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Safety_Series_025_1968 - gnssn - International Atomic Energy ...

Safety_Series_025_1968 - gnssn - International Atomic Energy ...

This publication is no

This publication is no longer validPlease see http://www.ns-iaea.org/standards/As fa r as public hygiene is con cern ed, the contam ination of thepopulation at large o c c u r s m ainly through food , a little throughdrinking w ater, p ra ctica lly not at all by a ir.Owing to the com plexity of the problem no standards have beenlaid down for food. It would in fact be n ecessary to take into accountthe contribution each nuclide m akes to the total radionuclide intake,which w ill vary depending on the diet; and although m en all breathethe sam e way, they have not the sam e dietary habits.3. 3. 3. 2. M axim um p e rm issib le con cen tration sHow is it p o s s ib le to p ass fr o m m axim u m p e r m is s ib le bod yburdens to m axim um p e rm issib le con cen tration s in air and w ater?A num ber o f assum ptions a re m ade about the occu pation al activityo f w o rk e rs. The w ork in g sch edule a ccep ted fo r g en eral p u rp osesis 40 hours a week, divided into 5 days of 8 hours each and 50 weeksa year. E xposure is assum ed to be continuous. F rom these data arelationship can be established between m aximum p erm issib le bodyburdens and m axim um p erm issib le concentrations in air and w ater.The relation sh ip is sim p le enough. The m axim um p e r m is s ib lecon cen tration in air (M P C a) is given by the form u la(3)where k a is a unit con version factor; qf is the quantity of ra d ionuclidein the critic a l organ; T is the effectiv e h a lf-life ; fa is thefra ctio n o f inhaled ra d ion u clide w hich rea ch es the c r it ic a l organ ;and t is the p eriod o f e x p osu re.T h ere is a s im ila r form u la fo r the M PC value in w ater:qf(4)where fw is the fraction of ingested radionuclides which reach es thecritica l organ.Thus the M PC valu es a re lim itin g v alu es fo r the p ollu tion o fa ir o r drinking w ater corresp on d in g to the m axim um p e rm is s ib le7 8

This publication is no longer validPlease see http://www.ns-iaea.org/standards/d oses to the v a riou s organ s. T hey rep resen t average valu es o v e ra y ea r and m ay o cca sio n a lly be ex ceed ed .U nder th ese form u la s an in dividual cannot at any tim e in hislife be su b je ct, as a resu lt o f the in co rp o ra tio n o f ra d io n u clid e s,to an exposure o f the whole body o r a particu lar organ in e x ce ss ofthe b a sic standards laid down. T his does not m ean that the m a x i­mum p e rm issib le con cen tration o r body burden w ill alw ays involvean exposu re equal to the m axim um p e rm is s ib le d ose but on ly thatthe maximum p erm issible dose w ill never be exceeded. The follow ­ing exam ples w ill make the point cle a r.On a graph tim e t is plotted as an a b s c is s a and the d ose rateas the ord in ate. If we take a su bstan ce such as sod iu m -2 4 , eq u i­librium w ill be reached very quickly because the h a lf-life of sodiumis on ly a few h ou rs. Thus, applying the MPC form ulae (3) and (4),we find that an individual who from tim e z e ro inhales a ir con ta m i­nated with sodium in this con cen tra tion w ill r e c e iv e in his bod y asodium concentration which in creases slow ly so as to reach the lim itat a given tim e . A cu rv e o f the type show n in F ig .1 8 is ob ta in ed .DAYSFIG. 18. Internal contam ination due to exposure to sodium -2 4 .T he lim it w ill be rea ch ed , within 1%, at the beginning o f thefifth day. F o r a few d ays, until su ch tim e as the b od y bu rd en o fsodiu m re a ch e s eq u ilib riu m , the exposu re w ill be l e s s .A correspon din g curve w ill be obtained fo r io d in e -131 (F ig . 19);assuming that an individual starts work when 18 years old and is subjectedto continuous exposure to iodine, the lim it w ill be reached onthe fifty-third day.F o r ca lc iu m -45 the lim it w ill be p r a c tic a lly rea ch ed after1000 days, so that even though the m axim um p e r m is s ib le con ce n -79

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