10 METEOROLOGY---FALLOUT AND WEATHERING THE SHORTER-THERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD SOIL ERODIBILITY AS A FUNCTION OF PARTICLE SIZE Diameter (microns) Less than 20 0 - 50 Relative erodibility Non-erodible except at wind speeds greater than 50 mph, 6 inches above ground, Difficult to erode. 50 - 500 500 - 1000 More than 1000 Highly erodible. Difficult to erode. Non-erodible except at wind speeds greater than 50 mph, 6 inches above ground, Figure 6.~—Sotl Erodibility as a Function of Particle Size. studies of local irregularities as they affectthe fallout pattern? Mr. Nacurr. The question of irregularities in the fallout pattern? Mr. Spsncer. Inthe groundcontouras they affect the fallout pattern. . Mr. Nacter. We knowthat these irregu Jarities exist, but to my knowledge, there bas been no good quantitative study of them. The observed Nevada fallout pattern which I showed is probably oversimplified, since most of the monitoring runs are made in fairly broad, flat valleys, Some features of rougher terrain must act like snow fences and cause an irregular piling up of the radioactive particles. Dr. Werner, Are there any other questions from the audience? Dr. Stannarp (University of Rochester). Could you give us some very average figures for the fraction of activity on particle sizes too small to settle out? Mr. Nacrer. Thefraction not settling out is quite dependent on the type of burst. We can get an idea of this fraction by considering the measurements of what fraction settles out. For Nevada tower bursts perhaps 5 to 20 percent of the total radioactivity falls out within the first 200 miles or so. For a surface explosion, where a great many morelarge particles are formed, a much higher percentage mayfall down, perhaps as much as 80 percent, within this distance. For an air burst, this percentage falling down is almost negligible. In each case some of the reniaining activity reaches ground which had not heen affected. T don’t think this would be a very dangerous effect. As far as structures are coneerned, the airborne particles would gradually infiltrate into homes and so forth, much as dust does. 1 don’t know quantitatively how important an effect this would be. I would not imagine that it would be too great an effect. Dr. Werner. As I recall at Redwing, there were some effects of this sort noticed on the ships that were ont in the fallout pattern, Passing through a rainstorm did have quite a significant effect in reducing the level of radiation. However, here there was a convenient waste disposal tank available which would not be available in the case of land installations, For land installation, decreased by weathering processes. if you would comment. on that. think that T wonder Mr. Naaurr. I can cite an example of this. We drove in very close to the remains of one tower—that from the explosion on May 5, 1955—-just. a few davs after the test, The levels of radioactivity were rather low on the asphalt pad almost underneath the toweritself. There had been rather strong winds. J would suspect that from smooth surfaces like city streets and buildings the wind erosion of these particles will be rather large. On rough terrain and in vegetation it would be rather small, It must be a tremendously variable thing. It would also depend upon how damp Dr. Werner. Are there any further quesI would like to ask a question, if I could. What would you expect the effect would be of weathering on redistribution on a rather large-scale fallout field and also on structures? Mr, Naauer. On a large seale field the general effect would be to diminish the fallout in the places where it was most dangerous. Weathering would not have a concentrating effect normally. It would tend to distribute it and bring small amounts to other places would perhaps the intensity would not be appreciably in a few days, but muchis on particles with no significant settling velocity. tions? I 448029 O—58——~-2 the ground was. 11 On very dry ground,particles maybe picked up moreeasily. Dr. Werner. Perhaps we have time for one other question. Dr. Newcomae (USNRDL). Do you have any information on the possible screening effect of vegetation in determining the amount of fallout on the ground? 1 have in mind availability io the animals on the floor of a forest, for instance, as contrasted with a desert area or grassland area. Mr. Nauter. T believe that Mr. Larson of UCLA has had some data on that, that the leaves of plants do tend (o selectively collect small particles, due to the rough structure, the tinyhairs on theleaves, and eo forth. So there is actually a collecting mechanism which is probablyiniportant in some types of vegetation in intercepting and holding these particles, making them more available to the animals. This is an important effect. In Nevada we don’t have the best place for measuring the effect on foliage, but we feel this is an important effect. Dr. Werner. Thank you. T believe there is one thing that impresses those of us who have been concerned with the matter of predicting fallout field that Mr. Nagler has been discussing and that is the variability. Even under best conditions as you can see where the impute data is determined, one canstill expect rather large variations.