Induced swelling in radiation damaged ZrSiO4

538

Nuclear

Instruments

and Methods

in Physics Research Bl (1984) 538-541 North-Holland, Amsterdam

INDUCED SWELLING IN RADIATION DAMAGED ZrSiO, G.J. EXARHOS Pacific Northwest Laboratory

Richlanci, Washington 99352, USA

A hydrothermal gelation method was used to prepare phase pure polycrystalline ZrSiO, which was sintered to 95% theoretical density. Actinide doped samples containing 10 wt8 23sPu were prepared by an analogous procedure and incurred bulk radiation damage through internal alpha-decay processes. Undoped samples were subjected to external irradiation from 5.5 MeV alpha sources, and from a 6oCo gamma source. Actinide doped ZrSiO, exhibits dose dependent swelling caused by displacement processes leading to ingrowth of amorphous regions. Bulk density and XRD measurements, as a function of dose, showed first order exponential ingrowth behavior similar to that observed in other actinide doped materials. Results are compared with reported data for naturally damaged crystals subjected to significantly lower alpha decay rates. No significant dose rate dependence on damage ingrowth has been observed. Kinetic models for the observed dose dependent swelling are proposed and rate constants for damage ingrowth in synthetic and natural crystals are compared. To study localized damage induced by both external alpha and gamma irradiation, vibrational Raman measurements were obtained for several accumulated doses. Results indicate that the initial stage of damage ingrowth is confined to the silicate sublattice. Vibrational results will be discussed in terms of microstructural changes which result from irradiation.

1. Introduction The interaction of high energy radiation with crystalline oxides results in bulk property changes which can be evaluated to assess the radiation stability or instability of that material. Formation of color centers by electron, gamma, or external alpha irradiation is a manifestation of ionization damage resulting from bound electron excitation within localized molecular orbitals. Atomic displacements caused by internal alpha decay processes or heavy-ion irradiation can result in a disordering of the solid (amorphization). Disorder induced swelling can be easily followed by bulk density and XRD measurements as a function of incurred dose, while molecular spectroscopic techniques (ESR, optical absorption, Raman spectroscopy) may be used to elucidate the nature of radiation induced color centers in nonmetallic solids. The natural mineral zircon, ZrSiO,, has been a classic material on which to base radiation damage investigations. Since the early diffraction studies on naturally damaged zircon by Holland and Gottfried [l], many techniques, including TEM, have been applied to understanding the amorphization process in this mineral [2-41. Amorphization is accompanied by a significant swelling resulting in a density decrease of ca. 16% [l]. Significant questions regarding disordering processes in nonmetallic solids concern effects of radiation types and dose rates on induced swelling and are addressed in this manuscript. Both actinide doped (23sPu) and undoped zircon have been exposed to radiation damage from internal 0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

alpha decay processes, and external gamma and alpha particle bombardment, respectively. Bulk density and XRD changes with cumulative dose were measured for the actinide doped material and are compared with results reported for naturally damaged zircon, which had incurred damage at a significantly lower dose rate. Vibrational Raman spectroscopy is used to probe locally induced damage in polycrystalline zircon that has been exposed to external gamma or alpha irradiation. This technique has proved valuable in identifying localized induced defects in irradiated alkaline earth fluorides [5], and for following radiation induced amorphization in neutron-irradiated crystalline quartz [6] and silicon carbide [7], and radiation effects in vitreous silica [6,8].

2. Experimental Polycrystalline phase pure zircon was synthesized in both actinide doped and actinide free compositions using a hydrothermal gelation procedure. Stoichiometric amounts of Zr(NO,), - xH ,O dissolved in C,H,OH/H,O and TEOS, (C,H,O),Si, in C,H,OH were combined and allowed to nucleate at 40°C for 90 h. Slow addition to 3M NH,OH yielded a flocculent precipitate which was oven dried at llO”C, denitrated in air at 800°C cold pressed at 30000 psi, and sintered at 1800°C for 2 h in argon. 238Pu, present in an aqueous nitrate solution, was introduced prior to the nucleation step. The acidity of the zirconium nitrate precursor is crucial to the successful isolation of phase pure zircon.

