Nuclear Power

1. Nuclear Power

2. 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

3. Nuclear fusion and fission

4. 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

5. 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

6. 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

7. Fission reactions

8. 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

9. 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

10. A nuclear reactor

11. 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

12. 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

13. Pressurized nuclear power plant