|Date||March 28, 1979|
|Time||04:00 (Eastern Time Zone UTC−5)|
|Location||Londonderry Township, Dauphin County, Pennsylvania near Harrisburg|
|Outcome||INES Level 5 (accident with wider consequences)|
|Designated||March 25, 1999|
The Three Mile Island accident was a partial meltdown of the Three Mile Island, Unit 2 (TMI-2) reactor in Pennsylvania began at 4 a.m.on March 28, 1979. It is the most significant accident in U.S. commercial nuclear power plant history. On the seven-point International Nuclear Event Scale, it is rated Level 5 - Accident with Wider Consequences.
The accident began with failures in the non-nuclear secondary systemfollowed by a stuck-open pilot-operated relief valve (PORV) in the primary system that allowed large amounts of nuclear reactor coolant to escape. The mechanical failures were compounded by the initial failure of plant operators to recognize the situation as a loss-of-coolant accident. TMI training and procedures left operators and management ill-prepared for the deteriorating situation. During the event, these inadequacies were compounded by design flaws, including a cacophony of alarms, an inconvenient arrangement of instruments and controls, and the absence of clear indicators for coolant inventory or the position of the stuck open PORV.
The accident crystallized anti-nuclear safety concerns among activists and the general public and resulted in new regulations for the nuclear industry. It has been cited as a contributor to the decline of a new reactor construction program, a slowdown that was already underway in the 1970s.The partial meltdown resulted in the release of radioactive gases and radioactive iodine into the environment.
Anti-nuclear movement activists expressed worries about regional health effects from the accident. billion.However, epidemiological studies analyzing the rate of cancer in and around the area since the accident determined there was not a statistically significant increase in the rate and thus no causal connection linking the accident with these cancers has been substantiated. Cleanup started in August 1979, and officially ended in December 1993, with a total cleanup cost of about $1
In the nighttime hours before the incident, the TMI-2 reactor was running at 97% power, while the companion TMI-1 reactor was shut down for refueling.The main chain of events leading to the partial core meltdown on March 28, 1979, began at 4:00:37 a.m. EST in TMI-2's secondary loop, one of the three main water/steam loops in a pressurized water reactor (PWR).
The initial cause of the accident happened eleven hours earlier, during an attempt by operators to fix a blockage in one of the eight condensate polishers, the sophisticated filters cleaning the secondary loop water. These filters are designed to stop minerals and impurities in the water from accumulating in the steam generators and decrease corrosion rates on the secondary side.
Blockages are common with these 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. The operators decided to blow the compressed air into the water and let the force of the water clear the resin. When they forced the resin out, a small amount of water forced its way past a stuck-open check valve and found its way into an instrument air line. This would eventually cause the feedwater pumps, condensate booster pumps, and condensate pumps to turn off around 4:00 a.m., which would, in turn, cause a turbine trip.
As the steam generators were no longer receiving feedwater, heat transfer from the reactor coolant system(RCS) was greatly reduced, and RCS temperature rose. The rapidly heating coolant expanded and surged into the pressurizer, compressing the steam bubble at the top. When RCS pressure rose to 2,255 psig (155.5 bar), the pilot-operated relief valve (PORV) opened, relieving steam through piping to a tank in the containment building basement. RCS pressure continued to rise, reaching the reactor protection system (RPS) high-pressure trip setpoint of 2,355 psig (162.4 bar) eight seconds after the turbine trip. The reactor automatically tripped, its control rods falling into the core under gravity, halting the nuclear chain reaction and stopping the heat generated by fission. However, the reactor continued to generate decay heat, initially equivalent to approximately 6% of the pre-trip power level. Because steam was no longer being used by the turbine and feed wasn’t being supplied to the steam generators, heat removal from the reactor's primary water loop was limited to steaming the small amount of water remaining in the secondary side of the steam generators to the condenser using turbine bypass valves.
When the feedwater pumps tripped, three emergency feedwater pumps started automatically. An operator noted that the pumps were running, but didn't notice that a block valve was closed in each of the two emergency feedwater lines, blocking emergency feed flow to both steam generators. The valve position lights for one block valve were covered by a yellow maintenance tag. Why the operator missed the lights for the second valve is not known. The valves may have been left closed during a surveillance test two days earlier.With the block valves closed, the system was unable to pump any water. The closure of these valves was a violation of a key Nuclear Regulatory Commission (NRC) rule, according to which the reactor must be shut down if all auxiliary feed pumps are closed for maintenance. This was later singled out by NRC officials as a key failure.
After the reactor tripped, secondary system steam valves operated to reduce steam generator temperature and pressure, cooling the RCS and lowering RCS temperature, as designed, resulting in a contraction of the primary coolant. With the coolant contraction and loss of coolant through the open PORV, RCS pressure dropped as did pressurizer level after peaking fifteen seconds after the turbine trip. Also, fifteen seconds after the turbine trip, coolant pressure had dropped to 2205 psig (152 bar), the reset setpoint for the PORV. Electric power to the PORV's solenoid was automatically cut, but the relief valve was stuck open with coolant water continuing to be released.
In post-accident investigations, the indication for the PORV was one of many design flaws identified in the operators' controls, instruments and alarms.There was no direct indication of the valve’s actual position. A light on a control panel, installed after the PORV had stuck open during startup testing, came on when the PORV opened. When that light—labeled Light on - RC-RV2 open —went out, the operators believed that the valve was closed. In fact, the light, when on, only indicated that the PORV pilot valve’s solenoid was powered, not the actual status of the PORV. While the main relief valve was stuck open, the operators believed the unlighted lamp meant the valve was shut. As a result, they did not correctly diagnose the problem for several hours.
The operators had not been trained to understand the ambiguous nature of the pilot-operated relief valve indicator and to look for alternative confirmation that the main relief valve was closed. A downstream temperature indicator, the sensor for which was located in the tail pipe between the pilot-operated relief valve and the pressurizer relief tank, could have hinted at a stuck valve had operators noticed its higher-than-normal reading. It was not, however, part of the "safety grade" suite of indicators designed to be used after an incident, and personnel had not been trained to use it. Its location behind the seven-foot-high instrument panel also meant that it was effectively out of sight.
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 pilot-operated relief valve, 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 pilot-operated relief valve stuck and core coolant loss began, due to fears the system was being overfilled.
With the pilot-operated relief valve still open, the pressurizer relief tank that collected the discharge from the pilot-operated relief valve overfilled, causing the containment building sump to fill and sound an alarm at 4:11 a.m. This alarm, along with higher than normal temperatures on the pilot-operated relief valve discharge line and unusually high containment building temperatures and pressures, were clear indications that there was an ongoing loss-of-coolant accident, but these indications were initially ignored by operators. At 4:15 a.m., the relief diaphragm of the pressurizer relief tank ruptured, and radioactive coolant began to leak out into the general containment building. This radioactive coolant was pumped from the containment building sump to an auxiliary building, outside the main containment, until the sump pumps were stopped at 4:39 a.m.
