Radiation Decay Using the Seven-Tenths Rule
The level of radioactivity in the fallout from detonation of a ground burst nuclear weapon “decays” quite rapidly.
For most purposes the ‘seven-tenths rule’ enables the making of a quick approximate calculation of the radiation level at any given time, from a single measurement at a known time.
This rule states that the intensity of radiation falls by a factor (multiple) of 10 as the time lengthens by a factor (multiple) of 7. Its application is illustrated in the following table:
After the nearby explosion of a nuclear bomb assume the level of radiation at particular location is 2000 rads per hour. (At 200 rads/hr most people will be seriously ill with “radiation sickness” and many will eventually die from its effects).
Time-after a Nuclear Detonation — Level of Nuclear Radiation at That Time
@1 hour — 2000 rads/hr
@7×1 hour = 7 hours — 1/10th of 2000=200 rads/hr
@7x7x1 hour = 49 hours, that is about 2 days later — 1/100th of 2000=20 rads/hr
@7x7x7x1 hour = 343 hours, about 2 weeks later – 1/1000th of 2000=2 rads/hr
@7x7x7x7x1 hour = 2401 hrs, about 100 days later — 1/10,000th of 2000=0.2 rph
