S4.11.8 Chapter 8

Weighing up nuclear power

People disagree about nuclear power. What is true about it, and what do we have to weigh up?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A It gives out carbon dioxide while making electricity, like coal.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It cannot make steady electricity.
You picked B. Read on. At the end, the chapter comes back to your guess.
C It makes no waste.
You picked C. Read on. At the end, the chapter comes back to your guess.
D It gives almost no carbon dioxide, but it leaves radioactive waste that must be kept safe for a very long time.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Compare nuclear power with fossil fuels and renewables
Explain how nuclear waste is stored safely

This book has described the science. This chapter asks you to use it to judge. There is no perfect way to make electricity. Every source has gains and costs, and sensible people weigh them differently. Here are the facts that go on the scales.

Start with the fuel. Burning coal, oil or gas gives out carbon dioxide, which adds to climate change, and also soot and other pollution. These fuels will run out, because they took millions of years to form. Their power stations are quicker to build than nuclear ones.

Renewable sources, such as wind, sun and water, need no fuel and give out no carbon dioxide while working. But wind and sunshine change with the weather and the time of day. So the supply is not steady, and backup or storage is needed. Hydroelectric power needs a suitable river and valley.

Nuclear power sits between these. A power station gives out almost no carbon dioxide when it runs. The fuel takes little room. One uranium fuel pellet, the size of a fingertip, holds as much energy as about a tonne of coal.

A reactor also runs almost all the time, in any weather, day and night. A typical one is working for more than 8 hours in every 10. That makes it a steady supply, which suits a country whose use never stops.

To make a fair comparison we must count carbon dioxide over the whole life of a source, not only while it runs. Building, mining, making and taking apart all add some. The table gives rounded middle values from reviews of many studies by the Intergovernmental Panel on Climate Change.

Grams of carbon dioxide equivalent for each kWh of electricity, over the whole life. Rounded middle values. Real values vary.

Worked example

A home uses 3 000 kWh of electricity in a year. How much carbon dioxide, in kg, is given out if all of it came from coal, at 820 g for each kWh?

carbon dioxide = grams for each kWh × kWh.
carbon dioxide = 820 × 3 000 = 2 460 000 g.
1 000 g = 1 kg, so 2 460 000 g ÷ 1 000 = 2 460 kg.

Answer: 2 460 kg

The same home on nuclear power, at 12 g for each kWh, would cause 36 kg, which is far less. On wind it would be the same. The table shows that nuclear power and wind have among the lowest values, with solar next, and coal and gas far higher.

Cost is a second item. A nuclear power station is expensive and slow to build, often taking many years. Once built, its fuel is cheap, and it can run for decades. At the end, taking it apart safely, called decommissioning, takes years and money too.

Safety is the third. Serious accidents at nuclear power stations are very rare. The ones at Chernobyl in 1986 and Fukushima in 2011 spread radioactive material, forced people to leave their homes and contaminated land. Yet nuclear power has caused far fewer deaths than coal, oil and gas for each unit of energy. This counts deaths from air pollution. Estimates differ.

The fourth is waste. Burning fuel leaves ash and gives out gases. A reactor gives out no carbon dioxide, but it leaves nuclear waste, which is radioactive. The amount is small, but it must be kept safe for as long as it stays dangerous.

Waste comes in three groups. Low level waste is gloves, overalls and tools that are lightly contaminated. It is only weakly radioactive, so it can be buried in shallow sites. These sites are watched for a few hundred years. Intermediate waste, such as reactor parts and filters, is sealed in concrete or steel.

The three groups of waste. Most of the volume is low level.

High level waste is the used fuel. It holds the fission products and some heavy nuclei made in the reactor. It is very radioactive and hot, but it is a small volume. Most of the waste is low level. Only a small part is high level.

It is stored in steps. First the used fuel is cooled for some years in deep pools of water. The water takes the heat away and shields the radiation. Then it is sealed in thick steel and concrete casks, which are kept above ground, guarded and watched.

The usual steps for high level waste. Deep stores are being built in a few countries.

Why a deep store? The decay of the waste depends on half-lives. Strontium 90 and caesium 137 have half-lives of about 30 years. Some other nuclei last far longer. Plutonium 239 has a half-life of about 24 000 years, and a few have half-lives of millions of years. So the waste must be kept apart for many thousands of years. It goes in a place that stays dry and still, deep in stable rock.

A sample of waste with a half-life of 30 years falls to an eighth in 90 years.

Worked example

A sample of waste has a half-life of 30 years. Its count rate is 800 counts/min. How long until it falls to 100 counts/min?

Halve the count rate until you reach 100: 800 → 400 → 200 → 100.
That is 3 half-lives.
time = 3 × 30 years = 90 years.

Answer: 90 years

There is a link to the half-life. A short half-life means strong radiation now, but it soon fades. A long half-life means weaker radiation, but for a very long time. The waste needs shielding for the first, and isolation for the second.

A wrong idea many people have

All nuclear waste stays dangerous for tens of thousands of years.

Why it feels right: We hear that waste lasts for ages, and the longest half-lives get the most attention.

The test that breaks it: Look at the three groups. Most waste, by volume, is low level and only weakly radioactive. Only the high level waste is strongly radioactive, with very long half-lives.

What is true: Most nuclear waste is low level and only weakly radioactive. A small part is high level, and some of that needs isolation for a very long time. This is why it goes to a deep store.

Try it

Count the carbon dioxide from a lamp

You need: paper, a pencil.

A 100 W lamp is on for 10 hours each day for 100 days. Change the power to kW: 100 W is 0.1 kW.
Work out the hours: 10 × 100 = 1 000 hours.
Energy = power × time. Work out the energy in kWh.
Carbon dioxide in grams = grams for each kWh × kWh. Do this for coal at 820, gas at 490 and wind at 12.
Change each answer to kg by dividing by 1 000. Write the three answers in a table.

What you will see: The energy is 0.1 × 1 000 = 100 kWh for every source. The carbon dioxide is very different: 82 kg for coal, 49 kg for gas and 1.2 kg for wind. The same lamp, the same light, a very different effect.

If you cannot try it, this is what happens.

So how do we decide? A country with plenty of wind, sun or rivers may choose one way. A country that needs a steady supply, with little space or sun, may choose another. Many countries use a mix. The science tells us the gains and costs. The choice is for people to make, with the facts in front of them.

CO₂: carbon dioxide given out while it works. Steady: the output does not depend on the weather.

Key words

nuclear waste: radioactive material left over from a nuclear reactor or from its use.
high level waste: the used fuel, which is very radioactive and needs long isolation.
decommissioning: taking a power station apart safely at the end of its life.

Back to your guess

The answer is D: It gives almost no carbon dioxide, but it leaves radioactive waste that must be kept safe for a very long time.

Nuclear power gives very little carbon dioxide, and a small amount of fuel gives a great deal of energy. But the waste stays radioactive for a long time, and a reactor is costly to build. Each source has gains and costs.

Keep it

Copy this into your exercise book.

Nuclear: almost no CO₂ in use, little fuel, steady, but costly, slow to build, and it leaves waste. Waste: low level, intermediate, high level. High level is cooled, sealed, then kept deep in rock. Every source has gains and costs.

Draw: Three boxes joined by arrows: used fuel in water, then in a steel cask, then in a deep store in rock.