At about 5:20 a.m., after almost 80 minutes of slow temperature rise, the primary loop's four main reactor coolant pumps began to cavitate as a steam bubble/water mixture, rather than water, passed through them. The pumps were shut down, and it was believed that natural circulation would continue the water movement. Steam in the system prevented flow through the core, and as the water stopped circulating it was converted to steam in increasing amounts. Soon after 6:00 a.m., the top of the reactor core was exposed and the intense heat caused a reaction to occur between the steam forming in the reactor core and the zircaloy nuclear fuel rod cladding, yielding zirconium dioxide, hydrogen, and additional heat. This reaction melted the nuclear fuel rod cladding and damaged the fuel pellets, which released radioactive isotopes to the reactor coolant, and produced hydrogen gas that is believed to have caused a small explosion in the containment building later that afternoon.
At 6:00 a.m. there was a shift change in the control room. A new arrival noticed that the temperature in the pilot-operated relief valve tail pipe and the holding tanks was excessive and used a backup — called a "block valve"— to shut off the coolant venting via the pilot-operated relief valve, but around 32,000 US gal (120,000 l) of coolant had already leaked from the primary loop. It was not until 6:45 a.m., 165 minutes after the start of the problem, that radiation alarms activated when the contaminated water reached detectors; by that time, the radiation levels in the primary coolant water were around 300 times expected levels, and the general containment building was seriously contaminated.
At 6:56 a.m. a plant supervisor declared a site area emergency, and less than 30 minutes later, station manager Gary Miller announced a general emergency.Metropolitan Edison (Met Ed) notified the Pennsylvania Emergency Management Agency (PEMA), which in turn contacted state and local agencies, Governor Richard L. Thornburgh and Lieutenant Governor William Scranton III, to whom Thornburgh assigned responsibility for collecting and reporting on information about the accident. The uncertainty of operators at the plant was reflected in fragmentary, ambiguous, or contradictory statements made by Met Ed to government agencies and to the press, particularly about the possibility and severity of off-site radioactivity releases. Scranton held a press conference in which he was reassuring, yet confusing, about this possibility, stating that though there had been a "small release of radiation...no increase in normal radiation levels" had been detected. These were contradicted by another official, and by statements from Met Ed, who both claimed that no radioactivity had been released. In fact, readings from instruments at the plant and off-site detectors had detected radioactivity releases, albeit at levels that were unlikely to threaten public health as long as they were temporary, and providing that containment of the then highly contaminated reactor was maintained.
Angry that Met Ed had not informed them before conducting a steam venting from the plant, and convinced that the company was downplaying the severity of the accident, state officials turned to the NRC. a.m. met with two other commissioners. However, the NRC faced the same problems in obtaining accurate information as the state, and was further hampered by being organizationally ill-prepared to deal with emergencies, as it lacked a clear command structure and did not have the authority either to tell the utility what to do or to order an evacuation of the local area.After receiving word of the accident from Met Ed, the NRC had activated its emergency response headquarters in Bethesda, Maryland and sent staff members to Three Mile Island. NRC chairman Joseph Hendrie and commissioner Victor Gilinsky initially viewed the accident as a "cause for concern but not alarm". Gilinsky briefed reporters and members of Congress on the situation and informed White House staff, and at 10:00
In a 2009 article, Gilinsky wrote that it took five weeks to learn that "the reactor operators had measured fuel temperatures near the melting point". a.m., roughly half of the uranium fuel had already melted."He further wrote: "We didn't learn for years—until the reactor vessel was physically opened—that by the time the plant operator called the NRC at about 8:00
It was still not clear to the control room staff that the primary loop water levels were low and that over half of the core was exposed. A group of workers took manual readings from the thermocouples and obtained a sample of primary loop water. Seven hours into the emergency, new water was pumped into the primary loop and the backup relief valve was opened to reduce pressure so that the loop could be filled with water. After 16 hours the primary loop pumps were turned on once again, and the core temperature began to fall. A large part of the core had melted, and the system was still dangerously radioactive.[ citation needed ]
On the third day following the accident, a hydrogen bubble was discovered in the dome of the pressure vessel and became the focus of concern. A hydrogen explosion might not only breach the pressure vessel but, depending on its magnitude, might compromise the integrity of the containment vessel leading to a large-scale release of radioactive material. However, it was determined that there was no oxygen present in the pressure vessel, a prerequisite for hydrogen to burn or explode. Immediate steps were taken to reduce the hydrogen bubble and, by the following day, it was significantly smaller. Over the next week, steam and hydrogen were removed from the reactor using a catalytic recombiner and, controversially, by venting straight to the atmosphere.[ citation needed ]
The release occurred when the cladding was damaged while the pilot-operated relief valve was still stuck open. Fission products were released into the reactor coolant. Since the pilot-operated relief valve was stuck open and the loss of coolant accident was still in progress, primary coolant with fission products and/or fuel was released, and ultimately ended up in the auxiliary building. The auxiliary building was outside the containment boundary.
This was evidenced by the radiation alarms that eventually sounded. However, since very little of the fission products released were solids at room temperature, very little radiological contamination was reported in the environment. No significant level of radiation was attributed to the TMI-2 accident outside of the TMI-2 facility. According to the Rogovin report, the vast majority of the radioisotopes released were the noble gases xenon and krypton. The report stated, "During the course of the accident, approximately 2.5 MCi (93 PBq) of radioactive noble gases and 15 Ci (560 GBq) of radioiodines were released." This resulted in an average dose of 1.4 mrem (14 μSv) to the two million people near the plant. The report compared this with the additional 80 mrem (800 μSv) per year received from living in a high altitude city such as Denver. As further comparison, a patient receives 3.2 mrem (32 μSv) from a chest X-ray—more than twice the average dose of those received near the plant. Measures of beta radiation were excluded from the report.
Within hours of the accident, the United States Environmental Protection Agency (EPA) began daily sampling of the environment at the three stations closest to the plant. Continuous monitoring at 11 stations was not established until April 1, and was expanded to 31 stations on April 3. An inter-agency analysis concluded that the accident did not raise radioactivity far enough above background levels to cause even one additional cancer death among the people in the area, but measures of beta radiation were not included, because the EPA found no contamination in water, soil, sediment, or plant samples.
