CHAPTER 3 EFFECTS OF SOIL TYPE AND BUILDING MATERIAL ON THE PORTION OF RADIOACTIVITY DEPOSITED IN LOCAL FALLOUT The effects of soil type and building material on the amount and levels of radioactivity deposited in local fallout are of three types: (1) induced activities can enhance the radiation levels over those of fission products alone, and can change the shapeof the decay curve; (2) the substrate material can influence fractionation effects among the fission products through variation of melting and vaporization temperatures and chemical reactivity with the various radionuclides ; and (3) differences in particle size, density, and other physical properties of the soil or matrix material can influence specific activity/particle-size distributions and hence fall-rate variables. U. S. experience and hence data are limited to three substrates: Nevada dry desert alluvium, wet coral rock and sand, and sea water. In addition there is the experience with tower bursts (mostly steel, but occasionally of aluminum or wood) referred to in Chapter 2. Comparative analyses of the data are difficult because detonations on coral have generally been larger in yield than those over Nevada soil; the larger yield detonations were also in tropical atmospheres having much higher humidity and other differences from temperate-zone atmospheres, Extensive work in the days of atmospheric testing 1-5 indicated that the principal soil constituents that influence the production of induced activities are sodium, aluminum, iron, and manganese contained in the soil (these being the principal elements activated by bomb neutrons), and the water content of the soil (a non-activated competitor for neutrons). The longer-lived of the first two induced activities, Na-24, has a half life of only 15 hours, so it significantly affects decay only up to about 4 days after the detonation. It has, however, a penetrating component of radiation that makes its presence important while it does last. At later times, Fe-59 and Mn-54 may be found in relatively large amounts in fallout from large-yield detonations. In general, soil-activation products are not important contributors to local fallout except for weapons with very low fission-fusion ratios. With respect to the effects of soil type or substrate, no definitive sets of data are available for comparing gross radioactivity/particle- size distributions of the fallout from comparable detonations on dry desert soil and wet coral. In principle, the fallout from detonations over deep water or over wet substrates such as coral should have a smaller K-factor (K,) than fallout from detonations on dry soil, since the radionuclides would be carried initially by smaller particles and by particles with variable fall rates due to evaporation and condensation es RY ys ra pi a ‘ rst F4 wy ry bs Ba 12