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Isotopic disequilibrium effects in leaching of natural uraninite and thorianitc
~eochrmxa pl Cosmrxhtmrca .4c10 Vol. 5 i. pp. 2593-2594 0 Pergamon Journals Ltd. 1987. Pnnted I” U.S.A.
0016.7037/87133.00
+ .oo
LETTER
Isotopic disequilibrium effects in leaching of natural uraninite and thorianite E. R. VANCEand M. GASCOYNE Atomic Energy of Canada Limited, Pinawa, Manitoba, Canada, ROE I LO (Received May 13, 1987; accepted in revisedform
August 12,
1987)
Abstract-Fractional leach rates of “‘Th that are greater than those of 232Thfram natural uraninite and thorianite have been interpreted by Eyal and Reischer in terms of a-decay damage to the crystal lattice. An alternative interpretation proposed here is that the enhanced leaching of 228This due to its presence as interstitial ions.
EYALAND FLEISCHER( 1985a.b) recently observed that the fractional leach rate of 22RTh(fH = 1.9 a), derived from the a-decay of 232Th, was greater than that of 232Th from naturai uraninite and tho~anite, whereas the fractional leach rate of 23@Th(the a-decay product of 234U; fnh= 8 X IO4 a) in the same materials, was approximately equal to that of 232Th.These results were explained in terms of a-decay damage to the crystal lattice, with self-annealing of the damage taking place in a time + 1.9 a but 48 X lo4 a. Thus the enhanced leachability of the **‘Th ions was argued to derive from structural disorder created along the recoil trajectories of the 228Ra ions from which the 22BThions are subsequently formed by B-decay. On this model, structural disorder associated with the recoil trajectories of the 2qh ions would be largely dissipated by elf-ann~n~ before Lvdecay of *‘@Thtook place. An alternative interpretation for the leaching results is proposed here, namely that for uraninite and thorianite, enhanced leaching of 228This due to its presence as interstitial ions rather than its enhanced accessibility via radiation-damaged pathways. This difference, although subtle, has important implications for the stability of used U02 nuclear fuel following disposal at depth in a geological formation. Arguments against the interpretation of EYAL and FLEISCHER( 1985a.b) are first presented. ( 1) Etchable tracks due to e-decay processes are not observed in UOz and U02 is not susceptible to structural amorphization by a-irradiation (WEBER 198 1, 1983). Therefore, U02 is unlikely to sustain radiationdamaged pathways. The smail lattice expansions that are observed on a-particle irradiation are attributed to Frenkel defects. The lattice expansion is observed to saturate at -0.81, at an 4luence which corresponds to - 10-l displacements per atom (dpa); here, dpa refers to instantaneous displacements rather than quasipermanent displacements. The samples of EYAL and FLEISCHER(1985a,b) were deduced to have incurred 150 to 450 dpa from a-decay processes. X-ray diffrac-
tion showed that, though the samples were highly crystalline, there was a slight contraction of the lattice parameter on annealing at 8 lO”C, in agreement with the results of WEBER( 198 1, f 983) on UOz irradiated with cu-particles. Alpha-recoils induce a smaller lattice expansion in U02 than cu-particles even though the numbers of (instantaneous) atomic displacements per event are - 100 for an cu-particle, compared with - 1SO0 for an alpha recoil. Thorianite is isomorphous with UOz and should display the same radiation behaviour. (2) In radiation-sensitive refractory silicates such as zircon, X-ray amo~hism and density decreases of about 10% are observed at only a few dpa (HOLLAND and GOTTFRIED, 1955; KARIORIS et al., 1982), so the lack of such effects in irradiated uraninite presumably shows that almost all of the ions, displaced instantaneously along the trajectory of an a-particle or rx-recoil. subsequently return to their original, or equivalent, lattice sites. If the self-annealing time at ambient temperatures of adecay damage in UO2 is in the range of 1.9 to 8 X 1O4 a, as argued by Eyal and Fleischer for natural samples, then in laboratory a-particle or LYrecoil irradiation experiments, of durations in the order of hours, no such elf-annealing would be expected. Thus, contrary to experimental observations (WEBER, 198 1, 1983; KARIORISet al., 1982), amorphism in U02 would be predicted at a particle fluence corresponding to a few dpa. This kind of argument has been used (KARIORS et al., 1982) to explain the fact that natural monazites show very little evidence of structural damage even when they contain considerable quantities of U and/or Th, while, in the laboratory, monazite can readily be rendered amorphous by heavy ion irradiation. (3) Alpha-particle and a-recoil damage effects that are induced more or less permanently in uraninite and thorianite, would be expected to mainly affect the anion sublattice because the oxygen ions are more abundant and lighter than the cations, and because the Coulomb energy associated with a Frenkel defect on the cation sublattice wilt be considembly greater than that on the