Researchers at nearby Dickinson College—which had radiation monitoring equipment sensitive enough to detect Chinese atmospheric atomic weapons-testing—collected soil samples from the area for the ensuing two weeks and detected no elevated levels of radioactivity, except after rainfalls (likely due to natural radon plate-out, not the accident). 50 mi (80 km) from the reactor subsequent to the accident were found to have significantly higher levels of cesium-137 than in deer in the counties immediately surrounding the power plant. Even then, the elevated levels were still below those seen in deer in other parts of the country during the height of atmospheric weapons testing. Had there been elevated releases of radioactivity, increased levels of iodine-131 and cesium-137 would have been expected to be detected in cattle and goat's milk samples. Yet elevated levels were not found. A later study noted that the official emission figures were consistent with available dosimeter data, though others have noted the incompleteness of this data, particularly for releases early on.Also, white-tailed deer tongues harvested over
According to the official figures, as compiled by the 1979 Kemeny Commission from Metropolitan Edison and NRC data, a maximum of 480 PBq (13 MCi) of radioactive noble gases (primarily xenon) were released by the event. However, these noble gases were considered relatively harmless, and only 481–629 GBq (13.0–17.0 Ci) of thyroid cancer-causing iodine-131 were released. Total releases according to these figures were a relatively small proportion of the estimated 370 EBq (10 GCi) in the reactor. It was later found that about half the core had melted, and the cladding around 90% of the fuel rods had failed, with 5 ft (1.5 m) of the core gone, and around 20 short tons (18 t ) of uranium flowing to the bottom head of the pressure vessel, forming a mass of corium. The reactor vessel—the second level of containment after the cladding—maintained integrity and contained the damaged fuel with nearly all of the radioactive isotopes in the core.
Anti-nuclear political groups disputed the Kemeny Commission's findings, claiming that other independent measurements provided evidence of radiation levels up to seven times higher than normal in locations hundreds of miles downwind from TMI. [ verification needed ]Arnie Gundersen, a former nuclear industry executive and anti-nuclear advocate, said "I think the numbers on the NRC's website are off by a factor of 100 to 1,000".
Gundersen offers evidence, based on pressure monitoring data, for a hydrogen explosion shortly before 2:00 p.m. on March 28, 1979, which would have provided the means for a high dose of radiation to occur. Gundersen cites affidavits from four reactor operators according to which the plant manager was aware of a dramatic pressure spike, after which the internal pressure dropped to outside pressure. Gundersen also claimed that the control room shook and doors were blown off hinges. However official NRC reports refer merely to a "hydrogen burn". [ verification needed ] The Kemeny Commission referred to "a burn or an explosion that caused pressure to increase by 28 pounds per square inch (190 kPa) in the containment building", while The Washington Post reported that "At about 2:00 pm, with pressure almost down to the point where the huge cooling pumps could be brought into play, a small hydrogen explosion jolted the reactor." Work performed for the Department of Energy in the 1980s determined that the hydrogen burn (deflagration), which went essentially unnoticed the first few days occurred 9 hours and 50 minutes after initiation of the accident, had a duration of 12 to 15 seconds and did not involve a detonation.
Twenty-eight hours after the accident began, William Scranton III, the lieutenant governor, appeared at a news briefing to say that Metropolitan Edison, the plant's owner, had assured the state that "everything is under control".Later that day, Scranton changed his statement, saying that the situation was "more complex than the company first led us to believe". There were conflicting statements about radioactivity releases. Schools were closed and residents were urged to stay indoors. Farmers were told to keep their animals under cover and on stored feed.
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. 663,500 population within the 20-mile radius remained in that area. [ full citation needed ] According to a survey conducted in April 1979, 98% of the evacuees had returned to their homes within three weeks.Within days, 140,000 people had left the area. More than half of the
Post-TMI surveys have shown that less than 50% of the American public were satisfied with the way the accident was handled by Pennsylvania State officials and the NRC, and people surveyed were even less pleased with the utility (General Public Utilities) and the plant designer.
Several state and federal government agencies mounted investigations into the crisis, the most prominent of which was the President's Commission on the Accident at Three Mile Island, created by Jimmy Carter in April 1979. [ full citation needed ] Kemeny said that the actions taken by the operators were "inappropriate" but that the workers "were operating under procedures that they were required to follow, and our review and study of those indicates that the procedures were inadequate" and that the control room "was greatly inadequate for managing an accident".The commission consisted of a panel of twelve people, specifically chosen for their lack of strong pro- or anti-nuclear views, and headed by chairman John G. Kemeny, president of Dartmouth College. It was instructed to produce a final report within six months, and after public hearings, depositions, and document collection, released a completed study on October 31, 1979. The investigation strongly criticized Babcock & Wilcox, Met Ed, GPU, and the NRC for lapses in quality assurance and maintenance, inadequate operator training, lack of communication of important safety information, poor management, and complacency, but avoided drawing conclusions about the future of the nuclear industry. The heaviest criticism from the Kemeny Commission concluded that "fundamental changes were necessary in the organization, procedures, practices 'and above all—in the attitudes' of the NRC [and the nuclear industry.]"
The Kemeny Commission noted that Babcock & Wilcox's pilot-operated relief valve had previously failed on 11 occasions, nine of them in the open position, allowing coolant to escape. The initial causal sequence of events at TMI had been duplicated 18 months earlier at another Babcock & Wilcox reactor, the Davis-Besse Nuclear Power Station owned at that time by Toledo Edison. The only difference was that the operators at Davis-Besse identified the valve failure after 20 minutes, where at TMI it took 80 minutes, and the Davis-Besse facility was operating at 9% power, against TMI's 97%. Although Babcock engineers recognized the problem, the company failed to clearly notify its customers of the valve issue.
The Pennsylvania House of Representatives conducted its own investigation, which focused on the need to improve evacuation procedures.[ citation needed ]
In 1985, a television camera was used to see the interior of the damaged reactor. In 1986, core samples and samples of debris were obtained from the corium layers on the bottom of the reactor vessel and analyzed.
According to the IAEA, the Three Mile Island accident was a significant turning point in the global development of nuclear power. U.S. nuclear reactors were canceled from 1980–1984.From 1963–1979, the number of reactors under construction globally increased every year except 1971 and 1978. However, following the event, the number of reactors under construction in the U.S. declined from 1980–1998, with increasing construction costs and delayed completion dates for some reactors. Many similar Babcock & Wilcox reactors on order were canceled; in total, 51
The 1979 TMI accident did not initiate the demise of the U.S. nuclear power industry, but it did halt its historic growth. Additionally, as a result of the earlier 1973 oil crisis and post-crisis analysis with conclusions of potential overcapacity in base load, forty planned nuclear power plants already had been canceled before the TMI accident. At the time of the TMI incident, 129 nuclear power plants had been approved, but of those, only 53 (which were not already operating) were completed. During the lengthy review process, complicated by the Chernobyl Disaster seven years later, Federal requirements to correct safety issues and design deficiencies became more stringent, local opposition became more strident, construction times were significantly lengthened and costs skyrocketed.Until 2012, no U.S. nuclear power plant had been authorized to begin construction since the year before TMI.
