CHAPTER 17
2.
Underwater Bursts.
The Ref. tS
method of predicting the
deposit dose resulting from underwater bursts employs the same basic
theory as that for predicting deposit dose from surface bursts, but
defines the parameters differently. Times t; and ty are estimated
from the base-surge dimensions and rate of motion.
Such values mst
be obtained from a base-surge model (see Vol. 1 of the Handbook).
Fur-
ther test data are required to determine whether deposit dose is
significant for an underwater burst.
17.6.5 Theoretical Colculotions for Shielded Locations
A computational method has been developed at NRDL”™ to calculate the
effectiveness of a ship's structure in attenuating the gamma radiation
from activity deposited on the weather deck. Results of the calculations,
in terms of the shielding factor, can be obtained for any specific location
within the ship.
The method, essentially a means of calculating the value
of the ratio of the dose rate at a given location within the ship to the
dose rate at a given exterior location, is independent of the quantitative
value of either dose rate.
The NRDL computational method employs an idealized concept of the
interactions of radiations with a ship's structure, and is based on
several simplifying assumptions: (1) deck-deposited activity is a uniform
distribution of isotropically
emitting point sources on horizontal
surfaces only; (2) buildup factors computed for infinite media are
applicable for the finite shielding layers of a ship; (3) material in
separate layers, like decks of a ship, has the same scattering char-
acteristics as a single elab of the total thickness; (4) a deck-platingthickness miltiplying factor of 2 accounts for shielding material other
than deck plating (bulkheads, beams, machinery, etc.); (5) pseudospectra,
consisting of five energies, can be used in calculations to replace
actual fission-product spectra for given times after fission, and can
be weighted for each time to give virtually the same attenuation as the
more complex actual spectrum would give.
A brief discussion of the
method followa; details of the method are given in Refs. 53 and 70.
The theory was developed from the basic expression (Eq. 17-13 in 17.5.5)
for the exposure dose rate dy (r/hr) at a distance x (cm) from a point
isotropic source emitting 1 photon/sec, nis of energy E; in a homogeneous
medium:
d
.
“yx
kuai nyEqBye a
i”
hax
r/hr
(17-13)
The exposure dose rate due to a polyenergetic point source is found by
summing the above equation over all the emitted energies.
;
;
The on fase factor, as defined in Section 17.55,
dose from
both
from unscattered
is the 7atie of the
scattered and unscattered radiations to
radiations only.
4
“
the dose
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