( although the enitted radiation is somewhat clifferent). fhis activity decaysrapidly, hovever I during the first two weeks it decreases to one thousand.th ofvhat it was one hour after the €xplosion, rncluded in the ctoud is also much ofthe origi.nal fissil-e material used in the nuclear weapon, notabl-y uranium_235or plutonium-z3g, al:rd activities ind.uced in weapon materials or in surround,ingmatter ' including the air. These nay constitute more long-lasting hazards as thehalf-lives of some of these nuclides range from tens of thous,Jrd.s to hundreds ofrni]lions of years. As the cloud develops, the radioactive atons are incorporatedin larger particles forroed by condensing vapours and mixed-in dust and- dirt. Therange of the radiation is relatively short compared to either the height of thecloud base o" the size of the devastated area. For this reason, the rad.ioactiveparticles in the cloud do not constitute a health hazard. until they are depositedon the ground as radioactive fal-l-out.Al 35 /392 ,UNE-L 1S h Annex rag" )a 158. The rad.ioactive cloud drifts, changes shape and eventually disintegrates under the action of the vinds at those altitudes where it stabilized, At the same time, the particles carrying the activity subside vith speeils which depend. strongly on their size. In the case of an air burst, most particles wilL be very sna1l and it nay take from days to years for them to reach the ground. By that tine they have J.ost nost of their rad.ioactirity and have been scattered over a wide area. Fall-out over intermediate tines may be denoted troposptrerlc, while the very slow deposition of particles injected into the stratosphere is usually referred to as g1oba1 fallout. This fa"Ilout rad.i ation does not cause any acute i1l- effects, but over the decad.es to follow, some tens of cases (for a f0-20 kt. yield) of rtfate effectsi' (additiona] callcers and genetic inJuries) nay occur' In other instances, as in Nagasaki, sone radioactivity may be deposi.ted on the €lToundof the target area by rainfall inrluced. by or coincident with the explosion. An increase in the acute and late effects is then to be expected.. 159. I,ihen the nucl-ear weapon expfodes at o? close to the Sround, vith the firebafl in direct contact raith the surface, thoussrds of tons of soil are iniected into the hot vapours. l,arge (diameters utr) to one nilfimetre or no"e) particles then
**Analysis**
The text discusses the effects of a nuclear explosion on the environment, including the radiation released, the formation of a radioactive cloud, and the subsequent deposition of radioactive particles on the ground. The focus is on the scientific and environmental aspects of the explosion, rather than its economic, social, or cultural impacts.

**Category**
Natural resources and environment, Health