A Comparison of the Heft Subsurface and Delfic Particle Size Distributions and Effects in HPAC

The Heft subsurface three component lognormal fallout particle size distribution is compared and contrasted with the single lognormal fallout particle size distribution used by the Defense Land Fallout Interpretive Code (DELFIC). Comparison of the two distributions is accomplished with results from the AFIT smear code and the Hazard Prediction and Assessment Capability (HPAC). The effect of the distributions is explored in HPAC for varying yield weapons, varying surfaces, precipitation conditions, varying wind effects and varying dose rate times. The results from the two distributions are quantitatively compared using the concepts of grounded source normalization constant and the rate at which activity is being deposited on the ground everywhere at time t. The Heft subsurface three component lognormal fallout particle size distribution results in significantly less activity on the ground than does the DELFIC single lognormal particle size distribution. The grounded source normalization constant resulting from the Heft distribution is up to three times smaller than that observed when using the DEFLIC distribution.