Question

Uranium-235 undergoes fission to produce two neutrons per fission event. A neutron-absorbing material is mixed in with this sample. This causes the number of neutrons that can initiate fission events to decrease. Which of the following will be effective at achieving a critical mass? Check all that apply.

Surround the sample with a deflective material to prevent neutrons from escaping to the surroundings.

Add more absorbent material.

Compress the uranium-235 sample.

Answer 1

Effective:
- Surround the sample with a deflective/reflective material — Yes. A reflector reduces neutron leakage by scattering or returning neutrons to the fuel, increasing the fraction of neutrons available to cause further fissions and so lowers the amount of fuel needed to reach criticality.
- Compress the uranium-235 sample — Yes. Increasing the fuel density raises the probability a neutron will hit a fissile nucleus before escaping, so compression increases the neutron multiplication and helps reach criticality.

Not effective:
- Add more absorbent material — No. Absorbers capture neutrons that would otherwise induce fission, reducing the neutron multiplication factor and making it harder (or impossible) to reach criticality.

(High-level physics explanation only; no instructions or specifications for constructing nuclear devices.)