Globally, the end of the increase in nuclear power plant construction came with the more catastrophic Chernobyl disaster in 1986 (see graph).
Three Mile Island Unit 2 was too badly damaged and contaminated to resume operations; the reactor was gradually deactivated and permanently closed. TMI-2 had been online for only 3 months but now had a ruined reactor vessel and a containment building that was unsafe to walk in. Cleanup started in August 1979 and officially ended in December 1993, with a total cleanup cost of about $1 billion. Benjamin K. Sovacool, in his 2007 preliminary assessment of major energy accidents, estimated that the TMI accident caused a total of $2.4 billion in property damages.
Initially, efforts focused on the cleanup and decontamination of the site, especially the defueling of the damaged reactor. Starting in 1985, almost 100 short tons (91 t) of radioactive fuel were removed from the site. In 1988, the Nuclear Regulatory Commission announced that, although it was possible to further decontaminate the Unit 2 site, the remaining radioactivity had been sufficiently contained as to pose no threat to public health and safety. The first major phase of the cleanup was completed in 1990, when workers finished shipping 150 short tons (140 t) of radioactive wreckage to Idaho for storage at the Department of Energy's National Engineering Laboratory. However, the contaminated cooling water that leaked into the containment building had seeped into the building's concrete, leaving the radioactive residue too impractical to remove. Accordingly, further cleanup efforts were deferred to allow for decay of the radiation levels and to take advantage of the potential economic benefits of retiring both Unit 1 and Unit 2 together.
In the aftermath of the accident, investigations focused on the amount of radioactivity released by the accident. In total approximately 2.5 megacuries (93 PBq) of radioactive gases, and approximately 15 curies (560 GBq) of iodine-131 was released into the environment. According to the American Nuclear Society, using the official radioactivity emission figures, "The average radiation dose to people living within ten miles of the plant was eight millirem (0.08 mSv), and no more than 100 millirem (1 mSv) to any single individual. Eight millirem is about equal to a chest X-ray, and 100 millirem is about a third of the average background level of radiation received by US residents in a year."
Based on these emission figures, early scientific publications, according to Mangano, on the health effects of the fallout estimated no additional cancer deaths in the 10 mi (16 km) area around TMI. Disease rates in areas further than 10 miles from the plant were never examined. Local activism in the 1980s, based on anecdotal reports of negative health effects, led to scientific studies being commissioned. A variety of epidemiology studies have concluded that the accident had no observable long term health effects.
The Radiation and Public Health Project, an organization with little credibility amongst epidemiologists, medical journal articles and a book on Low Level Radiation and Immune Disease—that reported a spike in infant mortality in the downwind communities two years after the accident. [ archive verification needed ] Anecdotal evidence also records effects on the region's wildlife. For example, according to one anti-nuclear activist, Harvey Wasserman, the fallout caused "a plague of death and disease among the area's wild animals and farm livestock", including a sharp fall in the reproductive rate of the region's horses and cows, reflected in statistics from Pennsylvania's Department of Agriculture, though the Department denies a link with TMI.cited calculations by its member Joseph Mangano—who has authored 19
John Gofman used his own, non-peer reviewed low-level radiation health model to predict 333 excess cancer or leukemia deaths from the 1979 Three Mile Island accident.A peer-reviewed research article by Dr. Steven Wing found a significant increase in cancers from 1979–1985 among people who lived within ten miles of TMI; in 2009 Dr. Wing stated that radiation releases during the accident were probably "thousands of times greater" than the NRC's estimates. A retrospective study of Pennsylvania Cancer Registry found an increased incidence of thyroid cancer in some counties south of TMI (although, notably, not in Dauphin County itself) and in high-risk age groups but did not draw a causal link with these incidences and to the accident. The Talbott lab at the University of Pittsburgh reported finding only a few, small, mostly statistically non-significant, increased cancer risks within the TMI population, such as a non-significant excess leukemia among males being observed. The ongoing TMI epidemiological research has been accompanied by a discussion of problems in dose estimates due to a lack of accurate data, as well as illness classifications.
The TMI accident enhanced the credibility of anti-nuclear groups, who had predicted an accident,and triggered protests around the world. (President Carter—who had specialized in nuclear power while in the United States Navy—told his cabinet after visiting the plant that the accident was minor, but reportedly declined to do so in public in order to avoid offending Democrats who opposed nuclear power. )
Members of the American public, concerned about the release of radioactive gas from the accident, staged numerous anti-nuclear demonstrations across the country in the following months. The largest demonstration was held in New York City in September 1979 and involved 200,000 people, with speeches given by Jane Fonda and Ralph Nader. The New York rally was held in conjunction with a series of nightly "No Nukes" concerts given at Madison Square Garden from September 19–23 by Musicians United for Safe Energy. In the previous May, an estimated 65,000 people—including California Governor Jerry Brown—attended a march and rally against nuclear power in Washington, D.C.
In 1981, citizens' groups succeeded in a class action suit against TMI, winning $25 million in an out-of-court settlement. Part of this money was used to found the TMI Public Health Fund. In 1983, a federal grand jury indicted Metropolitan Edison on criminal charges for the falsification of safety test results prior to the accident. Under a plea-bargaining agreement, Met Ed pleaded guilty to one count of falsifying records and no contest to six other charges, four of which were dropped, and agreed to pay a $45,000 fine and set up a $1 million account to help with emergency planning in the area surrounding the plant.
According to Eric Epstein, chair of Three Mile Island Alert, the TMI plant operator and its insurers paid at least $82 million in publicly documented compensation to residents for "loss of business revenue, evacuation expenses and health claims". Also according to Harvey Wasserman, hundreds of out-of-court settlements have been reached with alleged victims of the fallout, with a total of $15 million paid out to parents of children born with birth defects. However, a class action lawsuit alleging that the accident caused detrimental health effects was rejected by Harrisburg U.S. District Court Judge Sylvia Rambo. The appeal of the decision to U.S. Third Circuit Court of Appeals also failed.
The Three Mile Island accident inspired Charles Perrow's Normal Accident Theory, in which an accident occurs, resulting from an unanticipated interaction of multiple failures in a complex system. TMI was an example of this type of accident because it was "unexpected, incomprehensible, uncontrollable and unavoidable".
