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Nuclear Power

Nuclear Fission

• We convert mass into

energy by breaking

large atoms (usually

Uranium) into smaller

atoms.

A slow moving neutron induces fission in Uranium 235

Expanding Chain Reaction

• The fission reaction

produces more

neutrons which can

then induce fission in

other Uranium atoms.

• Mouse Trap Chain

Reaction

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Linear Chain Reaction• Obviously, an expanding chain reaction cannot

be sustained for long (bomb). For controlled nuclear power, once we reach our desired power level we want each fission to produce exactly one additional fission

Tricks of the trade

• Slow moving (thermal) neutrons are more

effective at inducing fission, but, fissions

produce fast moving electron. We need

to slow neutrons down.

• Fissions typically produce several

neutrons but a linear chain reaction only

needs one. We need to get rid of a good

fraction of our neutrons.

Moderator• Neutrons are slowed

down by having them collide with light atoms (Water in US reactors).

• Highest level of energy transfer occurs when the masses of the colliding particles are equal (ex: neutron and hydrogen)

Control Rods

• Control rods are

made of a material

that absorbs excess

neutrons (usually

Boron or Cadmium).

• By controlling the

number of neutrons,

we can control the

rate of fissions

https://www.youtube.com/watch?v=LCKsD

8ZLHxw

Basic Ideas

• The Uranium is both the fuel and the

source of neutrons.

• The neutrons induce the fissions

• The Water acts as both the moderator and

a heat transfer medium.

• Control rods regulate the energy output by

“sucking up” excess neutrons

Practicalities

• Processing of Uranium

• Each ton of Uranium ore produces 3-5 lbs of Uranium compounds

• Uranium ore is processed near the mine to produce “yellow cake”

• Only 0.7% of U in yellow cake is 235U. Most of the rest is 238U which does not work for fission power.

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US Uranium Deposits World Distribution of Uranium

Enrichment• To be used in US

reactors, fuel must be 3-5% 235U.

• Yellow cake is converted into UF6

and this compound is enriched using gaseous diffusion and/or centrifuges.

• There are some reactor designs that run on pure yellow cake.

• NOTE: A nuclear bomb requires nearly 100% pure 235U or 239Pu. The

3% found in reactor grade Uranium CANNOT create a nuclear explosion!

Fuel Pellets• The enriched UF6 is

converted into UO2

which is then made into fuel pellets.

• The fuel pellets are collected into long tubes. (~12ft).

• The fuel rods are collected into bundles (~200 rods per bundle

• ~175 bundles in the core

• Two common US

reactor types: Boiling

Water Reactor and

Pressurized Water

Reactor.

• BWR: P=1000 psi

T=545°F

• PWR P=2250 psi

T=600°F

• PWR is most common

and is basis of marine

nuclear power.

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Reactor is inside a large containment building

Nuclear Power in the US

• We currently generate approximately 20%

of our electricity using nuclear power.

• No new nuclear power plants have been “ordered” since the late 1970’s.

• Even “new” plants are nearing 20 years

old and will start to need replacing.

US Nuclear Power Plants

World Nuclear Power

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