Nuclear reactor Study Guide

📖 Core Concepts Nuclear reactor – a device that sustains a controlled fission chain reaction to produce heat. Fissile fuel – isotopes that undergo fission when they capture a neutron (e.g., U‑235, Pu‑239). Moderator – material that slows neutrons to thermal energies, enabling fission of low‑enriched fuel (water, heavy water, graphite). Coolant – fluid that removes heat from the core (water, CO₂, helium, liquid Na, Pb, molten salt). Chain reaction – each fission releases neutrons; if on average ≥ 1 neutron causes another fission, the reaction is self‑sustaining. Delayed neutrons (0.65 % of total) give operators a time buffer to control power; prompt critical occurs when only prompt neutrons sustain the reaction → rapid power rise. Xenon‑135 poisoning – a strong neutron absorber produced from I‑135 decay; can shut down the reactor after shutdown (the “iodine pit”). 📌 Must Remember Energy density: 1 kg of low‑enriched U‑235 ≈ 120 000 ×  the energy of 1 kg of coal. Typical enrichment: ≈ 4 % U‑235 for commercial Light‑Water Reactors (LWRs). Reactivity control: Insert control rods (neutron absorbers) → ↓ power; withdraw rods → ↑ power. Reactor fleet: 9 % of world electricity; 90 % of that from Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Generation timeline: Gen I – prototypes & early research reactors Gen II – commercial plants built 1965‑1996 Gen III – evolutionary designs 1996‑2016 Gen III+ – enhanced safety 2017‑present Gen IV – under development (safety, waste, sustainability). Burnup unit: MW·d / tHM (megawatt‑days thermal per metric ton of initial heavy metal). Typical plant life: 40‑60 yr (license), often extended 10‑20 yr after safety review. 🔄 Key Processes Fission event Neutron captured by fissile nucleus → splits → kinetic fragments + γ‑rays + 2‑3 neutrons. Fragment kinetic energy → collisions → thermal heat. Heat removal Coolant circulates, picks up thermal energy → either directly generates steam (BWR) or transfers heat to a secondary loop (PWR). Power conversion Steam drives turbine → turbine spins generator → electrical power. Reactivity control loop Measure neutron flux → adjust control‑rod position → change neutron population → stabilize power. Xenon poisoning cycle I‑135 (t₁/₂ ≈ 6.6 h) → decays to Xe‑135 (strong absorber) → builds up after shutdown → suppresses re‑criticality for 1‑2 days. 🔍 Key Comparisons Thermal vs. Fast reactors Thermal: uses moderator → neutrons ≈ 0.025 eV; fuel can be low‑enriched or natural uranium. Fast: no moderator → neutrons stay high‑energy; requires ≈ 20 % enriched fuel; can breed more fissile material. Water‑cooled vs. Liquid‑metal‑cooled Water: excellent moderator, high heat capacity, but limited to ≤ ≈ 330 °C (PWR) or boiling (BWR). Liquid metal (Na, Pb, LBE): no moderation, allows higher temperature operation, but chemically reactive (Na) or heavy (Pb). Pressurized vs. Boiling Water Reactors PWR: coolant kept liquid at high pressure; separate steam‑generator loop. BWR: water boils inside core; steam goes directly to turbine. Heavy‑water vs. Light‑water reactors Heavy‑water: low neutron absorption → can run on natural uranium. Light‑water: higher absorption → needs enriched uranium (4 %). ⚠️ Common Misunderstandings “All reactors are the same.” → Reactor designs differ markedly in moderator, coolant, and neutron spectrum, affecting fuel, safety, and waste. “Prompt criticality is normal operation.” → It is a dangerous condition; reactors rely on delayed neutrons for stable control. “Xenon poisoning only happens during operation.” → It peaks after shutdown, creating the iodine pit that delays restart. “Molten‑salt reactors are just water‑cooled.” → They dissolve fuel in salt; the salt itself is the coolant and may or may not include a moderator. 🧠 Mental Models / Intuition Chain‑reaction balance: Neutrons in = neutrons out. Control rods act like a “tap” on the neutron flow. Coolant as a “heat‑pipe”: Imagine coolant as a conveyor belt carrying hot bricks (fuel) away; if the belt stops, bricks overheat → meltdowns. Xenon pit as “post‑shutdown fog”: After you turn off a furnace, residual heat still fuels a brief, invisible fog (Xe‑135) that blocks new fire (neutrons). 🚩 Exceptions & Edge Cases Fuel types that can be refueled on‑line: Pebble‑bed, RBMK, molten‑salt, Magnox, advanced gas‑cooled, CANDU reactors. Supercritical water reactors: Operate above water’s critical point (≈ 22 MPa, 374 °C); water behaves like a single-phase supercritical fluid, not distinct liquid/steam. Fast breeder reactors: Can produce more fissile material than they consume, but require very high enrichment or plutonium fuel. 📍 When to Use Which Choose Light‑Water (PWR/BWR) → most mature tech, abundant fuel supply, proven safety record. Choose Heavy‑Water (CANDU) → natural‑uranium fuel, flexibility for on‑line refueling, useful where enrichment infrastructure is limited. Choose Fast‑reactor / breeder → when the goal is waste minimization or plutonium utilization. Choose Gas‑cooled (He/CO₂) → high‑temperature operation for process heat or hydrogen production. Choose Molten‑Salt → inherent safety (fuel drain), potential for thorium cycle, high thermal efficiency. 👀 Patterns to Recognize “Steam → turbine → electricity” appears in every commercial design; the difference lies in where steam is generated (primary vs secondary loop). “Moderator + coolant = water” in LWRs → leads to coupled neutron‑moderation and heat‑removal constraints. Accident narratives often involve loss of coolant → emergency core cooling activation → containment breach risk. Fuel enrichment level ↔ reactor type: natural U → heavy‑water; low‑enriched → LWR; high‑enriched → naval or fast reactors. 🗂️ Exam Traps “All reactors use water as coolant.” – Liquid‑metal, gas‑cooled, and molten‑salt reactors are also commercial concepts. “Xenon poisoning only reduces power.” – It can shut down a reactor and delay restart for days. “Prompt criticality is safe because reactors are designed for it.” – Prompt criticality is a runaway condition; safety systems rely on delayed neutrons. “Generation IV reactors are already in operation.” – They are still under development; only prototypes exist. “Fuel enrichment is always > 5 %.” – Heavy‑water reactors use natural uranium (≈ 0.7 % U‑235). --- Study this guide in short bursts; focus on the bolded keywords and the “must‑remember” facts for rapid recall before the exam.