Response to Dcpa Questions on Fallout (Dcpa Research Report No. 20)

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RESPONSE TO DCPA QUESTIONS ON FALLOUT DCPA Research Report No. 20 Nov. 1973 Prepared by Subcommittee on Fallout Advisory Committee on Civil Defense National Academy of Sciences with Notes and Comments by J. C. Greene, DCPA This report has been. reviewed by the Defense Civil Preparedness Agency and approved for publication. Approval does not signify that the contents necessarilyy reflect the views and policies of the Defense Civil Preparedness Agency. This document has been approved for public release. Its distribution is unlimited.

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The May 1973 report of the Advisory Committee on Civil Defense of the National Academy of Sciences (ACCD/NAS), which discusses varims aspects of the fallout hazard, is considered to be of sufficient importance to warrant further distribution. For this reason, and with the ACCD/NAS concurrence, this report is herein reissued in the format of a DCPA Research Report. There is a further advantage in that an opportunity is provided for the addition of notes and comments to help define the implications of the report. These notes and comments by Jack C. Greene of DCPA, listed by chapter, appear as Part II of this Research Report. Part I is the ACCD Subcommittee document in its original form.

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SOME ASPECTS OF FALLOUT OF CONCERN TO CIVIL DEFENSE Subcommittee on Fallout Advisory Committee on Civil Defense NATIONAL ACADEMY OF SCIENCES

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SOME ASPECTS OF FALLOUT OF CONCERN TO CIVIL DEFENSE A report of the ad hoc Subcommittee on Fallout Advisory Cmumittee on Civil Defense The work of the Advisory Ccnmnittee on Civil Defense is approved by the Governing Board of the National Research Council, acting in behalf of the National Academy of Sciences. Such approval reflects the Board’s judgment that the work of the Committee is of national importance and appropriate with respect to both the purposes and resources of the National Research Council. The members of the Subcommittee selected to prepare this report were chosen for recognized scholarly competence and with due consideration for the balance of disciplines appropriate to the project. Responsibility for the detailed aspects of this report rests with the Advisory Committee on Civil Defense. Each report issuing from a committee of the Natioml Research Council is reviewed by an independent group of qualified individuals according to procedures established and monitored by the Report Review Committee of the National Academy of Sciences. Distribution of the report is approved, by the President of the Academy, upon satisfactory completion of the review process.

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TABLE OF CONTENTS THE PORTION OF ACTIVITY DEPOSITED IN LOCAL FALLOUT .......... .0 ..,00. 2 EFFECTS OF SMALL CHANGES IN BURST HEIGHT ON THE EFFECTS OF SOIL TYPE AND BUILDING MATERIAL ON THE PORTION OF RADIWCTIVITY DEPOSITED IN LOCAL FALLOUT ..........0......12 DETECTION OF FALLOUT BY THE PHYSICAL SENSES ...........l ..0....0.. l o. 19 FEASIBILITY OF DEVELOPING FALLOUT PREDICTION TECHNIQUES INTERACTION OF SIMULTANEOUS SURFACE BURSTS .........................027

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THE PORTION OF ACTIVITY DEPOSITED IN LOCAL FALLOUT A typical deterministic fallout-prediction system is based upon a forecast of climatological winds and a postulated (or calculated) initial distribution of radioactivity on particles oflvarious sizes locatedat various positions within a radioactive cloud. The amount and kinds of radioactivity postulated depend on the yield of the nuclear explosion, its fission-fusion ratio, the type of fissionable material used, and the kinds of induced activities produced. The height and other dimensions of the cloud depend on the yield and on ambient atmospheric conditions , particularly on the variation of temperature and relative humidity above the ground. During their fall the radioactive particles move laterally under the influence of the wind field. If the above factors are properly accounted for , one can predict levels of deposition of radioactivity on the ground, from which radiation exposure rates can be derived. DCPA and hence this paper is concerned primarily with surface and near-surface bursts. Possibly important perturbations, which will be taken up later, are small changes in the height of burst (Chapter 2) , the chemical and physical properties of the soil or other substrate over which the explosion takes place (Chapter 3), and the influence of adjacent, nearly simultaneous bursts (Chapter 8). A central problem in fallout prediction is that of relating radiation exposure rates at various locations to the yield of the detonation that produced the fallout. Sophisticated models can, at least in principle, rigorously compute this relation nuclide by nuclide for each point on the ground, subject to the accuracy of the fission-product data base, the assumed relation between radioactivity and particle size, and available wind and weather information. however, predict only the gross deposition of mixed-fission products. Any model implies , and one of the models used by DCPA explicitly uses , an empirical factor called the K-factor* to relate deposition to radiation intensity. In the literature, this term has referred to at least two different but related things: (1) the ratio of exposure rate measured at a particular place in the fallout field to the density of deposition of radioactivity there; and (2) an integrated, weighted average of this ratio over the “local” fallout field. The confusion caused by the various uses of the concept has been well reviewed by Ra p2 and Cane. R/hr per kt/mi at H + 1 hour. Since this is a rather unwieldy unit, we shall not repeat it hereafter. An idealized limit of the K-factor corresponds to unfractionated fission products uniformly spread over a smooth ideal plane, and measured with an ideal detector 3 feet above the plane. This limit, here called Ko, varies depending on what particular fission process is being considered. According to Tompkins, 4~= 3067 for U-235 fissioned by *AIsO called the Normalization )j’actor,the Magic Number, and the Exposure Rate Conversion Factor. 2

