The Three Mile Island accident, TMI, is one of the most heavily written-about nuclear incidents in history. An internet search returns more than 25 million webpages, or hits using the search term “Three Mile Island.” More than 14,000 books are listed on a popular internet book selling site. With that, this article strives to provide a different perspective – a perspective based on principles of safety and process engineering. For comparison, a search for the Fukushima meltdown produces only 12 million webpages (about half) and only 1500 books on the same book-selling site.
Three Mile Island nuclear plant consists of two almost-identical units, number 1 and number 2, located on an island in the Susquehanna River in Pennsylvania, about 80 miles west of Philadelphia and 50 miles north of Baltimore. (see photos) It is very near the heavily populated northeast corridor running from New York City through Philadelphia, Baltimore, and
|Three Mile Island Nuclear Plant|
Unit 1 (top) and Unit 2 (bottom)
Showing operating cooling towers
at unit 1, idle towers at Unit 2. Containment
domes can be seen at left center.
credit: from google maps
This accident at TMI showed clearly and emphatically that a meltdown was possible even without the very rare events that caused the problem at Chernobyl, performing an unauthorized test with safety systems disabled, and at Fukushima, where an earthquake was followed immediately by a tsunami, both of which were greater in magnitude than the plant was designed to handle.
TMI was caused by a routine mechanical failure of a pump. Nobody can claim that a pump failure is a rare event. The problem at TMI was made much, much worse by a valve that stuck open. It is inexcusable that nuclear plant designers, operators, and oversight agencies failed to recognize that valves sometimes stick. The fact that valves sometimes stick in the open position, sometimes closed, and sometimes in-between is well-known to those in the
|Three Mile Island Units 1 and 2 in 1979|
Unit 1 is on the left, Unit 2 is on the right
Nuclear proponents frequently argue that the reason nuclear plants cost so much is due to needless design changes by the NRC during plant construction, and costly retrofits to those plants already in operation. The argument is invalid. We would indeed be a stupid society to allow plants to operate with known safety deficiencies such as existed at TMI before the accident. In fact, if not for the existence of all three required containment systems, deadly nuclear radiation would have spewed all over the northeastern corridor of the United States. Those three levels of containment are the fuel tube, the reactor vessel, and the containment building. Ultimately, the fuel tubes failed and melted, the reactor vessel barely contained the melted fuel, and the containment building contained most, but not all, of the gaseous radioactive particles.
With the passage of time, more than 3 decades now, TMI has faded into the background. Yet, the lessons from that incident are serious, and point to what we can expect going forward.
Summary of Events from NRC website see link
“The accident began about 4 a.m. on Wednesday, March 28, 1979, when the plant experienced a failure in the secondary, non-nuclear section of the plant (one of two reactors on the site). Either a mechanical or electrical failure prevented the main feedwater pumps from sending water to the steam generators that remove heat from the reactor core. This caused the plant's turbine-generator and then the reactor itself to automatically shut down. Immediately, the pressure in the primary system (the nuclear portion of the plant) began to increase. In order to control that pressure, the pilot-operated relief valve (a valve located at the top of the pressurizer) opened. The valve should have closed when the pressure fell to proper levels, but it became stuck open. Instruments in the control room, however, indicated to the plant staff that the valve was closed. As a result, the plant staff was unaware that cooling water was pouring out of the stuck-open valve. (emphasis added) (design flaw – operators relied on bad information. Also, when water pours out of a relief valve, there should be an indication – a flow measurement – of that water. The receiving vessel also should have a level indicator that can be observed as increasing. -- RES )
As coolant flowed from the primary system through the valve, other instruments available to reactor operators provided inadequate information. There was no instrument that showed how much water covered the core. (design flaw) As a result, plant staff assumed that as long as the pressurizer water level was high, the core was properly covered with water. As alarms rang and warning lights flashed, the operators did not realize that the plant was experiencing a loss-of-coolant accident. They took a series of actions that made conditions worse. The water escaping through the stuck valve reduced primary system pressure so much that the reactor coolant pumps had to be turned off to prevent dangerous vibrations. To prevent the pressurizer from filling up completely, the staff reduced how much emergency cooling water was being pumped in to the primary system. These actions starved the reactor core of coolant, causing it to overheat.
