Explain why enriched uranium, rather than natural uranium, is typically used as fuel in most nuclear fission reactors.

IB DP Physics Standard Level (2023 syllabus) — E.4 Fission · Explain · 4 marks · View as Markdown

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

Natural uranium consists of approximately 99.3% uranium-238 and only 0.7% uranium-235. Reactor-grade enriched uranium typically contains between 3% and 5% uranium-235.

Model answer (4 marks)

1. In a thermal reactor, fission is mainly caused by slow neutrons captured by U‑235.
2. U‑238 absorbs many of these slow neutrons without fission, removing them from the chain reaction.
3. Natural uranium contains only 0.7 % U‑235, giving too little fissile material to sustain a chain reaction in most reactor designs.
4. Enriching the fuel increases the U‑235 proportion, raising the probability of fission per neutron and enabling a self‑sustaining chain reaction.

Examiner tips

Use the exact terminology: ‘thermal neutrons’, ‘U‑235’, ‘U‑238’, ‘enrichment’.
Show the logical sequence: initiation → loss by U‑238 → insufficient U‑235 → enrichment solves the problem.
Keep each point concise and directly linked to the mark scheme points.

Common mistakes

Confusing U‑238 with U‑235 as the fissile isotope.
Failing to mention that U‑238 absorbs neutrons without fission.
Giving extra detail about reactor types that is not required for the 4‑mark answer.

Mark scheme (4 marks)

Fission in a thermal reactor is initiated primarily by slow (thermal) neutrons absorbed by uranium-235 nuclei.
Uranium-238 tends to absorb slow neutrons without undergoing fission, removing them from the chain reaction.
The low natural abundance of U-235 (0.7%) means there is insufficient fissile material to sustain a chain reaction in natural uranium in most reactor designs.
Increasing the proportion of U-235 through enrichment raises the probability of fission events per neutron, allowing a self-sustaining chain reaction to be maintained.

Key terms in this question

enriched uranium