Method of fallout prediction for tower bursts at the Nevada Test Site

Page 1

A METHOD OF FALLOUT' PREDICTION FCR TOWER BIRSTS AT T® NEVADA TEST SITE Available from the Office of Technical Services Department of Commerce Special Projects Section Scientific Services Division Prepared under contract to the Santa Fe Operations Office of the Atomic Energy Commission Memorandum of Understanding SF-5h,321

Page 2

Ubder contract to the Santa Fe Operations Office of the Atomic Energy Commission, the Special Projects Section of the Weather Bureau has devised a method of forecasting radioactive fallout for tower bursts at the Nevada Test Site. In the past, fallout forecasts have been made by various approximate empirical methods. It was felt, how ever, that enough data had accumulated during the Snapper-Tumbler and Upshot-Knothole test series to provide a sounder physical basis for a The first phase of our work involved the preparation of maps of the observed deposited radioactivity and fallout, or radex, plots based on the observed winds for the nine 300-foot-tower bursts, identified in Table 1. The yields shown are the fireball yields determined by Edgerton, Genaeshausen, and Greer (l). The heights used for the top and mushroom base of each cloud are those considered most reliable after an evaluation of the various available estimates. There is an average error of perhaps as much as 2,000 feet in the assumed height of the top of the visible cloud; and, furthermore, it is not always clear whether or not the visible cloud coincides with the cloud of radioactivity. The uncertainty in the height of the base is considerably greater, both because there were fewer measurements made of the mushroom base and because in seme clouds, or in some wind fields, the mushroom base is difficult to define. Note: Except for the discussion on the adaptation of the model to fore casting, a large part of the material in this report was presented at the Fallout Symposium conducted by the Armed Forces Special Weapons Project in Washington, D. C., January 10-12, 1955* The basic model given at that meeting, however, differed significantly from the present model, in that generally greater activities were given in the earlier model. The reason for the difference is that the anomolously high fallout of the ninth Upshot-Knothole burst was considered in the preparation of the earlier model, but ignored with the present model.

Page 3

OBSERVATIONS OF DEPOSITED ACTIVITY In the preparation of maps of the observed radioactivity, the ground monitoring data (2, 3, U, £) have been used primarily. The aircraft estimate of radioactivity at the ground have been considered merely as approximate information which can provide a rough independent check on our method of determining fallout concentrations. For most of the data outside the test site itself, the monitors drove along the roads taking readings at intervals of 1-5 miles. Where only a single monitoring run was made along a road, the values reported •were accepted as the final values; but for most roads, there were several runs by different monitors, and the concentrations used on the maps of observed radioactivity were a compromise - often a difficult one to make. The discrepancies between two or more monitoring runs along the same road may be due to several causes: redeposition of debris that originally landed elsewhere, removal of debris by wind, instrumental errors, a different rate of decay than that assumed, errors in positioning, or to various other human errors. Figure 1 shows the results of several monitor runs along one road, following the 9th Upshot-Knothole burst. This illustrates many of the uncertainties which are encountered in using the observational data. Two runs give similar peak values but differ markedly to the left of the peaks, probably because the fallout was not complete when the one run was made. A third peak suggests much lower concentration and is displaced from the other two. The two partial runs obviously do not fit the others very well. One seemed to indicate that the reported positioning was wrong, due to a faulty speedometer on the monitoring automobile. The circles indicate the valves selected as "observed" for the fallout study. This series of monitor runs is somewhat more confusing than the average, but not as confusing as some. It must be remembered that these observations of deposited activity were not always made with the care and accuracy desirable in a scientific experiment. They were made to determine whether there was any radiological hazard to personnel.