2593
2594
E. R. Vance and M. Gascovne
anion sublattice (JACKSON et ai.. 1986). Thus, again it is difficult to visualize sufficient damage to the cation sublattice occurring to cause enhanced leaching of 228Th from uraninite or thorianite. We now consider the alternative possibility in which enhanced leaching of decay products is due to their presence as interstitial ions rather than their accessibility via radiation-damaged pathways, When the LYrecoil comes to rest at the end of its trajectory (-20 nm long), it is unlikely that there will be any available cation vacancies, so it will be constrained to occupy an interstitial site. In this location it will be much more loosely bound, and therefore more leachable, than if it were in a substitutional site. We recognize that there will be a small probability that it can migrate to fill temporary cation vacancies back along the trajectory. or any cation vacancies arising from previous a-decays in the vicinity. However, enhanced leaching of such interstitials relative to substitutional Th would occur only when - 1atomic layer of cations is leached, since the leachant cannot contact interstitial ions lying below the crystal surface in the absence of radiation-damaged pathways. For their thorianite samples, Eyal and Fleischer observed the maximum leaching enhancement of 22RTh at the shortest leach times, at which the fraction of 232Th leached was 50.1%; for a particle size of -0.4 pm (EYAL and FLEISCHER, 1985b) the thickness of the surface layer leached from the particle is calculated as 61 A. This is in keeping with the above requirement that - 1 atomic layer would need to be leached to observe enhanced leaching of interstitials. The fractional “*Th leach enhancement with respect to that of 232Th (substitutional impurity) in the uraninite sample was observed at higher fractional Th extraction than was the case for the thorianites, but the enhancement itself was less than for the thorianites, and no particle size measurements were made (EYAL and FI.EISCHER, 1985b). Solid-state diffusion processes will allow the interstitial ion to eventually relocate in its original site (or
an equivalent.
vacant site produced tn a separate crdecay event). On this basis. the results of Eyal and Fleischer may be understood qualitatively. of the ttme for this diffusion process, of interstitial 22SThand 23@fh ions. at ambient temperatures is ~1 3 a and 6 X JO4 a. This alternative explanation, of leachmg of interstitial recoil ions, is relevant to the prediction of leaching of cudecay and fission products from used 1 K$ nuclear fuel in an underground disposal vault. Any enhanced leaching of ions by invading groundwater would. on our above arguments. occur by removal of ions displaced from the U02 lattice rather than along radiation damaged u-tracks and should. therefore. be dependent solely on surface area; moreover the enhanced leach rate would vanish after the first few layers of substitutional cations had been dissolved. Acknowledgements-We thank 1. Eyal, R. 1.. i+3SCher, .i. Davies. H. Schwartz, A. G. Latham and M. Puls for helpful discussion and correspondence. Editorial handling: H. P. Schwarcr
REFERENCES EYALY. and FLEI~CHERR. L. (1985a) Timescale of natural annealing in radioactive minerals affects retardation of radiation-damage-induced leaching. Nature 314, 5 18-520 EYAL Y. and FLEBCHERR. L. (1985b) Preferential leaching and the age of radiation damage from alpha decay in minerals. Geochim. et Cosmochim. Acta 49, I 1.5%1164. HOLLANDH. D. and GOTTFRIEDD. (1955)The effect of nuclear radiation on the structure of zircon. Arfn I’rwr g* 29 I-300. JACKSON R. A., MURRAY A. D., HARDINC~J. t-l. and CATLOWC. R. A. (1986) The calculation of defect parameters in U02. Phil. Msg. AS, 27-50. KARIORIS F. G., GOWDA K. A., CARTZ L. and LABBE4. (’ ( 1982) Damage cross-sections of heavy ions in crystal structures. J. Nucl. Mater. 108/109, 748-750. WEBERW. J. (1981) Ingrowth of lattice defects m alpha nradiated U02 single crystals. J NM/. Muter. 98, 206-Z IS WEBERW. J. (1983) Thermal recovery of lattice defects in alpha-irradiated U02 crystals. / Nucl. Mater 114, 2 13
??I.
