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128-755: The Three Mile Island accident was a partial nuclear meltdown of the Unit 2 reactor (TMI-2) of the Three Mile Island Nuclear Generating Station on the Susquehanna River in Londonderry Township , near Harrisburg, Pennsylvania . The reactor accident began at 4:00 a.m. on March 28, 1979, and released radioactive gases and radioactive iodine into the environment. It is the worst accident in U.S. commercial nuclear power plant history. On

256-418: A loss-of-pressure-control accident , a loss-of-coolant accident (LOCA), an uncontrolled power excursion. Failures in control systems may cause a series of events resulting in loss of cooling. Contemporary safety principles of defense in depth ensure that multiple layers of safety systems are always present to make such accidents unlikely. The containment building is the last of several safeguards that prevent

384-508: A respirator —the two navigated the reactor auxiliary building to draw the sample. However, Houser had lost his pocket dosimeter while taking measurements. Houser had noted the sample he drew looked "like Alka-Seltzer " and was highly radioactive, with readings as high as 1,000 rem/h. The two spent five minutes in the building, then withdrew. Houser had gone past the NRC's quarterly dose limit for radiation exposure (3 rem/qtr in 1979) by one and

512-451: 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. 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

640-485: A CANDU rather than a meltdown, such as deformation of the calandria into a non-critical configuration. All CANDU reactors are located within standard Western containments as well. One type of Western reactor, known as the advanced gas-cooled reactor (or AGR), built by the United Kingdom, is not very vulnerable to loss-of-cooling accidents or to core damage except in the most extreme of circumstances. By virtue of

768-496: A backup—called a block valve—to shut off the coolant venting via the PORV, 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

896-403: 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. 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". He further wrote: "We didn't learn for years—until the reactor vessel was physically opened—that by the time

1024-727: A coolant with very high heat capacity, sodium metal. As such, they can withstand a loss of cooling without SCRAM and a loss of heat sink without SCRAM, qualifying them as inherently safe. Soviet-designed RBMK reactors ( Reaktor Bolshoy Moshchnosti Kanalnyy) , found only in Russia and other post-Soviet states and now shut down everywhere except Russia, do not have containment buildings, are naturally unstable (tending to dangerous power fluctuations), and have emergency cooling systems (ECCS) considered grossly inadequate by Western safety standards. RBMK emergency core cooling systems only have one division and little redundancy within that division. Though

1152-416: A decade for fission products to decay, the containment can be reopened for decontamination and demolition. Another scenario sees a buildup of potentially explosive hydrogen, but passive autocatalytic recombiners inside the containment are designed to prevent this. In Fukushima, the containments were filled with inert nitrogen, which prevented hydrogen from burning; the hydrogen leaked from the containment to

1280-697: A disc or piston, and a seat. The volume above the piston is called the dome. The pressure is supplied from the upstream side (the system being protected) to the dome often by a small pilot tube. The downstream side is the pipe or open air where the PORV directs its exhaust. The outlet pipe is typically larger than the inlet. 2 in × 3 in (51 mm × 76 mm), 3 in × 4 in (76 mm × 102 mm), 4 in × 6 in (100 mm × 150 mm), 6 in × 8 in (150 mm × 200 mm), 8 in × 10 in (200 mm × 250 mm) are some common sizes. The upstream pressure tries to push

1408-460: 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

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1536-435: A material that slows neutrons and thus speeds up the chain reaction. Water is used as a coolant, but not a moderator. If the water boils away, cooling is lost, but moderation continues. This is termed a positive void coefficient of reactivity. The RBMK tends towards dangerous power fluctuations. Control rods can become stuck if the reactor suddenly heats up and they are moving. Xenon-135, a neutron absorbent fission product, has

1664-411: A maximum of 480 PBq (13 MCi) of radioactive noble gases, primarily xenon , were released by the event. 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

1792-470: A negative void coefficient and a fast-acting rapid shutdown system. The passive emergency cooling system uses reliable natural phenomena to cool the core, rather than depending on motor-driven pumps. The containment structure is designed to withstand severe stress and pressure. In the event of a pipe break of a cooling-water channel, the channel can be isolated from the water supply, preventing a general failure. The greatly enhanced safety and unique benefits of

