Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite

Transcription

1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 41, No. 10, p (October 2004) ORIGINAL PAPER Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite Yoshito NAKASHIMA Exploration Geophysics Research Group, National Institute of Advanced Industrial Science and Technology (AIST), Higashi Central 7, Tsukuba-shi, Ibaraki (Received April 24, 2004 and accepted in revised form August 5, 2004) Kunigel-V1 bentonite was analyzed by proton nuclear magnetic resonance (NMR) spectroscopy over a wide range of water content and temperature at 0.47 T. Kunigel-V1 is a bentonite clay consisting of 50 wt% Na-rich montmorillonite from Yamagata, Japan, and represents a candidate for engineered barriers of underground nuclear waste disposal sites in Japan. The NMR-related properties of bentonite are also important with respect to application of the material as a mud in boreholes for NMR well logging in the geophysical exploration of disposal sites. The proton relaxation times, surface relaxivity of montmorillonite, and H 2 O self-diffusion coefficient, were determined in this study for water-rich gel samples of bentonite at 11.0 to 70.0 C and for bentonite weight fractions of 0 to 37.7 wt%. The proton relaxation times (T1 and T2) were measured by the inversion recovery method and Carr Purcell Meiboom Gill method, respectively, and the self-diffusion coefficient of H 2 O molecules (D) was measured by the pulsed-gradient spin-echo method. The results showed that T1, T2, and D increased with increasing temperature, and decreased with increasing bentonite weight fraction (w). The T1 and T2 surface-relaxivities were on the order of 10 7 m/s and also decreased with temperature. The activation energies of the T1 and T2 relaxation for the bentonite gels were significantly lower than those for bulk water (i.e., about 50 to 70%), whereas the activation energy of the diffusion process for the gels was nearly equal to that for bulk water. As a result, the normalized H 2 O self-diffusivity, D=D 0, obeys a temperature-independent master curve described by lnðd=d 0 Þ¼1:54½expð 0:0377wÞ 1Š, where D 0 is D in bulk water and w is in wt%. KEYWORDS: bound water, clay mineral, CPMG, JCSS3101, NMR logging, relaxation rate, PGSE NMR, porous media, smectite, tortuosity, Tsukinuno I. Introduction Bentonite, which consists primarily of montmorillonite, is being promoted for use in the underground disposal of highlevel nuclear waste. One possible application is as the buffer materials and back-filling materials against the diffusion of radioactive nuclides. 1,2) The dry high-density bentonite in the engineered barriers is likely to swell due to the diffusive invasion of groundwater to become a less-dense water-rich gel during re-submergence of the shafts and galleries. The swollen bentonite with low density probably causes the increase of the mobility of harmful radioactive nuclides because the diffusivity of the radioactive nuclides increases with decreasing bulk density of the bentonite. 3,4) This undesirable enhanced mobility should be considered, for example, in the design of the thickness of the buffer materials. Because the time required for the compacted bentonite to change into the water-rich gel by the diffusive invasion of water molecules depends on the H 2 O diffusivity in bentonite, the data on the H 2 O diffusivity in bentonite is essential to the safe design of the engineered barriers. While H 2 O self-diffusivity has been measured for dense compacted bentonite samples, 3 8) there are few studies on the less-dense water-rich bentonite gels. Therefore, the H 2 O self-diffusivity in gels was measured in the present study to provide fundamental data for the safe design of the buffer and back-filling materials. Corresponding author, Tel , Fax , nakashima.yoshito@aist.go.jp Another potential application is as the drilling mud for boreholes used for geophysical well logging at the disposal sites. The proton nuclear magnetic resonance (NMR) well logging is a promising method for the in-situ measurement of porosity, pore-size distribution, and permeability of strata. 