Sphere kyun? Sphere ki unit volume per sabse kam surface area hoti hai → sabse kam leakage → sabse chhoti critical mass. (Bare 235U sphere ≈ 52 kg; neutron reflector ke saath, kaafi kam.)
Recall Quick self-test
Q: Do identical subcritical half-spheres ek saath thaok di jaati hain. k ka kya hota hai?
A: Halves milane se ek ball banti hai jiska surface-to-volume ratio kam hota hai → kam leakage → k 1 se upar jump karta hai → supercritical (yahi "gun-type" bomb ka principle hai).
Ek heavy nucleus ka lighter fragments mein tootna + 2–3 neutrons + ~200 MeV energy.
Fission energy release kyun karta hai?
Fragments ka binding-energy-per-nucleon parent se zyada hota hai (iron peak ke kareeb); gain kinetic energy ke roop mein release hota hai.
U-235 fission mein energy release?
~200 MeV.
Mass defect ko energy se link karne wala formula (unit shortcut ke saath)?
E=Δmc2; 1u→931.5MeV.
Multiplication factor k define karo.
Ek generation mein fission-causing neutrons ka pichli generation se ratio; govern karta hai chain badhegi (k>1), rahegi (k=1), ya khatam hogi (k<1).
k=1 ka physically matlab kya hai?
Critical: steady self-sustaining chain — reactor ka operating point.
Chain reaction ke liye minimum (critical) mass kyun chahiye?
Production volume se scale hoti hai (r3), leakage surface se (r2); sirf ek minimum size se upar hi itne neutrons andar rehte hain ki k≥1 ho.
Minimum critical mass ke liye sphere kyun?
Volume per minimum surface area → sabse kam neutron leakage.
Moderator ka role?
Fast neutrons ko thermal speeds par slow karta hai (light nuclei se elastic collisions) taaki U-235 unhe efficiently capture kare.
Control rods ka role?
Excess neutrons absorb karte hain (Cd, B) k=1 rakne ke liye.
Thermal vs fast reactor — ek key difference?
Thermal slowed (moderated) neutrons + low enrichment use karta hai; fast unmoderated fast neutrons + high enrichment use karta hai, fuel breed kar sakta hai.
Breeder reactor kya karta hai?
Fertile 238U ko fast neutrons use karke fissile 239Pu mein convert karta hai, consume se zyada fuel banata hai.
U-238 → Pu-239 ka reaction chain?
238U + n → 239U →(β−) 239Np →(β−) 239Pu.
Recall Feynman: 12-saal ke bacche ko samjhao
Ek kamra socho jo mousetraps se bhara hua hai, har ek mein do ping-pong balls loaded hain. EK ball andar fenko. Woh ek trap spring karti hai, jo do aur balls fenkti hai, jo do aur traps spring karti hain... jaldi hi poora kamra balls se bhara hua explode karta hai. Yahi chain reaction hai. Har uranium atom ek loaded mousetrap hai; neutron ping-pong ball hai. Agar kamra bahut chhota hai, toh zyaatar balls khule darwaze se bahar ud jaati hain trap hit karne se pehle, aur fizzle ho jaata hai — tumhe itna bada pile chahiye (critical mass). Power plant mein, hum kuch catchers (control rods) lagate hain exactly extra balls pakdne ke liye, taaki yeh ek calm, steady snapping rahi aaye — kabhi explosion nahi.
