Radiation Safety
Radiation Safety tests dose quantities and units, biological effects, ALARA, shielding, contamination control, radioactive waste, and the regulatory role of AERB. NPCIL expects a plant-aware understanding: dose is controlled by time, distance, and shielding, and contamination is prevented from spreading.
Core rules
- Absorbed dose is measured in gray (Gy), equivalent dose in sievert (Sv); 1 Gy = 1 J/kg.
- Occupational dose limit under AERB practice: 20 mSv per year averaged over five consecutive years, not exceeding 30 mSv in any one year; public limit 1 mSv per year.
- ALARA means keeping dose as low as reasonably achievable, social and economic factors considered.
- Point-source dose rate follows the inverse-square law: doubling distance reduces dose rate to one quarter.
- Half-value layer (HVL) is the shield thickness that halves the dose; shielding is most effective with high-density materials for gamma rays.
- Contamination is radioactive material on surfaces or in the body; irradiation is exposure to radiation without becoming radioactive.
Worked example. A point source gives 100 µSv/h at 1 m. At 2 m, dose rate = 100 × (1/2)² = 25 µSv/h. At 4 m it falls to 100 × (1/4)² = 6.25 µSv/h. The inverse-square relationship applies to an unshielded point source in air, not to a broad beam with heavy shielding.
Traps. Gy and Sv are not interchangeable for all radiation; contamination and irradiation are different; half-life is not half-value layer; inverse-square does not apply to a plane source; do not treat the public dose limit as the occupational limit.