U-235 slow-neutron fission: approximately how many secondary neutrons are emitted per fission event (average prompt + delayed, rounded to a typical textbook integer)?

Introduction / Context:
The number of neutrons emitted per fission governs chain reaction feasibility, reactor criticality, and breeding potential. While the exact average depends on isotope and neutron energy, standard figures are widely used in introductory reactor physics.

Given Data / Assumptions:

• Thermal (slow) neutron fission of U-235 is considered.
• The average number of neutrons emitted per fission is slightly above two.
• Exam questions often ask for a rounded integer.

Concept / Approach:
For U-235, the average number of prompt neutrons per thermal fission is around 2 to 3 (commonly cited ~2.4). Including the small contribution from delayed neutrons (crucial for control), a rounded figure of “about 3” is frequently accepted in GK-type questions. This number is high enough to sustain a chain reaction once non-leakage and parasitic captures are accounted for via the four/five-factor formulae in reactor theory.

Step-by-Step Solution:
State typical average: roughly 2–3 neutrons per U-235 fission.Recall that problems often ask for a nearest integer.Choose 3 as the standard rounded answer.

Verification / Alternative check:
Reactor physics tables list ν (nu) for U-235 thermal fission ≈ 2.4; rounding to 3 is customary for quick GK questions when no decimal option is provided.

Why Other Options Are Wrong:
235/200/92: These are mass or atomic numbers and not neutron yields.1: Too low to sustain chain reactions after losses.

Common Pitfalls:
Confusing atomic number (92) or mass number (235) with neutron yield; these are different concepts entirely.

Final Answer:
3