9,10) An NMR sensor in the borehole detects a transverse relaxation process of 1 HinH 2 O molecules in the water-saturated porous strata. Because the NMR signal amplitude increases with increasing number of 1 H atoms in the sensed region, it is possible to estimate the porosity by the signal amplitude. The pore size can be estimated by the relaxation times (spin lattice relaxation time (T1) and spin-spin relaxation time (T2)) because the protons relax faster in smaller pores. T1 refers to the characteristic time required for the excited nuclear magnetization to return to the thermal equilibrium; T2 is that required for the rotation phase of each nuclear spin to be randomized. The permeability prediction is also possible using the porosity and pore-size data by assuming the Kozeny Carman model. However, as the bentonite mud will invade into the strata, there will be implications for the use of NMR well logging. As the bentonite mud is likely to affect the region sensed by the NMR logging tool, 11) it is necessary to understand the NMR properties of bentonite mud such as proton relaxation time, surface relaxivity of clay minerals, and H 2 O self-diffusivity, in order to correctly interpret logging data. Bentonite, Kunigel-V1, is now widely used as drilling mud in Japan, 11,12) and is also one of candidates as an engineered barrier in high-level nuclear waste disposal in Japan. 1,2) 981

2 982 Y. NAKASHIMA Although the NMR properties of water-rich gels of Kunigel- V1 have been reported in one instance, 12) the data set only covers a relatively narrow range of conditions (temperature 20.6 to 39.8 C, bentonite weight fraction 0 to 24.2 wt%). The NMR properties for a wider temperature and bentonite fraction range are needed because (i) computer simulations have indicated that the temperature of the buffer and backfilling materials reaches to about 60 C after 1,000 years since the burial, 1) (ii) NMR well logging may be performed at below 20.6 C, and (iii) the compacted bentonite used for the engineered barrier is more dense than 24.2 wt%. 1 8) Therefore, in the present study, proton NMR experiments were conducted for a mixture of water and Kunigel-V1 powder over a wide range of conditions (11.0 to 70.0 C, 0 to 37.7 wt%) in order to obtain the fundamental properties of bentonite gels (T1, T2, surface relaxivity of montmorillonite, and H 2 O self-diffusivity) essential to the interpretation of NMR logging data and engineered barrier design for nuclear waste disposal. The results are also compared with those of previous studies on Kunigel-V1 7) and Kunipia-F 3,4,6 8,13) (purified and cation-exchanged montmorillonite samples mined from the same locality as Kunigel-V1). II. Experimental Kunigel-V1 is a Na-rich bentonite mined at Tsukinuno, Yamagata, Japan and was provided by Kunimine Industries Co., Ltd. ( The chemical composition of the sample was as follows: 14) SiO , Al 2 O , Fe 2 O , FeO 0.62, TiO , MnO 0.22, Na 2 O 2.56, K 2 O 0.33, CaO 2.30, MgO 2.26, P 2 O , H 2 O 19.33, CO , S 0.29 (total wt%). The Kunigel- V1 sample contained 47 wt% montmorillonite, 37 wt% chalcedony, 4 wt% plagioclase, 3 wt% analcime, 2 wt% calcite, 2 wt% dolomite, 0.6 wt% quartz, and 0.6 wt% pyrite (total 96.2 wt%). 14) The cation exchange capacity (CEC) was 56 meq/100 g. 14) Thermogravimetry revealed that the weight loss of the bentonite powder used in the present study was 10.2 wt% at 110 C and 14.3 wt% at 1,000 C. Water-rich bentonite gel (not compacted bentonite) samples were prepared by adding deionized water to the bentonite powder. Pre-existing H 2 O in the powder sample (weight loss 10.2 wt% at 110 C) was corrected in the calculation of the bentonite weight fraction in the gel sample (w). The bulk densities ( bulk ) of the samples were obtained by weighing samples of known volume (Tables 1 3). NMR measurements were performed using a proton spectrometer (NMS120, Bruker, Karlsruhe, Germany) at a resonant frequency of 20 MHz (0.47 T). Each 1-cm 3 sample was placed in a separate 10 mm-external diameter glass tube. No pressure vessel was employed, allowing the clay to swell freely in the glass tube. The relaxation times and self-diffusion coefficients of bulk water were also measured using deionized water to compare with the results of the gel samples. The temperature dependence of T1, T2, and the H 2 O selfdiffusion coefficient was measured by systematically changing the temperature of the circulating water in the NMR spectrometer (11.0, 20.6, 30.0, 39.8, 51.3, 61.1, and 70.0 C). T1 and T2 for protons associated with H 2 O in the gel samples and deionized water were measured by the conventional inversion recovery method and the Carr Purcell Meiboom Gill (CPMG) method, respectively. 15) The echo spacing in the CPMG method for the gel samples was as short as 0.2 ms, identical to the condition in actual NMR well logging. 