SHIELDING, OR SCREENING FROM BLAST

SHIELDING, OR SCREENING FROM BLAST
In any explosion, a certain amount of protection from blast
may be gained by having any large and substantial object
between the protected object and the center of the explosion.
This shielding effect was noticeable in the atomic
explosions, just as in ordinary cases, although the magnitude
of the explosions and the fact that they occurred at a
considerable height in the air caused marked differences
from the shielding which would have characterized ordinary
bomb explosions.
The outstanding example of shielding was that afforded
by the hills in the city of Nagasaki; it was the shielding of
these hills which resulted in the smaller area of devastation
in Nagasaki despite the fact that the bomb used there was
not less powerful. The hills gave effective shielding only at
such distances from the center of explosion that the blast
pressure was becoming critical—that is, was only barely
sufficient to cause collapse -for the structure. Houses built
in ravines in Nagasaki pointing well away from the center
of the explosion survived without damage, but others at
similar distances in ravines pointing toward the center of
explosion were greatly damaged. In the north of Nagasaki
there was a small hamlet about 8,000 feet from the center
of explosion; one could see a distinctive variation in the
intensity of damage across the hamlet, corresponding with
the shadows thrown by a sharp hill.

The best example of shielding by a hill was southeast of
the center of explosion in Nagasaki. The damage at 8,000
feet from X consisted of light plaster damage and destruction
of about half the windows. These buildings were of
European type and were on the reverse side of a steep hill.
At the same distance to the south-southeast the damage
was considerably greater, i.e., all windows and frames,
doors, were damaged and heavy plaster damage and cracks
in the brick work also appeared. The contrast may be illustrated
also by the fact that at the Nagasaki Prefectural office
at 10,800 feet the damage was bad enough for the
building to be evacuated, while at the Nagasaki Normal
School to which the Prefectural office had been moved, at
the same distance, the damage was comparatively light.
Because of the height of the bursts no evidence was expected
of the shielding of one building by another, at least
up to a considerable radius. It was in fact difficult to find
any evidence at any distance of such shielding. There appeared
to have been a little shielding of the building behind
the Administration Building of the Torpedo Works in
Nagasaki, but the benefits were very slight. There was also
some evidence that the group of buildings comprising the
Medical School in Nagasaki did afford each other mutual
protection. On the whole, however, shielding of one building
by another was not noticeable.
There was one other peculiar type of shielding, best exhibited
by the workers’ houses to the north of the torpedo
plant in Nagasaki. These were 6,000 to 7,000 feet north of
X. The damage to these houses was not nearly as bad as
those over a thousand feet farther away from the center of
explosion. It seemed as though the great destruction caused
in the torpedo plant had weakened the blast a little, and the
full power was not restored for another 1,000 feet or more.