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Nuclear microprobe applications to radioactive waste management basic research
Nuclear Instruments and Methods in Physics Research B 158 (1999) 511±516
www.elsevier.nl/locate/nimb
Nuclear microprobe applications to radioactive waste management basic research a,*
, V. Badillo a,b, N. Barre a, L. Bois a, C. Cachoir S. Guilbert a, F. Mercier d, C. Tireau e
P. Trocellier
a,c
, J.P. Gallien a,
a
e
CEA-CNRS, Laboratoire Pierre S ue, CE Saclay, 91191 Gif sur Yvette, Cedex, France b CEA, DCC/DESD/SESD, CE Saclay, 91191 Gif sur Yvette Cedex, France c SCK-CEN, Waste and Disposal Unit, Boeretang 200, B2400 Mol, Belgium d CEA-CNRS, UMR 172, CE Saclay, 91191 Gif sur Yvette, Cedex, France Centre National de Recherche sur les Sites et Sols Pollu es, BP 537, 59505 Douai, Cedex, France
Abstract Radioactive waste management is one of the major technical and scienti®c challenge to be solved by industrialized countries near the beginning of the 21st century. Relevant questions arise about the extrapolation of the long termbehavior of materials from waste package, engineered barriers and near ®eld repository. Whatever the strategical option might be, wet atmosphere or water intrusion through the dierent barriers constitute the two main remobilization factors for radionuclides in the geosphere and the biosphere. The study of solid alteration processes and elemental sorption phenomena on mineral surfaces is one of the most ecient basic research approaches to assess the long term performance of waste materials. Ion beam analysis and more recently nuclear microprobe techniques have been applied to investigate exchange mechanisms near representative solid/liquid interfaces such as glass/deionized water, uranium dioxide/granitic or clay water or mineral surface/aqueous solution doped with chemical elements analogue to actinide or ®ssion products. This paper intends to describe the dierent works that have been carried out in Saclay using the nuclear microprobe facility. The coupling of lRBS, lPIXE and lNRA permits to determine the evolution of the surface composition induced by chemical reactions involved. Complementary observation of solid morphology and solution analysis allows to obtain a complete elemental balance on exchange processes. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The extrapolation of the long time behavior (106 years or more) of solid materials from a nuclear waste repository needs a complete de*
Corresponding author.
scription at a microscopic level of both the structure and the composition of each constituant of the whole system. More and more sophisticated analytical methods have been used for many years to characterize waste package, back®lling and near ®eld materials, as published in the Materials Research Society Symposium Proceedings Series [1]. Microanalysis techniques such as
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 9 1 - 2
512
P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
electron microprobe, ion microprobe and nuclear microprobe give access to the volumic distributions of elements within near-surface and bulk regions of solids and allow the identi®cation of aging phenomena by comparing microstructure and composition [2±4]. Nuclear microprobe investigations on nuclear waste materials started at the beginning of the 1980s, using the Bruyeres le Ch^ atel facility [5]. They are carried out now with the Saclay nuclear microprobe facility. A new growth of activity will occur in the near future with the development of the hot microbeam line, able to receive nuclear spent fuel rods or waste glass or ceramics samples [6]. Other groups have also performed nuclear microprobe measurements in the frame of waste management research programs as for example the Harwell group who studied the sorption of actinides (uranium and plutonium) on granite minerals [7], but the use of nuclear microprobe in this ®eld is still very scarce. 2. Corrosion mechanisms of nuclear waste glasses The ®rst generation of nuclear waste glass that has been investigated, corresponded to alkali± borosilicate glasses able to contain 15±20 wt.% of ®ssion product and actinide oxides. The complex alteration layers formed during aqueous corrosion test re¯ect the succession of several important chemical processes such as ion exchange, silica network dissolution, hydrolysis of highly insoluble cations, precipitation of amorphous species from saturated leachate and surface crystal growth. Fig. 1 gives the concentration pro®les of some constituents of the French PWR nuclear glass (R7T7), its composition is provided in Table 1, after leaching in deionized water for 28 days at 100°C in a Soxhlet column. The glass sample was embedded in an epoxy resin and then a polished transverse cross section was prepared. Distributions of Ca, Fe and Nd have been measured by lPIXE [8]. The thickness of the hydrated layer is around 20 lm. The concentration pro®les permit to distinguish two main types of glass constituents: those exhibiting a strong depletion in the corrosion layer as Na, Si or Ca and those able to
Fig. 1. Elemental distributions of Ca, Fe and Nd obtained on a polished cross section of the French simulated nuclear waste glass leached at 100°C during 28 days, measured by lPIXE (Ep 3 MeV, beam spot area 3 ´ 3 lm2 , current 250 pA).