11) The data obtained for T1 and T2 were analyzed to estimate the surface relaxivity of montmorillonite and the activation energy of the relaxation process using the following equations: 9,16 18) 1 T1 ¼ 1 S V 1 T2 ¼ S 2 V pore pore 1 ¼ A 1 exp E ; ð1þ 1 RT 1 ¼ A 2 exp E ; ð2þ 2 RT where 1 and 2 are the T1 surface relaxivity and T2 surface relaxivity of montmorillonite, respectively, ðs=vþ pore is the surface-to-volume ratio of the pore space of the porous bentonite gel, A 1 and A 2 are constants, E 1 and E 2 are activation energies, R is the gas constant ( J/molK), and T is the absolute temperature. The parameters 1 and 2 are fundamental properties used to discuss the w-dependence of T1 and T2, and E 1 and E 2 are essential to predict the temperature-dependence of T1 and T2. The H 2 O self-diffusion coefficient D was measured by pulsed-gradient spin-echo (PGSE) NMR (Fig. 1). The selfdiffusion coefficients were calculated by measuring the decrease in the NMR signal intensity with increasing magnetic field gradient. The NMR spin-echo signal intensity I (Fig. 1) is given by: I=I 0 ¼ expð bdþ; where b ¼ðGÞ 2 ð =3Þ: The quantity I 0 is the signal intensity in the absence of a pulsed field gradient, is the gyromagnetic ratio of a proton (2: rad/ts), G is the strength of the field gradient pulses, is the duration of the field gradient pulse, and is the interval between two gradient pulses. 19,20) In the PGSE NMR experiments, the echo time, T E, was set shorter for water-poor gel samples with small T2 in order to improve the signal-to-noise ratio of the spin-echo signal. Thus, the pulse parameters were as follows: ¼0:7 ms, T E ¼28 ms, and ¼14 ms for samples of 0w16:2 wt% (Fig. 1); and ¼1:0 ms, T E ¼12 ms, and ¼6 ms for samples of 19:7 w37:7 wt%. Signal stacking was set at 16, and the repetition time for the pulse sequence, T R, was set at T R ¼5T1 to satisfy the full relaxation condition. A range of G (0 to 2.1 T/m) was examined for each specific bentonite fraction and temperature to measure the dependence of the ratio I=I 0 on b. D was then calculated by regression analysis of the data sets (I=I 0 vs. b) combining Eqs. (3) and (4). The activation energy of the diffusion process (E diff ) was calculated as follows: ð3þ ð4þ JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

3 Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite 983 Table 1 Summary of experiments for T1 of water protons in Kunigel-V1 gels w (wt%) bulk T1 T1 T1 T1 T1 T1 T1 20 C 11.0 C 20.6 C 30.0 C 39.8 C 51.3 C 61.1 C 70.0 C (g/cm 3 ) (ms) E 1 (kj/mol) ,882 2,452 3,066 3,785 4,752 5,690 6, Table 2 Summary of experiments for T2 of water protons in Kunigel-V1 gels w (wt%) bulk T2 T2 T2 T2 T2 T2 T2 20 C 11.0 C 20.6 C 30.0 C 39.8 C 51.3 C 61.1 C 70.0 C (g/cm 3 ) (ms) E 2 (kj/mol) ,718 2,165 2,665 3,101 3,584 4,020 4, Table 3 Summary of experiments for H 2 O self-diffusion coefficient in Kunigel-V1 gels. D 0 is D for w¼0:00 wt%. w (wt%) bulk D D D D D D D 20 C 11.0 C 20.6 C 30.0 C 39.8 C 51.3 C 61.1 C 70.0 C (g/cm 3 ) (10 9 m 2 /s) (ms) E diff (kj/mol) VOL. 41, NO. 10, OCTOBER 2004

4 984 Y. NAKASHIMA Fig. 1 Pulse sequence for diffusion measurement by PGSE NMR method Pulse parameters used for samples of 0w16:2 wt% are indicated. The spin-echo intensity I obeys Eq. (3). D ¼ A 3 exp E diff ; ð5þ RT where A 3 is the constant. III. Results 1. Relaxometry Examples of T1 and T2 measurements are shown in Fig. 2, demonstrating that the fitting using a single decay component is reasonable. This is a remarkable contrast to porous sedimentary rocks, 9) to which multi-exponential fitting should be applied. The pore size of the porous clay gels (i.e., characteristic distance between clay platelets) is submicrometer 21) and is much smaller than that of sedimentary rocks. Averaging by the fast diffusion of water molecules traveling on a micrometer scale (much longer than the pore size of the clay gel) during the spin relaxation is probably responsible for this mono-exponential decay. 17) The results of the relaxation experiments are listed in Tables 1, 2, and Table 4, and are illustrated in Figs T1 and T2 decrease with increasing w (Figs. 3(a) and 4(a)). This negative dependence of T1 and T2 on w has been reported for other smectites (montmorillonite from Wyoming, 22) synthetic stevensite, 23) hectorite, 22) and saponite 24) ). The surface of mineral grains is a strong sink of magnetization. 