Nuclear Energy & Reactor Design

NucE=Energy from nuc rxns, primarily fission in U-235, Pu-239. Fission yields ~200 MeV/event, releases kinetic E of frag, n, γ. Chain rxn req k-eff ≥1. Criticality: subcritical (k<1), critical (k=1), supercritical (k>1). Moderator (H2O, D2O, graphite) slows n via elastic scatt to thermal E (~0.025 eV) to ↑ σ_fiss. Coolant (H2O, Na, He, Pb-Bi) removes heat, may also act as mod. Fuel: UO2 pellets in Zr-alloy cladding (zircaloy), assy in fuel rods → fuel assemblies. Burnup measured in GWd/tHM. Depletion: fissile ↓, FP (fission prod) ↑, some FP (Xe-135, Sm-149) have high σ_abs → poison reactivity. Refuel cycles: batch refuel (PWR, ~1/3 every 12–24 mo), online (CANDU). Reactivity control: CR (control rods, B, Ag, In, Cd), chem shim (boric acid in PWR), burnable poisons (Gd2O3, B4C). Temp coeffs: Doppler (fuel, prompt neg), mod temp coeff (mod density ↓ → ↓moderating → neg in LWRs). Safety: inherent (passive) stability via neg feedback, engineered SCS (safety cooling sys), containment struct. Gen classification: Gen I (early prototype), Gen II (comm LWRs, incl PWR, BWR; ~90% oper reactors), Gen III/III+ (enh safety, std design, e.g., AP1000, EPR), Gen IV (future, high eff, waste min, non-prolif, 6 types). PWR: pressurized H2O primary loop (15–16 MPa, ~315°C), steam gen via SG, secondary H2O → turbine. BWR: lower P (~7 MPa), boiling in core, direct cycle, steam to turbine. CANDU: D2O mod & cool, natU fuel, PHWR, online refuel. HTGR: TRISO fuel (UO2 kernel, C/SiC layers), graphite mod, He cool, >700°C out, high eff gen or process H2. SFR: Na cool (fast n spectrum), no mod, high fissile conv, closed fuel cycle, metallic or MOX fuel. MSR: fuel (UF4, ThF4) dissolved in molten fluorides (LiF-BeF2), online proc, neg temp coeff, low P op. Lead-cooled (LFR): Pb or Pb-Bi eutectic, fast spect, passive safety, high Z → gamma shield. GFR: He cool, fast spect, ceramic fuel, high T, high eff. Fast reactors ↑ Pu breeding (η>2), enable fuel breeding (U-238 → Pu-239), reduce actinide waste. Fuel cycle: open (once-thru, store spent fuel), closed (reprocess: PUREX extracs U/Pu → MOX), Gen IV targets full recycle. Transmutation: convert long-lived MAs (Np, Am, Cm) → short-lived via n capture/fission in fast flux. Challenges: NIMBY, high cap cost, dec comm (decades), waste disp (geol repos, e.g., Onkalo), prolif risk (Pu separation), accident legacy (TMI, ChN, Fuku). Passive safety: AP1000 uses grav, condens, nat circ for ECCS. SMRs: <300 MWe, mod fab, enhanced safety (integral PWR, NuScale), flexible deploy. Microreactors: <20 MWe, transportable, remote/military use. Fusion: not fission; D-T → He + n + 17.6 MeV, req T > 100 MK, confin: magnetic (tokamak, stellarator), inertial (laser). ITER: int’l tokamak, Q>10, net power demo. DEMO: post-ITER, grid-connected. Materials: radiation damage (displ per atom, dpa), embrittlement, swell (voids), creep. Zr-alloy: H pickup → hydriding, risk brittle fail. SiC cladding R&D for accident-tol fuel (ATF). Coolant chem control: PWR: pH via LiOH, H2 for O2 suppr; BWR: H2 water chem (HWC) to ↓ RCS dose. Severe accident mgmt: core melt, H2 gen (Zr + H2O → H2 + ZrO2), containment venting, core catchers (EPR). Digital I&C: modern reactor ctrl sys, cybersec critical. Licensing: NRC (US), IAEA safety standards, WENRA. Economics: LCOE high vs. gas/solar, but high CF (>90%), low var cost, long op life (60+ yr). Waste: HLW (spent fuel), ILW (cladding, resins), LLW (tools, cloth). Reprocessing ↓ vol but ↑ cost/complexity. Deep geol repos: multi-barrier (waste form, canister, buffer, geo). Actinide recycle in fast reactors ↓ radiotox lifetime from 10^5 to ~10^3 yr. Future: hybrid (fusion-fission), ADS (accelerator-driven subcritical sys for waste transmutation), thorium cycle (²³²Th → ²³³U), MSR with Th fuel. Fusion-fission hybrids use fusion n to drive subcritical fission blanket for energy amplification or waste burnup. Key metrics: capacity factor, thermal eff (30–40% for LWRs, >45% for Gen IV), neutron economy, doubling time (fast breeders).