Nuclear Power — AP® Environmental Science Notes

A nuclear power plant works like a coal plant from the steam onward: steam spins a turbine, the turbine turns a generator. What is different is the heat source. Instead of burning fuel, it splits atoms.

Fission and the chain reaction

The fuel is uranium, mined as ore. Natural uranium is only about 0.7 percent uranium-235, the form that splits easily, so it is enriched to about 3 to 5 percent and packed into pellets inside long fuel rods.

In nuclear fission, a neutron strikes a uranium-235 nucleus, which splits into two smaller nuclei and releases a large amount of heat and 2 or 3 more neutrons. Those neutrons can split more nuclei: a chain reaction. Control rods made of neutron-absorbing material are raised or lowered between the fuel rods to keep the reaction steady, or to stop it.

A nuclear power plant. In the reactor, green fuel rods of uranium-235 stand between dark control rods. Hot water flows to a steam generator; steam spins a turbine connected to a generator that sends out electricity. Used steam goes to a condenser cooled by a cooling tower, and cooler water returns to the reactor. Notes say control rods absorb neutrons to slow or stop the chain reaction, and that in fission a neutron splits U-235, releasing heat and 2 or 3 more neutrons. — **Figure 1.** A nuclear power plant. LevlPrep original diagram.

Water flowing through the core carries the heat away to make steam. A condenser and often a cooling tower remove the waste heat; the white plume above a cooling tower is water vapor. Like coal plants, nuclear plants turn only about a third of their heat into electricity.

Radioactive decay and half-life

Some nuclei are unstable. In radioactive decay they give off radiation and change into other elements. The half-life is the time it takes for half of the radioactive atoms in a sample to decay. It is fixed for each isotope: about 8 days for iodine-131, about 30 years for cesium-137, about 24,000 years for plutonium-239.

Each half-life halves what is left, so the amount never quite reaches zero: 100% → 50% → 25% → 12.5% → 6.25%.

Amount left. A sample holds 200 g of cesium-137 (half-life 30 years). How much is left after 90 years?

Number of half-lives: $90 \div 30 = 3$ .

Setup: $200\,\text{g} \times 0.5^{3}$ , or halve three times: 200 → 100 → 50 → 25.

Answer: $25\,\text{g}$ left. Dividing 200 by 3 is the classic mistake.

Time to a fraction. How long until strontium-90 (half-life 29 years) falls to one sixteenth?

One sixteenth is $\tfrac{1}{2} \times \tfrac{1}{2} \times \tfrac{1}{2} \times \tfrac{1}{2}$ : 4 half-lives.

Answer: $4 \times 29 = 116$ years.

Nuclear waste

Used fuel rods, called spent fuel, are full of fission products, some with half-lives of thousands of years. This nuclear waste is first kept in pools of water at the plant to cool, then sealed in steel and concrete dry casks. The United States has no permanent storage site; the long-planned repository at Yucca Mountain in Nevada was never opened. Waste must be isolated from people and groundwater for thousands of years.

Accidents

Fission products keep making heat even after the chain reaction stops. If cooling water is lost, the fuel can overheat and melt: a meltdown. Three Mile Island in Pennsylvania (1979) had a partial meltdown with little release. At Chernobyl in Ukraine (1986), a reactor exploded and burned, spreading radioactive material across Europe. At Fukushima in Japan (2011), an earthquake and tsunami cut the cooling, and three reactors melted down.

Weighing nuclear power

Advantages	Disadvantages
no carbon dioxide or smoke while running	uranium is nonrenewable; mining disturbs land and leaves radioactive tailings
huge energy from a small mass of fuel	spent fuel stays dangerous for thousands of years
steady output day and night	rare but severe accidents; costly to build
small land area for the energy produced	warm cooling water returned to rivers lowers dissolved oxygen

Nuclear fuel is uranium, not water or steam. Nuclear power is low carbon while running but nonrenewable.