Perrow concluded that the failure at Three Mile Island was a consequence of the system's immense complexity. Such modern high-risk systems, he realized, were prone to failures however well they were managed. It was inevitable that they would eventually suffer what he termed a 'normal accident'. Therefore, he suggested, we might do better to contemplate a radical redesign, or if that was not possible, to abandon such technology entirely.
"Normal" accidents, or system accidents, are so-called by Perrow because such accidents are inevitable in extremely complex systems. Given the characteristic of the system involved, multiple failures which interact with each other will occur, despite efforts to avoid them.Events which appear trivial initially cascade and multiply unpredictably, creating a much larger catastrophic event.
Normal Accidents contributed key concepts to a set of intellectual developments in the 1980s that revolutionized the conception of safety and risk. It made the case for examining technological failures as the product of highly interacting systems, and highlighted organizational and management factors as the main causes of failures. Technological disasters could no longer be ascribed to isolated equipment malfunction, operator error or acts of God.
Following the Three Mile Island (TMI) power plant's partial core melt on March 28, 1979, President Jimmy Carter commissioned a study, Report of the President's Commission on the Accident at Three Mile Island (1979).Subsequently, Admiral Hyman G. Rickover was asked to testify before Congress in the general context of answering the question as to why naval nuclear propulsion (as used in submarines) had succeeded in achieving a record of zero reactor-accidents (as defined by the uncontrolled release of fission products to the environment resulting from damage to a reactor core) as opposed to the dramatic one that had just taken place at Three Mile Island. In his testimony, he said:
Over the years, many people have asked me how I run the Naval Reactors Program, so that they might find some benefit for their own work. I am always chagrined at the tendency of people to expect that I have a simple, easy gimmick that makes my program function. Any successful program functions as an integrated whole of many factors. Trying to select one aspect as the key one will not work. Each element depends on all the others.
On March 16, 1979, twelve days before the accident, the movie The China Syndrome premiered, and was initially met with backlash from the nuclear power industry, claiming it to be "sheer fiction" and a "character assassination of an entire industry."
In the film, television reporter Kimberly Wells (Jane Fonda) and her cameraman Richard Adams (Michael Douglas) secretly film a major accident at a nuclear power plant while taping a series on nuclear power. The operating crew notices a high-pressure measurement on a gauge and begins to reduce coolant flow to lower the pressure. This does not appear to work, and they continue reducing the flow until an emergency indicator lamp warns of extremely low pressure. Confused by the conflicting indications, an operator taps the gauge, at which point the needle becomes unstuck and swings over to indicate extremely low pressure. (This is based on a 1970 incident at Dresden Generating Station.) The reactor is SCRAMed. In the aftermath, the plant supervisor, Jack Godell (Jack Lemmon) discovers potentially catastrophic safety violations at the plant and with Wells' assistance attempts to raise public awareness of these violations. At one point in the film, an official tells Jane Fonda's character that an explosion at the plant "could render an area the size of the state of Pennsylvania permanently uninhabitable."
After the release of the film, Fonda began lobbying against nuclear power. In an attempt to counter her efforts, Edward Teller, a nuclear physicist and long-time government science adviser best known for contributing to the Teller–Ulam design breakthrough that made hydrogen bombs possible, personally lobbied in favor of nuclear power.Teller suffered a heart attack shortly after the incident and joked that he was the only person whose health was affected.
Currently, Unit 1—which was not involved in the 1979 accident—is owned and operated by Exelon Nuclear, a subsidiary of Exelon. Unit 1 was sold to AmerGen Energy Corporation, a joint venture between Philadelphia Electric Company (PECO), and British Energy, in 1998. In 2000, PECO merged with Unicom Corporation to form Exelon Corporation, which acquired British Energy's share of AmerGen in 2003. In 2009, Exelon Nuclear absorbed AmerGen and dissolved the company. Exelon Nuclear operates TMI Unit 1, Clinton Power Station and several other nuclear facilities. TMI Unit 1 shut down on September 20, 2019.
Unit 1 had its license temporarily suspended following the incident at Unit 2. Although the citizens of the three counties surrounding the site voted by an overwhelming margin to retire Unit 1 permanently in a non-binding resolution in 1982, it was permitted to resume operations in 1985 following a 4–1 vote by the Nuclear Regulatory Commission. General Public Utilities Corporation, the plant's owner, formed General Public Utilities Nuclear Corporation (GPUN) as a new subsidiary to own and operate the company's nuclear facilities, including Three Mile Island. The plant had previously been operated by Metropolitan Edison Company (Met-Ed), one of GPU's regional utility operating companies. In 1996, General Public Utilities shortened its name to GPU Inc and in 1998, it sold Unit 1 to AmerGen.
General Public Utilities was legally obliged to continue to maintain and monitor the site, and therefore retained ownership of Unit 2 when Unit 1 was sold to AmerGen in 1998. GPU Inc. was acquired by FirstEnergy Corporation in 2001 and subsequently dissolved. FirstEnergy then contracted out the maintenance and administration of Unit 2 to AmerGen. Unit 2 has been administered by Exelon Nuclear since 2003, when Exelon Nuclear's parent company, Exelon, bought out the remaining shares of AmerGen, inheriting FirstEnergy's maintenance contract. Unit 2 continues to be licensed and regulated by the Nuclear Regulatory Commission in a condition known as Post Defueling Monitored Storage (PDMS).
The TMI-2 reactor has been permanently shut down with the reactor coolant system drained, the radioactive water decontaminated and evaporated, radioactive waste shipped off-site, reactor fuel and core debris shipped off-site to a Department of Energy facility, and the remainder of the site being monitored. The owner planned to keep the facility in long-term, monitoring storage until the operating license for the TMI-1 plant expired, at which time both plants would be decommissioned.In 2009, the NRC granted a license extension which allowed the TMI-1 reactor to operate until April 19, 2034. In 2017 it was announced that operations would cease by 2019 due to financial pressure from cheap natural gas, unless lawmakers stepped in to keep it open. When it became clear the subsidy legislation wouldn't pass within the next month Exelon decided to retire the plant, with TMI-1 shut down by September 30, 2019.