9) The pore size (characteristic distance between clay particles) decreases with increasing w, and the collision frequency between diffusing water molecules and the clay particles increases with decreasing pore size. It is therefore reasonable that T1 and T2 decrease with increasing w. Arrhenius plots of T1 and T2 are shown in Figs. 3(b) and 4(b). The magnitude of the activation energy of the bentonite gels ( kj/mol for T1, kj/mol for T2) is similar to that for water-saturated or oil-saturated sedimentary rocks, 18) and significantly smaller than that for bulk water (16.8 kj/mol for T1, 13.7 kj/mol for T2). The T1 and T2 of Fig. 2 Example of T1 and T2 measurements at 30.0 C The bentonite fraction (w) of each sample is indicated. A mono-exponential fitting for the data points is shown by solid curves. (a) Time-series data from the inversion recovery method for T1 measurement. (b) Time-series data from the CPMG method for T2 measurement. Table 4 Summary of surface relaxivity ( 1 and 2 ) of montmorillonite for water protons in Kunigel-V1 gels 1 (mm/s) 2 (mm/s) 11.0 C 20.6 C 30.0 C 39.8 C 51.3 C 61.1 C 70.0 C bulk water increase with temperature, which can be readily explained by motional narrowing or by an increase in molecular motion activated by the thermal energy. 25) On the JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

5 Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite 985 Fig. 3 Results of T1 measurement, corresponding to the data in Table 1 (a) T1 as a function of bentonite weight fraction (wt%). The sample temperature ( C) is indicated. (b) Arrhenius plot of (a). Data points were fitted to Eq. (1) by a least-squares method. Fig. 4 Results of T2 measurement, corresponding to the data in Table 2 (a) T2 as a function of bentonite weight fraction (wt%). The sample temperature ( C) is indicated. (b) Arrhenius plot of (a). Data points were fitted to Eq. (2) by a least-squares method. VOL. 41, NO. 10, OCTOBER 2004

6 986 Y. NAKASHIMA Fig. 5 Estimate of surface relaxivity of montmorillonite The sample temperature ( C) is indicated. (a) T1 surface relaxivity. (b) T2 surface relaxivity. Equations (1) and (2) were fitted to (a) and (b), respectively. Relaxation data for bulk water were omitted in this fitting because the fast-diffusion approximation breaks down owing to the infinite pore size. other hand, the temperature dependence of relaxation times of pore fluid in porous media is not simple, depending on fluid and rock chemistry. The increase in T1 and T2 with increasing temperature (Figs. 3(b) and 4(b)) is similar to the behavior of water-saturated calcite grains and oil-saturated sandstone, 18) implying that the binding energy between water molecules and the clay surface is low. This implication is consistent with the argument that hydrogen bonding between clay and water molecule is weaker than that between water molecules. 26) The surface relaxivity of montmorillonite ( 1 and 2 ) was estimated by analyzing the w-dependence of T1 and T2. The assumption of a fast diffusion regime 9) allows Eqs. (1) and (2) to be used for estimating 1 and 2, respectively. The term ðs=vþ pore in Eqs. (1) and (2) is expressed in terms of w as follows: S V pore ¼ ¼ 100 S V ¼ solid solid S solid 100 solid M solid S 100 solid V solid solid ; ð6þ where is the porosity of the porous bentonite gels (vol%), solid ¼100 is the volume fraction of solid mineral grains (also in vol%), ðs=vþ solid ¼ðS=VÞ pore =ð100 Þ is the surface-to-volume ratio of the solid mineral grains (clay and non-clay minerals), solid is the density of the mineral grains, and ðs=mþ solid is the specific surface area of the mineral grains (m 2 /g). The parameter solid can be related to w by solid ¼ solid water w þ 1 solid water w 100 ; ð7þ where water is the density of bulk water; 27) solid ¼2:7 g/cm 3 and water ¼1 g/cm 3 were assumed in the present study. The value of ðs=mþ solid for Kunipia-F (purified and cationexchanged product of Kunigel-V1) is 700 m 2 /g according to the EGME (ethylene glycol monoethyl ether) method. 6) The contribution of non-clay minerals in Kunigel-V1 such as chalcedony to ðs=mþ solid is negligible owing to the grain size as large as a few micrometers. Thus, ðs=mþ solid for Kunigel-V1 was estimated to be 7000:47=0:99330 m 2 / g, where 0.47 and 0.99 are the weight fractions of montmorillonite in Kunigel-V1 14) and Kunipia-F, 13) respectively. It was assumed in the estimate that ðs=mþ solid of montmorillonite in Kunigel-V1 is equal to that in Kunipia-F. It should be noted that the ðs=mþ solid value of clay minerals measured by the BET (Brunauer Emmett Teller) method 17) using N 2 gas adsorption is misleading because N 2 molecules cannot probe the narrow interlayer space, 6) resulting in an underestimate of ðs=mþ solid. The results are shown in Fig. 5, and are listed in Table 4. The estimated values of 2 in Table 4 are of the same order as those for Wyoming montmorillonite, illite, kaolinite, and chlorite. 