-The accident at Three Mile Island was sparked by a maintenance problem. 0:00- During a maintenance operation to clear a resin blockage, water was inadvertently forced into the plants instrument air system. Although it was not a serious problem, the system interpreted it as such and shut down the feedwater pumps. -Operators do not react to alarm because water pressure and temp are normal and there is no cause for concern. -PORV releases steam to a holding tank -Two days prior to accident, maintenance personnel erroneously left the valves shut. -Incorrect mental model because operator training made the operators mental model incorporate the valves as being open. The error by the maint. Crew made the situation different from the mental model and the operators had no reason to suspect the valves were closed since the pumps were operating correctly.
-The light indicated that the signal for the valve to close had been sent, not that the valve was actually closed. The operators could have closed a cutoff valve but they didn’t know that the PORV was open. -Steam and water continued to be released through the PORV which created a Loss of Coolant Accident (LOCA) -Confirmation Bias – The operators assumed that because the light went off, the valve was shut - Operators dismiss EIW because it has come on many times before when there was no leak.
-0:04:30 After turning off the EIW, the water level still appears to be rising but is actually dropping. Water and steam are being released through the PORV. Had the operators left the EIW on, the accident could have been avoided Confirmation Bias - Thought valve was closed but it wasn’t so they were led to an incorrect conclusion that the water level was too high so they shut off the EIW. -0:08:00 Tags were covering the valve position indicators. The secondary side is now operating normally. -0:15:00 The instrument that checks radioactivity is on a secondary control panel and unless they cleared all auditory alarms from the principle panel they would not be able to hear that alarm, so the operators still have no reason to suspect a LOCA. - Decision Complexity Advantage – The advantage of a few complex decisions over several simple ones For one LOCA, the operators experienced over 500 annunciators change status in the first minute, and more than 800 within the second minute. The operators at TMI complained that the bombardment of signals prevented them from obtaining good information about the initial conditions that led to secondary failures. -0:45:00 Gauges in control room erroneously indicate that the water level is up.
-1:20:00 Shaking is caused by the steam passing through the pumps. Operators had not determined a LOCA so they expected natural circulation of coolant to protect the core in the absence of forced pumping. -1:40:00 Steam within the primary loop, now no longer circulating with water, rises. No circulating water => core heats up and converts more water to steam. -2:15:00 The heat from the exposed core turns steam => superheated steam. Control rods + superheated steam = hydrogen and radioactive gases.
-2:20:00 More than a quarter million gallons of radioactive cooling water has been discharged since the PORV first opened. The water level is still low in the primary loop. This water is boiling and damaging the core. -2:30:00 This is the first indication that the operators receive about abnormal radiation levels. -2:45:00 A site emergency is declared. Radioactivity levels are 350 times the normal level. At this time, 50-60 people are in the control trying to resolve the situation. This is not TMI but a typical control room which are usually designed for 5-6 people at most.
-3:00:00 – Temp. scale goes up to 620 degrees F. Later, a digital voltmeter indicated temperatures as high as 2500 degrees F. Overconfidence – The operators that did not believe the temperature reading were overconfident in their own assessment of the situation. 3:40:00 – The EIW starts putting water back into the primary loop. 6:27:00 – All remaining personnel are wearing protective gear. This impairs their communication at some points and some people even removed their mask in order to talk to each other.
-7:30:00 – They have to depressurize the reactor in order to release the heat. -15:50:00 – The core’s temperature is now under control although half of it is melted and part of it has disintegrated.
