Failure Knowledge Database / 100 Selected Cases

Three Mile Island Nuclear Accident
March 28, 1979 at Three Mile Island, Pennsylvania, USA
Masayuki Nakao (Institute of Engineering Innovation, School of Engineering, The University of Tokyo)
An acc ident oc curred a t T hree M ile I sland site, Unit 2 nucl ear power pla nt 1 0 mil es ( 16 km) southe ast ofHarrisburg, Pennsylvania (See Figure 1). Failure of the coo ling sy stem of t he Unit 2 nuclear reactor le d tooverheating and partial melting of the pressurized-water reactor’s uranium core and release of radioactive gas
and contaminated water. The accident had a number of primary causes, related both to technical malfunction in
the c ondensation sy stem and huma n error. Thr ee day s af ter th e a ccident, the official iss ued a n advis ory toevacuate pregnant women and preschool ch ildren liv ing within a 5- mile rad ius of Three Mile Island, raisi ng
fears an explosion and dispersal of radioactivity among residences. The accident increased public concern overthe dangers of nuclear power and slowed construction of other reactors in the US.

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Failure Knowledge Database / 100 Selected Cases
Figure 1. Location of Three Mile Island Generating Station [4]1. EventAn accident occurred at Thr ee Mile Island site of a nuclear pow er pla nt 10 miles (16 k m) sout heast ofHarrisburg, Pennsylvania. Failure of the cooling system of the Unit 2 nuclear reactor led to overheating andpartial melting of the pr essurized-water r eactor’s ur anium core a nd r elease of r adioactive gas andcontaminated wa ter. Thr ee days aft er the ac cident, t he of ficial issu ed an a dvisory to e vacuate pregnant
women and preschool children in a 5-mile radius of Three Mile Island, raising fears of a n explosion anddispersal of radioactivity among residences.
2. Course (Figure 2, 3 and 4)(1) The Three Mile Island Generating Station had two pressurized water reactors. Unit 2 was rated at 959
MWe. The accident in the Unit 2 reactor began when the plant’s main feedwater pumps in thesecondary non-nuclear cooling system failed (4 in Figure 2). In response to the failure, the auxiliary
feedwater pumps kicked in, however, the water did not reach the steam generator because the outlet
valves were closed (5 in Figure 2). The valve was discovered closed about eight minutes into theaccident. Responding to the increase in temperature and pressure in the primary system, the pressurizer
relief valve automatically opened in the line between the pressurizer and the quench tank (7 in Figure2). After the nuclear reactor automatically shut down (8 in Figure 2), pressure in the reactor dropped.
The valve, which was supposed to close after the pressure dropped below the set-point for closure,failed to re-close (11 in Figure 2). The valve was left open for 2 hours and 20 minutes, leaking 80 tonof primary coolant water from the quench tank. At this point, over 100 warnings tripped in the control
room causing a big panic.
(2) Pressure drop in the reactor caused the Emergency Core Cooling System (ECCS) to pour water into thereactor system at 4ton/min (13 in Figure 3). As water and steam escaped through the relief valves,
coolant water surged into the pressurizer, raising the water level in it. Unaware of the stuck valve andthe false indicator readings, operators manually turned off the ECCS (15 in Figure 3). Temperaturerose, steam then formed in the reactor primary cooling system (22 in Figure 3). The pumping of themixture of steam and water caused the reactor cooling pumps to vibrate, and the operators shut down
the pumps to stop severe vibrations (24 in Figure 3). As the reactor cooling water boiled away, the topof the reactor core was exposed (26 in Figure 3).
(3) Coolant water stopped flowing into the pressurizer, the temperature in the primary system reached2,000 degrees Celsius, and approximately 45% of the reactor core melted down (32 in Figure 4). The
coated tubing and water reacted to generate hydrogen gas, and ten hours later, a hydrogen explosion
occurred. Water from the pressure relief valves overfilled the quench tank (18 in Figure 4) and flooded
the containment vessel floor. The floor pump sent the radioactive coolant to the auxiliary building
ventilation system (20 in Figure 4) where the radioactivity escaped to the outside. Eventually, coolingwater was added and the reactor (33 in Figure 4) which then started to cool with natural circulation,
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Failure Knowledge Database / 100 Selected Cases
however, hydrogen gas and radioactive gas kept generating and approximately 10 million curies ofradioactive gas were released into the atmosphere by the event.