be retained in the hydrated layer likely as insoluble compounds (3d, 4d, 4f and 5f series) with among them transition, rare earth and actinide elements. The second generation of glass matrix to be explored nowadays corresponds to selective materials able to encapsulate speci®c long-life radionuclides and mainly actinides (U, Np, Pu, Am and Cm). Actinides are generally simulated by rare earth cations having similar chemical properties (La, Ce and Nd). Speci®c glasses with highly improved mechanical properties have been prepared as for example oxynitride silicoaluminate glasses [9] (see composition given in Table 2). For leach tests conducted in deionized water, the glass surface exhibits strong local enrichment of Y and La coupled with depletion of Si and Al [10]. Fig. 2 shows typical results obtained after 28 days at 100°C by coupling lPIXE and lRBS. Moreover, the composition of the aqueous leachant controls the surface composition of the leached glass. For example, after leaching in a diluted sodium hydrogenophosphate solution at 96°C, samples exhibit a simultaneous and strong enrichment in both P and rare earths. Thus, it can be assumed that the leached glass surface is uniformly covered with a 50±80 nm rare earth phosphate layer.
P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
513
Table 1 Composition of the French PWR nuclear glass áR7T7ñ Oxide
Content (wt.%)
Oxide
Content (wt.%)
SiO2 B2 O3 Na2 O Al2 O3 Li2 O CaO P2 O5 Fe2 O3 MnO2 Cr2 O3 ZnO CoO NiO ZrO2 MoO3
45.48 14.02 9.86 4.91 1.98 4.04 0.28 2.91 0.72 0.51 2.50 0.12 0.74 2.65 1.70
SrO BaO Cs2 O Ce2 O3 La2 O3 Nd2 O3 Y2 O3 Pr2 O3 Ag2 O CdO SnO2 Sb2 O3 TeO2 ThO2 UO2
0.33 0.60 1.42 0.93 0.90 1.59 0.20 0.44 0.03 0.03 0.02 0.01 0.23 0.33 0.52
Table 2 Composition of the oxynitride glass Compound SiO2 Al2 O3 AlN
Content (mol. %)
Compound
55.3 9.9 16.6
La2 O3 Y2 O3
Content (mol. %) 9.1 9.1
3. Nuclear fuel dissolution The alternative of nuclear fuel reprocessing and geological surface or deep storage is the direct storage of spent fuel. This option has been chosen by several countries like Canada, Sweden, Finland and Spain [11]. France has decided to explore this way in order to compare with nuclear waste vitri®cation. The alteration behavior of uranium dioxide is basicallly dierent under oxidizing or reducing conditions as it was shown recently by Cachoir [12]. Fig. 3a clearly shows the growth of a crystalline secondary phase on a UO2 leached surface under oxidizing conditions in a simulated granitic groundwater at 96°C. lRBS measurement using the 3.45 MeV scattering resonance on 16 O permits to identify partially dehydrated schoepite (UO2 ,
Fig. 2. Nuclear microprobe investigation of a leached oxinitride glass sample (1.8 MeV 4 He microbeam, 10 ´ 10 lm2 , 600 pA): (a) Y and La enrichment shown by lRBS; (b) Y enrichment and Al, Si depletion shown by lPIXE.