0016.7037/87133.00
+ .oo
LETTER
Isotopic disequilibrium effects in leaching of natural uraninite and thorianite E. R. VANCEand M. GASCOYNE Atomic Energy of Canada Limited, Pinawa, Manitoba, Canada, ROE I LO (Received May 13, 1987; accepted in revisedform
August 12,
1987)
Abstract-Fractional leach rates of “‘Th that are greater than those of 232Thfram natural uraninite and thorianite have been interpreted by Eyal and Reischer in terms of a-decay damage to the crystal lattice. An alternative interpretation proposed here is that the enhanced leaching of 228This due to its presence as interstitial ions.
EYALAND FLEISCHER( 1985a.b) recently observed that the fractional leach rate of 22RTh(fH = 1.9 a), derived from the a-decay of 232Th, was greater than that of 232Th from naturai uraninite and tho~anite, whereas the fractional leach rate of 23@Th(the a-decay product of 234U; fnh= 8 X IO4 a) in the same materials, was approximately equal to that of 232Th.These results were explained in terms of a-decay damage to the crystal lattice, with self-annealing of the damage taking place in a time + 1.9 a but 48 X lo4 a. Thus the enhanced leachability of the **‘Th ions was argued to derive from structural disorder created along the recoil trajectories of the 228Ra ions from which the 22BThions are subsequently formed by B-decay. On this model, structural disorder associated with the recoil trajectories of the 2qh ions would be largely dissipated by elf-ann~n~ before Lvdecay of *‘@Thtook place. An alternative interpretation for the leaching results is proposed here, namely that for uraninite and thorianite, enhanced leaching of 228This due to its presence as interstitial ions rather than its enhanced accessibility via radiation-damaged pathways. This difference, although subtle, has important implications for the stability of used U02 nuclear fuel following disposal at depth in a geological formation. Arguments against the interpretation of EYAL and FLEISCHER( 1985a.b) are first presented. ( 1) Etchable tracks due to e-decay processes are not observed in UOz and U02 is not susceptible to structural amorphization by a-irradiation (WEBER 198 1, 1983). Therefore, U02 is unlikely to sustain radiationdamaged pathways. The smail lattice expansions that are observed on a-particle irradiation are attributed to Frenkel defects. The lattice expansion is observed to saturate at -0.81, at an 4luence which corresponds to - 10-l displacements per atom (dpa); here, dpa refers to instantaneous displacements rather than quasipermanent displacements. The samples of EYAL and FLEISCHER(1985a,b) were deduced to have incurred 150 to 450 dpa from a-decay processes. X-ray diffrac-
tion showed that, though the samples were highly crystalline, there was a slight contraction of the lattice parameter on annealing at 8 lO”C, in agreement with the results of WEBER( 198 1, f 983) on UOz irradiated with cu-particles. Alpha-recoils induce a smaller lattice expansion in U02 than cu-particles even though the numbers of (instantaneous) atomic displacements per event are - 100 for an cu-particle, compared with - 1SO0 for an alpha recoil. Thorianite is isomorphous with UOz and should display the same radiation behaviour. (2) In radiation-sensitive refractory silicates such as zircon, X-ray amo~hism and density decreases of about 10% are observed at only a few dpa (HOLLAND and GOTTFRIED, 1955; KARIORIS et al., 1982), so the lack of such effects in irradiated uraninite presumably shows that almost all of the ions, displaced instantaneously along the trajectory of an a-particle or rx-recoil. subsequently return to their original, or equivalent, lattice sites. If the self-annealing time at ambient temperatures of adecay damage in UO2 is in the range of 1.9 to 8 X 1O4 a, as argued by Eyal and Fleischer for natural samples, then in laboratory a-particle or LYrecoil irradiation experiments, of durations in the order of hours, no such elf-annealing would be expected. Thus, contrary to experimental observations (WEBER, 198 1, 1983; KARIORISet al., 1982), amorphism in U02 would be predicted at a particle fluence corresponding to a few dpa. This kind of argument has been used (KARIORS et al., 1982) to explain the fact that natural monazites show very little evidence of structural damage even when they contain considerable quantities of U and/or Th, while, in the laboratory, monazite can readily be rendered amorphous by heavy ion irradiation. (3) Alpha-particle and a-recoil damage effects that are induced more or less permanently in uraninite and thorianite, would be expected to mainly affect the anion sublattice because the oxygen ions are more abundant and lighter than the cations, and because the Coulomb energy associated with a Frenkel defect on the cation sublattice wilt be considembly greater than that on the
