Operational Reactor Safety
22.091 /22.903
Professor Andrew C. Kadak Professor of the Practice
Lecture 19
Three Mile Island Accident
Primary system
Pilot operated relief valve Secondary System
Emergency Cooling Pump
Feedwater line
Pressurizer
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September 24, 1977 - 9 :24 PM
Davis Besse Event:
Power Level 9%
Transient initiated - Loss of “feedwater” to steam generator
Temperature increased in reactor
Water level in pressurizer increased
Pilot Operated Relief Valve (PORV) opened
PORV did not close but stuck open
Reactor Automatically Shutdown
Emergency Feedwater pumps automatically started
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Primary coolant system cooled and shrunk
Primary system
Pilot operated relief valve Secondary System
Emergency Cooling Pump
Feedwater line
Pressurizer
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Event Sequence Continued
Primary circuit pressure dropped rapidly
Coolant leakage out of PORV
Coolant shrinkage (cooldown)
Emergency Core Cooling System (ECCS) automatically starts to provide more water
Operators monitoring water level in pressurizer which was changing until with ECCS water level became normal.
Operators shutdown ECCS cooling water system
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However PORV still open - “ small break loss of coolant accident”
Event Sequence Continued
Since PORV was open - pressure kept dropping reaching “saturation pressure” of coolant which allowed the water to boil - f orming steam voids which caused the pressurizer water level to rise.
22 Minutes later, operators determined that there was a continuing primary coolant leak and closed the block valve downstream of the pressurizer.
Operators then restored cooling water by ECCS and water make up system of reactor and returned reactor to normal shutdown condition.
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Consequences
Core damage accident avoided by timely operator action.
NRC, Babcock and Wilcox and the utility investigated the incident
Neither NRC nor the nuclear steam supplier - B &W shared this information with other B&W plants or the rest of the industry.
1975 Rasmussen (MIT Prof) Report analyzed probabilistic risks (PRA) of nuclear plants
Conclusion - s mall break LOCAs are dominant accident contributors
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NRC and critics did not want to use PRA is safety determinations
18 MONTHS LATER
March 28, 1979
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Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
Three Mile Island
What Happened and Why ?
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Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
What Is Three Mile Island ?
TMI is a two unit nuclear plant site in Middletown, Pennsylvania
Unit 1- 792 Mwe: Unit 2 - 8 80 Mwe
Babcock & Wilcox Designed PWR
2 Steam Generators - once through
4 Main Coolant Pumps
Condensers cooled by Cooling Towers
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Primary system
Pilot operated relief valve Secondary System
Emergency Cooling Pump
Feedwater line
Pressurizer
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Steam Generators
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B&W Once T h rough Steam Generator Reci rculating U Tube Steam Generator
Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
Reactor Shutdown Signals
Overpower
High Temperature Outlet
High Pressure
Reactor Building Pressure
Low Pressure
Power imbalance/flow
Reactor Coolant Pumps
ARTS - anticipatory reactor trip system
(not at TMI) - f or feedwater and turbine trip for Integrated Control System)
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Precursors
TMI Unit 2 - Newest Unit on Site
Small coolant leak in the pressurizer relief valve raises temperature downstream of valve which drains into the drain tank
Operators unaware that two valves on emergency feedwater valves were closed following maintenance (were supposed to be open).
Small blockage in a transfer line for demineralizer resins which could not be cleared.
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Initiation
Plant operating normally at 97% power
4:00:37 (AM) loss of condensate flow due to a condensate pump trip (shutdown).
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Key Sequence of Events
Courtesy of Three Mile Island Nuclear Power Station. Used with permission. 18
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Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
Courtesy of Three Mile Island Nuclear Power Station. Used with permission. 20
Courtesy of Three Mile Island Nuclear Power Station. Used with permission. 21
Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
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Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
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Bubble Trouble
Hydrogen Production
Due to zirconium water reaction
Oxygen generation due to
Radiolysis
Boiling
Possible explosive mixture ?
Why or why not ?
Why missed in analysis ?
Was reactor vessel in danger of exploding ?
What was missing in the approach ?
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NRC - I ndustry
Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
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Inside TMI Reactor Vessel Post Accident
Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
1979
Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
Today
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Public Health and Environmental Consequences
Studies by EPA, Health and Human Services, DOE, NRC and the State of Pennsylvania
Thousands of environmental samples
Conclusions
Average dose to 2 million people in area < 1 mrem
Max dose at site boundary full time - 1 00 mrem
Natural background in area - 1 00-125 mrem/yr
1 Chest x-ray - 6 millirem
No adverse Health Effects
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Lessons Learned (Good)
Water helps even if core is severely damaged
Vessel did not fail even with molten core in the bottom
No energetic steam explosion
Most fission products contained inside
Containments work
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Lessons Learned (not so good)
Inadequate operator training - f ormed INPO
Instrumentation needs upgrading - d id it
Added Safety Parameter Display System
Added new instrumentation
Reactor Water Level
Post Accident Sampling System
Added Shift Technical Advisor to all shifts
Emergency planning needed upgrading - d one
Focused attention on severe accident research
Need new operator emergency response procedures - c ritical safety functions - d one
Risk focus should not be on Large Break LOCA but more likely events
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Use of Probabilistic Risk Analysis to understand safety of plants not just depend on regulations
Courtesy of Three Mile Island Nuclear Power Station. Used with permission.
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Botto m Lines
Precursors are important indicators of problems - need to be addressed not ignored
Industry Complacency is a risk.
Attention to detail and understanding how plants work is vital
On-line risk monitors very helpful in gauging plant status
Knowledgeable and well trained staff and engineers very important
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Management and Safety Culture hugely important to safe operations.
Homework
Read Rogovin Report Executive Summary
Analyze event from the perspective of why the accident happened and why it became a such serious problem. (Beyond the obvious of failing to recognize the open PORV). Given your experience in the simulators (recall that the simulator you were in did not represent control rooms of the 1970’s).
Consider operator training
Regulations
Technical response
Knowledge
Control room Design and instrumentation
Fundamental design of reactor
Management culture
Read Steam Generator Report for background on designs
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3 pages of analysis (11 font - 1.5 line spacing).
MIT OpenCourseWare http://ocw.mit.edu
22.091 Nuclear Reactor Safety
Spring 2008
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