Page 4

The selected observations of radioactivity for each tower burst in the Snapper-Tumbler and Upshot-Knothole series were extrapolated to a common time (12 hours after the burst) and are shown (along with other information to be discussed below) in Figures 2 to lU in units of ndlliroentgens per hour. The fallout plots used in the study differ from the type used previously in Nevada in two basic ways. In the first place, rather than a uniform rate of fall, an aerodynamic fall velocity has been used, devised by means of the formulae given in the Rand report on Project Aureole (6). The fall rates used for selected particle sizes, expressed in terms of the time of fall through each 5;000-foot layer, are given in Table 2. The air temperatures and densities assumed for the various elevations are based on a mean atmosmheric sounding for the Upshot-Knothole series; however, the rates of fall applicable to the individual bursts in either the Upshot-Knothole or Snapper-Tumbler series are within about 5 percent of those given in Table 2. The second basic difference from most earlier fallout plots is that in confuting the path of a falling particle, analyses of the space and time variability of the wind were used, not merely the wind reported at the time and place of the burst. In brief, maps were prepared showing the mean winds for each 5,000-foot layer at three-hourly intervals. On the maps each individual wind observation (for a 5,000-foot layer) was plotted at the point over which the balloon actually was located when it passed through the center of that layer, rather than at the station from which the balloon was launched. Also, the drift of the rising atomic cloud was taken into account in detenaining the starting point for particles at the various elevations. Trajectories of particles of six sizes ranging from 60 to 833 microns were drawn, starting from each 5,000-foot layer - 10,000, 15,000, 20,000

Page 5

feet, etc., and from the reported top of the cloud. Using the space and time changes in wind along the descending trajectory, particles of each size were tracked to the position on the ground, or to the 5,000-foot surface (MSL) which with a few exceptions was close to the height of the terrain along the roads where radiological observations were made. By interpolation from the initial fallout plot convenient intervals of height and particle size were obtained. The height lines are at 2500-foot intervals starting with the height of the top of the cloud. The particle size interval was chosen such that the time of fall from 35,000 feet to 5,000 feet is in multiples of an hour. Thus, a spherical particle 291 microns in diameter (with a specific gravity of 2.5) takes 1 hour to fall through this layer, a 162 micron particle takes 2 hours, etc. Errors in fallout plots. Considerable care was exercised in pre paring the fallout plots for the various bursts; nevertheless there are still several sources of error. The trajectories of the falling particle may be in error, due to errors in the wind observations and the analysis. This error is probably less than 10$ of the distance away from Ground Zero for a calculated position of debris initially in the mushroom. However, the relative error between adjacent points on the fallout plot should be much less. Calculated ground positions for debris from the lowest part of the cloud would have a greater percentage A further possible source of error is that the rate of fall may be incorrect because the particlea are not spherical, have a specific gravity different from the 2.5 assumed, or follow a different settling law than the one assumed. However, the meager existing evidence on particle shape - such as the photographs in WT-811 (ii) - indicates that most of the radioactive particles produced by a tower burst are nearly spherical. Moreover, the few measurements of arrival time (derived from data in (3)) agree reasonably well with the fallout calculations. Also, for the

Page 6

second Upshot-Knothole burst, there are particle size observations (U) that seem to verify the aerodynamic law better than Stoke's lav or CONSTRUCTION OF A MODEL The primary step in the development of a fallout forecasting scheme is the determination (for various yields and burst heights) of the distribution of the debris within the initial atomic cloud according to height and particle size. Since this distribution is not measured directly, it must be estimated from the measured radioactivity and whatever particle size or time of fall data are available at the ground and from the fallout plot. Significance of the width of the cloud. One complicating factor in the construction of a model is the width of the cloud. The usual fallout plot represents the position where particles of various sizes from various heights , originating along the central axis of the cloud, reach the ground. The cloud has a significant width, however, which varies from one level to another and from one atomic cloud to another. For simplicity in our investigation, all clouds were assumed to be $ miles in diameter at all levels. As will be pointed out later, the exact diameter chosen is not usually important in fallout computations for the Nevada tests, so that the use of a fixed width, the same for both mushroom top and stem is With a vertical line source (no width to the cloud), each initial level and particle size corresponds uniquely to one point on the ground - as given by the appropriate intersection of height line and particle size line on the fallout plot. With a finite cloud, however, a point on the ground can receive fallout from a range of initial heights and particle sizes. This range can be seen by drawing a circle of the assumed diameter (in this case, 5 miles) around the point in question on the fallout plot. Those initial elevations and particle sizes lying inside the circles contribute to the activity at the point.

