The Three Mile Island nuclear accident ---------------------------------------- On March 28th, 1979, a partial meltdown of a nuclear reactor at a Pennsylvania power station caused a radiation leak that remains America’s most serious nuclear incident to date ---------------------------------------- Three Mile Island Nuclear Generating Station (TMI) 200m 650 ft Susquehanna River 100km 60 miles PA New York Philadelphia Washington D.C. ---------------------------------------- Containment building Turbine building 1970s: United States riding swell of enthusiasm for what was then viewed as perfect energy source (cheaper and cleaner than oil or coal) – nuclear power Vent stack: Releases radiation Auxiliary building Unit 1: Still powering 800,000 homes Damaged nuclear waste sent by rail to be stored at Idaho National Engineering Laboratory TMI 1000km 600 miles UNIT 2 UNIT 1 50m 160 ft ---------------------------------------- Nuclear fallout* Harrisburg TMI 10km 6 miles York ---------------------------------------- ACCIDENT SEQUENCE 1. Water pump stops 2. PORV jams open 3. Water level drops 4. Pump turned off Pressure tank Turbine Pressuriser Containment building (primary system – nuclear) Turbine building (secondary system – non-nuclear) ---------------------------------------- HOW HUMAN ERROR, DESIGN DEFICIENCIES AND COMPONENT FAILURES CAUSED NEAR CATASTROPHE 1. Mar 28, 1979 – 4am: Pump fault stops circulating water coolant through reactor (running at 97%). System overheats and computer shuts it down 2. Pressure Operated Relief Valve (PORV) auto- matically opens to vent pressure but fails to close when pressure normalises Control room instrument wrongly indicates PORV has closed, when in reality radioactive steam and water are escaping 5am: Gauges falsely show water is filling pressure tank, potentially bursting cooling system – event known as going solid 3. Water level is actually dropping as reactor heats up and vaporises liquid (there is no control room instrument to show water level inside pressure tank) 4. Supervisor turns off reactor water pumps, thinking it will prevent it going solid – instead makes matters far worse 6am: Worker discovers stuck PORV – 113,000kg of coolant has evaporated or leaked into plant basement. Parts of reactor are 2,200°C but operators are unaware as core monitoring meters only read up to 370°C (if core reaches 2,760°C, it will melt through containment building and reach outside) Babcock & Wilcox (B&W, reactor designers) try to contact TMI control room but its single phone line is constantly busy 7:30am: Station Manager Gary Miller declares state of emergency 11am: Radiation leak outside plant detected Levels inside reactor Containment building reach 10,000 rems† 7:30pm: B&W tell workers to restart pumps to send water through core again. Reactor finally stabilises. There are no fatalities In following days, build-ups of toxic gas are vented into atmosphere to alleviate growing pressure inside Apr 27: Cold shutdown achieved – reactor core being cooled by natural movement of water ---------------------------------------- Sept 2019: Still functioning (but loss-making) Unit 1 scheduled to permanently close down 2040: Entire plant to be decommissioned, when Unit 2 radioactive decay levels have decreased sufficiently ---------------------------------------- Sources: U.S. NRC, GOA, Union of Concerned Scientists, History, World Nuclear Association, Encyclopaedia Britannica, ABC, StateImpact Pennsylvania, AP, Google Maps Picture: Apple Maps *Residents received radiation dose about 1 millirem higher than usual background dose (area’s natural radioactive background level is about 100-125 millirem per year) †Unit of radiation dosage (humans can safely be exposed to 5 rems per year) © GRAPHIC NEWS