A Comprehensive Overview of Nuclear Reactor Types and Neutron Physics

Introduction

The lecture by Michael Short provides a contextual foundation for studying neutrons, reactor physics, and the wide variety of nuclear reactor designs. It emphasizes a pedagogical shift toward learning applications before theory, aiming for better retention of complex concepts.

Neutron Fundamentals

• Neutron neutrality: All interactions discussed are charge‑neutral, simplifying the physics compared to charged particles.
• Average neutrons per fission (ν̅): Typically around 2.4 for U‑235 at thermal energies; increases with incident neutron energy.
• Fission timeline:
• Neutron absorption → compound nucleus (≈10⁻¹⁴ s).
• Splitting into two primary fission products.
• Prompt neutron emission (10⁻¹⁷ s) and subsequent beta, gamma, or alpha decays (10⁻¹³ s to 10⁻⁶ s).
• Cross sections: Total, scattering (elastic/inelastic), capture, fission, and multi‑neutron emission are energy‑dependent and tabulated in databases such as JANIS.
• Chi spectrum: Describes the energy distribution of neutrons born from fission, peaking near 2 MeV.

Reactor Types Overview

Light‑Water Reactors (LWRs)

• Boiling Water Reactor (BWR): Water acts as both coolant and moderator; steam directly drives the turbine.
• Pressurized Water Reactor (PWR): Water is kept under high pressure to remain liquid; heat is transferred to a secondary loop that powers the turbine.
• Neutron moderation: Hydrogen (A=1) can transfer up to 100 % of neutron kinetic energy, rapidly thermalizing neutrons.

Gas‑Cooled Reactors

• Advanced Gas‑Cooled Reactor (AGR): Uses CO₂ coolant and graphite moderator; operates with natural or low‑enriched uranium.
• Pebble‑Bed Modular Reactor (PBMR): Spherical fuel pebbles containing UO₂ kernels coated with SiC; helium coolant transfers heat to a secondary water/steam cycle.
• High‑Temperature Reactor (HTR): Targets outlet temperatures of 800–850 °C; challenges include material corrosion and helium impurity control.

Heavy‑Water Reactors

• CANDU: Uses natural uranium and heavy water (D₂O) as moderator; low neutron absorption of deuterium enables operation without enrichment. Heavy water is costly and toxic if ingested.

Graphite‑Moderated Reactors

• RBMK: Graphite moderator with light‑water coolant; infamous for the Chernobyl accident due to a positive void coefficient and graphite‑tipped control rods.
• Windscale (UK): Early graphite‑moderated plant where accumulated Wigner energy caused a fire in 1957.

Supercritical Water Reactors (SCWR)

• Operate water above its critical point, providing high density and efficient heat transfer while retaining good moderation.

Liquid‑Metal Cooled Reactors

• Sodium‑cooled Fast Reactor: Sodium’s excellent thermal conductivity and low neutron moderation enable fast‑neutron spectra; challenges include chemical reactivity and fire safety (sand extinguishers).
• Lead‑Bismuth Eutectic (LBE) Reactor: Low melting point (≈123 °C) alloy; corrosion is a major issue, but it avoids sodium’s fire hazard.
• Fast Reactor Physics: Utilizes U‑238 fission at high neutron energies; provides breeding of Pu‑239 but requires rapid neutron lifetimes, making control more demanding.

Molten‑Salt Reactors (MSR)

• Coolant and fuel are the same molten salt (e.g., LiF‑UF₄); offers inherent safety—leakage leads to solidification and subcriticality. High melting points (~450 °C) are a design challenge.

Safety and Operational Insights

• Control rod design: Materials with high capture cross sections (e.g., boron carbide) must avoid unintended moderation effects.
• Heat removal: Dense coolants (water, liquid metals) provide efficient heat extraction; low‑density gases require higher pressures.
• Accident scenarios: Loss‑of‑flow in sodium reactors can freeze coolant; graphite reactors can accumulate Wigner energy; RBMK’s void coefficient caused rapid power spikes.
• Practical experience: MIT Research Reactor operates safely with hourly log checks; modern reactors incorporate passive safety systems and automated alarms.

Pedagogical Approach

• The course adopts a "context first, theory second" methodology, presenting real‑world reactor designs before deriving neutron transport and diffusion equations. This mirrors the department’s broader curriculum redesign.

Key Variables for Neutron Transport

• Position vector r (x, y, z)
• Energy E
• Direction (solid angle Ω, defined by θ and φ)
• Time t
• Reaction cross sections σ (total, scattering, capture, fission, (n,2n), etc.)
• Neutron source term χ(E) (birth spectrum)

These variables feed into the neutron balance equation, which will be simplified in subsequent lectures for analytical and computational solutions.

Conclusion

Understanding the diverse reactor technologies and the underlying neutron physics equips engineers to design safer, more efficient nuclear systems and to apply the neutron transport equation effectively across all reactor types.