1920-432: A nuclear reaction to run a generator . If the heat from that reaction is not removed adequately, the fuel assemblies in a reactor core can melt. A core damage incident can occur even after a reactor is shut down because the fuel continues to produce decay heat . A core damage accident is caused by the loss of sufficient cooling for the nuclear fuel within the reactor core. The reason may be one of several factors, including

2048-450: A panel of 12 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. According to the official figures, as compiled by the 1979 Kemeny Commission from Met Ed and NRC data,

2176-479: A reactor coolant. Because of the similar densities of the fuel and the HLM, an inherent passive safety self-removal feedback mechanism due to buoyancy forces is developed, which propels the packed bed away from the wall when certain threshold of temperature is attained and the bed becomes lighter than the surrounding coolant, thus preventing temperatures that can jeopardize the vessel’s structural integrity and also reducing

2304-456: A reactor that uses uranium hydride as a moderator and fuel, similar in chemistry and safety to the TRIGA, also possesses these extreme safety and stability characteristics, and has attracted a good deal of interest in recent times. The liquid fluoride thorium reactor is designed to naturally have its core in a molten state, as a eutectic mix of thorium and fluorine salts. As such, a molten core

2432-589: 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. Efforts focused on the cleanup and decontamination of

2560-558: A significant turning point in the global development of nuclear power. From 1963 to 1979, the number of reactors under construction globally increased every year except in 1971 and 1978. However, following the event, the number of reactors under construction in the U.S. declined from 1980 to 1998, with increasing construction costs and delayed completion dates for some reactors. Many similar Babcock & Wilcox reactors on order were canceled. In total, 52 U.S. nuclear reactors were canceled between 1980 and 1984. The accident did not initiate

2688-406: A situation where the two parameters went in opposite directions. The water level in the pressurizer was rising because the steam in the space at the top of the pressurizer was being vented through the stuck-open PORV, lowering the pressure in the pressurizer because of the lost inventory. The lowering of pressure in the pressurizer made water from the coolant loop surge in and created a steam bubble in

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2816-470: A sudden extreme overpressure and consequent gross structural failure of the primary system or RPV. Though most modern studies hold that it is physically infeasible, or at least extraordinarily unlikely, Haskin, et al state that there exists a remote possibility of an extremely violent FCI leading to something referred to as an alpha-mode failure , or the gross failure of the RPV itself, and subsequent ejection of

2944-465: A tendency to build up in the core and burn off unpredictably in the event of low power operation. This can lead to inaccurate neutronic and thermal power ratings. The RBMK does not have any containment above the core. The only substantial solid barrier above the fuel is the upper part of the core, called the upper biological shield, which is a piece of concrete interpenetrated with control rods and with access holes for refueling while online. Other parts of

3072-504: A theoretical person standing at the plant property line during the entire event would have received a dose of approximately 2 millisieverts (200 millirem), between a chest X-ray's and a CT scan's worth of radiation. This was due to outgassing by an uncontrolled system that, today, would have been backfitted with activated carbon and HEPA filters to prevent radionuclide release. In the Fukushima incident, however, this design failed. Despite

3200-465: A year." According to health researcher Joseph Mangano, early scientific publications estimated no additional cancer deaths in the 10 mi (16 km) area around TMI, based on these numbers. Disease rates in areas farther than 10 miles from the plant were not 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

3328-441: A yellow maintenance tag. The reason why the operator missed the lights for the second valve is not known, although one theory is that his own large belly hid it from his view. 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 water. The closure of these valves was a violation of a key Nuclear Regulatory Commission (NRC) rule, according to which

3456-513: Is a modern Russian-engineered channel type reactor that is a distant descendant of the RBMK, designed to optimize the benefits and fix the serious flaws of the original. Several unique features of the MKER's design make it a credible and interesting option. The reactor remains online during refueling, ensuring outages only occasionally for maintenance, with uptime up to 97-99%. The moderator design allows