xH2 O, with 0 6 x 6 2) as the phase controlling uranium solubility (Fig. 3b). Complementary investigations have been carried out to study the average surface stoichiometry of UO2x using the 16 O(d,p) nuclear reaction microspectrometry. Under oxidizing conditions, the O/U ratio increases up to 2.67 (equivalent to U3 O8 ) and the alteration process is oxydation±hydratation followed by dissolution of the oxidized layer prior to precipitation of secondary phases. Under reducing conditions, it decreases to around 2.00 [12, 13]. The driving force of UO2 alteration can thus be identi®ed as a reduction±hydration process; uraninite itself is probably the phase that controls the uranium release but no experimental evidence has been yet obtained [13]. A new study is actually in progress in Saclay concerning UO2 alteration behavior in clay water, in collaboration with the French National Agency
514
P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
Fig. 3. (a) Formation of uranyl hydrate at the surface of UO2 leached under oxidizing conditions; (b) Microbeam resonant backscattering on the uranyl hydrate crystal and on the underlying uranium dioxide.
for Nuclear Waste Management (ANDRA). Its objective is to assess the role of the composition of the leachate on the dissolution mechanism sequence by combining solution analytical chemistry (ICP-MS, capillary electrophoresis, ion chromatography), near surface analysis (XPS spectrometry), X-ray and nuclear microanalysis [14]. 4. Aqueous ion sorption In addition to chemical durability, it is also important to know the mechanisms by which aqueous radioactive species released by waste materials could be transported through the repository near ®eld and what type of water/rock interactions would be able to trap them eciently [15]. This approach is analogous to those conducted to study the contamination processes of soils by heavy metals, as developed in a recent paper by Tireau [16]. Two near ®eld minerals known for their anity for aqueous cations or anions are hydroxyapatite and calcite. Nuclear microprobe investigations of individual grains of these minerals have been
conducted after contact with cadmium in aqueous solution. lPIXE allows the determination of the elemental concentration of Cd retained by the two minerals and lRBS permits to evidence that cadmium is not only localized at the surface of the grain (see Fig. 4) [17, 18]. The presence of natural organic substances (such as humic and fulvic acids) may aect the behavior of radioelements in the geosphere. These compounds have strong complexation properties towards trace elements and exhibit a high anity for mineral surfaces. Moreover, due to their small size (1 nm±1 lm), they can act as a migration support for radionuclides in the geosphere [19]. lPIXE has been used for the examination of humic acids colloids of dierent granulometries (15±100 nm) collected on a Nuclepore ®lter in combination with the observation of the 12 C (p, p)12 C reaction. The ®rst results show the presence of trace elements important for the problematic of nuclear waste storage on the surface of humic acid (Ti, Cr, Mn, Co, Ni, Zr, Mo, In). Trace elements such as Ti, Fe, Cu and Zn (Fig. 5) seem to be associated with the organic colloidal matter having the smallest granulometry [20, 21].
P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
Fig. 4. Study of Cd incorporation in individual hydroxyapatite (a) or calcite grains; (b) by lRBS [17,18].
515
mental analyses in the ®eld of radioactive waste management basic research, as well to study the corrosion behavior of nuclear materials as to investigate elemental remobilization processes in the near ®eld. The main advantages oered by nuclear microanalysis in comparison with other microanalytical techniques are: · the access to light element distributions simultaneously with heavy elements by combining IBA methods (PIXE, RBS and NRA); · the obtention of quantitative results in a relatively easy way; · the possibility to control the evolution of the composition of the sample under investigation; · the depth selectivity of IBA methods (PIXE excepted); Nevertheless, some limitations clearly appear: · the lack of spatial resolution ( P 1 lm2 ); · the absence of structural and chemical informations; · the lack of sensitivity for one part of the analyzed elements in the case of IBA method coupling (lPIXE and lRBS or lPIXE and lNRA). Acknowledgements The authors are very grateful to Christian Morel, Michel David, Laurent Daudin, Marc Billon, Didier Guillier, Joel Jehanneuf and Francßois Saillant for their respective ecient management of nuclear microbeams and related devices in Bruyeres-le-Ch^atel and Saclay. References
Fig. 5. lPIXE investigation of trace element content of a humic acid colloids of dierent granulometries (50±80 nm) collected on a nuclepore ®lter (Ep 1.725 MeV, beam spot 100 lm2 , current 50 pA, surface coating 10 nm Au).
