Safety_Series_015_1965 - gnssn - International Atomic Energy ...

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This publication is not longer validPlease see http://www-ns.iaea.org/standards/perim ents com paring th eir rates of m ovem ent relative to that oftritium m ay a ssist in ch oosin g the m ost suitable tra ce r com patiblewith a particular soil and ground w ater. To avoid disrupting theground w ater stream lin es b y in jection , fro z e n s o u r c e s have beenin trod u ced [19]'that thaw slow ly to r e le a s e a plum e o f ra d ioa ctiv etra ce r. C hem isorption techniques have been investigated to su p ersedewater sampling, which itself inevitably causes som e interferenceto the natural flow . W here w ater and so il sam pling have been n e-.cessary, tools have been developed to sam ple these sim ultaneouslyfrom several p re-selected horizons in sm all quantities to cause m inimalin terference with ground water flow. M ore details of these p ro cedures are given in Appendix IV.M ost of these refinem ents have been adopted for use in relativelyshallow unconsolidated saturated form ation s. In deeper form ationsthe tasks are m ore difficult. A ll equipment has to be heavier, m orecom plicated and expensive and at presen t the standard o f m ea su rementsis le ss p re cise . Pumping tests have proved useful when con fined within the boundary of a particular stratum. By packing a casedw ell above and below the stratum , p erfora tin g the ca sin g betw een,and then pumping from this section, som e hydraulic ch a ra cteristicsmay be m easured. Flow m easurem ents within a single well are beinginvestigated electronically with the object of m easuring the horizontalflow through the w ell and the v ertical flow within it.III.CHEMICAL REACTIONS OF WASTES IN THE GROUNDAND THEIR PHYSICAL BEHAVIOUR(1) Chemical reactions with mineralsWhen solu tion s o f ra d ioa ctiv e w astes are r e le a s e d into theground the d isso lv e d m a teria l w ill often re a ct ch e m ica lly with thecla y -m in e ra l o r organ ic constituents of the s u b -s o ils . The resu lto f such rea ction is to reta rd the m ovem en t o f the ra d ioa ctiv e m a te ria l rela tiv e to that o f the solven t. Since the tim e req u ired fo rm ovem ent of the m aterial to a point where it is accessible to the publicdeterm in es the fra ctio n that w ill d ecay en rou te, the ch em ica lrea ction s are im portant. In m ost c a s e s , the solu tion s r e le a s e d to12
This publication is not longer validPlease see http://www-ns.iaea.org/standards/the ground are com plex and contain many chem ical constituents, bothstable and radioactive. The m edia through which the solutions floware also com plex m ixtures of various m inerals and organic m aterials.Under these circu m stances it is im p ractical to consider the chem icalbehaviour o f the solution in term s o f individual ch em ica l rea ction sand a m o re e m p irica l view is n e c e s s a r y [20], It is often p o ssib leto lim it the con sideration to a few im portant ra d ioisotop es, e. g.stron tium -90, c a e s iu m -137, ru th en iu m -106 o r cob a lt-6 0 .Studies [10, 21, 22] have shown that som e m in e ra ls d isp la y am arked sorption p re fe re n ce o r "s e le c tiv ity " fo r certa in ion s. Theeffect appears to be related to the ion ic dim ensions and som e stru ctural dim ensions in the cry sta l lattice of the m in eral. Pronouncedse le ctiv e sorption o f caesiu m has been dem on strated fo r m ost m in eral constituents of the soil and the effect of this selectivity may beo f m o re sig n ifica n ce than the m easu red total exchange ca p a city o fthe m in era l. F o r exam p le, illitic cla y s have a h igh er affinity butlow er capacity than m on tm orillon itic cla ys. Thus if the con cen trationof caesiu m ion is low the affinity fa cto r o v e rrid e s the capacityfa cto r and the illite w ill so rb the g rea ter amount o f ca esiu m . Butif the con cen tration o f ca esiu m ion is in crea sed , the exchange c a pacity becom es predom inant and the m ontm orillonite w ill sorb m orecaesium than the illite.N early all sorption reaction s with natural m in erals are lim itedto cation ic s p e cie s , so that th ose elem ents of ra d ioactiv e w aste incationic form are su bject to retarded m ovem ent through the soil. Incon trast, elem en ts in anionic fo r m exp e rie n ce no such d elay andm ove re la tiv e ly unhindered through the s o il at a sp eed rela ted to,but rath er le s s than, that o f the natural ground w ater. T h ere are,as m ight be expected , excep tion s to th ese g e n era liza tion s. Whencertain, cations are brought into contact with chelating agents, theyare com p lexed and h enceforth m ove rapid ly, e x p erien cin g no e x change reaction s with the soil. An exam ple of this [23] is the com plex form ed by E. D. T. A. with co b a lt-60. Again, anionic behaviouris frequ en tly changed if org a n ic m a te ria ls are p resen t in the soil;such m a teria ls m ay have som e anion exchange capacity and a c cordingly retard m igration. Fortunately m ost of the hazardous radionuclides p resen t in w aste are n orm a lly in cation ic fo rm but amongthe notable excep tion s is ru th e n iu m -106, w hich m ay be p resen t incationic, anionic or neutral form accord in g to its pre-treatm en t andits h istory after d isp osa l. With such u npredictable beh aviou r it isdifficult to know whether this radionuclide is likely to m igrate or not.13
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