Without the proper water flow, the nuclear fuel overheated to the point at which the zirconium cladding (the long metal tubes that hold the nuclear fuel pellets) ruptured and the fuel pellets began to melt. It was later found that about half of the core melted during the early stages of the accident. Although TMI-2 suffered a severe core meltdown, the most dangerous kind of nuclear power accident, consequences outside the plant were minimal. Unlike the Chernobyl and Fukushima accidents, TMI-2's containment building remained intact and held almost all of the accident's radioactive material.
Federal and state authorities were initially concerned about the small releases of radioactive gases that were measured off-site by the late morning of March 28 and even more concerned about the potential threat that the reactor posed to the surrounding population. They did not know that the core had melted, but they immediately took steps to try to gain control of the reactor and ensure adequate cooling to the core. The NRC's regional office in King of Prussia, Pa., was notified at 7:45 a.m. on March 28. By 8 a.m., NRC Headquarters in Washington, D.C., was alerted and the NRC Operations Center in Bethesda, Md., was activated. The regional office promptly dispatched the first team of inspectors to the site and other agencies, such as the Department of Energy and the Environmental Protection Agency, also mobilized their response teams. Helicopters hired by TMI's owner, General Public Utilities Nuclear, and the Department of Energy were sampling radioactivity in the atmosphere above the plant by midday. A team from the Brookhaven National Laboratory was also sent to assist in radiation monitoring. At 9:15 a.m., the White House was notified and at 11 a.m., all non-essential personnel were ordered off the plant's premises.
By the evening of March 28, the core appeared to be adequately cooled and the reactor appeared to be stable. But new concerns arose by the morning of Friday, March 30. A significant release of radiation from the plant's auxiliary building, performed to relieve pressure on the primary system and avoid curtailing the flow of coolant to the core, caused a great deal of confusion and consternation. In an atmosphere of growing uncertainty about the condition of the plant, the governor of Pennsylvania, Richard L. Thornburgh, consulted with the NRC about evacuating the population near the plant. Eventually, he and NRC Chairman Joseph Hendrie agreed that it would be prudent for those members of society most vulnerable to radiation to evacuate the area. Thornburgh announced that he was advising pregnant women and pre-school-age children within a five-mile radius of the plant to leave the area.
Within a short time, chemical reactions in the melting fuel created a large hydrogen bubble in the dome of the pressure vessel, the container that holds the reactor core. NRC officials worried the hydrogen bubble might burn or even explode and rupture the pressure vessel. In that event, the core would fall into the containment building and perhaps cause a breach of containment. The hydrogen bubble was a source of intense scrutiny and great anxiety, both among government authorities and the population, throughout the day on Saturday, March 31. The crisis ended when experts determined on Sunday, April 1, that the bubble could not burn or explode because of the absence of oxygen in the pressure vessel. Further, by that time, the utility had succeeded in greatly reducing the size of the bubble. “ (this was an acceptable conclusion. Basic chemistry shows that the hydrogen was formed by catalytic and heat-driven reactions from hot zirconium in the fuel tubes with water; basically water (H2O) was split into hydrogen and oxygen. The hydrogen formed a gas, but the oxygen combined with the zirconium to form ZrO2, zirconium dioxide. )
Also, see the Report of the President's Commission on Three Mile Island see link
Also "Three Mile Island; A Report to the Commissioners and to the Public," by Mitchell Rogovin and George T. Frampton, NUREG/CR-1250 see link
Plant operators on shift that night were all ex-navy nuclear submarine. Yet, with all their vaunted training, they made one critical mistake after another.