HF redesign – Compatibility, salience of cues, work design of space so that all controls/displays are accessible to operator w/o having to look behind or stand on a chair. Training program – Better understanding of nuclear physics and better understanding of the equipment (EIW). Indicators – Indicates whether valves are open or closed (PORV)
"Three Mile Island Accident"
The Three Mile Island The Biggest Nuclear Accident in the U.S. Pawitra Masa-at 4937092 SIRS/M 15-Dec-06
Location <ul><li>Three Mile Island Nuclear Generating Station sits on an island of area of 3.29 km2 ( 814 acres ) in the Susquehanna River Pennsylvania , </li></ul><ul><li>near Harrisburg . </li></ul>
Introduction <ul><li>March 28, 1979 – 4 AM </li></ul><ul><li>Minor malfunction caused the second reactor to shut down almost immediately </li></ul><ul><li>A relief valve was supposed to close, but it did not, contrary to what the instrumentation showed </li></ul><ul><li>Operators struggled to determine the problem and an appropriate solution </li></ul><ul><li>After almost 16 hours and the collaboration of 60 or more people, the situation was under control </li></ul>
Timeline of Events <ul><li>0:00:00 A maintenance problem causes the feedwater pumps to shut down. </li></ul><ul><li>0:00:01 An alarm sounds in the control room. </li></ul><ul><li>0:00:03 Pressure relief valve (PORV) o p ens automatically. </li></ul><ul><li>0:00:04 Backup pumps in the secondary loop come on. However, the pumps are disconnected from the system because of a maintenance error two days before. </li></ul><ul><li>Incorrect mental model. </li></ul>
Timeline of Events <ul><li>0:00:09 The PORV light goes out indicating that the valve had closed. However, the valve is still open. This led to a LOCA (Loss of Coolant Accident). </li></ul><ul><li>Confirmation Bias/Incorrect Mental Model </li></ul><ul><li>0:02:00 EIW (Emergency injection water) comes on, allowing water to flow into primary loop. This is a normal safety feature in the event of a LOCA. </li></ul>
Timeline of Events <ul><li>0:04:30 Operators turn off the EIW because the water level is rising and the pressure is dropping. </li></ul><ul><li>Confirmation Bias </li></ul><ul><li>0:08:00 An operator finally notices that the valves for the backup pumps are closed. He opens them. </li></ul><ul><li>0:15:00 3,000 gallons have escaped from the primary loop. </li></ul><ul><li>Hick-Hyman Law </li></ul><ul><li>0:45:00 Water level in primary loop still dropping. </li></ul>
Timeline of Events <ul><li>1:20:00 Pumps pushing water through the primary loop shake violently. 2 of 4 are turned off. </li></ul><ul><li>1:40:00 The other 2 pumps turn off. </li></ul><ul><li>2:15:00 Water no longer covers the top of the core. Control rods release hydrogen and radioactive gases through the PORV. </li></ul>
Timeline of Events <ul><li>2:20:00 An operator from the next shift comes on duty. He notices that the PORV discharge temperature is abnormally high. He shuts the PORV’s backup valve. </li></ul><ul><li>2:30:00 Operators receive first indication that radioactivity levels are going up. </li></ul><ul><li>2:45:00 The radiation alarm sounds. </li></ul>
Timeline of Events <ul><li>3:00:00 High temperatures in the core lead some to believe that the core is uncovered. The temperature is reading off of the scale which leads others to believe that the reading is wrong. </li></ul><ul><li>Overconfidence </li></ul><ul><li>3:40:00 EIW is turned back on. </li></ul><ul><li>6:27:00 All non-essential personnel are evacuated. </li></ul>
Timeline of Events <ul><li>7:30:00 PORV backup valve is opened to lower the pressure in the primary loop. </li></ul><ul><li>15:50:00 The pumps in the primary loop are turned back on. </li></ul>
Amount of contamination. <ul><li>Hydrogen & some radioactive gases were released into atmosphere. </li></ul><ul><li>A maximum of 13 million Curie of radioactive gases release. </li></ul><ul><li>The maximun dosage to a person at the site boundary have been less than 100 mrem </li></ul>
Cleanup <ul><li>Around 12 years to complete. </li></ul><ul><li>Cost of cleanup estimated at $ 973. </li></ul><ul><li>Plant was not reopened until 1985. </li></ul>
Health Effects <ul><li>Government claimed no injuries or adverse health effects from accidents . </li></ul><ul><li>Only one additional cancer death from radiation absorbed folowing the accident would result to those living within 50 miles of the plant. </li></ul>
Health Effects <ul><li>Studies found </li></ul><ul><ul><li>An increase in infant death. </li></ul></ul><ul><ul><li>An increase in babies born with hypothyroidism </li></ul></ul><ul><li>By late 1990, there had been no peer-reviewed articles that present any data on rates of cancer or other diseases. </li></ul>
Iodine - 131 <ul><li>Radiation form of iodine. </li></ul><ul><li>Affects thyroid gland. </li></ul><ul><li>Children have much smaller thyroid gland. </li></ul><ul><li>More energy per gram = higher dose. </li></ul><ul><li>Milk is the main route of contamination. </li></ul>
Fallout from TMI <ul><li>Measured level of airborne Xenon-133 in Albany, NY at three time higher than normal. </li></ul>
Recommendations <ul><li>Use human factors principles to design the control panels. </li></ul><ul><li>Better training program for operators. </li></ul><ul><li>Better indicators for status of valves. </li></ul><ul><li>Shift of plant priorities from continuing power production to safety. </li></ul>
References <ul><li>Gonyeau, Joseph. (2001). Main Control Rooms . Retrieved May 28, 2003. The Virtual Nuclear Tourist. http://www.nucleartourist.com/systems/control_rooms.htm. </li></ul><ul><li>Mason, John F. (Nov. 1979). An Analysis of Three Mile Island. IEEE Spectrum. 33-69. </li></ul><ul><li>PBS Online. (1999). Three Mile Island: What Happened. Retrieved April 22, 2003. Meltdown at Three Mile Island. http://www.pbs.org/wgbh/amex/three/sfeature/index.html </li></ul><ul><li>World Nuclear Association. (2000). Three Mile Island: 1979. Retrieved April 22, 2003. Three Mile Island. </li></ul><ul><li>http://www.world-nuclear.org/info/inf36.htm. </li></ul>