2593
2594
E. R. Vance and M. Gascovne
anion sublattice (JACKSON et ai.. 1986). Thus, again it is difficult to visualize sufficient damage to the cation sublattice occurring to cause enhanced leaching of 228Th from uraninite or thorianite. We now consider the alternative possibility in which enhanced leaching of decay products is due to their presence as interstitial ions rather than their accessibility via radiation-damaged pathways, When the LYrecoil comes to rest at the end of its trajectory (-20 nm long), it is unlikely that there will be any available cation vacancies, so it will be constrained to occupy an interstitial site. In this location it will be much more loosely bound, and therefore more leachable, than if it were in a substitutional site. We recognize that there will be a small probability that it can migrate to fill temporary cation vacancies back along the trajectory. or any cation vacancies arising from previous a-decays in the vicinity. However, enhanced leaching of such interstitials relative to substitutional Th would occur only when - 1atomic layer of cations is leached, since the leachant cannot contact interstitial ions lying below the crystal surface in the absence of radiation-damaged pathways. For their thorianite samples, Eyal and Fleischer observed the maximum leaching enhancement of 22RTh at the shortest leach times, at which the fraction of 232Th leached was 50.1%; for a particle size of -0.4 pm (EYAL and FLEISCHER, 1985b) the thickness of the surface layer leached from the particle is calculated as 61 A. This is in keeping with the above requirement that - 1 atomic layer would need to be leached to observe enhanced leaching of interstitials. The fractional “*Th leach enhancement with respect to that of 232Th (substitutional impurity) in the uraninite sample was observed at higher fractional Th extraction than was the case for the thorianites, but the enhancement itself was less than for the thorianites, and no particle size measurements were made (EYAL and FI.EISCHER, 1985b). Solid-state diffusion processes will allow the interstitial ion to eventually relocate in its original site (or
an equivalent.
vacant site produced tn a separate crdecay event). On this basis. the results of Eyal and Fleischer may be understood qualitatively. of the ttme for this diffusion process, of interstitial 22SThand 23@fh ions. at ambient temperatures is ~1 3 a and 6 X JO4 a. This alternative explanation, of leachmg of interstitial recoil ions, is relevant to the prediction of leaching of cudecay and fission products from used 1 K$ nuclear fuel in an underground disposal vault. Any enhanced leaching of ions by invading groundwater would. on our above arguments. occur by removal of ions displaced from the U02 lattice rather than along radiation damaged u-tracks and should. therefore. be dependent solely on surface area; moreover the enhanced leach rate would vanish after the first few layers of substitutional cations had been dissolved. Acknowledgements-We thank 1. Eyal, R. 1.. i+3SCher, .i. Davies. H. Schwartz, A. G. Latham and M. Puls for helpful discussion and correspondence. Editorial handling: H. P. Schwarcr
REFERENCES EYALY. and FLEI~CHERR. L. (1985a) Timescale of natural annealing in radioactive minerals affects retardation of radiation-damage-induced leaching. Nature 314, 5 18-520 EYAL Y. and FLEBCHERR. L. (1985b) Preferential leaching and the age of radiation damage from alpha decay in minerals. Geochim. et Cosmochim. Acta 49, I 1.5%1164. HOLLANDH. D. and GOTTFRIEDD. (1955)The effect of nuclear radiation on the structure of zircon. Arfn I’rwr g* 29 I-300. JACKSON R. A., MURRAY A. D., HARDINC~J. t-l. and CATLOWC. R. A. (1986) The calculation of defect parameters in U02. Phil. Msg. AS, 27-50. KARIORIS F. G., GOWDA K. A., CARTZ L. and LABBE4. (’ ( 1982) Damage cross-sections of heavy ions in crystal structures. J. Nucl. Mater. 108/109, 748-750. WEBERW. J. (1981) Ingrowth of lattice defects m alpha nradiated U02 single crystals. J NM/. Muter. 98, 206-Z IS WEBERW. J. (1983) Thermal recovery of lattice defects in alpha-irradiated U02 crystals. / Nucl. Mater 114, 2 13
??I.