Nuclear Reactor Physics and Radiation Transport in Advanced Fission Systems: Neutron Moderation, Criticality, and Isotopic Burn-up in Thermal and Fast Reactor Environments (2000–2026)

This comprehensive review synthesizes six decades of theoretical and experimental progress in nuclear reactor physics, with particular emphasis on developments from 2000 to 2026 spanning thermal, fast, and advanced Generation-IV reactor concepts. The article covers neutron cross-sections, moderation, and thermalization; criticality theory and the four-factor formula; reactivity feedback mechanisms (Doppler, moderator void, and temperature coefficients); neutron transport methods from diffusion theory to Monte Carlo (MCNP6, Serpent 2, OpenMC); fuel depletion and isotopic burn-up via Bateman equations; fission product poisoning (Xe-135, Sm-149); plutonium breeding and MOX fuel; decay heat (ANS-5.1); radiation shielding; major nuclear accidents (TMI, Chernobyl, Fukushima); the complete nuclear fuel cycle; and global reactor fleet statistics. Six original data figures and six comprehensive data tables are included. A forward-looking section covers SMR deployment, AI-assisted neutronics, and fusion-fission hybrid prospects through 2035.