3584-421: Is designed to remain dry, several NUREG-class documents advise operators to flood the cavity in the event of a fuel melt incident. This water will become steam and pressurize the containment. Automatic water sprays will pump large quantities of water into the steamy environment to keep the pressure down. Catalytic recombiners will rapidly convert the hydrogen and oxygen back into water. One debated positive effect of

3712-408: Is held shut by piping a small amount of the fluid to the rear of the sealing disk, with the pressure balanced on either side. A separate actuator on the piping releases pressure in the line if it crosses a threshold. This releases the pressure on the back of the seal, causing the valve to open. The essential parts of a PORV are a pilot valve (or control pilot), a main valve , a pilot tube, the dome,

3840-610: 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 core meltdown accident occurs when the heat generated by a nuclear reactor exceeds the heat removed by

3968-500: Is reflective of the normal and safe state of operation of this reactor type. In the event the core overheats, a metal plug will melt, and the molten salt core will drain into tanks where it will cool in a non-critical configuration. Since the core is liquid, and already melted, it cannot be damaged. Advanced liquid metal reactors, such as the U.S. Integral Fast Reactor and the Russian BN-350 , BN-600 , and BN-800 , all have

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4096-452: Is the bulk heavy-water moderator (a separate system from the coolant), and the second is the light-water-filled shield tank (or calandria vault). These backup heat sinks are sufficient to prevent either the fuel meltdown in the first place (using the moderator heat sink), or the breaching of the core vessel should the moderator eventually boil off (using the shield tank heat sink). Other failure modes aside from fuel melt will probably occur in

4224-523: The American Nuclear Society , using the official radioactivity emission figures, "The average radiation dose to people living within 10 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

4352-625: The Deployable Electrical Energy Reactor , a larger-scale mobile version of the TRIGA for power generation in disaster areas and on military missions, and the TRIGA Power System, a small power plant and heat source for small and remote community use, have been put forward by interested engineers, and share the safety characteristics of the TRIGA due to the uranium zirconium hydride fuel used. The Hydrogen Moderated Self-regulating Nuclear Power Module ,

4480-503: The Fukushima incident the emergency cooling system had also been manually shut down several minutes after it started. If such a limiting fault were to occur, and a complete failure of all ECCS divisions were to occur, both Kuan, et al and Haskin, et al describe six stages between the start of the limiting fault (the loss of cooling) and the potential escape of molten corium into the containment (a so-called "full meltdown"): At

4608-424: The cooling systems to the point where at least one nuclear fuel element exceeds its melting point . This differs from a fuel element failure , which is not caused by high temperatures. A meltdown may be caused by a loss of coolant , loss of coolant pressure, or low coolant flow rate or be the result of a criticality excursion in which the reactor is operated at a power level that exceeds its design limits. Once

4736-419: 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 was not being supplied to the steam generators, heat removal from the reactor's primary water loop was limited to steaming

4864-606: The corium layers on the bottom of the reactor vessel and analyzed. On Wednesday, March 28, hours after the accident began, Lieutenant Governor Scranton appeared at a news briefing to say that Met Ed 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. Based on

4992-401: 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 . Given that 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

5120-464: The rate of cancer in and around the area since the accident did determine that there was a statistically significant increase in the rate of cancer, while other studies did not. Due to the nature of such studies, a causal connection linking the accident with cancer is difficult to prove. Cleanup at TMI-2 started in August 1979 and officially ended in December 1993, with a total cost of about $ 1 billion (equivalent to $ 2 billion in 2023). TMI-1

5248-529: The 1975 Rasmussen ( WASH-1400 ) study, asserted steam could produce enough pressure to blow the head off the reactor pressure vessel (RPV). The containment could be threatened if the RPV head collided with it. (The WASH-1400 report was replaced by better-based newer studies, and now the Nuclear Regulatory Commission has disavowed them all and is preparing the overarching State-of-the-Art Reactor Consequence Analyses [SOARCA] study - see