539

G.J. Exarhos / Induced swelling =Pu-+a

U (94 keV) + a (5.5 MeVJ

-

c 0.16

B s

* 0.12 8 3

0.08 -

3

A*=Pu DOPED ZIRCON I10 WT%l l

IRRADIATION DAMAGE ZONE

’ ALUMINUM COVER FOIL (5 pm1

IRRAbIATION DAMAGE ZONE 0 0

Fig. 1. External

alpha-irradiation

technique. Fig. 2. Induced

The measured density of synthetic ZrSiO, was found to be 95% of the calculated theoretical density. The 10 wt.% *38Pu doped zircon incurs damage at a rate of 4.02 X 10” alpha decay events per milligram per day and possesses an equilibrium temperature of ca. 5O’C. Actinide free samples were externally alpha irradiated using the arrangement depicted in fig. 1. The aluminium cover foil served to prevent heavy ion recoils from contaminating the sample. Dose rates were calculated to be 1.14 X lOi alpha particles per cm2/day, and sample temperatures did not exceed 40°C. Actinide free samples were subjected to gamma irradiation from a 6oCo source to an accumulated dose of 1.5 x 10” R. Sample temperatures during irradiation were found to be 13O’C. Immersion density and XRD measurements on actinide doped samples were determined periodically. Actinide free samples exposed to ionizing radiation were examined by Raman spectroscopic techniques. Molecular vibrational spectra were excited from sarnples mounted in a 180’ backscattering geometry using 200 mW of 4880 A light from an argon ion laser. Scattered light was focused onto the entrance slit of a SPEX 1403 double monochromator and analyzed photometrically. Slit widths were maintained at 50 pm. 3. Results and discussion 3.1. Displacement damage - dose rate effects The 10 wt.% 238Pu doped

subjected

to self-irradiation

zircon (ZrSiO,) has beeu from alpha decay for ap-

NATURALLY DAMAGED ZIRCON 1H.D. Holland. 1965)

axio16 12x10’5 4x10’5 DOSE (ALPHASlmgm) swelling in ZrSiO,.

proximately one year and has incurred a decrease in density of ca. 4%. Based on data reported by Holland and Gottfried [l] for naturally radiation damaged zircon, the structure saturates and becomes X-ray amorphous as the decrease in density approaches 16%. Relative changes in zircon density as a function of cumulative dose are compared with data for naturally damaged zircon in fig. 2. The natural zircon data was acquired from numerous samples that had been age dated and chemically analyzed for actinide decay products. Fully amorphous natural zircon samples are nearly 600 million years old, while the expected time for complete amorphization in 238Pu doped zircon is but several years, as shown in table 1. To quantitatively compare density changes resulting from actinide decay processes for these two sets of data, the functional forms of the density change must be known. Previous work at PNL on radiation damaged [UO,, PuO,, Ca,Nd,(SiO,),O,, CaTiO,] materials indicated that density changes, as a function of dose, followed a simple first order exponential dependence [9,10]. The rare earth silicate showed exponential behavior of swelling until saturation damage was reached and the structure became amorphous. It is not unlikely, then, that zircon should also follow this behavior. Zircon data was fit to the following equation by a nonlinear least squares routine. Ap =Ap,(l

- eeklDN),

(1)

Table 1 Dose rate comparison Sample

Dose rate

Time to complete amorphization

10 wt.% *%‘u doped zircon Naturally damaged zircon [l]

4.02 x lo’* alpha/mg.d 5.4~ lo4 alpha/mg.d

Several years 570 million years

IX. NUCLEAR

WASTE

MATERIALS

G.J. Exarhos / Induced swelling

540

where A p = measured change in density, D = cumulative dose, k, = rate constant for damage ingrowth, N = order of damage ingrowth, and Ap, = saturation density change. The routine converged on optimum values for the fitting parameters k,, N, and Ap,. N was found to be 1.02 which signifies first order damage ingrowth and is in accord with prediction. Ap, was found to be 0.16 in excellent agreement with the saturation density change reported by Holland and Gottfried [l]. Apparently, dose rate effects do not alter the saturation density value provided that the starting materials have comparable initial densities. The normalized damage ingrowth rate constant was found to be 1.5 X lOWI6 mg/a. The sigmoidal appearance of dose dependent density changes exhibited by Holland and Gottfried’s data may be described by several models including the consecutive first order reaction model discussed in their paper [l]. One other model must be mentioned, particularly because it gives a substantially better fit to the data than the consecutive first order reaction model originally proposed. This model, based on Avrami growth kinetics for devitrification of amorphous phases [ll], relates the time dependent degree of crystallization A,+,(t), to the morphology of the resulting crystallites and an induction period for nucleation. A n+1=

1 - exp( -g,P,Z,r”“),

(2)

with g, = structure constant, V, = crystallization rate, I, = nucleation rate, and n = dimensionality of crystal growth. Eq. (2) can be rewritten in terms of density changes and proves to be an excellent representation of Holland and Gottfried’s data as shown in fig. 1. Ap =Apr(l

- eekNDN),

(3)

with Ap = relative change in density, Ap, = maximum density change (saturation density), N = growth parameter, and k, = rate constant for damage ingrowth. A nonlinear least-squares fit of the data indicated that Ap, = 0.16 and n = 2.01 suggesting one-dimensional morphology of ingrowth of amorphization. The normalized rate constant for damage ingrowth was determined to be k, = 1.7 x lo-l6 mg/cY. Regardless of which model is used to fit the data, calculated normalized rate constants are in good agreement.

