Transcript Accidents Happen - Welcome to Mr. Everett's Webpage!

Accidents Happen
But Nuclear Accidents Require Special Skill!
Nuclear Power Overview
• Fission of U235 releases excess nuclear Binding Energy.
• E=MC2
•
0n
1
+ 92U235 = Fission Energy + 2 0n1 +
54Xe
131
+
36Kr
84
+…
• The above nuclear reaction is an example; many
different fission fragments result from fission.
• Fission Energy takes the form of kinetic energy of the
fission fragments (fission products) & gamma radiation.
• Friction converts the kinetic energy into heat.
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Nuclear Power Overview
• A nuclear power reactor is designed to
manage the fuel geometry and neutron
population to sustain a given power level.
• Fuel geometry is established by a core
design which properly positions fuel rods.
• Neutron population and neutron kinetic
energy is controlled by control rods and
reactor materials.
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Nuclear Power Plant Fundamentals
• Reactor Coolant System includes the reactor,
water, primary piping, reactor coolant pumps, a
pressurizer and steam generator tubes.
– Water flows by the fuel rods and removes heat of
fission while slowing down (moderating) neutrons so
they are available to the fuel for more fission.
– Water transports the heat to the steam generator
tubes where it is transferred to water on the boiler
side (secondary side) of the tubes.
– The pressurizer maintains pressure in the system to
keep the water from boiling. Heaters , Spray 
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Nuclear Power Plant Fundamentals
• Secondary System includes steam
generator (boiler), steam piping, turbine,
condenser, condensate/feed piping and
condensate/feed pumps.
• Secondary water boils in steam generator
and steam drives turbine for electric
generator. Steam condenses in
condenser and is pumped back to steam
generator to repeat cycle.
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Emergency Systems
• High and low pressure injection systems
supply water to reactor coolant system in
the event of a loss of coolant accident.
• Reactor Containment building seals in
radioactivity and prevents releases to the
environment.
• Radiation Monitoring Systems detect air
and water radioactive contamination.
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Nuclear Safety Principles
• Nuclear reaction terminates immediately
by reactor SCRAM. All control rods
inserted.
• Energy balance must be maintained.
Energy produced must be less than or
equal to energy removed.
• Reactor fuel (core) must be covered by
water to provide sufficient heat transfer to
control fuel temperature.
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Nuclear Safety Principles
• Design must provide inherent safety,
redundancy, defense in depth and good
human factors. Must handle worst case
scenarios.
• Emergency procedures must be symptom
based not event based and provide for
multiple failures.
• Training must develop emergency skills
and team skills via full scale simulation.
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Nuclear Safety Principles
• Staff selection must include crossdiscipline engineering experience.
• Emergency preparedness must be
frequently exercised by large scale drills.
• External oversight (INPO & NRC) and
industry event awareness must be
maintained.
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Three Mile Island Unit-2
• Accident pre-conditions
– Control Room Design - Engineering
Convenience Focus rather than Operator
Effectiveness Focus
– Procedures - Event Based and only Single
Cause
– Training - Operating Crew Included None with
Engineering Degrees, NO Simulator Training
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Three Mile Island Unit-2
• Accident Events
– Condensate valve failure  Turbine Trip
– T↑, P↑ Pilot Operated Relief Valve fails open
– Pzr Level ↓, P↓ Crew Confusion - Procedures
& Training didn't handle multiple failures,
Instrumentation led to false conclusions
– Crew Errors - Loss of Coolant recognized
late, Reactor Coolant pumps turned OFF, Rx
Building Floor Drains not isolated
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Three Mile Island Unit-2
• Accident Aftermath
– Reactor core uncovered  melted down
– Radioactivity released to the environment but
no deaths, illness or injuries
– Re-design of control room, procedures,
training and staffing
– Increased Industry Oversight (INPO & NRC)
– World Record Safety and Performance
– No new plant construction in 27+ years
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Chernobyl Accident – Pre-conditions
• Engineering Arrogance
– No containment building
– No fear of positive feedback in reactivity
– Not designed for worst case accident
• Operational Arrogance
– No concern for safety during tests
– No concern for violating written procedures
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Chernobyl Accident – Pre-conditions
• RBMK 1000 reactor
– Use carbon blocks as moderator – poor choice since
carbon (charcoal) is flammable
– Have positive reactivity coefficient when at low power
– Have no containment structure
• Conducting low-power experiments
– Have safety systems bypassed
• Public told “a nuclear accident is impossible”
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Chernobyl Accident – Events
• High power operation followed by medium
power →Xenon transient →control rods
withdrawn (violate safety procedure)
• Low power operation in manual control
where Rx is unstable due to positive
reactivity void coefficient
• Within seconds power jumps 10x
• Steam explosion destroys core and roof
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Chernobyl Accident – Events
•
•
•
•
•
Carbon moderator ignites and burns
Core ejected into atmosphere
Firemen respond and die
More firemen respond and die
Rx buried in >5000 tons of borated
material by helicopter material drops
• Widespread contamination
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Chernobyl Accident – Aftermath
• Delay in reporting event to the world
• Foodstuffs (livestock and crops) are
radiologically contaminated
• 135000 people evacuated from 30km area
• 31 people died; 134 treated for acute
radiation syndrome
• Substantial increase in thyroid cancer
• RBMK Rx’s are closed
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Nuclear Accident Summary
• Nuclear accidents require
– Errors in design
– Errors in procedures
– Errors in training
– Errors in operator performance
• Nuclear accidents have improved the
industry
– Better designs, procedures, training &
planning
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Nuclear Accident Questions
• What role did poor design play in
– the TMI-2 accident?
– the Chernobyl accident?
• What role do procedures and training play
in nuclear accidents?
• Why are nuclear accidents less likely
today?
• What could make nuclear accidents more
likely in the future?
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