5. Conclusion Nuclear microprobe analysis has proved to be a very ecient method to carry out reliable experi-
[1] I.G. McKinley, C. McCombie (Eds.), Scienti®c Basis for Nuclear Waste Management, Materials Research Society Symposium Proceedings Series, vol. 506, 1998; W.J. Gray, I.R. Triay (Eds.), vol. 465, 1997; W.M. Murphy, D.A. Knecht, vol. 412, 1996; T. Murakami, R.C. Ewing (Eds.), vol. 353, 1995, Materials Research Society, Pittsburgh. [2] G. Malow, W. Lutze, R.C. Ewing, J. Non-Cryst. Solids 67 (1±3) (1984) 305. [3] J.K. Bates, W.L. Ebert, J.P. Bradley, W.L. Bourcier, in: ANS and ASCE (Eds.), Proceedings of the Second High Level Radioactive Waste Management Conference, Las Vegas, Nevada, 28 April±3 May 1991, p. 720.
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P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
[4] C. Cachoir, J.P. Gallien, P. Trocellier, Ann. Chim. Fr. 21 (1996) 567. [5] P. Trocellier, in: D. Cohen (Ed.), Proceedings of the Second French-Australian Workshop on the Applications of IBA, Sydney, 1±3 February 1995. [6] B. Berthier, Communication to the Accelerator Conference, Denton (Texas), November 1998. [7] J.A. Berry, H.E. Bishop, M.M. Cooper, P.R. Fozard, J.W. McMillan, S.A. Mountfort, Analyst 118 (1993) 1241. [8] P. Trocellier, TECHNE 6 (1997) 85. [9] J. Lang, P. Verdier, R. Pastuszak, R. Marchand, Ann. Chim. Fr. 5 (1980) 663. [10] L. Bois, N. Barre, S. Guillope, M.J. Guittet, M. GautierSoyer, J.P. Duraud, P. Trocellier, P. Verdier, Y. Laurent, J. Nuc. Mater. (1999) in press. [11] E. Lutze, R.C. Ewing (Eds.), Radioactive Waste Forms for the Future, North Holland, Amsterdam, 1988. [12] C. Cachoir, M.J. Guittet, J.P. Gallien, P. Trocellier, Radiochimica Acta 74 (1996) 59. [13] P. Trocellier, C. Cachoir, S. Guilbert, J. Nucl. Mater. 256 (1998) 197.
[14] S. Guilbert, Ph.D. thesis progress reports, July 97, December 97, September 98. [15] N. Toulhoat, V. Potocek, C. Neskovic, M. Fedoro, J. Jeanjean, U. Vincent, Radiochimica Acta 74 (1996) 257. [16] C. Tireau, P. Trocellier, Nucl. Instr. and Meth. B 136±138 (1997) 308. [17] V. Badillo, J. Ly, F. Mercier, N. Toulhoat, P. Trocellier, Communication to the International Congress Interfaces in Environmental Chemistry and Toxicology, Bordeaux, France, 14±18 April 1998. [18] V. Badillo, J. Ly, N. Toulhoat, Communication to Journees de Radiochimie et de Chimie Nucleaire, Saint Germain au Mont Dore, France, 3 October 1996. [19] V. Moulin, Ph.D. thesis, Lyon, France, 1985. [20] N. Labonne-Wall, V. Moulin, J.P. Vilarem, Radiochimica Acta 79 (1997) 37. [21] F. Mercier, Communication to the International Congress Interfaces in Environmental Chemistry and Toxicology, Bordeaux, France, 14±18 April 1998.