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5376-525: The Disclaimer in NUREG-1150 .) By 1970, there were doubts about the ability of the emergency cooling systems of a nuclear reactor to prevent a loss-of-coolant accident and the consequent meltdown of the fuel core; the subject proved popular in the technical and the popular presses. In 1971, in the article Thoughts on Nuclear Plumbing , former Manhattan Project nuclear physicist Ralph Lapp used

5504-460: 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 from natural radon plate-out, not

5632-400: The MKER design enhance its competitiveness in countries considering full fuel-cycle options for nuclear development. Pilot-operated relief valve In conventional PRVs, the valve is normally held closed by a spring or similar mechanism that presses a disk or piston on a seat, which is forced open if the pressure is greater than the mechanical value of the spring. In the PORV, the valve

5760-484: 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 said that "... fundamental changes will be necessary in the organization, procedures, and practices—and above all—in

5888-406: The PORV discharge line and unusually high containment building temperatures and pressures, were clear indications that there was an ongoing LOCA, 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 into the general containment building . This radioactive coolant was pumped from

6016-519: The PORV 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

6144-449: The RBMK were shielded better than the core itself. Rapid shutdown ( SCRAM ) takes 10 to 15 seconds. Western reactors take 1 - 2.5 seconds. Western aid has been given to provide certain real-time safety monitoring capacities to the operating staff. Whether this extends to automatic initiation of emergency cooling is not known. Training has been provided in safety assessment from Western sources, and Russian reactors have evolved in response to

6272-403: 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 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

6400-689: The accident had no observable long-term health effects. A peer-reviewed research article by Dr. Steven Wing found a significant increase in cancers between 1979 and 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 the Pennsylvania Cancer Registry found an increased incidence of thyroid cancer in some counties south of TMI (although, notably, not in Dauphin County where

6528-651: The accident). Also, the tongues of white-tailed deer harvested over 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 nuclear 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. Elevated levels were not found. A later study noted that

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6656-400: 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 confused, about this possibility, stating that though there had been

6784-403: The actual control pilot valve. There are many designs but the control pilot is essentially a conventional PRV with the special job of controlling pressure to the main valve dome. The pressure at which the control pilot relieves is the functional set pressure of the PORV. When the pilot valve reaches set pressure it opens and releases the pressure from the dome. The piston is then free to open and

6912-553: 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 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

7040-524: The advice of the Chairman of the NRC and in the interest of taking every precaution, I am advising those who may be particularly susceptible to the effects of any radiation, that is, pregnant women and pre-school aged children, to leave the area within a five-mile radius of the Three Mile Island facility until further notice. We've also ordered the closings of any schools within this area. Governor Thornburgh, on

7168-494: The attitudes" of the NRC and the nuclear industry. 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 Kemeny Commission noted that Babcock & Wilcox's PORV had previously failed on 11 occasions, nine of them in

7296-405: The cladding was damaged while the PORV was still stuck open. Fission products were released into the reactor coolant. Since the PORV 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

7424-515: 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

7552-439: 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 with a growing steam bubble in the reactor pressure vessel head, 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

7680-427: The containment could be credibly challenged; some of these modes are not applicable to core melt accidents. If the melted core penetrates the pressure vessel, there are theories and speculations as to what may then occur. In modern Russian plants, there is a "core catching device" in the bottom of the containment building. The melted core is supposed to hit a thick layer of a "sacrificial metal" that would melt, dilute

7808-400: The control room shook and doors were blown off hinges. However, official NRC reports refer merely to a "hydrogen burn". 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

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7936-403: The coolant-inventory level or the position of the stuck-open PORV. The accident heightened anti-nuclear safety concerns among the general public and led to new regulations for the nuclear industry. It accelerated the decline of efforts to build new reactors. Anti-nuclear movement activists expressed worries about regional health effects from the accident. Some epidemiological studies analyzing

8064-401: The core and increase the heat conductivity, and finally the diluted core can be cooled down by water circulating in the floor. There has never been any full-scale testing of this device, however. In Western plants there is an airtight containment building. Though radiation would be at a high level within the containment, doses outside of it would be lower. Containment buildings are designed for

