Accidents Happen

1. Accidents Happen But Nuclear Accidents Require Special Skill!

2. Nuclear Power Overview • Fission of U235 releases excess nuclear Binding Energy. • E=MC2 • 0n1 + 92U235 =Fission Energy+20n1+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 >5000 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 Rx’s 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?