A nuclear reactor core requires continued cooling even after a shutdown – it requires days to cool the tons of nuclear material in the core down to a long-term safe temperature. Circulating water, with the water externally cooled is the means of cooling the core.
Each time an incident occurs in a nuclear plant, the industry advocates insist the plants are safe. They insist that the event was an anomaly, it cannot happen elsewhere. Yet, another accident happens.
It is significant in the TMI meltdown saga that industry experts had witnessed a similar minor loss of coolant accident at a different plant only a year or so earlier. No meltdown occurred, but a sharp analyst noted that such an incident could easily result in a meltdown. A written warning was sent to the NRC, but nothing came of it. In short, the “dots” were collected, but nobody connected the “dots.”
In this case, a cooling water pump failed. This particular pump was located on the steam-generator side. It is important to know that there are three primary water circulating loops in a pressurized-water nuclear plant such as the design at TMI. The first loop is of radioactive water, this circulates through the core at very high pressure, and releases its heat in the steam generator. The water in the first loop remains a liquid at all times under normal operation. The second loop, the one with the pump failure at TMI, has non-radioactive water at high pressure that is turned to steam in the steam generator. This steam then spins the turbine. Exhaust steam from the turbine is condensed back to water in the condenser. The condensate is then pumped as liquid water back to the steam generator. It was the pump for the second loop, sending water to the steam generator that failed at TMI. The third water loop is the cooling water, usually from a cooling tower but sometimes from the ocean or a lake or river. The third water loop circulates cool water through the condenser, and other areas of the plant that require cooling.
Damage to the reactor vessel was extensive. A 1998 report shows that 45 percent of the fuel – 62 metric tonnes -- melted. It is important to note that TMI unit 2 was not a large reactor by today’s standards. It produced only about 900 MWe. More modern plants produce 1200 MWe, and some are designed for 1600 MWe. This means that approximately twice as much core material exists in the largest designs. It is questionable (doubtful?) that a larger reactor would withstand a similar core meltdown. The reason for this is the reactor vessels are made in the form of a vertical cylinder with a closed head at top and bottom. The diameter is only a bit larger (approximately 40 percent greater) for double the volume. TMI 2 Vessel Investigation Project Integration Report, Idaho National Engineering Laboratory, June 1998. (note: this report was 19 years after the accident)
Almost 1 million gallons of radioactive water accumulated in storage tanks and in the bottom of the containment building (700,000 gallons were reported).
Upon eventual opening the reactor and removing the melted mess, the still-radioactive fuel was shipped across the entire US – from Pennsylvania to Idaho – for treatment and disposal. It must have been a comfort to all those citizens along the route to know that radioactive, melted core material from TMI was passing by their homes.
One final note: the TMI meltdown occurred during the showing of one of the most-watched movies ever made on nuclear plants, The China Syndrome, starring Jack Lemmon and the infamous Jane Fonda.
The Three Mile Island meltdown, due to a minor loss of coolant accident, was not caused by a rare event such as an earthquake, tsunami, or other natural disaster as happened at Fukushima. It was not caused by plant operators who violated a planned test as happened at Chernobyl. TMI meltdown was caused by a combination of bad design, a normal equipment failure, a stuck valve that should have been recognized immediately but was not (even by the vaunted former-Navy submarine nuclear operators), improper training, misinterpretation of available data, and general confusion. In the TMI meltdown, operators had perfectly good equipment ready to inject water into the reactor to prevent a meltdown. Instead, they stopped the water flow long enough for the meltdown to occur. Only by sheer good luck was the water flow re-started when it was.
The reactor wall and bottom head were badly damaged by the melted core, and only good luck intervened to provide adequate cooling to the core in time to prevent a breach of the reactor itself. Had a reactor breach happened, melted core material would have flowed onto the containment building floor, and vast quantities of explosive hydrogen gas would have mixed with air in the containment building. The hydrogen most likely would have exploded with devastating consequences, exactly like the explosions at Fukushima 30 or so years later.