8192-513: The core can lead to fuel–coolant interactions , hydrogen explosions , or steam hammer , any of which could destroy parts of the containment. A meltdown is considered very serious because of the potential for radioactive materials to breach all containment and escape (or be released) into the environment, resulting in radioactive contamination and fallout , and potentially leading to radiation poisoning of people and animals nearby. Nuclear power plants generate electricity by heating fluid via

8320-465: The corium falling into water is that it is cooled and returns to a solid state. Extensive water spray systems within the containment along with the ECCS, when it is reactivated, will allow operators to spray water within the containment to cool the core on the floor and reduce it to a low temperature. These procedures are intended to prevent release of radioactivity. In the Three Mile Island event in 1979,

8448-416: The damaged fuel. There are several possibilities as to how the primary pressure boundary could be breached by corium. As previously described, FCI could lead to an overpressure event leading to RPV fail, and thus, primary pressure boundary fail. Haskin et al report that in the event of a steam explosion, failure of the lower plenum is far more likely than ejection of the upper plenum in the alpha mode. In

8576-456: 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 , 40 planned nuclear power plants already had been canceled before the accident. At the time of the incident, 129 nuclear power plants had been approved, but of those, only 53 which were not already operating were completed. During

8704-500: The efforts of the operators at the Fukushima Daiichi nuclear power plant to maintain control, the reactor cores in units 1–3 overheated, the nuclear fuel melted and the three containment vessels were breached. Hydrogen was released from the reactor pressure vessels, leading to explosions inside the reactor buildings in units 1, 3 and 4 that damaged structures and equipment and injured personnel. Radionuclides were released from

8832-405: The end of the increase in nuclear power plant construction came with the more catastrophic Chernobyl disaster in 1986 (see graph). Initially, GPU planned to repair the reactor and return it into service. However, TMI-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 three months but now had

8960-418: The entire mass of corium dropping into a pool of water (for example, coolant or moderator) and causing extremely rapid generation of steam. The pressure rise within the containment could threaten integrity if rupture disks could not relieve the stress. Exposed flammable substances could burn, but there are few, if any, flammable substances within the containment. Another theory, called an "alpha mode" failure by

9088-419: The environment at the three stations closest to the plant. Continuous monitoring at 11 stations was established on 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

9216-508: The evacuees had returned to their homes within three weeks. 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. According to the IAEA, the Three Mile Island accident was

9344-445: The event of lower plenum failure, debris at varied temperatures can be expected to be projected into the cavity below the core. The containment may be subject to overpressure, though this is not likely to fail the containment. The alpha-mode failure will lead to the consequences previously discussed. It is quite possible, especially in pressurized water reactors, that the primary loop will remain pressurized following corium relocation to

9472-418: The fuel elements of a reactor begin to melt, the fuel cladding has been breached, and the nuclear fuel (such as uranium , plutonium , or thorium ) and fission products (such as caesium-137 , krypton-85 , or iodine-131 ) within the fuel elements can leach out into the coolant. Subsequent failures can permit these radioisotopes to breach further layers of containment. Superheated steam and hot metal inside

9600-698: The general containment building was seriously contaminated with radiation levels of 800  rem / h . 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 , which in turn contacted state and local agencies, Pennsylvania Governor Richard L. Thornburgh and Lieutenant Governor William Scranton III , to whom Thornburgh assigned responsibility for collecting and reporting on information about

9728-503: 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 for 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 . The investigation strongly criticized Babcock & Wilcox , Met Ed, General Public Utilities, and

9856-403: 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

9984-479: The initial failure of plant operators to recognize the situation as a loss-of-coolant accident (LOCA). TMI training and operating procedures left operators and management ill-prepared for the deteriorating situation caused by the LOCA. During the accident, those inadequacies were compounded by design flaws, such as poor control design, the use of multiple similar alarms, and a failure of the equipment to indicate either