Page 7

For simplicity, the activity was assumed to be horizontally uniform throughout each 2500-foot layer (starting at the top of the cloud) for each particle size range and throughout the width of the Determination of activity in a cloud from radioactivity measure ments on the ground. On each fallout plot lines were drawn for the selected ranges of height (designated 1, 2, 3, etc. starting from the starting with the largest oarticles). Circles of 5 miles in diameter were drawn around each of a manber of selected points where observa tions of deposited activity were available. Where the circle lay entirely within the polygon denoting a range of particle size and initial height, the ratio of the area of the circle to the area of the polygon (determined by p la ninetaring) was obtained. This ratio indicates the dilution which occurred within the polygon. Thus, from this ratio and from the observed activity at the point it is possible to determine the concentration of activity in the cloud in that range of particle size and height. For convenience, the concentration thus computed was expressed in terms of the dose rate (in ailliroentgens per hour at 12 hours after burst time) which the particular range of height and particle size would give on the ground if there were no dilution by wind shear, i.e., if the activity from the range fell uniformly within a 5-mile circle. Where the selected observation point was in such a position on the fallout plot that a 5-mile diameter circle around it Intersected two or more polygons, the ratio of the included part of the circle to each polygon was obtained. Then, from a consideration of the various ratios for each of the observation points in an area, concentrations could be assigned the ranges such that the best fit of all observed activities was obtained.

Page 8

Thus, from the fallout plot and observed fallout concentrations for each of several bursts , an estimate of the was obtained for some of the ranges of particle In any one case, however, the observations were concentrations for only a small fraction of the amount of radioactivity size and initial height, such as to yield total desired number Scaling for yield. The synthesis of the various partial models into a composite model required some method of taking into account the From a quantitative study of the nine 300-foot-tower shots of the Snapper-Tumbler and tfoshot-Knothole series and from a qualitative con sideration of surface and air bursts, a scaling scheme was developed which seemed physically feasible and which fitted the observed fallout reasonably well. The fallout is assumed to be proportional to the product of the yield and tie ratio of the area of tie fireball inter secting the ground to the area of the great circle of the fireball. The radius of the fireball is assimed to be that given in the Rand report on Project Aureole (6), namely r 18I|I^^, where r is the radius in feet and Y the yield in kilotons. Thus, the fallout is where h is the tower height. Actually, this technique could not be used where the height lines were nearly coincident (little directional wind shear), since the areas of the polygons are very small in such cases and impossible to measure with sufficient accuracy. These cases (Upshot-Knothole 1, 6, 7, and a part of Snapper-Tumbler 6) were treated in a different manner to give an indication of the distribution of debris in each of them and were used in preparing the composite model, as will be described below.

Page 9

By aeans of a hazard or scaling factor derived from the above scaling scheme, the amounts of radioactivity from the various ranges of particle size and Initial height for each hurst vere converted to the amounts which would apply to a 20-kiloton hurst from a 300->foot The hazard factor is defined as the ratio of the quantity for the actual yield (and tower height) to that for a 20 kiloton yield for a 300-foot tower. A list of these factors for yields up to 55 kilotons and for 300-, 400-, and 500-foot towers, is given in Table 3. From the very incomplete fallout data from the Individual shots, a coatposlte model of the activity according to particle size was determined subjectively for a 20-kiloton hurst from a 300-foot tower, using the above scaling principle to adjust for yield. This model was then used to compute fallout patterns for all nine cases and changed several times until the best fit of the observed fallout data was attained. The resulting basic model is shown in Table 4, in units that give dose rate in milllroentgens per hour …