Nuclear power Study Guide

📖 Core Concepts Nuclear Power – Electricity generated from nuclear reactions (mainly fission of \(^{235}\)U or \(^{239}\)Pu). Fission Chain Reaction – A neutron splits a fissile nucleus, releasing energy and 2–3 neutrons that can start new fissions. Control Rods & Delayed Neutrons – Rods absorb neutrons; delayed neutrons (ms–s after fission) give operators time to adjust rods and keep the reaction stable. Reactor Types – PWR (Pressurized Water Reactor) – 63 % of civilian fleet. BWR (Boiling Water Reactor) – 18 %. PHWR (Heavy‑water) – 11 %. Fuel Cycle – Mining → Milling → Enrichment (0.7 % U‑235 → 3.5‑5 % for LWR) → Fabrication → In‑reactor use → Spent‑fuel cooling → Dry‑cask storage (or reprocessing). High‑Level Waste (HLW) – 95 % U, 4 % fission products, 1 % transuranics; radioactivity drops 99.5 % after 100 yr, to natural‑ore levels after 100 kyr. Economic Metrics – Capital‑intensive; median LCOE ≈ $69 \text{USD/MWh}$ (2025, 7 % discount). Lifetime‑extension LCOE ≈ $32 \text{USD/MWh}$. Safety Features – Negative void coefficient, emergency core‑cooling system (ECCS), multi‑barrier containment, high capacity factor (> 90 %). --- 📌 Must Remember Global Share (2023) – Nuclear 9 % of electricity, 4 % of total energy. Capacity Factor – World‑average 89 %; US reactors 92 %. Enrichment Level – Light‑water reactors need 3.5–5 % \(^{235}\)U. Spent‑Fuel Decay – 99.5 % drop in radioactivity after 100 yr; < natural‑ore after 100 kyr. LCOE Comparisons – Nuclear $69/MWh$ vs on‑shore wind $50/MWh$ vs utility solar $56/MWh$. Mortality Rate – 0.03 deaths / TWh (second‑safest after solar). INES Scale – 0 (no risk) → 7 (major accident). Chernobyl & Fukushima = 7. Breeding – Converts fertile \(^{238}\)U or \(^{232}\)Th into fissile \(^{239}\)Pu or \(^{233}\)U. --- 🔄 Key Processes Fission Chain Reaction Neutron → fissile nucleus → split → energy + 2–3 neutrons → repeat. Reactor Power Control Insert control rods → absorb excess neutrons → lower reactivity. Remove rods → increase reactivity. Fuel Fabrication Yellowcake \(U3O8\) → UF₆ → enrichment → UF₆ → conversion to UO₂ → sinter → pellets → load into rods. Spent‑Fuel Management 6–10 yr cooling pool → transfer to dry cask → (optional) reprocessing → MOX fuel. Economic Evaluation (LCOE) $$\text{LCOE} = \frac{\text{Present value of total costs}}{\text{Present value of electricity generated}}$$ Inputs: capital cost, O&M, fuel cost (< 1 % of total), discount rate, capacity factor. --- 🔍 Key Comparisons PWR vs BWR – PWR keeps water pressurised (heat transferred to secondary loop); BWR boils water directly in the core. Fission vs Fusion – Fission splits heavy nuclei (U, Pu) releasing 200 MeV per event; Fusion combines light nuclei (D‑T) needing > 100 million °C, still experimental. Reprocessing vs Direct Disposal – Reprocessing recovers 95 % U/Pu and cuts HLW volume 80 % but adds cost & proliferation risk; disposal stores waste unchanged. Generation III vs Generation IV – Gen III improves safety & efficiency; Gen IV adds waste‑burning, higher temperature operation, and proliferation resistance. --- ⚠️ Common Misunderstandings “Nuclear = high CO₂” – Operational emissions are essentially zero; life‑cycle emissions ≈ 12 g CO₂‑eq/kWh. “All spent fuel is highly dangerous forever” – Radioactivity declines sharply; after 100 kyr it’s comparable to natural ore. “Fusion will replace fission soon” – Commercial fusion is still targeted for 2034; many technical hurdles remain. “More reactors automatically mean more waste” – Waste volume is tiny compared with renewable waste; advanced cycles can reduce it further. --- 🧠 Mental Models / Intuition “Neutron economy” – Think of a bakery: each neutron is a baker. You need enough bakers (neutrons) to keep making bread (fission), but not so many they burn the kitchen (runaway reaction). Control rods are the fire‑extinguishers. “Capacity factor ≈ reliability” – A plant running at 90 % is like a student who studies 90 % of the semester; it delivers almost its full potential. “Fuel as a credit” – Enrichment is buying a higher‑interest loan: a small increase in \(^{235}\)U dramatically raises the “pay‑off” (energy per mass). --- 🚩 Exceptions & Edge Cases Negative Void Coefficient – Not all reactors have it; RBMK (Chernobyl) had a positive coefficient, contributing to the accident. Fast Breeder Reactors – Use fast neutrons; they can breed more fissile material but require different coolant (e.g., liquid sodium). SMR Licensing – Traditional site‑by‑site licensing may not apply; modular certification pathways are emerging. --- 📍 When to Use Which Choose PWR when high‑temperature steam and robust containment are priorities (most of world fleet). Choose BWR for simpler plant layout and lower water inventory (requires careful steam‑void management). Choose PHWR if natural‑uranium fuel is preferred (no enrichment needed). Select Reprocessing if uranium resources are scarce and a nation wants to reduce HLW volume, accepting higher cost & proliferation risk. Adopt SMRs for remote or low‑grid‑capacity sites, or when capital is limited and faster construction is needed. --- 👀 Patterns to Recognize High Capacity Factor + Low Variable Cost → Baseline “dispatchable” power → fits grid‑stability role. Accident Level (INES) + Reactor Type – Level 7 accidents involve design flaws + loss of containment (RBMK, GE‑type). Economic Viability – When carbon price ≥ $30/ton, nuclear LCOE becomes lower than coal/gas. Waste Longevity – Short‑lived isotopes dominate first centuries; long‑lived actinides dominate after ≈ 10⁴ yr. --- 🗂️ Exam Traps “Nuclear emits more CO₂ than renewables” – Wrong; life‑cycle emissions are an order of magnitude lower. “All reactors have a positive void coefficient” – Incorrect; modern designs are negative for safety. “Reprocessing eliminates all long‑lived waste” – Misleading; it removes most U/Pu but not the high‑half‑life fission products. “Fusion is already commercial” – Distractor; commercial operation is still projected for 2034 (first plant) and later for DEMO. “Higher enrichment always means better performance” – Over‑enrichment raises proliferation risk without proportional efficiency gains for LWRs. ---