Nuclear Science and Technology
What this domain tests
Nuclear Science and Technology in BARC OCES/DGFS probes reactor physics, the neutron life cycle, thermal hydraulics, radiation protection, fuel-cycle chemistry and safety philosophy. Questions are both numerical and conceptual, often framed around PHWR and fast-reactor practice: natural-uranium heavy-water lattices, pressure tubes and thorium utilisation.
Neutron balance
- Criticality: k_eff = k∞ · P_NL, where P_NL is the non-leakage probability. k_eff = 1 is critical; below 1 is subcritical; above 1 is supercritical.
- Thermal-reactor four-factor formula: k∞ = η f p ε. Here η is neutrons produced per absorption in fuel, f is thermal utilisation, p is resonance escape probability and ε is the fast-fission factor.
- Reactivity ρ = (k_eff − 1)/k_eff, usually quoted in pcm (1 pcm = 10^-5). Delayed neutrons in U-235 have β ≈ 650 pcm, which gives controllable margin.
Heat and radiation
- Decay heat must be removed after shutdown; residual heat removal is a safety-class function.
- Absorbed dose is measured in gray (J/kg); equivalent dose in sievert, using a radiation weighting factor.
- Gamma shielding favours high-Z lead; neutron shielding favours hydrogenous material.
Worked example
A lattice has k_eff = 1.005. Then ρ = (1.005 − 1)/1.005 = 0.004975. Multiply by 10^5 to obtain 497.5 pcm. The core is slightly supercritical, so a control rod worth about 500 pcm would restore criticality. If you instead use k − 1 = 0.005 = 500 pcm, you have only approximated; the exact value is 497.5 pcm.
Traps
- Using k∞ as k_eff and ignoring leakage.
- Reporting k − 1 instead of (k − 1)/k.
- Mixing pcm and dollars (1 dollar = β).
- Confusing gray and sievert.
- Treating decay heat as negligible immediately after scram.