established and the decay rate of the contamination determined. The procedure to be followed in
determining these two factors will depend upon the source of the contamination and the operational
situation existing at the time. In fast-moving division offensive operations, it is unlikely that detailed
processing will be required. A rough estimation of H-hour and an assumption of standard decay will be
sufficient to allow the evaluation of a radiological contamination hazard for a 6- to 18-hour period.
Detailed processing will be required later.
precise information concerning the hazard. When decay rates are assumed to be standard, calculations
made from these assumptions are estimates and should be so designated.
a. It is the responsibility of the NBCC to make precise calculations of H-hour, when unknown, and
the decay rate of the radiation in the fallout area. These factors must be known by a commander in order
to maintain accurate radiation exposure control. These factors are necessary to the normalization of dose
rates and the construction of radiological contours outlining areas of specific dose rate intensities.
b. Dose calculations and contaminated pattern evaluations depend upon a knowledge of the rate at
which the contamination is decaying; thus, the decay exponent n must be known. In fallout
contamination, the value of n will not necessarily be constant with time or even constant throughout a
particular contaminated area, although a fallout pattern caused by a single burst will have an average
value. This average value will vary from pattern to pattern. A fallout pattern caused by more than one
burst will not have an average value. Dose calculations in an overlapping fallout pattern must be based
on data from each burst.
4-4. Hazard Evaluation Factors. Evaluation of a radiological contamination hazard may be considered
sufficient when the following factors are known or reliably estimated:
a. The perimeter of militarily significant contamination.
(1) For new contamination (H-hour to H + 48 hours), this is as specified for induced
contamination; for fallout this is the one-hour-after-burst (H + 1) dose rate contour of 20 cGyph (cGy per
hour) for short-term (24-hour) occupancy. The 10-cGyph (H + 1) contour should be established locally,
as required, for the purposes of estimating doses associated with long-term occupancy of a contaminated
(2) For old contamination (greater than H + 48 hours) or for contamination of unknown age,
the perimeter is determined on the basis of conditions existing at the time. In the absence of specific
guidance, the 1-cGyph-dose-rate contour at the time of entrance into the area is used.
b. The H-hour or a reference time.
c. The decay rate.
d. Sufficient information about dose rates inside the perimeter.
PART B: DETERMINE H-HOUR
4-5. H-Hour Calculations. Nuclear calculations are based on a reference time for each nuclear burst. In
most cases, the time of burst (H-hour) will be known as it is reported by observing units in the NBC 1
(nuclear) report. However, there may be times when H-hour is unknown. In those cases where a detected
radiological hazard cannot be related to a specific nuclear burst, special monitoring reports will be
required to determine H-hour. Information necessary to determine H-hour is obtained from a series of