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 19