Nuclear Power

From mass to energy

• Nuclear energy is different from burning fuels or other chemical reactions

• Nuclear energy involves changes at the atomic level– Fission: a large atom of one element is split into two atoms of

different elements– Fusion: two small atoms join to form a larger atom of a

different element• The products of both have less mass than the starting

material– The small mass is multiplied by the speed of light squared,

resulting in a tremendous release of energy

Nuclear fusion and fission

The fuel for nuclear power plants

• All nuclear plants use fission (splitting) of uranium-235• Uranium occurs naturally in the Earth’s crust– It exists in two forms (isotopes): uranium-238 (238U) and

uranium-235 (235U)– Isotopes: contain different numbers of neutrons but the

same number of protons and electrons• Mass number = protons + neutrons– Different mass numbers come from different numbers of

neutrons (238U = 146, 235U = 143 neutrons)– 235U readily undergoes fission, but not 238U

Fission

• Fission occurs when a neutron hits the nucleus of 235U at just the right speed– Some atoms of 235U undergo radioactive decay and

release neutrons– These neutrons can hit other 235U atoms, producing highly

unstable 236U – 236U undergoes fission into lighter atoms (fission products)

• More neutrons are given off, releasing lots of energy– This domino effect causes a chain reaction

Nuclear bombs

• When 235U is highly enriched, spontaneous fission of an atom triggers a chain reaction

• Nuclear weapons have small amounts of pure 235U– Or other fissionable material– Two or three neutrons from a spontaneous fission

cause two or three other neutrons to undergo fission• The whole mass undergoes fission in a fraction of

a second– Releases all energy in one huge explosion

Fission reactions

The nuclear reactor

• A nuclear reactor has a continuous chain reaction – But does not amplify it into an explosion– Control is through enriching uranium to 3–5% 235U

• Faster neutrons absorbed by 238U convert it to 239Pu– Plutonium also undergoes fission and releases energy

• Moderators surround the enriched uranium– A moderator slows down neutrons to the right speed to

trigger another fission• Light-water reactors (LWRs): moderator is near-pure

water

Fuel rods• Enriched uranium is arranged in a suitable geometric

pattern surrounded by the moderator– Uranium pellets are inserted into long metal tubes (fuel

elements, fuel rods)• Fuel rods are placed close together to form a reactor

core– The core is inside a water-holding vessel (the moderator and

coolant)• Neutron-absorbing fission products accumulate in the

rods– They slow down the rate of fission and heat production– Radioactive spent-fuel rods are replaced with new ones

A nuclear reactor

Control rods

• Control rods: neutron-absorbing material inserted between the fuel elements– Control the chain reaction in the reactor core

• Withdrawing and inserting control rods starts and controls the chain reaction– The fuel rods and moderator become intensely hot

• A nuclear reactor is an assembly of fuel elements, moderator-coolant, and control rods

The nuclear power plant• Water is boiled to make steam to drive turbogenerators• Boiling-water reactors: water circulates through the

reactor• Pressurized-water reactors: high-pressure water circulates

through the reactor without boiling– The superheated water then circulates through a heat

exchanger and boils other, unpressurized water– Isolating hazardous reactor materials

• Both reactor types have a serious drawback– If a reactor cracks, there would be a sudden loss of water from

around the reactor

Pressurized nuclear power plant