6 - Nuclear Power Plant Incidents Flashcards
nuclear reactor basics
works by induced fissionMODERATOR: slows down reaction to keep free neutrons flowing; increase likelihood of further fissionCOOLANT: removes heat energy; maintains fuel temperatureCONTROL RODS: regulate fission by absorbing neutronsDO ONCE WHOLE CHAPTER COMPLETE
meltdown
uranium melts!
pressurized water reactor (PWR)
• WATER acts as moderator AND coolant• reactor heats water in primary system > heats water in secondary system > steam > drives turbine > generates electricity• steam condensed to water by tertiary system, which vents from cooling towers
three mile island
.partial nuclear meltdown.worse accident in US commercial nuclear power plant history.two units on susquehanna river, near Harrisburg, PA.built in response to 1970s energy crisis + petroleum shortages
march 28, 1979: 4:00:00am
.water leaks thru faulty seal into pneumatic AIR system-control valves for water pumps in secondary system to TRIP (shut down)-valve had failed 11 times before-yet no warnings from manufacturer, no changes to design.SO, secondary turbine + electrical generator shut down.tempe + pressure in primary system INCREASES,.reactor SCRAMS (emerg. shut down), control rods drop into reactor core > absorbing neutrons, stopping nuclear reaction.3 emerg. backup feedwater pumps start in secondary system (but were closed, and unable to pump any water!)
march 28, 1979: 4:00:09am
.no water reaches 3 emergency backup feedwater pumps (closed for maintenance 2 days before!).this cited as a key failure.light signalling these lines were closed was COVERED by a maintenance tag (literally saying it was under repair).later singled out as a key failure!! this part.PORV, pilot operated relief opened to relieve excess pressure but did not close after!-this allowed radioactive coolant water to escape drain tank, causing LOCA (loss of coolant accident) BAD!!
march 28, 1979: 4:02am
.EIW (emergency injection water) pumps start automatically, add water to primary system-BUT, guages show water rising, pressure falling, apparent paradox?? (b/c PORV open)-operators trained to never let reactor “go solid” (reaction stops) from overcooling (thought too much coolant available)-operators turn off EIW, problem grows-so EIW water rushes in and out through stuck open PORV, 30k litres of radioactive water discharged throughout building, no alarms triggered!-gauges show water level rising (but actualy falling) b/c of turbulence of water rushing out of PORV, sensors not reading properly
march 28, 1979: 4:08am
• supervisor notices cutoff valves for emergency backup feedwater pumps in secondarysystem are off- opens cutoff valves, AVERTING a major disaster (would have been a crater!)• secondary cooling system now functioning
march 28, 1979: 5:20am
• reactor coolant water  > steam > pumps shake violently- operators shut two pumps down; other pumps shut down 20 minutes later-shut down because scary, were shaking violently
march 28, 1979: 6:15am, 6:20am
6:15am• decay heat from nuclear core boils off coolant water- nuclear core EXPOSED; zirconium cladding and uranium dioxide fuel starts to melt- intense radiation field causes H2O to split apart- H2O bubble forms  prevents cooling (but not enough O2 for explosion) - control rods release radioactive gases6:20am• operator from next shift notices PORV discharge temperature is high; shuts the backup block valve
march 28, 1979: 6:45am
.radiation alarms sound (350x normal levels!).site emergency declared, general emergency 15m later (danger to surrounding communities).debate: over whether core is exposed, reliability of temperature readings-normal core temp is 600F, was 4000+!-instruments to measure core temp not standard equipment-as part of uni research of core performance, thermocouples installed 30cm above core-read ?????????, if temp above 600F
march 28, 1979: 7:50pm
• primary cooling system pumps turned back on; core temperature under control!-many hours later!
Three Mile Island: human factors errors: over-reliance on…
• operator over-reliance on Emergency Procedures Manuals- Event-Based procedures used: operators have to identify a particular event to find actions to fix it (assumes you know what happened, they didn’t so useless!)- Symptoms-Based procedures now used: procedural actions are linked to specific plant symptoms (wouldn’t have helped then either, because they had poor readings!!! LOLOLOL)
Three Mile Island: human factors errors:
-site emergency not declared until 3 hours after incident, as req. by NRC-maintenance checklists for auxiliary feedwater pumps had been thrown away-NRC: inadequacies of equipment, operator training, and design-president’s commission: incident would have been minor if not for human failures
Three Mile Island: human factors errors: primary cause of incident?
.human intervention primary cause: human error due mostly to control CONTROL ROOM DESIGN-relief valve safety shutoff located on BACK of control panel-computer printer overwhelmed (didn’t print error message about relief valve till 3 hours later)-control panel instruments conflicted-100+ alarm lights triggered in 2 minutes-turning off audible alarm disabled visual “annunciators” (e.g. for radioactive water leak)