Close agreement of k, and k, indicates that dose rate effects are negligible in controlling damage ingrowth and amorphization in crystalline zircon subjected to internal actinide decay processes. Acquired, XRD data as a function of cumulative dose are also in agreement with data reported by Holland and Gottfried for the naturally damaged mineral. Laboratory simulation of radiation damage in crystalline phases accurately simulates effects observed in naturally damaged materials. The question regarding energy release for each decaying nuclide in naturally and synthetically damaged zircon may be answered by referring to table 2; however, the important quantity for this damage/displacement process is the recoil ion momentum. Previous work in this laboratory comparing radiation induced swelling in externally alpha irradiated and actinide doped ceramic materials indicated that the heavy ion recoil is responsible for radiation induced amorphization [9,10]. Therefore, since the recoil ions have nearly the same energy and momenta, the 238Pu doped samples are a good simulation for naturally induced damage. The differences in functional form between both sets of density (dose) data may be explained in terms of long time annealing effects and age dating uncertainty for the naturally damaged zircon. 3.2. Ionization damage - Raman spectroscopy Externally alpha and gamma (1.5 x 10” R) irradiated polycrystalline zircon samples exhibit only slight swelling as a function of dose and show no tendency toward amorphization. Vibrational Raman spectra of both alpha and gamma irradiated zircon were measured to probe the effects of high energy radiation on localized molecular bonds in zircon. The vibrational spectrum of zircon and band assignments have previously been reported [13], based on a tetragonal unit cell of space group D4,, [19]. The allowed Raman modes can be separated into two groups involving motion of silicate anions alone, and vibrations of zirconium atoms against silicate tetrahedra. Raman spectra of alpha and gamma irradiated zircon are qualitatively alike and exhibit only slight band shifts from the unirradiated material. Data shown in table 3, however, show a uniform decrease in band position for the internal modes, but little change

Table 2 Nuclear decay parameters [12]. Reaction

Decay energy (MeV)

Alpha energy (MeV)

Recoil momentum [(amu.Mev)‘/*]

g2Th =ii8Ra+ CI ;;%J=, Y-h+ (x ;;“U =zTh+ a ~%I =;;4u+ a

4.080 4.681 4.268 5.592

3.994 4.597 4.195 5.499

198 197 185 209

G. J. Exarhos / Induced swelling

Table 3 Radiation

damaged zircon.

Vibrational

assignment

Internal

- 1.5 x 10” R y

modes (silicate anion)

“39 B,,

~1, *I, “2, Ai, v4> Bls V4T %

External E,

Observed frequency (cm-‘) (unirradiated)

1007.00 974.00 437.50

1006.50 973.25 437.00

391.50 356.50

391.00 356.00

modes (zirconium-silicate 223.50

B 1s Es (Egr B,,)

213.00 200.75 141.50

541

acquired at three week intervals and continues to fall on the predicted curve. While displacement damage causes marked swelling and eventual amorphization, ionization damage causes only slight swelling with no tendency toward amorphization. Raman results indicate that radiation induced damage is localized on ionic silicate sites in the structure.

223.50

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under contract DE-AC06-76RL0 1830. I am grateful to Mr. G.D. Maupin for assistance with sample preparation and experimental measurements.

212.75 200.75 140.50

References

interactions)

in frequency for the external modes. Irradiation produces a significant increase in Rayleigh scattering at low frequencies and causes an apparent bandshift of the Eg mode at 141.50 cm-‘. Bandshifts to lower frequency are observed for the internal modes which suggests that ionization damage is localized on silicate tetrahedra (regions of high electron density) in the structure.

4. Conclusions The 238Pu doped zircon has been undergoing damage from internal alpha decay for about one year, and accurately simulates damage incurred by the natural mineral over a time period of 100 million years. Dose rate effects for amorphization damage caused by alpha decay processes are not significant, and laboratory simulations mimic effects observed in the naturally damaged mineral, Zircon density/dose data is still being

[l] H.D. Holland, and D. Gottfried, Acta Cryst. 8 (1955) 291. [2] K. Yada, T. Tanji and I. Sunagawa, Phys. Chem. Min. 7 (1981) 47. [3] L.A. Bursill and AC. McLaren, Phys. Stat. Sol. 13 (1966) 331. [4] L. Cartz, and R. Foumelle, Rad. Eff. 41 (1979) 211. T.J. Headley, G.W. Arnold and C.J.M. Northrup, Scientific basis for nuclear waste management-V, ed. W. Lutge (North-Holland, 1982) p. 378. [5] G.J. Exarhos, J. Phys. Chem. 86 (1982) 4020. [6] J.B. Bates, R.W. Hendricks and L.B. Shaffer, J. Chem. Phys. 61 (1974) 4163. [7] R.B. Wright and D.M. Gruen, RAd. Eff. 33 (1977) 133. [8] R.H. Stolen, J.T. Krause and C.R. Kurkjian, Disc. Faraday Sot. 50 (1970) 103. [9] W.J. Weber, J.W. Wald and W.J. Gray, Scientific basis for nuclear waste management-III, ed., J.G. Moore (Plenum Press, 1981) p. 441. [lo] W.J. Weber, J. Am. Ceram. Sot. (1982) 544. [ll] M. Avrami, J. Chem. Phys. 7 (1939) 1103; 8 (1940) 212; 9 (1941) 177. [12] R.C. Weast (ed), Handbook of chemistry and physics (Chemical Rubber Co., 1980) p. B258. [13] P. Dawson, M.M. Hargreave and G.R. Wilkinson, J. Phys. C: Sol. Stat. Phys. 4 (1971) 240.

IX. NUCLEAR

WASTE MATERIALS