Nuclear safety aspectsAQA A-Level Physics: Revision notes

Thermal reactors use enriched uranium-235 as fuel (natural uranium has only about 0.7% uranium-235, raised to a few per cent). Some reactors also use plutonium-239. The fuel is made into pellets sealed in metal fuel rods.

Fresh fuel is only weakly radioactive, but spent fuel contains highly radioactive fission products. It emits intense ionising radiation (including gamma radiation) and releases thermal energy. It is therefore handled by remotely controlled machines, often under water, so that people are kept at a distance.

The reactor core produces gamma radiation and neutrons. It is enclosed in a thick steel pressure vessel and then several metres of concrete to absorb them, so that workers and the public are not exposed. Spent fuel stored in cooling ponds is shielded by the water above it, which also removes the thermal energy still being released by decay.

The thicker and denser the shield, the more radiation it absorbs.

In an emergency, such as an earthquake or loss of coolant, the control rods are dropped fully into the core, automatically if sensors trigger it. They absorb neutrons, so on average fewer than one neutron per fission causes another fission and the chain reaction stops within seconds.

The core still needs cooling afterwards, because the fission products continue to decay and release thermal energy (decay heat). Without cooling the fuel could overheat.

Waste is produced throughout the fuel cycle and is sorted by activity:

• Low-level: contaminated clothing and tools; buried in shallow sites.
• Intermediate-level: reactor components and fuel cladding; encased in concrete.
• High-level: spent fuel and fission products; the most dangerous.

High-level waste is handled remotely and kept in cooling ponds for years. It is then sealed in glass and steel containers and stored deep underground in stable rock. Its isotopes have long half-lives, for example plutonium-239 at 24 000 years, so it remains dangerous for thousands of years.

Worked example. Caesium-137 (half-life 30 years) falls to 0.10% of its activity when λt = ln 1000, so t = 6.91 × 30 ÷ 0.693 ≈ 299 years.

Benefits: a very large energy yield from a small mass of fuel (1.0 kg of uranium-235 releases about 8 × 10¹³ J, against about 3 × 10⁷ J for 1.0 kg of coal), no carbon dioxide produced in operation, and a reliable supply.

Risks: an accident could release radioactive material over a wide area, and high-level waste needs secure storage for thousands of years.

The safety measures above reduce the risks. Decisions about nuclear power weigh these benefits against the risks and costs and the alternatives. A good evaluation reaches a justified conclusion.