more quickly than coolant was lost. Because of the lack of a dedicated instrument to measure the level of water in the core, operators judged the level of water in the core solely by the level in the pressurizer. Since it was high, they assumed that the core was properly covered with coolant, unaware that because of steam forming in the reactor vessel, the indicator provided misleading readings. Indications of high water levels contributed to the confusion, as operators were concerned about the primary loop "going solid," (i.e. no steam pocket buffer existing in the pressurizer) which in training they had been instructed to never allow. This confusion was a key contributor to the initial failure to recognize the accident as a loss-of-coolant accident, and led operators to turn off the emergency core cooling pumps, which had automatically started after the PORV stuck and core coolant loss began, due to fears the system was being overfilled.

Later

The evacuation
Governor Dick Thornburgh, on the advice of NRC chairman Joseph Hendrie, advised the evacuation "of pregnant women and pre-school age children...within a five-mile radius of the Three Mile Island facility." The evacuation zone was extended to a 20-mile radius on Friday March 30. Within days, 140,000 people had left the area. More than half of the 663,500 population within the 20-mile radius remained in that area. According to a survey conducted in April 1979, 98% of the evacuees had returned to their homes within three weeks. Later, the Pennsylvania House of Representatives conducted an investigation, which focused on the need to improve evacuation procedures. About 50% of the population then and still support the idea of the evacuation.

At the accident site
The cleanup started in the August of 1979, and officially ended in the December of 1993, with a total cleanup cost of about $1 billion. Officially it was a Level 5 INES Scale event, but probably was actually a level 6 event.

Political fall-out
The U.S. 96th Congressional session passed U.S. legislation establishing a five-year nuclear safety, research, demonstration, and development program during the December of 1980.

Medical opinion
The partial meltdown resulted in the release of unknown amounts of slightly radioactive gases and radioactive iodine into the environment and cancer reportedly went up a bit (one acout said a total of 333 cases) for a few years.

Anti-nuclear people accused it causing cancer in a cancer free town and went in to a hysterical frenzy. The state denied there ever was any cancers at any time. The epidemiological studies analyzing the rate of cancer in and around the area since the accident, determined there was a 'small, statistically non-significant, increase' in the rate and so the came to the conclusion that no causal connection linking the accident with the slight gain in cancers can be made.

A greater understanding atomic and industrial accident theories
Charles Perrow’ s Normal Accident Theory was inspired by the accident. The theory proposes that an accident occurs, resulting from an unanticipated interaction of multiple failures in a complex system. "Normal" accidents, or system accidents, are so-called by Perrow because such accidents are inevitable in extremely complex systems.

Litigation
A citizens's groups won $25 million in a 1981 out-of-court settlement in a class action suit against TMI. Part of this money was used to found the TMI Public Health Fund. A federal grand jury indicted Metropolitan Edison over the criminal falsification of safety test results prior to the accident. Met Ed also agreed to pay a $45,000 fine and to set up a $1 million account to help with emergency planning in the area surrounding the plant. The plant's operator and its insurers then paid at least $97 million in compensation after that.

Also see

 * 1) The Windscale reactor fire and 5 Sellafield incidents
 * 2) Chernobyl disaster
 * 3) Corporate malfeasance at the Peach Bottom Nuclear Generating Station
 * 4) Leningrad Nuclear Power Plant accidents
 * 5) Kyshtym (Urals Mountains) disaster
 * 6) Chalk River nuclear accident
 * 7) SL-1 Experimental Power Station accident
 * 8) 1970s energy crises

Links

 * 1) http://www.wikiwand.com/en/Three_Mile_Island_accident
 * 2) http://www.world-nuclear.org/info/Safety-and-Security/Safety-of-Plants/Three-Mile-Island-accident/
 * 3) http://www.history.com/this-day-in-history/nuclear-accident-at-three-mile-island
 * 4) http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html
 * 5) http://www.britannica.com/event/Three-Mile-Island-accident
 * 6) http://threemileisland.org/
 * 7) https://en.wikipedia.org/wiki/Three_Mile_Island_Nuclear_Generating_Station
 * 8) http://www.history.com/topics/three-mile-island