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Failure Knowledge Database / 100 Selected Cases

Figure 2. Three Mile Island Nuclear Accident: Sequence of Events 1 [1]Figure 3. Three Mile Island Nuclear Accident: Sequence of Events 2 [1]4Failure Knowledge Database / 100 Selected Cases

Figure 4. Three Mile Island Nuclear Accident: Sequence of Events 3 [1]5Failure Knowledge Database / 100 Selected Cases

3. Cause(1) Inadequate failure information system
Problems with the fa ilure info rmation system in the control r oom caused ina dequate emergency
response by the op erators. A warning si gn bl ocked the view of a LED indicating that the auxiliary
feedwater v alve was closed, an d moreover, a gr een LED was lit when the v alve was closed. The
instruments in the control room did not have consistent LED color – some LEDs were green and somewere red under abnormal conditions.
The instruments only showed th at a CLOSE signal was sent to the relief valve without indicating thevalve’s actu al posi tion. The LED i nstruments wer e not designed t o warn m alfunctions, and theoperators did not know that the relief valve was stuck open.
Operators generally try to prevent filling up the pressurizer because them they will then not be able tocontrol pressure i n the pres surizer filled with water. Not e that the pr essurizer wat er level r ises withhigh-pressure in jection p umps p ushing replacem ent water into the reactor sy stem and cool ing water
surging into the pr essurizer while water and ste am escape thr ough th e open relief val ves. Gasgeneration fr om the hea ted r eactor cor e a lso l ifts the p ressurizer water lev el as wel l. The op erators
mistakenly j udged t hat the pr essurizer wat er level w as r ising under thes e si tuations. The water leve lindicator did not show the actual amount of water in the pressurizer.
The control panel had more than 1,200 LEDs and over 100 alarms went off during the emergency.
(2) Lack of reliability assurance
The malfunctioned pressurizer relief valve had repeated troubles earlier and was unr eliable. Despite of
the obvious issue, the plant instructed the operators t o “fool” the system without replacing the valves
with reliable ones.
(3) Inadequate training of operators
The operators of the Three Mile Island Nuclear Power Plant were not employees of the utilities firm.
The utilities firm had contracted out the plant operations, however, the contract operators lacked proper
knowledge about nuclear reactors and thermal phenomena. They had har dly been tr ained for accident
situation.
(4) Unexpected event not in the safety design standards
Safety devices of nuclear plants are designed to handle certain nuclear accidents. The chain of eventsin the Three Mile Island Nuclear accident was far out of the ran ge of assumed failures and no one hadthought about how to handle such cases.
4. Immediate Action
The ab ove sections des cribe actions ta ken by the operators. Because of conf used commun ication a mong
people uninformed about the plant’s status, officials concluded that the accident released hazardous amount6