16) The reciprocal of ðs=vþ pore gives a rough estimate of the pore diameter of the porous gel, 28) and the data in Fig. 5 show that the pore diameter varies from 5 to 80 nm. The fast diffusion approximation requires that a 1 =D 0 and a 2 =D 0 should be much less than unity, where a is the pore diameter. 9,29) The substitution of a¼ 5{80 nm, 1 2 0:3 mm/s (from Table 4), and D m 2 /s (from Table 3) yields a 1 =D 0 a 2 =D 0 ¼ to 810 6, much less than unity. This calculation indicates that the data analysis by Eqs. (1) and (2) assuming a fast diffusion approximation is reasonable. JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

7 Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite 987 Fig. 6 Example of diffusion measurements NMR signal intensities vs. b for 5 samples at 51.3 C. Each bentonite fraction (w) is indicated. Data points were fitted to Eq. (3) by a least-squares method. 2. Diffusometry Examples of normalized spin-echo intensities (I=I 0 ) are shown as a function of b in Fig. 6. The slope of each regression line represents the self-diffusion coefficient value. The clear regression line demonstrates the high accuracy of the self-diffusion coefficient value determined by this technique. It should be noted that only a few minutes were needed to acquire a single data point for the data shown in Fig. 6, which is shorter by several orders of magnitude than the time required for conventional diffusion tests using tritiated water. 3 7) Table 3 indicates that the diffusion distance measured by PGSE NMR, ð6dþ 1=2, is as short as 4 20 mm, while that by the conventional tests is about 1 cm. This contrast is responsible for the difference in the required experiment time. The results of the diffusion experiments are shown in Fig. 7 and summarized in Table 3. The data for D 0 (i.e., D at w¼0:00 wt%) were checked by comparison with the following literature data: 30) D 0 ¼2: m 2 /s at 25 C and E diff ¼17:6 kj/mol for C; D 0 ¼1: m 2 /s at 5 C and E diff ¼19:7 kj/mol for 1 15 C. This literature data predicts that D 0 ¼1: m 2 /s at 11.0 C, D 0 ¼2: m 2 /s at 20.6 C, D 0 ¼2: m 2 /s at 30.0 C, and D 0 ¼3: m 2 /s at 39.8 C. These results agree well with Table 3, demonstrating that the present PGSE NMR experiments were performed in a reliable manner. The H 2 O self-diffusion coefficients D decreased with increasing w (Fig. 7(a)), attributable to the obstruction effects of clay minerals with bound or less mobile water molecules on the surface. 22,24) According to Fig. 7(b), normalized D obeys the following phenomenological curve: ln D ¼ ½expð wþ 1Š; ð8þ D 0 where and are dimensionless constants, and w is the bentonite weight fraction of the gel (wt%). Equation (8) was originally found for Kunipia-F, 13) Wyoming montmorillonite, 22) and saponite 24) gels, but also appears to be applicable to Kunigel-V1 gels. Figures 7(a)(b) show that and are nearly independent of sample temperature. As a result, a temperature-independent master curve (Eq. (8)) can be readily determined. An Arrhenius plot of the diffusion data is shown in Fig. 7(c), and the corresponding activation energies E diff calculated using Eq. (5) are listed in Table 3. Although weak non-arrhenius behavior can be seen in Fig. 7(c), as reported previously, 31) the Arrhenius plot is yet reasonable. The activation energy of bulk water (18.2 kj/mol for Cin Table 3) is nearly equal to the reported result of 17.6 kj/mol for the temperature range C, 30) suggesting that the NMR experiments were also performed in a reliable manner. The activation energy of the relaxation process for bulk water is significantly larger than that for bentonite gels (Tables 1 and 2), rendering it impossible to discuss the w-dependence of T1 and T2 in terms of the normalization using relaxation times for bulk water. However, Table 3 indicates that E diff for each gel sample is nearly equal to the value for bulk water, which successfully leads to Eq. (8), as a simple temperature-independent relation between w and normalized self-diffusivity. IV. Discussion 1. Application to the Engineered Barrier Design The bentonite in the engineered barriers is dry and of high-density immediately after the burial. However, it probably swells and changes into a less-dense water-rich gel owing to the invasion of groundwater during re-submergence of the shafts and galleries. The mobility of harmful radioactive nuclides increases by this bentonite swelling because the diffusivity of the radioactive nuclides increases with decreasing bulk density of the bentonite. 