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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/The follow in g con sid eration s are th e re fo re v a lid / taking thevarious hazards into account sep arately. The secon d ary standardsare drawn up for the two main types o f irradiation , external ir r a d i­ation and radioa ctive contam ination. E xp osu res are e x p re ssed inrontgens w h ereas a b sorb ed d oses are e x p re ssed in rads and d oseequivalents are e x p re ssed in re m s.Maximum p erm issib le exposure means a single external exposurewhich, distributed in tim e and in sp ace through the body, d e liv e rsthe maximum p erm issib le dose to the individual. Maximum p e r ­m issible-con centration means a contamination such that the quantitieso f radionuclides present in the air and inhaled, o r present in drinkingw ater, leads to the m axim um p e rm issib le body burdens and thatthese m axim um p e rm issib le body burdens d eliver exactly the b a sicmaximum p erm issible dose. In practice, the maximum p erm issiblecon cen tration s fo r drinking water are not used in the health su p e r­v ision o f w ork ers..3. 3. 2. Secondary standards (derived lim its) foy external irradiationMaximum p erm issib le exposures are easy enough to define forexposure to electrom agn etic radiation (X and gamma rays). A ll thathas been said regarding the b asic standards can be applied d irectlyby replacing the rem unit by the rontgen unit. The resulting slighte r r o r o f 6% is quite accep table.Beta radiation is m ea su red in ion ization ch a m bers which areused for both beta and gamma radiation; and the m easurem ents canbe in terpreted in ra d s. The quality fa cto r (QF) fo r beta p a rticle sis assum ed to be equal to one. That being so the dose in rads c o r ­responds to the dose in rem s and the basic standards can be appliedd irectly to beta radiation by exp ressin g them in rads.T his cannot be done with neutrons; rem s m ust be con v ertedinto term s of maximum p erm issib le flux. In determ ining the m aximump e rm is s ib le neutron flux the follow in g co rre sp o n d e n ce isesta b lish ed .F o r th erm al neutrons with en erg ies of the o r d e r o f 0. 025 eV ,a flux of 670 n cm"2 s e c '1 is perm itted. The exposure resulting fromsuch a neutron flux is exactly equal to 5 rem s a year for w hole-bodyirra d ia tion . F o r partial ex p osu res corresp on d in g lim its m ust beapplied. The values given below co rresp on d to 5 rem s a y e a r fo ran occu p ation ally exposed w ork er.7 4

This publication is no longer validPlease see http://www.ns-iaea.org/standards/F o r neutrons other than therm al neutrons the p e rm issib le fluxwill vary in versely with the energy of the particles; thus for neutronso f 100 000 eV the perm itted flux falls to about one tenth o f 670, i.e.80 n cm '2 sec"1 . F or neutrons of 1 MeV the value is 18 n cm"2 sec"1and fo r neutrons above 3 M eV the value could fa ll fro m 18 to1.0 n cm -2 s e c -1 at 1000 MeV.When m easurem ents are made, it is found that it is not possiblein p ra ctice to establish the neutron spectru m to which a w ork er isexposed. F or this purpose threshold detectors are required, whichinvolves com p lica tion s. Consequently neutrons are usually dividedinto two o r m ore ca teg ories depending on th eir energy range.T herm al neutrons are m easured by m ethods which enable themto be m easured independently.F or fast neutrons, m easu red independently, the value adoptedis that which is m ost favourable from the point of.view of protection:all neutrons are assum ed to have the m axim um e n e rg ie s. The r e ­sulting e r r o r is on the safe sid e.3. 3. 3. 'Secondary standards' for radioactive contaminationThe corresp on d in g values fo r m axim um p e rm issib le con tam i­nations (e.g. m axim um p e rm issib le body burdens, m axim um p e r­m issib le con cen tration s, etc.) are m ore difficult to arriv e at. Theb a sic standards lay down that the average annual dose d eliv ered tothe whole body, the gonads and the red m a rrow should not exceed5 rem s. F or other organs, in a general manner, a value three tim esas high, i.e. 15 rem s a year, is allowed. F or the skin, where somesubstances such as arsenic get fixed, the thyroid, and bone the ICRPsp ecifies a lim it of 30 rem s a year.The following pages contain explanations with regard to determ i­nation of the m axim um p e rm issib le body burden and the m axim ump erm issib le con cen tration s. The only purpose o f including thisinform ation is to elucidate these con cep ts. In p ra ctice th ere is noneed to use the form u las given as the IC RP tables contain c o r r e s ­ponding values fo r all the m axim um p e rm issib le body burdens andcon cen tration s that have been calcu lated .3. 3. 3.1. M axim um p e rm issib le body burdensWhat m ust be determ ined then is the quantity o f ra d ion u clideswhich, permanently retained in the body at a given level, w ill deliver75

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