The answer most people want is a single number. The real answer is that the duration depends on where you are relative to the detonation, wind, weapon type, and your shelter's Protection Factor. This is the framework to understand it — based on established radiological science.
The 7-10 Rule
The 7-10 rule is the fundamental principle of nuclear fallout decay. For every sevenfold increase in time after a detonation, radiation intensity decreases by a factor of ten.
| Time After Detonation | Radiation Relative to Peak | Interpretation |
|---|---|---|
| 1 hour | 100% | Peak fallout period |
| 7 hours | 10% | First critical threshold |
| 49 hours (~2 days) | 1% | Manageable with monitoring |
| 343 hours (~2 weeks) | 0.1% | Swiss TWK 2017 baseline |
| 2,401 hours (~100 days) | 0.01% | Extended scenarios |
This decay rate is consistent across fission weapon fallout. The first 24 hours are the most dangerous, the first week is critical, and levels drop substantially over two weeks.
Recommended Shelter Durations by Scenario
Civil protection guidance from ICRP and national civil defence authorities distinguishes three scenarios:
Allow initial fallout to settle. Exit subject to official guidance and monitoring data.
Deposited fallout on ground is the primary risk. PF 1,000 shelter reduces dose to 0.1% of external.
Primary issue shifts to structural damage, fires, and infrastructure. Exit depends on surface conditions.
The 14-Day Specification Standard
Swiss civil protection standards (TWK 2017) specify 14-day self-sufficiency as the baseline. At 14 days, fallout has decayed to approximately 1% of peak. Exit with appropriate protective equipment and route planning is typically viable in most scenarios.
A shelter designed to support 14 days of occupancy needs:
- Power for all systems for 14 days (generator with rated fuel reserve)
- Water: minimum 3 litres per person per day for drinking
- Air filtration maintaining rated performance for the full duration
- Communication capability to receive external monitoring data
Why the Exit Decision Matters
A shelter that pushes occupants out early because supplies run out or power fails has failed its purpose. The exit decision should be made on the basis of external radiation monitoring data, not on when food runs out. This is why the 14-day life support specification is the floor, not the ceiling.
The exit decision should be made on the basis of external radiation data. A shelter that forces early exit has failed its purpose regardless of its protection factor.
What Affects Your Required Duration
- Distance from detonation
- Further distance means shorter fallout exposure. At 50km+ from a 1Mt detonation, 72 hours may be sufficient. At 10km, 14 days is appropriate.
- Wind and weather
- Fallout deposits downwind. Without monitoring data, assume the worst-case direction.
- Weapon type
- Surface detonations produce more fallout than air bursts, which maximise blast radius instead.
- Shelter Protection Factor
- A PF of 1,000 allows 14 days in a highly contaminated area. A PF of 10 (a modern brick house) does not support extended sheltering in contaminated zones.
Frequently Asked Questions
- What is the minimum safe time to shelter?
- 24 hours absolute minimum. 72 hours per most civil defence authorities. 14 days for a designed shelter in a fallout zone.
- Can I leave to check conditions?
- Only with dosimetry equipment that can tell you the current external dose rate. Without a calibrated instrument, exiting is guessing.
- How is a 14-day shelter different from a 72-hour shelter?
- Primarily in life support sizing: more fuel, water, provisions, and typically a larger footprint to maintain livable conditions.
We work on private shelter projects globally. If you have questions about translating a duration requirement into a shelter specification, get in touch.
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