4) The thickness or volume of the engineered barriers should be designed in consideration of this undesirable diffusion enhancement. Because the rate of the diffusive invasion of the water molecules depends on the H 2 O diffusivity in bentonite, the obtained results (Table 3) are essential to the safe design of the engineered barriers. The H 2 O self-diffusivity determined in the present study was compared in Fig. 8(a) with the literature data for HTO diffusivity in compacted Kunigel-V1 bentonite. Although there is a small discrepancy, Fig. 8(a) suggests that the D=D 0 data for water-rich, less-dense bentonite can be extrapolated to those for water-poor, more-dense compacted bentonite. Therefore, Eq. (8) is considered to be useful for predicting the H 2 O self-diffusivity in compacted Kunigel- V1 bentonite for arbitrary temperature and w. Kunipia-F, or the JCSS3101 reference sample of the Clay Science Society of Japan ( cssj2/english/reference clay.html) is purified and cationexchanged montmorillonite mined from the same locality as Kunigel-V1, and was compared with the present sample in Fig. 8(b). Because the montmorillonite purity of VOL. 41, NO. 10, OCTOBER 2004

8 988 Y. NAKASHIMA Fig. 7 Measured self-diffusion coefficient D for H 2 O in bentonite gels (data from Table 3) (a) Semi-log plot of D as a function of bentonite fraction (w) at various temperatures ( C). (b) Normalized H 2 O selfdiffusion coefficient, D=D 0, for 11.0 to 70.0 C. Note that D=D 0 is independent of temperature. The data point (w¼37:7 wt% at 11.0 C) was not used for curve fitting owing to the large discrepancy. (c) Arrhenius plot of D for various bentonite fractions. Parameters and in Eq. (8) obtained by a least-squares fit are shown in (a) and (b). Data points were fitted to Eq. (5) by a least-squares method for (c). Kunigel-V1 (purity 47 wt%) differs from that of Kunipia-F (purity 99 wt%), 13) the contribution of non-clay minerals to the w value was removed 7) as a correction to the Kunigel- V1 data in Fig. 8(b). The anisotropy, particle-size dependence, and E diff significantly larger than that for bulk water have been reported for compacted Kunipia-F. 6 8) It is therefore difficult to discuss the normalized water diffusivity using a single variable, such as the corrected montmorillonite weight fraction. This is one possible cause for the scatter of the data in Fig. 8(b). However, some important features can still be seen in Fig. 8(b). For example, although there is some data fluctuation in the data, it is reasonable to conclude JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

9 Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite 989 Fig. 8 Normalized apparent diffusion coefficients D=D 0 for water in Kunigel-V1 bentonite as a comparison of present data from Table 3 with literature data The effective water diffusivity for compacted bentonite of the literature was converted into the apparent diffusivity by dividing the effective diffusivity by the porosity of the porous bentonite sample. The dry bulk density data of the literature were converted into w by assuming that solid ¼2:7 g/cm 3 and water ¼1 g/cm 3. (a) Comparison of H 2 O diffusion (the present study) and HTO diffusion in compacted Kunigel-V1 bentonite at 25 C. 7) (b) Normalized H 2 O selfdiffusion in Kunigel-V1 (this study) compared with H 2 O, HTO and HDO diffusion in Kunipia-F. The horizontal axis is corrected considering the montmorillonite purity (i.e., 47 wt%) in the Kunigel-V1 powder sample to facilitate the comparison. 7) that the water diffusivity decreases with increasing montmorillonite fraction. Thus, dense bentonite is desirable for the safe design of nuclear waste repositories. Another feature is that the data points for water-rich gels of Kunigel-V1 agree well with those for Kunipia-F at corrected montmorillonite fractions of less than 10 wt%. This implies that the obstruction effects of non-clay minerals (53 wt% of Kunigel- V1) such as chalcedony on the ratio D=D 0 are negligible. This remarkable feature is a consequence of the 16.7-fold increase in the effective volume of the clay obstacles (montmorillonite grains) due to thick bound or immobilized water layers on montmorillonite surfaces. 22) The data points for Kunigel-V1 do not agree with those for Kunipia-F at corrected montmorillonite fractions higher than 10 wt%, attributable to differences in the clay microstructure between Kunigel-V1 and Kunipia-F for dense samples. 