What can go wrong?

• Nuclear power plants cannot explode like a nuclear bomb.

• A bomb needs a critical mass in a confiuration which is not present in the reactor core.

• Even a deliberate act of sabotage or terrorism could not cause such an explosion.

• The worst that can happen is a core melt down.• 2 classes of accidents-Criticality and Loss of

Coolant (LOCA) accidents

Criticality accident• If the control rods were removed and/or the control

systems failed, a runaway reaction would occur. • The tremendous heat produced would melt the

containment system and the reactor core would sink into the Earth

• Radioactive material would enter the ground and be released as steam (a radioactive cloud) into the air

• The area around the reactor would be highly contaminated with radioactivity

• The cloud could travel for hundreds or even thousands of miles, and could spread dangerous levels of radioactivity around the world.

Loss of coolant accident

• After a reactor is shut down, it is still hot enough to experience a core melt down if cooling system fails.

• Emergency coolant systems are in place to prevent this

• Big part of reactor design is the prevention of such accidents

Probability• To determine the likelyhood that such an

accident would occur something called an event tree is constructed.

• This determines the consequences of a particular event occurring

• Each component (pump, valves etc) has a failure probability assigned to it

• Bottom line-most recent studies indicate that for all 104 reactors operating the US, over their 30 year operating lifetime, there is a 1% probability of a large release of radioactivity

Core meltdown

Nuclear accidents

• 3 (that we know of…….)• Enrico Fermi reactor in Illinois• Three Mile Island-PA-Worst US accident• Chernobyl-Ukraine-Worst accident ever

Enrico Fermi Reactor• Enrico Fermi demonstration nuclear breeder

reactor, in Monroe, MI• Two fuel rod assemblies reached 700F(normal

operating temp is about 580 F). • Reactor was shut down, and it appeared fuel was

melting.• It was determined a piece of zirconium from the

meltdown cone (a structure designed to direct the flow of fuels should a meltdown occur) came loose and was blocking the coolant nozzles.

• The fuel in a breeder reactor is highly enriched, thus in molten form it could “reassemble” into critical assembly that could result in a runaway fission reaction which would generate enough heat to melt the material into the Earth. Some engineer remarked it could go all the way to China, and the phrase China Syndrome was born.

• Publically used by a Nuclear Physicist Richard Lapp in 1971

• No one was injured and no radiation was released.