Applications eg relevance to storage of radioactive waste, radioactive…

The idea

Radioactive decay has practical consequences because each nuclide has a characteristic half-life and type of emission. In waste storage, long half-life nuclides such as plutonium-239 remain hazardous for thousands of years, so they are vitrified in glass, sealed in steel canisters and buried deep underground to isolate them while activity falls. Short half-life waste can be stored until its activity is negligible. In radioactive dating, the ratio of parent to daughter nuclei is measured. For carbon-14 dating, living material exchanges carbon until death; afterwards carbon-14 decays with a half-life of about 5730 years, so the remaining fraction gives an age up to roughly 50000 years. For rocks, uranium-lead dating uses a very long half-life, so it works over billions of years. The same decay equations and half-life calculations underpin both applications.

What earns the marks

• State that the hazard of radioactive waste depends on half-life, activity and type of radiation emitted.
• Explain that long half-life waste remains radioactive for a very long time, so isolation from the environment is needed.
• Describe storage methods such as vitrification, sealing in canisters and deep geological disposal.
• Explain that short half-life waste can be stored until its activity has fallen to a safe level.
• Describe radioactive dating as using the ratio of parent to daughter nuclei and the known half-life.
• Apply the decay equation or half-life relationship to calculate an age or a remaining fraction.
• Compare carbon-14 dating with uranium-lead dating in terms of useful range and half-life.

Common mistakes and how to put them right

• Thinking that a long half-life means the waste is safe because activity is low.

  Put it right: A long half-life means it stays radioactive for a very long time, so isolation is needed for thousands of years.

• Confusing half-life with the time for all activity to stop.

  Put it right: Activity halves in each half-life and never reaches exactly zero in a finite time.

• Using carbon-14 dating for samples older than about 50000 years or for non-living material.

  Put it right: Carbon-14 has a relatively short half-life and only dates once-living material over a limited range.

Exam tips

• Link each application to the half-life value and the type of radiation emitted, not just to the word radioactive.
• Show the decay calculation clearly, including the fraction remaining and the number of half-lives.
• Use precise terms such as parent nuclei, daughter nuclei, activity, half-life and isolation when describing waste storage.