Upcoming:

Upcoming:

• Quiz Friday, Nov 3 on EFP chapter 11&12• Course evaluations:– October 29 through November 20.– Go to wku.evaluationkit.com and use your WKU N

etID and password.– Early access to grades and prizes for completing

all course evaluations

Nuclear reactor

• In a nuclear power plant, the energy to heat the water to create steam to drive the turbine is provided by the fission of uranium, rather than the burning of coal.

• Fuel is 3% 235U and 97% 238U. 235U is an isotope of 238U. The chain reaction will only occur in the 235U, but naturally occurring uranium has both present in it.

• The neutrons coming from a fission reaction have an energy of 2Mev. They are too energetic to sustain a nuclear reaction in 235U.

• Need to slow them down to energies on the order of 10-2 so they can sustain fission in the 235U

Slowing the neutrons down• A moderator is used to slow down the neutrons

and cause them to lose energy• The moderator could be water or graphite• The lower energy neutrons are called thermal

neutrons• Some of the neutrons will be absorbed by 235U

instead of causing a fission reaction or by 238U and resulting in the emission of a gamma ray in both cases.

• Absorption of a neutron by 238U can result in the creation of 239Pu which is also fissionable

Creating Plutonium• So: 238U captures a neutron creating 239U• 239U undergoes a beta decay (a neutron is converted to

a proton and an electron) with a half life of 24 minutes and becomes 239Np (Neptunium)

• 239Np then beta decays with a half life of 2.3 days into 239Pu.

• 239Pu has a half life of 24,000 years• 239Pu can also undergo fission by the slow neutrons in

the core, with an even higher probability• So as it builds up in the core, is contributes to the

fission reaction

Breeder reactor• A reactor designed to produce more fuel (usually 239Pu )

than it consumes.• 239Pu does not occur naturally, and it is more fissile than

235U.• Leads to the possibility of reactors that can create their

own fuel, and only need limited mounts of naturally occurring uranium to operate.

• Also leads to the danger of countries creating additional nuclear fuels for weapons development– Caution-reactor must be designed to produce weapons grade

plutonium, jut because someone has a nuclear reactor does not mean they create weapons grade plutonium

Reactor design• PWR – pressurized water reactor• Core – where the action is. Fuel assembly is

kept in here (fuel is usually in the form of fuel rods)

• Fuel rods are surrounded by the water which acts as the moderator. This water is kept under high pressure so it never boils-it heats a seconds water source which turns into steam

• Control rods are slid in and out from the top to control the fission rate-in an emergency they can be dropped completely into the reactor core, quenching the fission

• Once the steam is generated, this works just like a fossil fuel power plant

• Can run without refueling for up to 15 years if the initial fuel is highly enriched

• Used in submarines and commercial power systems

Reactor design• BWR –Boiling water reactor• Core – where the action is. Fuel

assembly is kept in here (fuel is usually in the form of fuel rods)

• Fuel rods are surrounded by the water which acts as the moderator and the source of steam

• Control rods are slid in and out from the bottom to control the fission rate-in an emergency they can be dropped completely into the reactor core, quenching the fission. Also, boron can be added to the water which also efficiently absorbs neutron

• Once the steam is generated, this works just like a fossil fuel power plant

Fuel Cycle

• Fuel rods typically stay in a reactor about 3 years• When they are removed, they are thermally and

radioactively hot• To thermally cool them they are put in a cooling

pond.• Initial idea was that they would stay in the

cooling pond for 150 days, then be transferred to a facility which would reprocess the uranium and plutonium for future use.

Nuclear waste disposal

• This idea ran into problems. • Fear that the plutonium would be easily

available for weapons use halted reprocessing efforts in 1977

• Note that it is very difficult to extract weapons grade plutonium from spent fuel rods

• Plan is now to bury the waste deep underground, in a place called Yucca Mountain, Nevada

Nuclear waste

• The spent fuel rods are radioactive• Radioactivity is measured in curies• A curie is 3.7x1010 decays per second• A 1000 MW reactor would have 70

megacuries(MCI) of radioactive waste once it was shut down

• After 10 years, this has decayed to 14 MCi• After 100 years, it is 1.4MCi• After 100,000 years it is 2000 Ci

Yucca Mountain

Transportation scenarios

Transportation scenarios

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