Failure Knowledge Database / 100 Selected Casesof radiation and iss ued an advis ory to evacu ate pregnant w omen a nd p reschool c hildren w ithin a 5-mileradius of Three Mile Island. Approximately 140,000 people evacuated the area in panic.
5. Countermeasure
Many agen cies a nd officials i nvestigated the Thr ee Mile Island Nucl ear ac cident. The U.S. Pr esident
ordered a full in vestigation of th e in cident, and th e Ac cident I nvestigation Boar d r eported inad equate
training of operators.
Nuclear Safety Commission of Japan submitted the first, the second and the third reports on the incident.
The reports provided 52 safety measures including safety standards, saf ety study, safety design, o peratingcontrol, disas ter pr evention and safety research. The local government r eviewed its nu clear disas ter
prevention plan.
6. Summary
The plant continued operation with unreliable equipment without through maintenance. Inadequate control
room instrumentation confused the operators and failed to pr ovide proper information about the acciden t.
The operators’ misunderstandings and inadequate responses to the events led to the worst accident.
7. Knowledge(1) Most of the time, an accident starts from a malfunction and gets aggravated with humanmisunderstandings and other problems further complicating the event..
It is important to install highly reliable equipment and maintain a backup system so that it functions
properly when needed.
(2) It is essential to design safety devices that has easy-to-read indicators accommodating human cognitive
constraints. It is also essential to develop safety systems for preventing inadvertent control inputs anderroneous operations.
(3) Outsourcing can involve risks to the operations. It is necessary to prevent outsourcing from degrading
operation efficiency and reliability.
8. BackgroundThe energy sources in the world at the time had shared of 70% oil, 20% coal, and a little less than 10%hydroelectric; oil by far supp lied most o f the electr icity to the wor ld. At the ti me, OP EC t urned i ts oil
pricing policy and the infrastructure of oil supply was vulnerable. Nuclear power then was the star alternate
energy source.
Commercial nuclear power generation involves containing and co ntrolling the fission reactions so that theheat can be us ed to make steam, which in turn drives a turbine and generates electricity. Basically the heatsource, which is oil or coal for thermal power stations, was replaced with fission.
Three Mile Island Nuclear Generating Station consists of two pressurized light water reactors, each with its
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Failure Knowledge Database / 100 Selected Cases
own containment building and cooling towers. The Uni t 2 r eactor, which su ffered a partial meltdown, had959MWe total capacity. It was co nstructed by Ba bcock an d Wilson, and its operations wer e run b yMetropolitan Edison Company. There we re nu mber o f malfunctions repo rted s ince its tes t op eration in
March 19 77 and dur ing th e co mmercial operati on until January 1979: 9 malfunctions in the feedwater
system, st uck-open main st eam rel ief valve, 9 activation of ECCS (1 manual activation). In particular in
March 1 978, a statio n black out o ccurred during low-p ower oper ation, and the pr essurizer r elief va lves
opened by accident. ECCS (high- pressure injection system) was activated automatically, but it was unable
to c ontrol the r eactor pr essure and t he pressurizer wa ter lev el b ecause of the st ation blackout. When t hepower cam e b ack on, the operators responded

to theemergency si tuation ap propriately, but not

immediately because the control room was not equipped with open/close indicator of the pressurizer relief
valve and it took time for the operators to understand the circumstances.
Prior to this accident, s imilar accidents were reported at a Swiss power station in August 1974, at Oconee
Nuclear Station in the US in June 19 75 and at Davis-Besse Nuclear Power Station in the US in Septe mber
1977, a nd the ir r eports in cluding a nalysis and evaluation on the events wer e ava ilable. T he ut ilities firmshould have studied these accident reports closely and have taken preventive measures.
Public confidence i n n uclear ener gy dec lined s harply following t he Three Mile Island accident. I t was amajor cause of the decline in nuclear construction through the 1980s and 1990s.
References[1] Yotaro Hatamura (Editor), Jissai-no Sekkei (Practical Design) Research Foundation (1996) Zoku-Zoku
Jissai-no Sekkei (Practical Design III), The Nikkan Kogyo Shimbun, LTD.
[2] Genshiryoku Hyakka Jiten (Nuclear Energy Encyclopedia),
http://mext-atm.jst.go.jp/atomica/02070401_1.html[3] Ku nio Yanagida (1986) Kyoofu-no Ni-jikan Juuhappun (2 Hours 18 Minutes in Terror), Bungeishunju,
LTD.
[4] “US Nuclear Power Plant Accident Analysis Report III”, Nuclear Safety Bureau Monthly Report, June
1981, 34, Nuclear Safety Bureau of Science and Technology Agency of Japan.

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