Answer: Average number of neutrons causing further fission > 1.
- A Average number of neutrons causing further fission > 1
- B Less than one neutron on average causes further fission
- C The fuel temperature exceeds 10<sup>8</sup> K, as needed for fusion
- D All released neutrons are absorbed without causing fission
Correct answer: A. Average number of neutrons causing further fission > 1
Explanation: Chain reaction: each fission releases 2-3 neutrons; if at least 1 causes further fission, the reaction is self-sustaining.
Binding energy per nucleon rises sharply for light nuclei, peaks around iron-56 (the most stable nucleus), then slowly declines for heavier nuclei - which is exactly why fusing light nuclei or splitting heavy nuclei both release energy: each moves the products toward this stability peak.
Concept context
Radioactivity, nuclear reactions, fission, fusion, and binding energy.