Modern reactors continue to have mechanical failures, electrical failures, security breaches, and emergency core shutdowns, as documented in article 16 of TANP (see link below). The major incidents amounted to 70 events in just the past four years – a rate of one every 3 weeks. Minor incidents number in the hundreds each year. It would not take much for a similar combination of operator confusion, lack of training, system replacement with different characteristics, and bad luck this time to have a much worse nuclear nightmare: a complete meltdown and reactor wall breach. It would not create the China Syndrome, but the results would be devastating.
The Truth About Nuclear Power emphasizes the economic and safety aspects by showing that (one) modern nuclear power plants are uneconomic to operate compared to natural gas and wind energy, (two) they produce preposterous pricing if they are the sole power source for a grid, (three) they cost far too much to construct, (four) use far more water for cooling, 4 times as much, than better alternatives, (five) nuclear fuel makes them difficult to shut down and requires very costly safeguards, (six) they are built to huge scale of 1,000 to 1,600 MWe or greater to attempt to reduce costs via economy of scale, (seven) an all-nuclear grid will lose customers to self-generation, (eight) smaller and modular nuclear plants have no benefits due to reverse economy of scale, (nine) large-scale plants have very long construction schedules even without lawsuits that delay construction, (ten) nuclear plants do not reach 50 or 60 years life because they require costly upgrades after 20 to 30 years that do not always perform as designed, (eleven) France has 85 percent of its electricity produced via nuclear power but it is subsidized, is still almost twice as expensive as prices in the US, and is only viable due to exporting power at night rather than throttling back the plants during low demand, (twelve) nuclear plants cannot provide cheap power on small islands, (thirteen) US nuclear plants are heavily subsidized but still cannot compete, (fourteen), projects are cancelled due to unfavorable economics, reactor vendors are desperate for sales, nuclear advocates tout low operating costs and ignore capital costs, nuclear utilities never ask for a rate decrease when building a new nuclear plant, and high nuclear costs are buried in a large customer base, (fifteen) safety regulations are routinely relaxed to allow the plants to continue operating without spending the funds to bring them into compliance, (sixteen) many, many near-misses occur each year in nuclear power, approximately one every 3 weeks, (seventeen) safety issues with short term, and long-term, storage of spent fuel, (eighteen) safety hazards of spent fuel reprocessing, (nineteen) health effects on people and other living things, (twenty) nuclear disaster at Chernobyl, (twenty-one) nuclear meltdown at Three Mile Island, (twenty-two) nuclear meltdowns at Fukushima, (twenty-three) near-disaster at San Onofre, (twenty-four) the looming disaster at St. Lucie, (twenty-five) the inherently unsafe characteristics of nuclear power plants required government shielding from liability, or subsidy, for the costs of a nuclear accident via the Price-Anderson Act, and (twenty-six) the serious public impacts of large-scale population evacuation and relocation after a major incident, or "extraordinary nuclear occurrence" in the language used by the Price-Anderson Act. Additional articles will include (twenty-seven) the future of nuclear fusion, (twenty-eight) future of thorium reactors, (twenty-nine) future of high-temperature gas nuclear reactors, and (thirty), a concluding chapter with a world-wide economic analysis of nuclear reactors and why countries build them. Links to each article in TANP series are included at the end of this article.
Part Fourteen - A Few More Reasons Nuclear Cannot Compete
Part Fifteen - Nuclear Safety Compromised by Bending the Rules
Part Sixteen - Near Misses on Meltdowns Occur Every 3 Weeks
Part Seventeen - Storing Spent Fuel is Hazardous for Short or Long Term
Part Eighteen - Reprocessing Spent Fuel Is Not Safe
Part Twenty Six - Evacuation Plans Required at Nuclear Plants
Part Twenty Seven - Power From Nuclear Fusion
Roger E. Sowell, Esq.
Marina del Rey, California