7) The bentonite in the buffer materials and back-filling materials is likely to swell due to infiltration of groundwater to become a less-dense, water-rich gel during re-submergence of the shafts and galleries. The temperature of the buffer and back-filling materials has been predicted to reach to about 60 C after 1,000 years since the burial. 1) The present study has measured the H 2 O self-diffusivity in water-rich gels of Kunigel-V1 at up to 70 C for the first time. The results showed that the diffusive mobility of H 2 O molecules increases dramatically as the water weight fraction approaches 100 wt% (i.e.,asw!0 wt% in Figs. 7(a)(b)). Thus, bentonite swelling upon re-submergence will produce an undesirable enhancement of H 2 O mobility. Careful design of the depository to prevent swelling, such as the through the use of concrete plugs, 1) is therefore needed for the safe disposal of nuclear waste. VOL. 41, NO. 10, OCTOBER 2004

10 990 Y. NAKASHIMA measurable by PGSE NMR is as short as ð6dþ 1=2 ¼ 4{20 mm, which renders it technically difficult to measure long-distance (1 mm) diffusion. 32) The obstruction effect by sand grains (diameter1 mm) can be estimated as follows. If the sand grains are randomly scattered in the bentonite, Eq. (10) of Nakashima and Mitsumori 24) for a random porous media model is applicable. For example, if the volume fraction of the sand grains in the mixture is 20 vol%, Eq. (10) predicts that the drop in self-diffusivity due to the obstruction effects of the sand grains (Fig. 9(b)) is 29%. On the other hand, if the sand grains are packed closely with a volume fraction of 63 vol%, computer simulation 33) predicts that the drop will be 38%. These calculations suggest that H 2 O self-diffusivity in the sand-bentonite mixture decreases by 29 38% compared to the prediction by Eq. (8). Fig. 9 H 2 O self-diffusion in a sand-bearing water-saturated bentonite Sand grains are assumed to be about 1 mm in diameter. (a) Schematic microstructure of sand-bentonite mixture. A random walker (H 2 O molecules, solid circle) diffuses by avoiding sand grains. (b) Hypothetical behavior of H 2 O self-diffusivity as a function of the diffusion distance. 32) The probable distance for the present PGSE NMR study (ð6dþ 1=2 ¼4{20 mm) is indicated. The geometrical tortuosity introduced by sand cannot be probed by PGSE NMR because the value of ð6dþ 1=2 is much smaller than the sand grain diameter. In real disposal practice, sand grains will be mixed with Kunigel-V1 to reduce the production cost of the clay barrier system. 1) As the sand grains will act as obstacles to diffusing H 2 O molecules in Kunigel-V1 gels (Fig. 9), the H 2 O selfdiffusivity of such a sand-bentonite mixture will decrease more than that predicted by Eq. (8). The diffusion distance 2. Application to the Well Logging Data Analysis The region sensed by the NMR logging tool extends 2 3 cm into the wall of the borehole. 9) Kunigel-V1 as drilling mud will likely affect such measurements, as it will probably penetrate into the sensed region, filling the pores and fractures with water-rich Kunigel-V1 gel. Thus, the NMR properties of bentonite mud are needed to make meaningful interpretations of logging data. Tables 1, 2, and 4, and Eqs. (1) and (2) are useful for predicting T1 and T2 at arbitrary temperature and bentonite weight fraction. The T1 value of drilling mud is important in NMR well logging. The repetition time of the pulse sequence, T R, should be set to T R T1 in order to obtain reliable logging data using a CPMG pulse sequence. The logging speed should also be sufficiently slow to allow proton magnetization to be polarized, which develops exponentially based on the time constant T1. Thus, extensive T1 data for mud will be needed for reliable well logging if pores and fractures in the sensed region become filled with mud. The data in Tables 1 and 4 represent essential information for the selection of optimal T R and logging speed. The T2 value of pore fluid is also critical in the discussion of pore size, fracture aperture, and permeability. 9) One possible application of the results obtained in the present study is for the aperture estimation for a mud-filled fracture. 11) For this calculation, it was necessary to determine the threshold value of T2 to distinguish mud in a large fracture with long T2 from mud in small pores with short T2 in the raw CPMG data. For example, if the mud temperature is 11.0 C and the Kunigel-V1 concentration is w¼10:8 wt%, the T2 of the bulk mud will be 42.3 ms (Table 4). This implies that the T2 of large mud-filled fractures (0:1 mm in aperture) will also be 42.3 ms. 11) Thus, the threshold value should be set at 42.3 ms in the analysis of the T2-distribution histogram. In diffusometry-based NMR logging, 10,33) the restricted self-diffusion of fluid molecules in rock pores is measured directly to estimate the tortuosity and permeability of strata. The self-diffusivity value for bulk fluid is therefore needed to obtain the tortuosity or normalized diffusivity. If the pore fluid is mud rather than pure water, the H 2 O self-diffusivity in bentonite gels (not in pure water) should be used for calculation of the tortuosity of the rock pore structure. Table 3 shows that the H 2 O self-diffusivity in bentonite gels is sig- JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

