w-moving (or thermal) neutron B1\n- The nucleus becomes unstable and splits (fissions) into two smaller, lighter daughter nuclei B1\n- This fission process releases two or three (fast) neutrons (along with energy) B1\n- These released neutrons are absorbed by other neighboring plutonium-239 nuclei, triggering subsequent fissions (to sustain the chain reaction) B1","marks":4,"order":0,"markingRubric":null,"explanation":{"methods":[{"id":"fission_chain_reaction","label":"Follow the fission sequence","steps":[{"type":"text","order":1,"title":"Start with absorption","content":"A slow-moving neutron is absorbed by a plutonium-239 nucleus. “Thermal neutron” is another name for a slow-moving neutron.\n\nThis precise starting statement earns the first mark.","highlights":[{"text":"Plutonium-239 is an isotope used as fuel in some breeder and nuclear reactors.","location":"specification"}]},{"type":"text","order":2,"title":"Describe the fission","content":"After absorbing the neutron, the plutonium-239 nucleus becomes unstable. It splits, or **fissions**, into two smaller, lighter daughter nuclei.\n\nWriting both the instability and the split gives the second mark."},{"type":"text","order":3,"title":"State what is released","content":"The fission releases energy and two or three fast neutrons. These neutrons are important because they can cause more fissions.\n\nStating the released neutrons earns the third mark."},{"type":"text","order":4,"title":"Explain the repetition","content":"The released neutrons are absorbed by other neighbouring plutonium-239 nuclei. Those nuclei then become unstable and fission too, releasing further neutrons.\n\nBecause this repeats without needing another starting neutron, it is a **self-sustaining chain reaction**. This earns the fourth mark."}]}],"generatedAt":"2026-07-22T09:13:52.439Z","modelVersion":"gpt-5.6-terra+diagrams-1","explanationVersion":"v3"},"options":[]}],"options":[],"explanation":null,"firstAttemptFullMarks":null},{"id":"2ff7ecde-e36a-4e3f-8a7b-06f180d5e68d","revisionId":"eff29e05-b344-4c7a-9526-7d4c67a6061a","questionNumber":null,"questionType":"LA","specification":"Inside a nuclear fission reactor, a moderator is used to control the speed of neutrons.","difficulty":6,"difficultyBand":"medium","totalMarks":3,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"d9af3218-d2e2-4625-bf50-337000e2178f","label":null,"content":"Explain what would happen to the rate of nuclear fission inside the reactor if the moderator were completely removed.","markscheme":"- States that neutrons released from fission are moving very fast / have high kinetic energy B1\n- Explains that without a moderator, these neutrons are not slowed down (to become thermal neutrons) B1\n- Concludes that fast-moving neutrons are less likely to be absorbed by uranium-235 nuclei, causing the rate of fission to decrease or the chain reaction to stop B1","marks":3,"order":0,"markingRubric":null,"explanation":{"methods":[{"id":"neutron_moderation","label":"Follow the neutron chain","steps":[{"type":"text","order":1,"title":"Start with released neutrons","content":"A fission event releases neutrons with **high kinetic energy**, so they are moving very fast.\n\nThis is the first marking point: identify the speed of the neutrons that are produced.","highlights":[{"text":"a moderator is used to control the speed of neutrons.","location":"specification"}]},{"type":"text","order":2,"title":"Remove the slowing effect","content":"A **moderator** is a material that slows neutrons down. Without it, the released neutrons are **not slowed down**, so they do not become slow, thermal neutrons.\n\nThat causal link earns the second marking point.","highlights":[{"text":"if the moderator were completely removed.","location":"specification"}]},{"type":"text","order":3,"title":"Link speed to fission rate","content":"Fast neutrons are less likely to be absorbed by uranium-235 nuclei. Therefore they trigger fewer new fissions, so the ra

Radiation safety and nuclear energy

Question 6

Radiation safety and nuclear energy Questions

Radiation safety and nuclear energy

Question 6

Radiation safety and nuclear energy Questions