10112-709: The integrity of the reactor vessel. In order to do this, someone needed to draw a boron concentration sample in order to ensure there was enough of it in the primary system to shut down the reactor entirely. Unit 2's chemistry supervisor, Edward "Ed" Houser, volunteered to draw the sample after his co-workers were hesitant. Shift supervisor Richard Dubiel asked Pete Velez, the radiation protection foreman for Unit 2, to join Houser. Velez would monitor airborne radiation levels and ensure that no overexposure would occur for either of them. Wearing excessive amounts of protective clothing—three pairs of gloves, one pair of rubber boots and

10240-411: The large core of the RBMK is less energy-dense than the smaller Western LWR core, it is harder to cool. The RBMK is moderated by graphite . In the presence of both steam and oxygen at high temperatures, graphite forms synthesis gas and with the water gas shift reaction , the resultant hydrogen burns explosively. If oxygen contacts hot graphite, it will burn. Control rods used to be tipped with graphite,

10368-469: 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, 1978. Globally,

10496-493: The level of the groundwater . It has not been determined to what extent a molten mass can melt through a structure (although that was tested in the loss-of-fluid-test reactor described in Test Area North 's fact sheet ). The Three Mile Island accident provided real-life experience with an actual molten core: the corium failed to melt through the reactor pressure vessel after over six hours of exposure due to dilution of

10624-526: The lower plenum. As such, pressure stresses on the RPV will be present in addition to the weight stress that the molten corium places on the lower plenum of the RPV; when the metal of the RPV weakens sufficiently due to the heat of the molten corium, it is likely that the liquid corium will be discharged under pressure out of the bottom of the RPV in a pressurized stream, together with entrained gases. This mode of corium ejection may lead to direct containment heating (DCH). Haskin et al identify six modes by which

10752-581: The melt by the control rods and other reactor internals, validating the emphasis on defense in depth against core damage incidents. Other types of reactors have different capabilities and safety profiles than the LWR does. Advanced varieties of several of these reactors have the potential to be inherently safe. CANDU reactors, Canadian-invented deuterium-uranium design, are designed with at least one, and generally two, large low-temperature and low-pressure water reservoirs around their fuel/coolant channels. The first

10880-405: The molecular profile of [thyroid cancer] in the population surrounding TMI", establishing a potential causal mechanism, although not definitively proving causation. Nuclear meltdown A nuclear meltdown ( core meltdown , core melt accident , meltdown or partial core melt ) is a severe nuclear reactor accident that results in core damage from overheating. The term nuclear meltdown

11008-423: 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 temperatures in the PORV tail pipe and the holding tanks were excessive, and used

11136-555: The official emission figures were consistent with available dosimeter data, though others have noted the incompleteness of this data, particularly for releases early on. 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 U.S. President Jimmy Carter in April 1979. The commission consisted of

11264-426: The open PORV, RCS pressure dropped as did pressurizer level after peaking 15 seconds after the turbine trip. Also, 15 seconds after the turbine trip, coolant pressure had dropped to 2,205 psi (152.0 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,

11392-533: 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 . The only differences were that the operators at Davis–Besse identified the valve failure after 20 minutes, where at TMI it took 80 minutes, and the fact that the Davis–Besse facility

11520-405: The orderly release of pressure without releasing radionuclides, through a pressure release valve and filters. Hydrogen/oxygen recombiners also are installed within the containment to prevent gas explosions. In a melting event, one spot or area on the RPV will become hotter than other areas, and will eventually melt. When it melts, corium will pour into the cavity under the reactor. Though the cavity

11648-403: The piston open but it is opposed by that same pressure because the pressure is routed around to the dome above the piston. The area of the piston on which fluid force is acting is larger in the dome than it is on the upstream side; the result is a larger force on the dome side than the upstream side. This produces a net sealing force. The pressure from the pilot tube to the dome is routed through

11776-405: The plant operator called the NRC at about 8:00 a.m., roughly half of the uranium fuel had already melted." 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

