AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/6/6-6)
Unit 6 · Topic 6.6
6.6 Nuclear Power
Nuclear power plants split uranium atoms to release heat, which boils water to spin a turbine. It is a large, steady, low-carbon source of electricity, but it creates radioactive waste that must be isolated for thousands of years and carries a small risk of serious accidents.
Key terms
- nuclear fission
- uranium-235
- fuel rods
- half-life
- radioactive waste
Fission and the chain reaction
Nuclear fission is the splitting of a large atom's nucleus into two smaller nuclei. In a reactor, a slow-moving neutron strikes a uranium-235 (U-235) nucleus. The nucleus splits, releasing a large amount of heat plus two or three new neutrons. Those neutrons can split more U-235 atoms, which release more neutrons, and so on. This self-sustaining process is a chain reaction.
Natural uranium is more than 99 percent U-238, which does not easily fission. Reactor fuel is enriched so that about 3 to 5 percent of it is U-235. The fuel is shaped into small pellets and stacked inside metal tubes called fuel rods.
Inside the plant
- The reactor core holds bundles of fuel rods surrounded by water.
- Control rods, made of a material that absorbs neutrons (such as boron or cadmium), are raised or lowered between the fuel rods to speed up or slow down the chain reaction. Fully inserted, they shut it down.
- Water around the core slows the neutrons so they can split U-235 (it acts as the moderator) and carries heat away (it acts as the coolant).
- The heat makes steam, which spins a turbine and generator, just as in a fossil fuel plant.
- The steam is cooled and condensed, often with a cooling tower. The cloud rising from a cooling tower is water vapor, not smoke or radiation.
Radioactive waste and half-life
Spent fuel rods are highly radioactive: they give off harmful radiation as unstable atoms decay. A half-life is the time it takes for half of the radioactive atoms in a sample to decay. After one half-life, half remains; after two, a quarter; after three, an eighth.
Some products in spent fuel have short half-lives, but others, such as plutonium-239 (half-life about 24,000 years), stay dangerous for many thousands of years. Spent fuel is first cooled in water pools, then often stored in steel and concrete dry casks at the plant. The United States still has no permanent deep underground storage site.
Uranium mining also leaves radioactive mine tailings, and the large volume of cooling water a plant uses can cause thermal pollution when it is returned warm to rivers or lakes (topic 8.6).
Accidents, pros and cons
Three accidents shaped public views of nuclear power. Three Mile Island (Pennsylvania, 1979) was a partial meltdown; very little radiation escaped, but it stalled new US reactor building for decades. Chernobyl (Soviet Ukraine, 1986) was a reactor explosion and fire that spread radioactive material across Europe and left a large area around the plant abandoned. Fukushima (Japan, 2011) began when an earthquake and tsunami knocked out power to the cooling systems, leading to meltdowns in three reactors and the evacuation of a large area.
| Advantages | Disadvantages |
|---|---|
| No CO₂ or air pollution while running | Radioactive waste needs safe storage for thousands of years |
| Huge energy from a small amount of fuel | Rare but serious accidents |
| Steady output day and night | Very expensive and slow to build |
| Small land footprint | Uranium is nonrenewable; mining harms land and water |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Basic half-life calculation
A sample of radioactive waste contains 800 g of an isotope with a half-life of 30 years. How much of the isotope is left after 90 years?
Show the solutionHide the solution
- Step 1: Number of half-lives = time ÷ half-life = 90 years ÷ 30 years = 3.
- Step 2: Halve the amount once for each half-life: 800 g → 400 g (30 years) → 200 g (60 years) → 100 g (90 years).
- Step 3: Shortcut: amount left = starting amount × (½)ⁿ, where n is the number of half-lives. 800 × (½)³ = 800 × ⅛ = 100 g.
Answer: 100 g remains after 90 years.
- Example 2
Working backward: how long until it drops to a fraction?
Strontium-90 has a half-life of about 29 years. How long does it take for a sample to fall to 1/16 of its original amount?
Show the solutionHide the solution
- Step 1: Figure out how many halvings get you to 1/16: 1 → ½ → ¼ → ⅛ → 1/16. That is 4 half-lives (because 2⁴ = 16).
- Step 2: Time = 4 half-lives × 29 years per half-life = 116 years.
Answer: About 116 years.
- Example 3
Trap: half-life is not 'half the time to disappear'
A student says: 'An isotope with a half-life of 10 years will be completely gone after 20 years.' Explain the error and find how much of a 1,000 g sample actually remains after 20 years.
Show the solutionHide the solution
- Step 1: Each half-life removes half of what is left, not half of the original amount. Decay never removes the 'second half' all at once.
- Step 2: 20 years ÷ 10 years = 2 half-lives. 1,000 g × (½)² = 1,000 × ¼ = 250 g.
- Step 3: So after 20 years a quarter of the sample is still there.
Answer: The student treated decay as linear. After two half-lives, 250 g (25%) remains.
Common mistakes
- Calling nuclear power renewable. Uranium is a finite mined resource.
- Saying cooling towers release radiation or smoke. The plume is water vapor.
- Treating half-life as linear (subtracting the same amount each period). Each half-life cuts the remaining amount in half.
- Mixing up control rods and fuel rods. Fuel rods hold the uranium; control rods absorb neutrons to slow the reaction.
On the exam
- Half-life calculations are a favorite. Show the number of half-lives and the halving steps or the (½)ⁿ setup.
- You may be asked for one advantage and one disadvantage of nuclear power compared with coal. Low CO₂ emissions versus long-lived radioactive waste is a reliable pair.
- Know the cause and rough outcome of each major accident; a question may ask what Fukushima revealed about siting plants in earthquake and tsunami zones.
Connected topics
Videos
Check yourself
4 questions on 6.6 Nuclear Power. Pick an answer to see if you got it, and why.
In a nuclear power plant, heat is produced when
A sample contains 80 grams of a radioactive isotope with a half-life of 30 years. How much of the isotope will remain after 90 years?
Which is the biggest long-term environmental challenge of nuclear power?
In 2011, an earthquake and the tsunami that followed knocked out cooling systems at a nuclear power plant, causing meltdowns and the release of radiation. Which accident was this?
0 of 4 answered