A nuclear power station produces radioactive waste. A safety officer states that one sample of waste has an activity of 5 000 000 Bq. State what is meant by the term 'activity' and explain what the unit becquerel (Bq) tells us about this sample.

Pearson Edexcel International GCSE Physics (4PH1) — 7.1 Units · State · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Radioactive waste from nuclear reactors must be disposed of carefully, usually by burying it in sealed drums deep underground. The activity of a radioactive source decreases over a period of time.

Model answer (4 marks)

Activity is the rate at which unstable nuclei decay.
One becquerel equals one decay per second.
The sample undergoes 5 000 000 decays every second.
The activity of the sample will decrease over time as the number of unstable nuclei decreases.

Examiner tips

  • Use the exact definition of activity and the conversion 1 Bq = 1 decay s⁻¹.
  • Show the numerical value in the correct form (5 000 000 Bq).
  • Mention the time‑dependence of activity to cover all points.

Common mistakes

  • Confusing activity with total number of nuclei.
  • Using the wrong unit (e.g. s⁻¹ instead of Bq).
  • Omitting the statement that activity decreases over time.

Mark scheme (4 marks)

  1. Activity is the rate at which unstable nuclei decay (accept: number of decays per unit time)
  2. One becquerel equals one decay per second (accept: 1 Bq = 1 decay per second)
  3. The sample undergoes 5 000 000 decays every second (accept: 5 000 000 nuclei decay per second)
  4. The activity of the sample will decrease over time (as the number of unstable nuclei in the sample decreases)

Key terms in this question

activity · becquerel

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