The parent article gives minimum standoff distances ranging from 10 m for a small sealed source to 30 km downwind of an exposed fuel pool, but these are planning figures chosen for caution, not distances derived from a measured dose. For a small source in open air the dose rate falls with the square of distance, which makes distance the cheapest protection, while for wide contaminated areas and plumes the inverse square law does not apply.
Part of Radiation Safety
A safe distance is the separation at which exposure from a source is acceptably low; for a settlement without instruments it is a conservative planning figure.
The Parent Article’s Table
From Nuclear Safety:
| Source type | Minimum distance | Parent’s note |
|---|---|---|
| Intact reactor building | 1 km | Monitor, approach only if necessary |
| Breached reactor | 10+ km | Stay upwind, map contamination |
| Exposed fuel pool | 30+ km downwind | Extreme hazard, area denial |
| Medical radiation source (sealed) | 10 m | Small but intense, do not handle |
| Industrial source (unsealed) | 100+ m | Needs specialised knowledge |
| Radiological dispersal area | Edge of contamination map | Defined by survey, not by guessing |
This guide does not alter these figures but does disagree with one phrase: “approach only if necessary” for an intact reactor. For the reasons in Reactor Status Assessment, read it as: do not approach. The safer reading of every row is to treat the number as a minimum.
For scale, the zones that authorities drew after the two largest reactor accidents were about 30 km in radius at Chernobyl and 20 km at Fukushima (recalled, not from the corpus; flagged for checking), so the parent’s 10 km for a breached reactor is smaller than real exclusion practice.
The Inverse Square Law For A Small Source
For a source small compared with the distance, in open air, rate is proportional to 1 / d^2. To keep a reading R2 from a source that gives R1 at d1: d2 = d1 x sqrt(R1 / R2).
- Doubling the distance gives 1/4; at 10 m the rate is 1/100 of that at 1 m (parent article).
- Hypothetical: to cut the rate at 1 m by a factor of 10,000, d2 = 1 x sqrt(10,000) = 100 m.
That means distance is cheap against a point source: a hundredfold distance cuts the rate 10,000-fold. See Shielding & Protection.
Where The Law Fails
- Large or wide sources. A contaminated field, a long wall or a pool does not fall as 1 / d^2; the rate drops far more slowly with distance.
- Plumes. Airborne material goes where the wind goes, not in all directions equally. Distance matters less than being downwind; see Weather Monitoring.
- Scatter in air. Radiation scatters, so some reaches you around obstacles (recalled general knowledge, not from the corpus).
- Contamination. Distance does not stop dust carried by wind, boots or water; see Contamination vs Irradiation.
How To Use Distances Without An Instrument
Treat each distance as the closest approach for a stated source, not as a safe place to live. Use the larger of two figures when unsure. Put the limit on the ground with markers (Exclusion Zones) and decide the type of source from Nuclear Facility Hazards and Waste Site Identification.
Common Mistakes
- Using 1 / d^2 for a plume or a field.
- Reading “10 m” as “safe to stand near for hours”. It is a minimum standoff, not a measured limit.
- Standing downwind of a suspected release.
- Treating the table as complete. A source type that is not listed is not safe.
Safety
Distance is not a measurement
No distance in this article is derived from a dose rate. If a source is strong, even the stated minimum may not be enough. When in doubt, go farther and cross wind or upwind; do not check by walking back.
What’s Next
- Learn next: Exclusion Zones — marking the distance on the ground
- Combine with: Reactor Status Assessment — why status cannot be judged from outside