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Some Aspects of Fallout of Concern to Civil Defense In October 1971, the Research Directorate of the Office of Civil Defense, now the Defense Civil Preparedness Agency (DCPA), posed questions on eight aspects of fallout of interest to civil defense. They were sent to the Fallout Subconnnittee of the Advisory Committee on Civil Defense in the form given in Appendix A. These topics encompass problems of the basic constraints used in fallout prediction, various perturbations on the standard surface-burst problem, the direct detection of heavy fallout without instruments, and the feasibility of extrapolating predictions on an operational basis. The Subcommittee itself and working groups of the Subcommittee have attempted to answer questions on these topics, several of which were rephrased in interaction and by agreement with DCPA representatives. The results of these deliberations are the subject matter of this report. May 1973 Members of Subcommittee

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neutrons with a fission spectrum, and for Pu-239, similarly fissioned, K. = 2692. From these and other values determined in the same manner, we conclude that 2900 is a good estimate of K. for most applications. The detonation products are not, of course, deposited uniformly. The ratio of exposure rate to deposition density has been observed to vary from point to point within the fallout field,5 tending to increase with increasing distance downwind from ground zero. This observation is consistent with the concensus that radiochemical fractionation causes this ratio to decrease with increasing particle 6 This problem has been customarily circumvented by using whatsize . amounts to an average of this ratio over the region of “local” fallout, where “local” was defined at the convenience of the author. This local averaged K-factor we call K1. Since local fallout (however defined) represents deposition of only a fraction of the total radioactivity produced by the detonation that produced the fallout, the ratio K1/Ko has been referred to as the fraction of the activity deposited in the local fallout, or simply “fraction down.” However, DCPA wants Kl, as well as the ratio. Two additional factors degrade the apparent value of the K-factor. Shielding by small-scale irregularities of terrain leads to a reduction in K1 of about 25% and measuring instruments used in the past have had built-in self-shielding factors that led to another reduction of about 25%. So-called measured values of the K-factor in the literature are nearly always this doubly degraded K-factor, here called K2. The numerical value of K1 or K2 depends on the definition of local fallout. Three definitions have been used: (1) all deposition out to the distance traveled by particles of a given size, say 45P, which fall from the top of the nuclear cloud, (2) fallout deposited up to a given time, say H + 24 hours , and (3) the region within a given fallout contour, say 0.5 R/hr at H + 1 hour. None of these leads to a K-factor completely independent of yield and meteorology, although the first comes closest. We focus here on the third which appears to be the most where A is the significant in fallout prediction systems used by DCPA. determinations of the K-factor make use of the intensity area (mi2) within the contour of intensity I (R/hr extrapolated from measurements back to H + 1 hour), Wf is the yield due to fission (kt), and Al is the area within the largest and least intense contour used.

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Problems in using this procedure have been particularly difficult very close to ground zero and very far away from it. At close-in locations, physical factors have often prevented the installation of recording instruments, and high radiation levels have denied entry for standard methods of measurement until decay and weathering have greatly decreased the levels of radiation. Airplaneand helicopter measurements over such areas have not been reliable. Often, however, because the area within the innermost measured contour is small, the resultant K-factor ha not been sensitive to the estimates needed in lieu of measurements within that contour. At great distances, the reliability of measurements is reduced because intensities are small, approaching background levels. Unfortunately their contribution to the integral can be large because of the large areas involved. Properly, Al (or rather the value of I(A1))sfiould be determined by the definition of local fallout. The tendency of many investigators to carry out the integration to the limit of reliability of the data results in an implicit definition of local fallout that varies from shot to shot and makes intercomparison of results difficult. A number of empirical values of K2 are listed in Table 1. (We cannot guarantee that these data do meet the criterion of consistent integration limits.) All the fallout patterns from which these data were obtained are uncertain to some degree. The Subcommittee believes that the best near-surface-burst data on this list are those from Johnie Boy, Buffalo 2, Zuni, Tewa, and Jangle Surface. Taking a mean of those average values, we get This mean represents average field-roughness conditions, and instruments as used in the past. In DCPA use, a K-factor is required which does not include corrections for surface roughness or instrument response, which The Subcommittee recommends that DCPA use this value of K1.