|April 1974||Reactor-1 online|
|February 1978||Reactor-2 online|
|March 1979||TMI-2 accident occurred. Containment coolant released into environment.|
|April 1979||Containment steam vented to the atmosphere in order to stabilize the core.|
|July 1980||Approximately 1,591 TBq (43,000 curies) of krypton were vented from the reactor building.|
|July 1980||The first manned entry into the reactor building took place.|
|November 1980||An Advisory Panel for the Decontamination of TMI-2, composed of citizens, scientists, and state and local officials, held its first meeting in Harrisburg, Pennsylvania.|
|December 1980||U.S. 96th Congressional session passes U.S. legislation establishing a five-year nuclear safety, research, demonstration, and development program.|
|July 1984||The reactor vessel head (top) was removed.|
|October 1985||Defueling began.|
|July 1986||The off-site shipment of reactor core debris began.|
|August 1988||GPU submitted a request for a proposal to amend the TMI-2 license to a "possession-only" license and to allow the facility to enter long-term monitoring storage.|
|January 1990||Defueling was completed.|
|July 1990||GPU submitted its funding plan for placing $229 million in escrow for radiological decommissioning of the plant.|
|January 1991||The evaporation of accident-generated water began.|
|April 1991||NRC published a notice of opportunity for a hearing on GPU's request for a license amendment.|
|February 1992||NRC issued a safety evaluation report and granted the license amendment.|
|August 1993||The processing of accident-generated water was completed involving 2.23 million gallons.|
|September 1993||NRC issued a possession-only license.|
|September 1993||The Advisory Panel for Decontamination of TMI-2 held its last meeting.|
|December 1993||Post-Defueling Monitoring Storage began.|
|October 2009||TMI-1 license extended from April 2014 until 2034.|
|May 2019||TMI-1 is announced to be closed in September 2019.|
|September 2019||TMI-1 shutdown at noon on September 30, 2019.|
A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world.
A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants. In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator. Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators.
A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR), which is also a type of light water nuclear reactor. The main difference between a BWR and PWR is that in a BWR, the reactor core heats water, which turns to steam and then drives a steam turbine. In a PWR, the reactor core heats water, which does not boil. This hot water then exchanges heat with a lower pressure water system, which turns to steam and drives the turbine. The BWR was developed by the Argonne National Laboratory and General Electric (GE) in the mid-1950s. The main present manufacturer is GE Hitachi Nuclear Energy, which specializes in the design and construction of this type of reactor.
A nuclear meltdown is a severe nuclear reactor accident that results in core damage from overheating. The term nuclear meltdown is not officially defined by the International Atomic Energy Agency or by the United States Nuclear Regulatory Commission. It has been defined to mean the accidental melting of the core of a nuclear reactor, however, and is in common usage a reference to the core's either complete or partial collapse.
A nuclear power plant is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a generator that produces electricity. As of 2018, the International Atomic Energy Agency reported there were 450 nuclear power reactors in operation in 30 countries around the world.
Davis–Besse Nuclear Power Station is a 894 megawatt (MW), nuclear power plant, located northeast of Oak Harbor, Ohio in Ottawa County, Ohio. It has a single pressurized water reactor. Davis–Besse is operated by Energy Harbor.
Passive nuclear safety is a design approach for safety features, implemented in a nuclear reactor, that does not require any active intervention on the part of the operator or electrical/electronic feedback in order to bring the reactor to a safe shutdown state, in the event of a particular type of emergency. Such design features tend to rely on the engineering of components such that their predicted behaviour would slow down, rather than accelerate the deterioration of the reactor state; they typically take advantage of natural forces or phenomena such as gravity, buoyancy, pressure differences, conduction or natural heat convection to accomplish safety functions without requiring an active power source. Many older common reactor designs use passive safety systems to a limited extent, rather, relying on active safety systems such as diesel powered motors. Some newer reactor designs feature more passive systems; the motivation being that they are highly reliable and reduce the cost associated with the installation and maintenance of systems that would otherwise require multiple trains of equipment and redundant safety class power supplies in order the achieve the same level of reliability. However, weak driving forces that power many passive safety features can pose significant challenges to effectiveness of a passive system, particularly in the short term following an accident.
Three Mile Island Nuclear Generating Station (TMI) is a closed nuclear power plant on Three Mile Island in Londonderry Township, Pennsylvania, on the Susquehanna River just south of Harrisburg. It had two separate units, TMI-1 and TMI-2. The plant was the site of the most significant accident in United States commercial nuclear energy when, on March 28, 1979, TMI-2 suffered a partial meltdown. According to the Nuclear Regulatory Commission (NRC) report, the accident resulted in no deaths or injuries to plant workers or in nearby communities. Follow-up epidemiology studies have linked no incidents of cancer to the accident.
Dresden Generating Station is the first privately financed nuclear power plant built in the United States. Dresden 1 was activated in 1960 and retired in 1978. Operating since 1970 are Dresden units 2 and 3, two General Electric BWR-3 boiling water reactors. Dresden Station is located on a 953-acre (386 ha) site in Grundy County, Illinois, at the head of the Illinois River, near the city of Morris. It is immediately northeast of the Morris Operation—the only de facto high-level radioactive waste storage site in the United States. It serves Chicago and the northern quarter of the state of Illinois, capable of producing 867 megawatts of electricity from each of its two reactors, enough to power over one million average American homes.
The Millstone Nuclear Power Station is a nuclear power plant in Connecticut and the only multi unit nuclear plant in New England. It is located at a former quarry in Waterford. With a total capacity of over 2 GW, the station produces enough electricity to power about 2 million homes. The operation of the Millstone Power Station supports more than 3,900 jobs, and generates the equivalent of over half the electricity consumed in Connecticut.
A containment building, in its most common usage, is a reinforced steel, concrete or lead structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radioactive steam or gas to a maximum pressure in the range of 275 to 550 kPa. The containment is the fourth and final barrier to radioactive release, the first being the fuel ceramic itself, the second being the metal fuel cladding tubes, the third being the reactor vessel and coolant system.
The Saxton Nuclear Experiment Station, also known as the Saxton Nuclear Generating Station or Saxton Nuclear Experimental Corporation Facility, was a small nuclear power plant located in Bedford County, near Saxton, Pennsylvania.
The Economic Simplified Boiling Water Reactor (ESBWR) is a passively safe generation III+ reactor design derived from its predecessor, the Simplified Boiling Water Reactor (SBWR) and from the Advanced Boiling Water Reactor (ABWR). All are designs by GE Hitachi Nuclear Energy (GEH), and are based on previous Boiling Water Reactor designs.
Carolinas–Virginia Tube Reactor (CVTR), also known as Parr Nuclear Station, was an experimental pressurized tube heavy water nuclear power reactor at Parr, South Carolina in Fairfield County. It was built and operated by the Carolinas Virginia Nuclear Power Associates. CVTR was a small test reactor, capable of generating 17 megawatts of electricity. It was officially commissioned in December 1963 and left service in January 1967.
Nuclear safety in the United States is governed by federal regulations issued by the Nuclear Regulatory Commission (NRC). The NRC regulates all nuclear plants and materials in the United States except for nuclear plants and materials controlled by the U.S. government, as well those powering naval vessels.