11904-489: The plant to the atmosphere and were deposited on land and on the ocean. There were also direct releases into the sea. As the natural decay heat of the corium eventually reduces to an equilibrium with convection and conduction to the containment walls, it becomes cool enough for water spray systems to be shut down and the reactor to be put into safe storage. The containment can be sealed with release of extremely limited offsite radioactivity and release of pressure. After perhaps

12032-409: The point at which the corium relocates to the lower plenum, Haskin, et al relate that the possibility exists for an incident called a fuel–coolant interaction (FCI) to substantially stress or breach the primary pressure boundary when the corium relocates to the lower plenum of the reactor pressure vessel ("RPV"). This is because the lower plenum of the RPV may have a substantial quantity of water -

12160-600: 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 noble gases xenon and krypton resulting in an average dose of 1.4 mrem (14 μSv) to

12288-412: 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. In total, approximately 2.5 megacuries (93 PBq) of radioactive gases and approximately 15 curies (560 GBq) of iodine-131 were released into the environment. According to

12416-422: 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. Arnie Gundersen , a former nuclear industry executive and anti-nuclear advocate, said "I think the numbers on the NRC's website are off by

12544-450: The reactor building, however, where it mixed with air and exploded. During the 1979 Three Mile Island accident, a hydrogen bubble formed in the pressure vessel dome. There were initial concerns that the hydrogen might ignite and damage the pressure vessel or even the containment building; but it was soon realized that lack of oxygen prevented burning or explosion. One scenario consists of the reactor pressure vessel failing all at once, with

12672-463: The reactor coolant - in it, and, assuming the primary system has not been depressurized, the water will likely be in the liquid phase , and consequently dense, and at a vastly lower temperature than the corium. Since corium is a liquid metal-ceramic eutectic at temperatures of 2,200 to 3,200 K (1,930 to 2,930 °C), its fall into liquid water at 550 to 600 K (277 to 327 °C) may cause an extremely rapid evolution of steam that could cause

12800-485: 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

12928-444: The reactor pressure vessel head, aided by the decay heat from the fuel. This steam bubble was invisible for the operators, and this mechanism had not been trained. Indications of high water levels in the pressurizer contributed to 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

13056-516: The reactor was located) and in high-risk age groups but did not draw a causal link between these incidences and the accident. The Talbott lab at the University of Pittsburgh reported finding a few, small increased cancer risks within the TMI population. A more recent study reached "findings consistent with observations from other radiation-exposed populations," raising "the possibility that radiation released from [Three Mile Island] may have altered

13184-473: The recriticality potential by limiting the allowable bed depth. Some design concepts for nuclear reactors emphasize resistance to meltdown and operating safety. The PIUS ( process inherent ultimate safety ) designs, originally engineered by the Swedes in the late 1970s and early 1980s, are LWRs that by virtue of their design are resistant to core damage. No units have ever been built. Power reactors, including

13312-412: The relatively inert coolant (carbon dioxide), the large volume and high pressure of the coolant, and the relatively high heat transfer efficiency of the reactor, the time frame for core damage in the event of a limiting fault is measured in days. Restoration of some means of coolant flow will prevent core damage from occurring. Recently heavy liquid metal, such as lead or lead-bismuth, has been proposed as

13440-421: The release of radioactivity to the environment. Many commercial reactors are contained within a 1.2-to-2.4-metre (3.9 to 7.9 ft) thick pre-stressed, steel-reinforced, air-tight concrete structure that can withstand hurricane -force winds and severe earthquakes . Before the core of a light-water nuclear reactor can be damaged, two precursor events must have already occurred: The Three Mile Island accident

13568-486: 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 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

13696-471: The seven-foot-high instrument panel also meant that it was effectively out of sight. Less than a minute after the beginning of the event, the water level in the pressurizer began to rise, even though RCS pressure was falling. With the PORV stuck open, coolant was being lost from the RCS, a loss-of-coolant accident (LOCA). Expected symptoms for a LOCA were drops in both RCS pressure and pressurizer level. The operators' training and plant procedures did not cover

13824-448: The seven-point logarithmic International Nuclear Event Scale , the TMI-2 reactor accident is rated Level   5, an "Accident with Wider Consequences". The accident began with failures in the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve (PORV) in the primary system, which allowed large amounts of water to escape from the pressurized isolated coolant loop. The mechanical failures were compounded by

