ustaining chain reaction.","markscheme":"- A neutron is absorbed by a uranium-233 nucleus, causing it to become unstable and split (undergo fission) into two lighter daughter nuclei. B1\n- This fission event releases energy and emits two or more further (fast) neutrons. B1\n- These emitted neutrons are subsequently absorbed by other neighboring uranium-233 nuclei, triggering further fission events. B1\n- To maintain a self-sustaining reaction, at least one of the emitted neutrons from each fission must, on average, go on to cause another fission event. B1","marks":4,"order":0,"markingRubric":null,"explanation":{"methods":[{"id":"neutron_chain_reaction","label":"Following the neutron chain","steps":[{"type":"text","order":1,"title":"Describe the first fission","content":"A free neutron is absorbed by a uranium-233 nucleus. This makes it unstable, so it undergoes fission and splits into two lighter daughter nuclei.\n\nThis complete absorption-to-splitting statement earns the first mark (B1).","highlights":[{"text":"fission of a single uranium-233 nucleus","location":"specification"}]},{"type":"text","order":2,"title":"Identify the fission products","content":"Fission releases energy and emits **two or more further fast neutrons**. “Fast” means that the neutrons have high kinetic energy.\n\nThis is the second marking point: energy release plus at least two further neutrons earns **B1**.","highlights":[{"text":"self-sustaining chain reaction","location":"specification"}]},{"type":"text","order":3,"title":"Trigger more fission","content":"The further neutrons travel to neighbouring uranium-233 nuclei. When a neutron is absorbed, it can make another nucleus unstable and cause another fission event.\n\nThis is B1: the emitted neutrons are absorbed by neighbouring uranium-233 nuclei and trigger further fission events.","highlights":[{"text":"how the fission of a single uranium-233 nucleus can initiate and maintain a self-sustaining chain reaction.","location":"specification"}]},{"type":"text","order":4,"title":"State the condition","content":"For the process to be **self-sustaining**, it must not die out. On average, at least one neutron emitted from each fission must be absorbed by another uranium-233 nucleus and cause a further fission.\n\nThis is B1: on average, at least one emitted neutron from each fission must cause a further fission.","highlights":[{"text":"self-sustaining chain reaction.","location":"specification"}]}]}],"generatedAt":"2026-07-22T09:33:09.600Z","modelVersion":"gpt-5.6-terra+diagrams-1","explanationVersion":"v3"},"options":[]}],"options":[],"explanation":null,"firstAttemptFullMarks":null},{"id":"2d31a84c-7ef5-4ccb-bb7d-ff0cec0b715b","revisionId":"69d84c85-ab8a-43d0-a54a-d710682c86a9","questionNumber":null,"questionType":"LA","specification":"Plutonium-239 is an isotope used as fuel in some breeder and nuclear reactors.","difficulty":4,"difficultyBand":"medium","totalMarks":4,"sourceTitle":null,"calculatorUse":null,"parts":[{"id":"911b00b1-c0fa-47a9-8770-7df36b0298b7","label":null,"content":"Explain how the fission of a single plutonium-239 nucleus can lead to a self-sustaining chain reaction.","markscheme":"- A plutonium-239 nucleus absorbs a slow-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

Radiation safety and nuclear energy

Question 5

Radiation safety and nuclear energy Questions

Radiation safety and nuclear energy

Question 5

Radiation safety and nuclear energy Questions