Accidents Happen

1
Accidents Happen

• But Nuclear Accidents Require Special Skill!

2
Nuclear Power Overview

• Fission of U235 releases excess nuclear Binding
  Energy.
• EMC2
• 0n1 92U235 Fission Energy 2 0n1 54Xe131
  36Kr84
• 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.

3
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.

4
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
  ?

5
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.

6
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.

7
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.

8
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.

9
Nuclear Safety Principles

• Staff selection must include cross-discipline
  engineering experience.
• Emergency preparedness must be frequently
  exercised by large scale drills.
• External oversight (INPO NRC) and industry
  event awareness must be maintained.

10
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

11
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

12
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

13
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

14
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

15
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

16
Chernobyl Accident Events

• Carbon moderator ignites and burns
• Core ejected into atmosphere
• Firemen respond and die
• More firemen respond and die
• Rx buried in gt5000 tons of borated material by
  helicopter material drops
• Widespread contamination

17
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 Rxs are closed

18
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

19
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?