NUCL 310 Oral Exam Prep

Why do radioactive materials decay?

Unstable nuclei seek a lower-energy, more stable configuration by emitting radiation or particles to correct energy or neutron–proton imbalance.

What are modes of decay?

Alpha, beta (β⁻, β⁺/electron capture), gamma decay, and spontaneous fission.

What are the three major types of radiation?

Alpha (heavy, low penetration), beta (light charged particles), gamma (highly penetrating photons).

What are the general characteristics of neutron transport in media?

Neutrons undergo scattering and absorption; motion is random and depends on material cross-sections and energy.

How does the energy content per gram compare for nuclear and chemical reactions?

Nuclear reactions release about a million times more energy per gram than chemical reactions.

What is the difference between fissile, fertile, and fissionable fuel?

Fissile fissions with thermal neutrons (U-235); fertile converts to fissile (U-238 → Pu-239); fissionable can fission but may need fast neutrons.

Why do thermal reactors employ moderators?

To slow neutrons to thermal energies where fission probability is higher.

Why do fast reactors employ heavy metals for cooling, e.g., liquid sodium?

To avoid neutron moderation while providing efficient heat transfer.

Why is fast reactor fuel enriched?

Fast neutrons are less likely to cause fission, so higher fissile content is needed.

Can thermal reactors operate on natural uranium? How?

Yes, using very efficient moderators like graphite or heavy water to minimize neutron losses.

Describe energy dependence of scattering and absorption cross-sections.

Absorption increases at low energies (~1/v); scattering changes more slowly with energy.

Give examples of threshold cross-sections.

Reactions like (n,2n) or fast fission that occur only above a certain neutron energy.

Explain Doppler broadening and its impact on reactor criticality.

Increased temperature broadens resonance peaks, increasing absorption and adding negative reactivity (stabilizing effect).

What does neutron energy distribution look like in thermal and fast reactors?

Thermal reactors have a low-energy Maxwellian spectrum; fast reactors have a broad high-energy spectrum.

Why is the moderator-to-fuel ratio important?

It controls neutron slowing-down vs absorption, strongly affecting reactivity.

How is coolant-to-fuel ratio minimized in fast reactor design?

By using compact cores and dense fuel to limit moderation.

What is spectral hardening? Spectral shift?

Hardening shifts neutrons to higher energies; spectral shift is any change in energy distribution due to conditions.

How does the spectrum shift/harden in LWRs?

Burnup or reduced moderation decreases thermal neutrons, increasing relative fast neutron fraction.

Why does kinf depend on flux-weighted cross-sections?

Reaction rates depend on neutron flux, so cross-sections are averaged over the energy spectrum.

What is a bare reactor? Why is it not used?

A reactor without a reflector; not used due to high neutron leakage.

Why is fuel split into small pins?

To improve heat removal and reduce temperature gradients.

Why is fuel cladded?

To contain fission products and protect fuel while allowing heat transfer.

Explain the basic assumptions of diffusion theory.

Flux is smooth and nearly isotropic; neutron transport approximated as diffusion.

Explain limitations of diffusion theory.

Inaccurate near boundaries or in low-scattering/anisotropic conditions.

What is the mean free path of fast and thermal neutrons in LWRs?

Fast: several cm; Thermal: about 1–2 cm.

What are the mean lifetimes of neutrons in reactors?

Thermal: ~10^-3 s; Fast: ~10^-7 s.

Why does flux reach steady state in a subcritical system with a source?

Source production balances absorption and leakage losses.

Why can’t a nuclear reactor be shut down instantly?

Delayed neutrons and decay heat continue after shutdown.

What design parameters control criticality?

Control rods, moderator density, fuel enrichment, coolant density, burnable poisons.

Name a prominent fission product and its impact.

Xenon-135 strongly absorbs neutrons, causing reactivity changes (poisoning).

What is energy and spatial self-shielding?

Energy: resonance absorption reduces flux at certain energies; Spatial: outer fuel absorbs neutrons, shielding inner regions.