11 Nuclear Magnetic Resonance Properties of Water-Rich Gels of Kunigel-V1 Bentonite 991 nificantly smaller than that in pure water. This information should be incorporated in the data analysis to avoid any overestimate of tortuosity. V. Conclusions In the present study, NMR technique was successfully applied to water-rich bentonite gels to measure the dependence of the proton relaxation and water self-diffusivity on temperature and bentonite weight fraction. The obtained data cover a wide range of condition (11.0 to 70.0 C in temperature and 0 to 37.7 wt% in bentonite fraction). The data are useful for the data analysis of NMR well logging and for the safe design of the engineered barriers against the re-submergence of shafts and galleries. The H 2 O diffusivity in bentonite was measured in the present study because the diffusion can be a dominant migration mechanism of water. Figure 5 implies that the pore diameter of water-rich bentonite gels used ranges from 5 to 80 nm. This is larger than that of water-poor compacted bentonite, 8) suggesting that the permeability of water-rich gels is significantly higher than that of compacted bentonite. Thus, Darcy flow (not diffusion) can be a dominant water-migration mechanism in bentonite gels if the pressure gradient of the pore water is considerably large around the disposal sites. Therefore, the permeability measurement of water-rich gel samples should be performed in future to evaluate the contribution of Darcy flow. The possible NMR-related subjects to be studied in future are (i) diffusometry of water in compacted bentonite (not water-rich gel), (ii) effects of the salinity of pore water on relaxation and diffusion, and (iii) diffusometry of heavy nuclides less sensitive than 1 H in terms of NMR spectroscopy. The compacted bentonite to be used at the real disposal sites is dense and water-poor compared with the gels used in the present study. Thus, the applicability of Eq. (8) to the dense and water-poor bentonite should be examined. Because the activation energy E diff for the compacted bentonite is possibly larger than that for water-rich gels, 8) the temperature-dependence of D also should be checked. The water-poor compacted bentonite samples characterized by short T2 are probably measurable by PGSE NMR method if the eddy current noise problem induced by strong field-gradient pulses is solved by the use of high-performance shielded gradient coils. The hydraulic permeability of compacted bentonite depends on the salinity of pore water. 1) This suggests that the results of the present study (deionized water was used) are not always applicable to the bentonite saturated with seawater. Thus, effects of the pore water salinity on the NMR properties should be examined. It is technically possible to perform NMR experiments of bentonite samples saturated with seawater. The NMR spectrometer used in the present study is a low-field type (permanent magnet of 0.47 T), and sufficient for the measurement of the sensitive nuclide, 1 H. The self-diffusion data on heavy nuclides (e.g., 127 I and 133 Cs) in bentonite are also important for the design of the buffer materials. The use of NMR for the diffusometry of heavy nuclides is beneficial because a measurement a few orders of magnitude quicker than the conventional method using radioisotopes is possible. Unfortunately these heavy nuclides are too less sensitive to measure the diffusion coefficient using the low-field magnet. However, they are probably measurable by PGSE NMR if a stronger magnet (e.g., superconducting electromagnet of 15 T) is available. Therefore, NMR is a promising and useful technique for diffusion study of the nuclear waste disposal as a non-destructive, quick, and accurate measurement method. Acknowledgments Comments by an anonymous reviewer were helpful. The author is grateful to Drs. 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