The three primary objectives of nuclear reactor safety systems as defined by the U.S. Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition and prevent the release of radioactive material.
The United States Government Accountability Office reported more than 150 incidents from 2001 to 2006 of nuclear plants not performing within acceptable safety guidelines. According to a 2010 survey of energy accidents, there have been at least 56 accidents at nuclear reactors in the United States. The most serious of these was the Three Mile Island accident in 1979. Davis-Besse Nuclear Power Plant has been the source of two of the top five most dangerous nuclear incidents in the United States since 1979. Relatively few accidents have involved fatalities.
Boiling water reactor safety systems are nuclear safety systems constructed within boiling water reactors in order to prevent or mitigate environmental and health hazards in the event of accident or natural disaster.
Nuclear power plants pose high risk if chemicals are exposed to those in surrounding communities and areas. This nuclear emergency level classificationresponse system was firstly developed by the US Nuclear Regulatory Commission to allow effective and urgent responses to ultimately control and minimise any detrimental effects that nuclear chemicals can have. These classifications come in four different categories – Unusual Event, Alert, Site Area Emergency (SAE), as well as General Emergency. Thus, each classification has differing characteristics and purposes, depending on the situation at hand. Every nuclear power plant has a different emergency response action plan, also depending on its structure, location and nature. They were developed by thorough discussion and planning with numerous authoritative parties such as local, state, federal agencies as well as other private and non-profit groups that are in association with emergency services. Today, nuclear emergency plans are continuously being developed over time to be improved for future serious events to keep communities and nuclear power plant working members safe. There is a high emphasis for the need of these emergency responses in case of future events. Thus, nuclear plants can, and have paid up to approximately $78 million to ensure that are required measurements are readily available, and that equipment is sufficient and safe. This is applicable for all nuclear power plants in the United States of America.
Nuclear power has been widely established in Pennsylvania since the 1950s, and has grown to provide almost 25% of the energy produced in PA. This is achieved through the four active reactors currently operating. There are five inactive reactors in PA, including Three Mile Island, which had a partial meltdown and caused a reevaluation of nuclear reactor safety practices.
A revised International Nuclear and Radiological Event Scale (INES) extends its reach.
Level 5: Accident with Wider Consequences; Three Mile Island, USA, 1979 — Severe damage to the reactor core.
The steam generator tubes, steam turbine, condenser, and associated pipes, pumps, and heaters used to convert the heat energy of the reactor coolant system into mechanical energy for electrical generation. Most commonly used in reference to pressurized water reactors.
The primary system (also called the Reactor Coolant System) consists of the reactor vessel, the steam generators, the reactor coolant pumps, a pressurizer, and the connecting piping. A reactor coolant loop is a reactor coolant pump, a steam generator, and the piping that connects these components to the reactor vessel. The primary function of the reactor coolant system is to transfer the heat from the fuel to the steam generators. A second function is to contain any fission products that escape the fuel.
(In 1979 Foreword:) "...we arrive at 333 fatal cancers or leukemias."
Thyroid cancer incidence has not increased in Dauphin County, the county in which TMI is located. York County demonstrated a trend toward increasing thyroid cancer incidence beginning in 1995, approximately 15 years after the TMI accident. Lancaster County showed a significant increase in thyroid cancer incidence beginning in 1990. These findings, however, do not provide a causal link to the TMI accident.
The NRC conducted detailed studies of the accident's radiological consequences, as did the Environmental Protection Agency, the Department of Health, Education and Welfare (now Health and Human Services), the Department of Energy, and the Commonwealth of Pennsylvania. Several independent groups also conducted studies. The approximately 2 million people around TMI-2 during the accident are estimated to have received an average radiation dose of only about 1 millirem above the usual background dose. To put this into context, exposure from a chest X-ray is about 6 millirem and the area's natural radioactive background dose is about 100–125 millirem per year for the area. The accident's maximum dose to a person at the site boundary would have been less than 100 millirem above background. In the months following the accident, although questions were raised about possible adverse effects from radiation on human, animal, and plant life in the TMI area, none could be directly correlated to the accident. Thousands of environmental samples of air, water, milk, vegetation, soil, and foodstuffs were collected by various government agencies monitoring the area. Very low levels of radionuclides could be attributed to releases from the accident. Comprehensive investigations and assessments by several well-respected organizations, such as Columbia University and the University of Pittsburgh, have concluded that in spite of serious damage to the reactor, the actual release had negligible effects on the physical health of individuals or the environment.
TMI1 was not operating because it had been shut down for routine refueling.
The chain of events that set off the severe accident at TMI-2 and melted a substatial portion of its core began innocently enough at 4:00 A.M.
The system used to remove energy from the reactor core and transfer that energy either directly or indirectly to the steam turbine.
A tank or vessel that acts as a head tank (or surge volume) to control the pressure in a pressurized water reactor.
The function of the turbine bypass system is to remove excess energy from the reactor coolant system by discharging a stated percentage of rated main steam flow directly to the main condensers, i.e. by bypassing the turbine. This heat is rejected to the condenser through the steam dump valves.
The closing of the secondary system caused heat and pressure to rise rapidly in the primary system, largely because the steam generators could no longer remove heat from the water that had come from the core. As a result, eight seconds after the polisher pumps tripped, the reactor scrammed automatically. The control rods entered the core and terminated the production of heat from nuclear fission. But the problem of dealing with decay heat remained...
When the feedwater flow stopped, the temperature of the reactor coolant increased. The rapidly heating water expanded. The pressurizer level (the level of the water inside the pressurizer tank) rose and the steam in the top of the tank compressed. Pressure inside the pressurizer built to 2,255 pounds per square inch, 100 psi more than normal. Then a valve atop the pressurizer, called a pilot-operated relief valve, or PORV, opened — as it was designed to do — and steam and water began flowing out of the reactor coolant system through a drain pipe to a tank on the floor of the containment building. Pressure continued to rise, however, and 8 seconds after the first pump tripped, TMI-2's reactor — as it was designed to do — scrammed: its control rods automatically dropped down into the reactor core to halt its nuclear fission.
(v) A 1978 revision in the TMI-2 surveillance procedure for the emergency feedwater block valves violated TMI-2's technical specifications, but no one realized it at the time. The approval of the revision in the surveillance procedure was not done according to Met Ed's own administrative procedures. <vi>Performance of surveillance tests was not adequately verified to be sure that the procedures were followed correctly. On the day of the accident, emergency feedwater block valves which should have been open were closed. They may have been left closed during a surveillance test 2 days earlier.