13952-540: 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. Planning and work was partially hampered by too-optimistic views about the damage. In 1988, the NRC 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

14080-476: 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 did not 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

14208-484: The steam bubble at the top. When RCS pressure rose to 2,255 psi (155.5 bar), the pilot-operated relief valve (PORV) opened, relieving steam through piping to the reactor coolant drain tank in the containment building basement. RCS pressure continued to rise, reaching the reactor protection system high-pressure trip setpoint of 2,355 psi (162.4 bar) eight seconds after the turbine trip. The reactor automatically tripped , its control rods falling into

14336-413: The term "China syndrome" to describe a possible burn through of the containment structures, and the subsequent escape of radioactive material(s) into the atmosphere and environment. The hypothesis derived from a 1967 report by a group of nuclear physicists, headed by W. K. Ergen . Some fear that a molten reactor core could penetrate the reactor pressure vessel and containment structure and burn downwards to

14464-569: 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. 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

14592-438: The three main water/steam loops in a pressurized water reactor . The initial cause of the accident happened 11 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 other impurities in the water from accumulating in the steam generators and to decrease corrosion rates on

14720-532: The two million people near the plant. In 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. On average, a U.S. resident receives an annual radiation exposure from natural sources of about 310 mrem (3,100 μSv). Within hours of the accident, the United States Environmental Protection Agency (EPA) began daily sampling of

14848-451: The upper plenum of the RPV as a missile against the inside of the containment, which would likely lead to the failure of the containment and release of the fission products of the core to the outside environment without any substantial decay having taken place. The American Nuclear Society has commented on the TMI-2 accident, that despite melting of about one-third of the fuel, the reactor vessel itself maintained its integrity and contained

14976-593: The use of less-enriched fuels, with a high burnup rate. Neutronics characteristics have been optimized for civilian use, for superior fuel fertilization and recycling; and graphite moderation achieves better neutronics than is possible with light water moderation. The lower power density of the core greatly enhances thermal regulation. An array of improvements make the MKER's safety comparable to Western Generation III reactors: improved quality of parts, advanced computer controls, comprehensive passive emergency core cooling system, and very strong containment structure, along with

15104-585: The weaknesses that were in the RBMK. Nonetheless, numerous RBMKs still operate. Though it might be possible to stop a loss-of-coolant event prior to core damage occurring, any core damage incidents will probably allow massive release of radioactive materials. Upon entering the EU in 2004, Lithuania was required to phase out its two RBMKs at Ignalina NPP , deemed totally incompatible with European nuclear safety standards. The country planned to replace them with safer reactors at Visaginas Nuclear Power Plant . The MKER

15232-467: Was a compounded group of emergencies that led to core damage. What led to this was an erroneous decision by operators to shut down the ECCS during an emergency condition due to gauge readings that were either incorrect or misinterpreted; this caused another emergency condition that, several hours after the fact, led to core exposure and a core damage incident. If the ECCS had been allowed to function, it would have prevented both exposure and core damage. During

15360-526: Was a key contributor to the initial failure to recognize the accident as a LOCA 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. With the PORV still open, the pressurizer relief tank that collected the discharge from the PORV 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

15488-507: 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

15616-401: 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

15744-403: 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 by venting directly into the open air. The release occurred when

15872-427: 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

16000-413: Was only admitted back to work the following quarter. 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 could breach the pressure vessel and, depending on its magnitude, might compromise the integrity of the containment building leading to a large-scale release of radioactive material. However, it

16128-455: 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. 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

16256-414: 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 dangerously radioactive. On the day following the accident, March 29, control room operators needed to ensure

16384-484: Was restarted in 1985, then retired in 2019 due to operating losses. It is expected to go back into service by 2028 as part of a deal with Microsoft to power its data centers. In the night 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 Wednesday, March 28, 1979, began at 4:00:36 a.m. EST in TMI-2's secondary loop, one of

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