A Method for Estimating the Deposition Density of Fallout on the Ground and on Vegetation from a Low-yield, Low-altitude Nuclear Detonation

Abstract

This paper describes a relatively simple model developed from observations of local fallout from US and USSR nuclear tests that allows reasonable estimates to be made of the deposition density (activity per unit area) on both the ground and on vegetation for each radionuclide of interest produced in a nuclear fission detonation as a function of location and time after the explosion. In addition to accounting for decay rate and in-growth of radionuclides, the model accounts for the fractionation (modification of the relative activity of various fission and activation products in fallout relative to that produced in the explosion) that results from differences in the condensation temperatures of the various fission and activation products produced in the explosion. The proposed methodology can be used to estimate the deposition density of all fallout radionuclides produced in a low yield, low altitude fission detonation that contribute significantly to dose. The method requires only data from post-detonation measurements of exposure rate (or beta or a specific nuclide activity) and fallout time-of-arrival. These deposition-density estimates allow retrospective as well as rapid prospective estimates to be made of both external and internal radiation exposure to downwind populations living within a few hundred kilometers of ground zero, as described in the companion papers in this volume.

Fig. 1

Measured vs. calculated N₀ vs. t_r = TOA/t_max for NTS tests [d = 1.8, (1-a) = 0.9].

Fig. 2

Condensation temperatures of various elements relative to melting temperature of soil as a function of time after detonation for a ~10 kt event (Based on Miller 1963). The time scale varies with fission yield, so for a yield of 20 kt, the time at which soil solidifies would be ~6 s and ~ 9 s for an 84 kt event (Appendix A).

Fig. 3

Example of dependence of X˙(t) on R/V, normalized to R/V = 0.5.

Fig. B1

Variation of (1-a) with extent of fireball (FB) interaction with ground. X = FB radius-HOB.

Fig. 4

Ratio of DD_Cs-137/X˙(12) (Bq m⁻²/mR h⁻¹) vs. R/V for a ²³⁹Pu-fueled device (Tesla), a ²³⁵U-fueled device (Smoky), and two tests (Trinity and Diablo) where the fission was due to a combination of ²³⁵U, ²³⁸U, and ²³⁹Pu (Appendix B, Table B1).

Fig. A1

Fission yields (%) for ²³⁵U, ²³⁸U, and ²³⁹Pu (from England and Ryder, 1994).

Fig. A2

Illustration of variation in activity median diameter (AMD) with t_r. Data from Baurmash et al. (1958) for NTS test Apple.

Fig. A3

Fallout particle size distribution (AMD) vs percentage of activity on particles less than 50 μm. Data from Baurmash et al. (1958) for NTS test Apple.

Fig. A4

Calculated relative volatility for an 84 kt fission detonation. Data from Miller 1963.

Fig. A5

Estimated variation of ¹³⁷Cs/⁹⁰Sr activity vs. average R/V (normalized to 1.0 at R/V = 1.0).

Fig. B2

Variation of d with estimated fallout pattern width.

Fig. B3

Best fit to N₀ for NTS test Smoky. Calculated/Measured: mean = 1.02, GM = 1.02, standard deviation = 0.12. d = 1.7, (1-a) = 0.85. Solid line indicates calculated = measured.

Fig. B4

Fit to N₀ for NTS test Boltzmann. Calculated/measured: mean = 1.04, GM = 1.05, standard deviation = 0.15. d = 1.8, 1-a = 0.85. Solid line indicates calculated = measured.

Fig. B5

Fit to N₀ for NTS test Tesla. Calculated/measured: mean = 1.31, GM = 1.44, standard deviation = 0.65. d = 1.4, 1-a = 0.91. Solid line indicates calculated = measured.

Fig. B6

Fit to N₀ data for SNTS test N242. Calculated/measured: mean = 0.94, GM = 0.9, GSD = 1.4. d = 1.6, 1-a = 0.96. Dotted line is best fit to data.

Fig. B7

Fit to N₀ data for SNTS test N148. Fit/measured: mean = 0.98, GM = 0.93, GSD = 1.3. d = 1.6, (1-a) = 0.99. Dotted line is best fit to data.

Fig. B8

Illustration of fitting N₀ measurements using TOA as opposed to distance.

Fig. B9

Example of increase in N₅₀ with distance from trace axis for NTS test Tesla, t_r ~ 0.1. (Data from Baurmash et al. 1958). Negative and positive distances represent different directions from axis.

Fig. B10

Example of change in median particle size with distance from axis reflecting the decrease in particle size leading to an increase in N₅₀, which implies a decrease in R/V. Data for NTS test Apple at t_r ~ 0.2 (Data from Baurmash et al. 1958).

Fig. B11

Comparison of model estimates of N₅₀ and measured N₅₀ vs. X˙/X˙_max for NTS test Tesla for t_r= 0.1.

Fig. B12

Comparison of model estimates of N₅₀ and measured N₅₀ vs X˙/X˙_max for NTS test Tesla for t_r= 0.33.

Fig. C1

X˙(t) vs. t for R/V = 0.5 vs R/V = 3 with and without activation products.

Fig. C2

CT height (above ground level) in km for NTS tests (Data from Hawthorne 1979).

Fig. C3

Illustration of uncertainty in N₀ (circles) and N₅₀ (triangles) at X˙_/X˙_max = 0.5 due to 10% error in CT or t_max.

Fig. C4

DD_Cs-137/X˙(12), Bq m⁻²/mR h⁻¹ measured downwind from NTS and SNTS vs. corresponding estimated t_r at sampling site.

Fig. C6

Dependence of N₀ on (1-a) as d is held fixed (d = 1.6).

Fig. C7

Dependence of N₀ on d as (1-a) is held fixed (1-a = 0.9).

Fig. C5

Estimated relationship between R/V and N₅₀ (data points) and proposed binned estimates (solid grey lines).