Apparently the valves were closed for routine maintenance, in violation of one of the most stringent rules that the Nuclear Regulatory Commission has. The rule states simply that auxiliary feed pumps can never all be down for maintenance while the reactor is running.
At TMI-2, the PORV opened three seconds after the condensate pumps tripped, exactly as designed. Unfortunately, ten seconds later, after the temperature and pressure in the primary system had diminished, it failed to close as designed. The open relief valve allowed growing quantities of reactor coolant to escape. This was not the first time that the PORV had stuck open at TMI-2, and it was a chronic problem at Babcock and Wilcox plants. The same sequence of events had occurred at Davis-Besse in 1977. In that case, an operator recognized that the valve was open and immediately blocked it.
At TMI-2, the PORV opened three seconds after the condensate pumps tripped, exactly as designed. Unfortunately, ten seconds later, after the temperature and pressure in the primary system had diminished, it failed to close as designed. The open relief valve allowed growing quantities of reactor coolant to escape. This was not the first time that the PORV had stuck open at TMI-2, and it was a chronic problem at Babcock and Wilcox plants. The same sequence of events had occurred at Davis-Besse in 1977. In that case, an operator recognized that the valve was open and immediately blocked it.
After an incident at TMI-2 a year earlier during which the PORV stuck open, an indicator light was installed in the control room. That light showed only that a signal had been sent to close the valve—it did not show whether the valve was actually closed—and this contributed to the confusion during the accident.
Invalid Information. The PORV status indicator is a single red light located on Panel 4. The light is designed to come on when an electrical signal is transmitted to the PORV to open, and go out when a signal is transmitted for the valve to close. As indicated in Figure 3 the light is labeled "Light on - RC-R V2 open." This design is a violation of basic HFE principles as referenced by the following provision of MIL-STD-1472B, paragraph 22.214.171.124.4-. "The absence or extinguishment of a signal or visual indication shall not be used to denote a 'go-ahead,' 'ready,' 'in-tolerance,' or completion condition ...Changes in display status shall signify changes in functional status rather than results of control actuation alone."CS1 maint: uses authors parameter (link) CS1 maint: others (link)
An indicator light in the control room shows when the PORV has been ordered to close - that is, when power to the valve opening solenoid is cut off - but does not show when the valve actually closes. It is now known that the valve did not, if fact, close as it was designed to do. The operators, however, had no direct means of knowing this.
Finally..., a shift supervisor who had just arrived at the plant..., concluded from the pressure and temperature readings in the primary loop that the PORV was at least partially open. He was not certain of what was happening in the core, but he reasoned that no harm and perhaps some benefit could be achieved by shutting the offending relief valve. At 6:22 a.m., he ordered that a backup for the PORV, called a block valve, be closed. By that time, about thirty-two thousand gallons of coolant, more than one-third of the volume in the primary system, had flowed out of the stuck-open PORV. None of the staff in the control room took action to determine how long the PORV had been open or to replace the coolant that had escaped. Closing the block valve was a sound decision but insufficient in itself to prevent the severe damage to the core that leaving the PORV open for about two hours and twenty minutes had caused.
At 6:56 A.M., one of the plant supervisors—accounts vary on which one—declared a site emergency. The emergency plan for TMI-2 directed that this action be taken if radiation alarms sounded in more than one area. A site emergency meant that there was a possibility of an "uncontrolled release of radioactivity" within the plant's boundaries. It required evacuation of the affected buildings, closure of the gates leading to the plant, and notification of the NRC and the state of Pennsylvania. Less than half an hour later..., the Three Mile Island station manager, declared a general emergency... New and alarmingly high radiation readings in the containment building persuaded him to announce a general emergency, which was defined as having the "potential for serious radiological consequences to the health and safety of the general public."
Thornburgh assigned responsibility for collecting and reporting information about the accident to Scranton because of his confidence in the ability and integrity of the lieutenant governor.
The levels measured outside TMI-2 on March 28 were, by any standard, very small and unlikely to threaten public health—as long as they did not occur continuously over an extended period. Met Ed had twenty instruments for measuring environmental radioactivity in locations surrounding the site. Although one stack monitor that was calibrated to measure very low levels of radiation went off-scale early in the accident, readings from other instruments provided reasonably reliable information about releases from the plant. The highest reading was 7 millirems per hour; most measurements were in the range of 1 millirem per hour or less. Those values were far below the amount of radiation normally present in the environment.
...Scranton held his second news conference of the day... "At this point, we believe there is still no danger to public health." Scranton's statements left no doubt that the state had written off the utility as a partner in responding to the accident. The state hoped that it would realize greater benefits in its efforts to guard public health by collaborating with the NRC. As Critchlow recalled, "I think we just almost instinctively preferred to deal with NRC people.".
Like Hendrie, Gilinsky regarded it as cause for concern but not alarm. He tried to keep his previously scheduled appointments to avoid the appearance of a crisis and, in his capacity as acting chairman, he spent much of the day responding to telephone queries from members of Congress and reporters.
By tradition, structure, and statutory mandate, the agency was ill prepared and ill equipped to deal with an emergency at a nuclear plant. It was not an operational agency that ran plants or carried out emergency procedures but a deliberative agency that made rules, considered license applications, and conducted inspections. It had no authority to tell the utility what it should do to stabilize the reactor, no capability for operating the plant, and no power to order an evacuation of the surrounding area. Its role was largely limited to collecting information and making recommendations. The NRC lacked a command structure for dealing with a major accident.
Arnie Gundersen—a nuclear engineer and former nuclear industry executive turned whistle-blower—has done his own analysis, which he shared for the first time at a symposium in Harrisburg last week. "I think the numbers on the NRC's website are off by a factor of 100 to 1,000," he said.
The hydrogen burn occurred throughout essentially all of the 2,033,000 ft3 (57,600 m3) containment during a period of approximately 12 s. Less than 5% of the burning took place in the first 6 s, less than 40% during the next 3 s, and more than half of the burning occurred during the last 3 s. There was no detonation. The hottest gas was the gas that burned at approximately 6 s prior to the end of the burn. Even though the gas was losing heat to the unburned gas and surrounding surfaces after it burned, compression heating was dominant and significantly increased its temperature until the pressure peaked.CS1 maint: uses authors parameter (link)
Prior to the burn (deflagration), the hydrogen was well mixed with the containment air. The average hydrogen concentration was calculated to be 7.9%, wet basis. The hydrogen burn occurred at all three levels in the containment. The burn was initiated somewhere in the lowest level; probably on the west side. Even though the burn time was about 15 sec, nearly all of the burning occurred during a 6-sec period. Over one-half of the burning occurred during the last 3-sec period.CS1 maint: uses authors parameter (link)
These findings, however, do not provide a causal link to the TMI accident
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