www.elsevier.nl/locate/nimb
Nuclear microprobe applications to radioactive waste management basic research a,*
, V. Badillo a,b, N. Barre a, L. Bois a, C. Cachoir S. Guilbert a, F. Mercier d, C. Tireau e
P. Trocellier
a,c
, J.P. Gallien a,
a
e
CEA-CNRS, Laboratoire Pierre S ue, CE Saclay, 91191 Gif sur Yvette, Cedex, France b CEA, DCC/DESD/SESD, CE Saclay, 91191 Gif sur Yvette Cedex, France c SCK-CEN, Waste and Disposal Unit, Boeretang 200, B2400 Mol, Belgium d CEA-CNRS, UMR 172, CE Saclay, 91191 Gif sur Yvette, Cedex, France Centre National de Recherche sur les Sites et Sols Pollu es, BP 537, 59505 Douai, Cedex, France
Abstract Radioactive waste management is one of the major technical and scienti®c challenge to be solved by industrialized countries near the beginning of the 21st century. Relevant questions arise about the extrapolation of the long termbehavior of materials from waste package, engineered barriers and near ®eld repository. Whatever the strategical option might be, wet atmosphere or water intrusion through the dierent barriers constitute the two main remobilization factors for radionuclides in the geosphere and the biosphere. The study of solid alteration processes and elemental sorption phenomena on mineral surfaces is one of the most ecient basic research approaches to assess the long term performance of waste materials. Ion beam analysis and more recently nuclear microprobe techniques have been applied to investigate exchange mechanisms near representative solid/liquid interfaces such as glass/deionized water, uranium dioxide/granitic or clay water or mineral surface/aqueous solution doped with chemical elements analogue to actinide or ®ssion products. This paper intends to describe the dierent works that have been carried out in Saclay using the nuclear microprobe facility. The coupling of lRBS, lPIXE and lNRA permits to determine the evolution of the surface composition induced by chemical reactions involved. Complementary observation of solid morphology and solution analysis allows to obtain a complete elemental balance on exchange processes. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The extrapolation of the long time behavior (106 years or more) of solid materials from a nuclear waste repository needs a complete de*
Corresponding author.
scription at a microscopic level of both the structure and the composition of each constituant of the whole system. More and more sophisticated analytical methods have been used for many years to characterize waste package, back®lling and near ®eld materials, as published in the Materials Research Society Symposium Proceedings Series [1]. Microanalysis techniques such as
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 9 1 - 2
512
P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
electron microprobe, ion microprobe and nuclear microprobe give access to the volumic distributions of elements within near-surface and bulk regions of solids and allow the identi®cation of aging phenomena by comparing microstructure and composition [2±4]. Nuclear microprobe investigations on nuclear waste materials started at the beginning of the 1980s, using the Bruyeres le Ch^ atel facility [5]. They are carried out now with the Saclay nuclear microprobe facility. A new growth of activity will occur in the near future with the development of the hot microbeam line, able to receive nuclear spent fuel rods or waste glass or ceramics samples [6]. Other groups have also performed nuclear microprobe measurements in the frame of waste management research programs as for example the Harwell group who studied the sorption of actinides (uranium and plutonium) on granite minerals [7], but the use of nuclear microprobe in this ®eld is still very scarce. 2. Corrosion mechanisms of nuclear waste glasses The ®rst generation of nuclear waste glass that has been investigated, corresponded to alkali± borosilicate glasses able to contain 15±20 wt.% of ®ssion product and actinide oxides. The complex alteration layers formed during aqueous corrosion test re¯ect the succession of several important chemical processes such as ion exchange, silica network dissolution, hydrolysis of highly insoluble cations, precipitation of amorphous species from saturated leachate and surface crystal growth. Fig. 1 gives the concentration pro®les of some constituents of the French PWR nuclear glass (R7T7), its composition is provided in Table 1, after leaching in deionized water for 28 days at 100°C in a Soxhlet column. The glass sample was embedded in an epoxy resin and then a polished transverse cross section was prepared. Distributions of Ca, Fe and Nd have been measured by lPIXE [8]. The thickness of the hydrated layer is around 20 lm. The concentration pro®les permit to distinguish two main types of glass constituents: those exhibiting a strong depletion in the corrosion layer as Na, Si or Ca and those able to
Fig. 1. Elemental distributions of Ca, Fe and Nd obtained on a polished cross section of the French simulated nuclear waste glass leached at 100°C during 28 days, measured by lPIXE (Ep 3 MeV, beam spot area 3 ´ 3 lm2 , current 250 pA).
