Safety_Series_025_1968 - gnssn - International Atomic Energy ...

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This publication is no longer validPlease see http://www.ns-iaea.org/standards/On the other hand ca esiu m -1 3 7 , produ ced a lso by the n u cleartests, is m o re u n iform ly distribu ted in the body, the la rg e st part(about 60%) being deposited in m u scles and the rest in v is c e r a l o r gans, brain, blood, bones and teeth in that o rd e r.Iodine-131 is se lectiv ely deposited in the thyroid. A fter inhalationabout 15% is retained and 20% after ingestion. R adioactive iodineis a ls o d ep osited to a slig h t d e g r e e in m u s c le s , b on es and sk in .Iodin e-131 m ay be a hazard if its con cen tration in the th yroidgland is in cre a sed . A fter the W in d scale r e a c to r incident in 1957la rg e quantities of 131I escaped to the environm ent. The ch ild 'sthyroid is very sen sitive to irradiation . A dose of 200 rads of X -ra y sto the neck may induce carcin om a in the thyroid in about 3% of ca ses.As the milk after W indscale incident contained m ore than 1 /jCi 1311/litre(this correspon ds to an integrated dose of about 130 rads to the ch ild 'sthyroid) much o f the m ilk produced in the area had to be d isca rd edfo r six w eeks.R adioactive ca rbon , im portant beca u se o f its long h a lf-life(5700 y ea rs), is produced by the in teraction of neutrons with atm osp h eric nitrogen. It is in corp ora ted in many im portant vital c o m pounds where it can act as a sou rce of soft beta radiation or ch em ically through its transm utation to nitrogen.Polonium and plutonium belong to the m ost toxic alpha sou rces.The toxicity of 210Po is about 120 tim es g rea ter than that o f 226Ra.P olon iu m is distribu ted quite u n iform ly in the body, plutonium iscon cen trated predom in an tly in bon es, liv e r and o v a ry .1.4.4. Elimination o f radionuclidesR adioactive nuclides introduced into the body may be elim inatedby kidneys, lungs through the gut, by sweat glands etc. Substancesd issolv ed in body fluids a re ex creted predom inantly in u rin e. Thebow els elim inate isotop es which have been partly absorbed and alsothose which have passed without absorption through the digestive tractowing to th eir low solu bility. R adioelem en ts a re m ost rea d ily r e m oved fro m n erve and m u scle tissu e, m uch m ore slow ly fro m thekidneys and the c e lls o f the reticu lo-en d oth elia l system , and m ostslow ly o f all fro m the b on es. In the lym ph nodes ra d ioa ctiv e e le m ents a re retained fo r a com p aratively long p eriod . G aseous is o top es such as radon a re elim in ated m ost ra p id ly by the lu n gs.The elim ination rate is governed by the age of the organism , byits p h y siolog ical condition, nature of the radionuclide, by the quantityof isotopes in the whole body and other fa ctors. The elim ination4 8
This publication is no longer validPlease see http://www.ns-iaea.org/standards/of caesiu m -137 by m ice is dem onstrated in F ig. 17. It must be r e m em bered that the exponential relation is not com m on. In m ost casesthe elim ination curve has a com plex ch aracter and only in restrictedtim e -in te rv a ls has an exponential ch a ra cte r. The b io lo g ic a l h a lflifeT b m ay be d ir e c tly d eterm in ed fro m the graph in F ig .,17 (T =7 d a y s). It can be e x p re sse d as fo llo w s:= ln_2 = 0. 693b " X b “ Xbw here Ab is constant of b io lo g ica l elim in ation . If Ar is the d isin teg ration constant o f the ra d ioisoto p e, then the e ffe ctiv e constant Xeff isX e ff = X r + X band T eff , the e ffe ctiv e h a lf-life , isIn 2 _ 0.693 _ Th Treff‘ X+Xb Xt + X„ T b + T r 'w h ere Tr is the ra d io lo g ica l h a lf-life .4 9
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