THE APPLICATION OF AUTOMATIC WASHDOWNTO PITCHED
ROOFS
By A. J. Bangin
Health and Safety Laboratory, New York Operations Office, United States Atomic Energy Commission
The classical approach to estimating the
dosage which would accrue to the occupants of
a building in a falloutsituation is to compute the
additive contributions to dose from contamina-
tion on the surrounding plane and the building
roof.
These dose contributions are functions
of the geometry and the attenuation properties
of the structure, One can conceive of many
combinations of these variables in which the
relative proportion of the dose originating from
the roof would vary over a wide range. However, in typical one-story residences and industrial buildings of moderate size, the variability of this proportion ranges from 20 to
60 percent. The relative roof contribution is
greater within a basement than atgrade.
Continuous removal of roof contamination
during a fallout event would result in an im-
portant reduction in dose to persons within a
building.
This method of dose reduction by
itself would not necessarily render building
occupants safe from harmful radiation in severe
fallout but when applied in combination with
measures to reduce the radiation emitted from
the plane source, the overall protection could
be made quite effective. When measures ere
taken which reduce the plane source component
the roof component assumes a governing sig-
nificance. Thus applying such a measure in
conjunction with others can result in effective
overall protection.
It has been suggested that continuous roof
decontamination might be accomplished with
an automatic washdown system. Such washdown systems have been used successfully on
warships. Tests of their effectiveness against
simulated fallout and actual field experience
have demonstrated that efficiencies approaching 99 percent are achieved. On the other
hand, consideration of the effectiveness of
washdown on roof surfaces has been primarily
one of conjecture. Not only are there uncertainties regarding removal efficiencies but
legitimate questions may also be raised con-
cerning water supply, contaminant disposal,
effectiveness relative to olher countermeasures,
etc, One can conceive of circumstances in
practice where these questions could be resolved making washdown feasible, provided
that the washdown mechanism per se is effective.
Certainly a fundamental question is whether
or not a water film will transport masses of
particulate matter over a sloped surface. The
magnitude of contaminant deposition in an
event producing lethal dose rates may be milligrams to hundreds of grams per square foot.
Large particles have been found in fallout of
this intensity. Particle diameters covering a
range of 150 to 4(0 # with a meanof 260 u were
found on the Fukuryu Maru. Analyses of
particle distribution in fallout collected on the
outer Marshall Islands during Pacific tests
indicate size medians of about 80 to 100 micra.
It would seem, therefore, that in roof washdown the problem is one of mass transport
rather than actual compound formation or
simple adsorption which have been shown to be
important. contamination mechanisms in small
seale laboratory tests.
At the Health and Safety Laboratory a pilot
experiment was conducted to determine whether
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