be retained in the hydrated layer likely as insoluble compounds (3d, 4d, 4f and 5f series) with among them transition, rare earth and actinide elements. The second generation of glass matrix to be explored nowadays corresponds to selective materials able to encapsulate speci®c long-life radionuclides and mainly actinides (U, Np, Pu, Am and Cm). Actinides are generally simulated by rare earth cations having similar chemical properties (La, Ce and Nd). Speci®c glasses with highly improved mechanical properties have been prepared as for example oxynitride silicoaluminate glasses [9] (see composition given in Table 2). For leach tests conducted in deionized water, the glass surface exhibits strong local enrichment of Y and La coupled with depletion of Si and Al [10]. Fig. 2 shows typical results obtained after 28 days at 100°C by coupling lPIXE and lRBS. Moreover, the composition of the aqueous leachant controls the surface composition of the leached glass. For example, after leaching in a diluted sodium hydrogenophosphate solution at 96°C, samples exhibit a simultaneous and strong enrichment in both P and rare earths. Thus, it can be assumed that the leached glass surface is uniformly covered with a 50±80 nm rare earth phosphate layer.
P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
513
Table 1 Composition of the French PWR nuclear glass áR7T7ñ Oxide
Content (wt.%)
Oxide
Content (wt.%)
SiO2 B2 O3 Na2 O Al2 O3 Li2 O CaO P2 O5 Fe2 O3 MnO2 Cr2 O3 ZnO CoO NiO ZrO2 MoO3
45.48 14.02 9.86 4.91 1.98 4.04 0.28 2.91 0.72 0.51 2.50 0.12 0.74 2.65 1.70
SrO BaO Cs2 O Ce2 O3 La2 O3 Nd2 O3 Y2 O3 Pr2 O3 Ag2 O CdO SnO2 Sb2 O3 TeO2 ThO2 UO2
0.33 0.60 1.42 0.93 0.90 1.59 0.20 0.44 0.03 0.03 0.02 0.01 0.23 0.33 0.52
Table 2 Composition of the oxynitride glass Compound SiO2 Al2 O3 AlN
Content (mol. %)
Compound
55.3 9.9 16.6
La2 O3 Y2 O3
Content (mol. %) 9.1 9.1
3. Nuclear fuel dissolution The alternative of nuclear fuel reprocessing and geological surface or deep storage is the direct storage of spent fuel. This option has been chosen by several countries like Canada, Sweden, Finland and Spain [11]. France has decided to explore this way in order to compare with nuclear waste vitri®cation. The alteration behavior of uranium dioxide is basicallly dierent under oxidizing or reducing conditions as it was shown recently by Cachoir [12]. Fig. 3a clearly shows the growth of a crystalline secondary phase on a UO2 leached surface under oxidizing conditions in a simulated granitic groundwater at 96°C. lRBS measurement using the 3.45 MeV scattering resonance on 16 O permits to identify partially dehydrated schoepite (UO2 ,
Fig. 2. Nuclear microprobe investigation of a leached oxinitride glass sample (1.8 MeV 4 He microbeam, 10 ´ 10 lm2 , 600 pA): (a) Y and La enrichment shown by lRBS; (b) Y enrichment and Al, Si depletion shown by lPIXE.
xH2 O, with 0 6 x 6 2) as the phase controlling uranium solubility (Fig. 3b). Complementary investigations have been carried out to study the average surface stoichiometry of UO2x using the 16 O(d,p) nuclear reaction microspectrometry. Under oxidizing conditions, the O/U ratio increases up to 2.67 (equivalent to U3 O8 ) and the alteration process is oxydation±hydratation followed by dissolution of the oxidized layer prior to precipitation of secondary phases. Under reducing conditions, it decreases to around 2.00 [12, 13]. The driving force of UO2 alteration can thus be identi®ed as a reduction±hydration process; uraninite itself is probably the phase that controls the uranium release but no experimental evidence has been yet obtained [13]. A new study is actually in progress in Saclay concerning UO2 alteration behavior in clay water, in collaboration with the French National Agency
514
P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
Fig. 3. (a) Formation of uranyl hydrate at the surface of UO2 leached under oxidizing conditions; (b) Microbeam resonant backscattering on the uranyl hydrate crystal and on the underlying uranium dioxide.
