Radiation Protection Principles and Dose Limits
The three principles
Justification (the practice must do more good than harm), optimisation (ALARA, with dose constraints and reference levels) and dose limitation (fixed limits for workers and the public). Know that dose limits apply to planned exposure situations; emergencies use reference levels, not limits.
Quantities and units
Distinguish absorbed dose (gray), equivalent dose (sievert, with radiation weighting factors), and effective dose (sievert, with tissue weighting factors). Committed effective dose integrates intake over 50 years for adults. Collective dose is the sum of individual effective doses, used for optimisation, never for proving compliance.
Limits to memorise
Occupational effective dose: 20 mSv per year averaged over five years, with no single year above 50 mSv. Public: 1 mSv per year. Lens of the eye: 20 mSv per year occupational. Skin and extremities: 500 mSv per year. These are ICRP values adopted in many national frameworks; confirm the values in the applicable national regulation before answering.
Traps
- Confusing dose constraint (a planning value below the limit) with dose limit.
- Applying public limits to a worker or vice versa.
- Forgetting that pregnant workers may declare pregnancy and receive additional protection.
- Treating collective dose as a compliance metric.
Rule of thumb
If a question mentions ALARA, dose constraint or optimisation, the answer is a value below the limit, not the limit itself.
Worked example
A worker receives 18 mSv, 22 mSv and 15 mSv in three consecutive years. The five-year average is 18.3 mSv, below 20 mSv, and no year exceeds 50 mSv, so the limit is met.