Explain why the products of a uranium-235 fission reaction are radioactive and why this radioactivity presents a long-term challenge for nuclear waste management.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When uranium-235 absorbs a slow-moving neutron it undergoes fission, splitting into two smaller nuclei (fission fragments) along with several fast neutrons and gamma radiation.
Model answer (4 marks)
Fission fragments are neutron‑rich because the original U‑235 nucleus contains many more neutrons than protons; the fragments therefore have a high neutron‑to‑proton ratio. To reach a stable configuration they undergo β⁻ decay (and often emit γ‑rays). The waste therefore contains a wide mixture of radioisotopes, some with half‑lives of thousands to millions of years. Because of these very long half‑lives, the waste must be safely contained for extremely long periods, which is why geological storage or deep underground repositories are required to protect people and the environment from ionising radiation.
Examiner tips
- Use the term ‘neutron‑rich’ and explain β⁻ decay; mention γ‑radiation. State the wide range of half‑lives and the need for long‑term containment. Keep answer concise and use correct terminology.
Common mistakes
- Confusing β⁺ decay with β⁻ decay; forgetting to mention γ‑radiation; not recognising the very long half‑lives of some waste isotopes.
Mark scheme (4 marks)
- Fission fragments have a high neutron-to-proton ratio because they originate from a heavy nucleus (U-235) which requires proportionally more neutrons for stability than lighter nuclei do.
- These neutron-rich fission fragments therefore undergo beta-minus decay (and may also emit gamma radiation) to reach a stable configuration.
- The waste contains a mixture of isotopes with a very wide range of half-lives, including some radioisotopes with half-lives of thousands to millions of years.
- Safe containment must therefore be maintained for extremely long periods, making geological storage / deep underground repositories necessary to protect living organisms from ionising radiation over these timescales.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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