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TABLE 1. SOME CALCULATED K-VALUES FROM WEAPON TESTS (Based on field measurements that include terrain roughness effects, and which were not corrected for instrument response.) Item Yield (}.) Heffter Miller DASA Tompkins Average Heffter -- urivate communication .- except Johnie Boy to 1.0 R/hr. * The scaled height of burst (A) is determined by dividing the actual height of burst in feet by the cube root of the total yield in kilotons. ~ Value > (0.75)2 x 2900: not included in average. * Reduced from Miller’s original 990 by excluding data at distances beyond the 0.5 R/hr

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1. Glasstone, S. (cd.) The Effects of Nuclear Weapons, U.S. Atomic Energy Commission (1962), Chapter IX. 2. Rapp, R. R. An Error in the Prediction of Fallout Radiation, 3. Cane, J. W. Fallout Phenomenology : Nuclear Weapons Effects Research Project at a Crossroads, Special Report 64, Santa 4. private Communications from R. C. Tompkins, 12 October 1972 and Program, AEC Symposium Series 5 (1965). 6. Tompkins, R. C. Sensitivity Analysis of the DELFIC Particle Local Fallout from Nuclear Test Detonations (U). Vol. V. Transport and Distribution of Local (Early) Fallout from Nuclear Weapons Radioactivities in Fallout from Nuclear Explosions, SRI-MU-6358, 9. Miller, C. F. The Analysis and Correlation of Fallout Pattern Data. Part One: Summary of Methods and Derived Values of Scale Parameters,

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EFFECTS OF SMALL CHANGES IN BURST HEIGHT ON THE AMOUNT OF LOCAL FALLOUT Data from weapons tests show that radiation-intensity levels of local fallout decrease as the height of burst increases. An estimate of the magnitude of this decrease is of interest to DCPA since detonations in a nuclear attack may occur on contact with urban structures or even in trees. Such heights of burst will, however, be small when scaled from megaton-yield weapons. Taking a one-megaton burst as an example, the scaled height of burst ranges from X = 12 (ft/ktl/3)~ for a tall tree or a ten-story building to A = 50 for a 40-story building. It is well known that the amount of local fallout is very low, almost insignificant, when the height of burst is greater than about A 1801, (except when the burst takes place In such detonations, the soil or dust swept up by rising fireball either does not reach the fireball or enters it only after most of the radionuclides have condensed. In this case the small amount of local fallout depends primarily on the mass of the warhead assembly. The bulk of the radionuclides is carried by very small particles which do not fall to the ground fast enough to contribute to local fallout. For detonations at small heights of burst, the coupling of energy to and the interaction of the fireball with the ground apparently decreases rapidly as the height increases, as manifested in the rapid decrease in crater volume. The particle-size distribution is also shifted to smaller sizes. In the absence of a theoretical foundation for describing these effects quantitatively, one must take recourse in observation. Table 1 integrals by a number of investigators, with data from bursts beyond the altitude range of immediate concern included for perspective. The averages have been plotted in Figures 1 and 2, along with the spread in various investigators ‘ interpretations. The mode of support of each burst is indicated as a basis for interpretation. Most of these bursts were over dry desert soil at the Nevada Test Site (NTS) and in the low kiloton range. Thus these data are intercomparable. Also shown are K-factors from a number of megaton bursts; although these fallou”~ patterns are much less well known due to the difficulties of obtaining and interpreting data over water, it is comforting to see that their resultant K-factors are consistent with those from NTS In Figure 2 , which extends the field of view to greater heights of burst, the horizontal part of line B represents the mean K-factor *See first footnote on Table 1. 7

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(~ = 220) for 30 tower shots with ~ z 100. The horizontal part of line A represents the mean K-factor (K2 = 25) of 40 airbursts. There is a substantial difference between detonations on steel towers and those that are air burst. We consider a burst on …