for Nuclear Waste Management (ANDRA). Its objective is to assess the role of the composition of the leachate on the dissolution mechanism sequence by combining solution analytical chemistry (ICP-MS, capillary electrophoresis, ion chromatography), near surface analysis (XPS spectrometry), X-ray and nuclear microanalysis [14]. 4. Aqueous ion sorption In addition to chemical durability, it is also important to know the mechanisms by which aqueous radioactive species released by waste materials could be transported through the repository near ®eld and what type of water/rock interactions would be able to trap them eciently [15]. This approach is analogous to those conducted to study the contamination processes of soils by heavy metals, as developed in a recent paper by Tireau [16]. Two near ®eld minerals known for their anity for aqueous cations or anions are hydroxyapatite and calcite. Nuclear microprobe investigations of individual grains of these minerals have been
conducted after contact with cadmium in aqueous solution. lPIXE allows the determination of the elemental concentration of Cd retained by the two minerals and lRBS permits to evidence that cadmium is not only localized at the surface of the grain (see Fig. 4) [17, 18]. The presence of natural organic substances (such as humic and fulvic acids) may aect the behavior of radioelements in the geosphere. These compounds have strong complexation properties towards trace elements and exhibit a high anity for mineral surfaces. Moreover, due to their small size (1 nm±1 lm), they can act as a migration support for radionuclides in the geosphere [19]. lPIXE has been used for the examination of humic acids colloids of dierent granulometries (15±100 nm) collected on a Nuclepore ®lter in combination with the observation of the 12 C (p, p)12 C reaction. The ®rst results show the presence of trace elements important for the problematic of nuclear waste storage on the surface of humic acid (Ti, Cr, Mn, Co, Ni, Zr, Mo, In). Trace elements such as Ti, Fe, Cu and Zn (Fig. 5) seem to be associated with the organic colloidal matter having the smallest granulometry [20, 21].
P. Trocellier et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 511±516
Fig. 4. Study of Cd incorporation in individual hydroxyapatite (a) or calcite grains; (b) by lRBS [17,18].
515
mental analyses in the ®eld of radioactive waste management basic research, as well to study the corrosion behavior of nuclear materials as to investigate elemental remobilization processes in the near ®eld. The main advantages oered by nuclear microanalysis in comparison with other microanalytical techniques are: · the access to light element distributions simultaneously with heavy elements by combining IBA methods (PIXE, RBS and NRA); · the obtention of quantitative results in a relatively easy way; · the possibility to control the evolution of the composition of the sample under investigation; · the depth selectivity of IBA methods (PIXE excepted); Nevertheless, some limitations clearly appear: · the lack of spatial resolution ( P 1 lm2 ); · the absence of structural and chemical informations; · the lack of sensitivity for one part of the analyzed elements in the case of IBA method coupling (lPIXE and lRBS or lPIXE and lNRA). Acknowledgements The authors are very grateful to Christian Morel, Michel David, Laurent Daudin, Marc Billon, Didier Guillier, Joel Jehanneuf and Francßois Saillant for their respective ecient management of nuclear microbeams and related devices in Bruyeres-le-Ch^atel and Saclay. References
Fig. 5. lPIXE investigation of trace element content of a humic acid colloids of dierent granulometries (50±80 nm) collected on a nuclepore ®lter (Ep 1.725 MeV, beam spot 100 lm2 , current 50 pA, surface coating 10 nm Au).
5. Conclusion Nuclear microprobe analysis has proved to be a very ecient method to carry out reliable experi-
[1] I.G. McKinley, C. McCombie (Eds.), Scienti®c Basis for Nuclear Waste Management, Materials Research Society Symposium Proceedings Series, vol. 506, 1998; W.J. Gray, I.R. Triay (Eds.), vol. 465, 1997; W.M. Murphy, D.A. Knecht, vol. 412, 1996; T. Murakami, R.C. Ewing (Eds.), vol. 353, 1995, Materials Research Society, Pittsburgh. [2] G. Malow, W. Lutze, R.C. Ewing, J. Non-Cryst. Solids 67 (1±3) (1984) 305. [3] J.K. Bates, W.L. Ebert, J.P. Bradley, W.L. Bourcier, in: ANS and ASCE (Eds.), Proceedings of the Second High Level Radioactive Waste Management Conference, Las Vegas, Nevada, 28 April±3 May 1991, p. 720.
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