After Fallout Has Stopped Coming Down near the Bomb Shelter
Forecasting Radiation Exposure
When the survey meter readings level off and then continue to decrease, the arrival of fallout from that particular cloud at your location has almost ended. If no more fallout clouds arrive, the radiation levels will continue to decrease rapidly.
The highest radiation exposure at a given place in a bomb shelter will accumulate during the first 24 hours after fallout arrives. After these first 24 hours have passed, there are two general rules which can be used to forecast the radiation exposure, as follows:
Rule 1: The radiation exposure at a given place during the entire week following the arrival of fallout is unlikely to be more than 2 1/2 times the exposure during the first 24 hours.
Rule 2: The radiation exposure at a given place during the entire month following the arrival of fallout is unlikely to be more than 3 3/4 times the exposure during the first 24 hours.
If the fallout comes from distant ground bursts and doesn’t arrive at your bomb shelter until 24 hours or more after the explosions, the numbers in Rules 1 and 2 may be slightly greater. For example, if the fallout takes about 36 hours to get to your bomb shelter, the number 2 1/2 in Rule 1 will be increased to 3.0 and the number 3 3/4 in Rule 2 will be increased to 4.5.
If the fallout takes about 48 hours to get to your bomb shelter, the corresponding numbers will be increased to about 3 1/3 and 5 1/3, respectively. When the fallout takes a long time to arrive, the radioactivity will have decayed a great deal. If the fallout comes from a large number of ground bursts of large-yield weapons, as might take place on military targets, the fallout may still be hazardous even though it may take 48 hours to arrive at your bomb shelter.
If the fallout comes from closer ground bursts and arrives at your bomb shelter in 12 hours or less after the explosions, the numbers in Rules 1 and 2 will be less. More than half of the total exposure in a week will accumulate in the first 24 hours after fallout arrives. The number 2 1/2 in Rule 1 will be decreased to between 1 1/2 and 1 3/4, and the number 3 3/4 in Rule 2 will be decreased to between 1 3/4 and 2 /12.
Exposure forecasts can be made using the seven-ten rule described earlier when all the fallout is the same age, when the time of the explosion is known fairly well, and when there are no weathering effects. These circumstances are unlikely in a modern, full-scale nuclear war. The Radiological Defense Officer in the local Emergency Operating Center (EOC) may be able to provide further guidance on estimated radiation exposure.
The general rules given above can be used to make forecasts for the possibility of radiation sickness among a group of people in a given bomb shelter. If the radiation exposure of an average adult is 60 R or less at the end of the 24 hours after fallout arrives and that person remains in the same place, that person’s accumulated radiation exposures will be expected to be less than 150 R in one week and less than 225 R in one month, providing no additional fallout arrives.
According to the Penalty Table (below), that person should require no medical care in the first week, but the exposure in a month would exceed the limits set in the Penalty Table for not requiring medical care.
If it appears that the radiation exposure of average adults will be more than a pre-selected value, such as 60 R, at the end of the first 24 hours after fallout arrives at the bomb shelter, the local EOC should be notified. Some emergency action may be possible which will reduce the accumulated radiation exposure and thus prevent radiation sickness among these people.
Again, let us look at the made-up example provided by the apartment bomb shelter. The radiation exposure record for a dosimeter mounted at location 1 is shown in the first Radiation Exposure Record (above), and the survey meter readings for that location are shown in the Survey Meter Readings Figure. The first detection of fallout was made outside the bomb shelter at 1009 hr on July 5. It was estimated that this fallout resulted from many large-yield ground bursts on military targets about 250 km (150 miles) upwind during the night before, at around 2100 hr on July 4.
The radiation level from this fallout reached a maximum value at around 1330 hr on July 5, indicating that most of the fallout destined for the apartment bomb shelter from these explosions had reached the ground by this time. The fallout took 13 hours to reach the apartment bomb shelter. It kept coming down for about 3 1/2 hours.
A distant explosion was heard at 1400 hr on July 5, in the direction of a city located about 50 km (30 miles) upwind. The fallout from this explosion began to arrive at the apartment bomb shelter at about 1645 hr, an hour and 45 minutes after the explosion was heard. This fallout was more radioactive than the older fallout from the distant explosions. Being fresher, it would decay faster. This fallout kept coming down for about 2 1/2 hours and added to the radiation levels which were already there from the older fallout.
At the end of the first 24 hours after fallout arrived, at 1000 hr on July 6, the accumulated radiation exposure by the dosimeter at location 1 was 81 R, as shown in the Radiation Exposure Record (above). After one week, the accumulated radiation exposure was 174 R, 2.15 times the exposure during the first 24 hours. After one month, it was 226 R, 2.79 times the exposure during the first 24 hours.
The Penalty Table
Medical Care Will Be Needed By:
Accumulated Radiation Exposure (R) in Any Period of: a b c
One Week One Month Four Months
A None 150 200 300
B Some 250 350 500
C Most 450 600 ----
Note: The Penalty Table here is taken from Radiological Factors Affecting Decision-Making in a Nuclear Attack, National Council on Radiation Protection and Measurements, Report No. 42.
An adult will not normally need medical care when the whole body is exposed to the quantities of radiation listed in Row A of the Penalty Table if the exposure is spread out over the listed periods of time. Rows B and C are intended to be used for making decisions on performing urgent missions which may involve the risk of increased radiation exposure.
Each person can tolerate a certain amount of sunshine on bare skin in an afternoon without getting a painful sunburn. Similarly, each person can be exposed to a certain amount of whole-body gamma radiation within a certain period of time without getting sick. The Penalty Table shows in Row A what exposures might be received by an average adult without requiring medical care, when the exposure is spread out over different periods of time.
Infants, small children, and pregnant women should be given special consideration when possible, because they are more likely to have radiation sickness at lower levels of radiation exposure than other individuals of the general population.
For most bomb shelter occupants, the exposures in Row A should not be exceeded. If the radiation levels reach 10 R/hr in the bomb shelter and continue to climb, it is possible that the accumulated exposure in one week will be greater than 150 R. In this case, the local EOC should be notified. Some emergency action may be possible which will reduce the accumulated radiation exposure and thus prevent radiation sickness in the bomb shelter.