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 + 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 >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?