Effects of Temperature on the Deposition Rate of Supersaturated Silicic Acid on Ca-type Bentonite

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1 Journal of Energy and Power Engineering 11 (2017) doi: / / D DAVID PUBLISHING Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite Tsuyoshi Sasagawa, Taiji Chida and Yuihi Niibori Department of Quantum Siene and Energy Engineering, Graduate Shool of Engineering, Tohoku University, Sendai , Japan Reeived: August 04, 2017 / Aepted: August 14, 2017 / Published: September 30, 2017 Abstrat: Na-type bentonite is ommonly used as a tunnel bakfilling material to prevent groundwater and radionulide migration during the onstrution of a geologial disposal system for high-level radioative waste in Japan. However, host rok fratures with strong water flow an develop groundwater paths in the bakfilling material. Espeially, the alteration to Ca-type bentonite auses degradation of the barrier performane and aelerates the development of groundwater paths. Additionally, using ementitious materials gradually hanges ph between 13 and 8. High alkaline groundwater results in high solubility of silii aid; therefore, silii aid is eluted from the host rok. Downstream, in the low alkaline area, the groundwater beomes supersaturated in silii aid. This aid is deposited on Ca-type bentonite, thus leading to the logging of the groundwater paths. In the present study, we investigate the silii aid deposition rate on Ca-type bentonite under K for depths greater or equal to 500 m. The results indiate that temperature does not affet the silii aid deposition rate up to 323 K. However, in this temperature range, the deposition of silii aid on Ca-type bentonite in bakfilled tunnels results in logging of the flow paths. Key words: Supersaturated silii aid, Ca-type bentonite, bakfilling material, apparent deposition rate onstant, geologial disposal system, flow paths. 1. Introdution 1.1 Senario of Radionulide Migration In Japan, highly radioative liquid wastes from a spent nulear fuel proessing plant that is vitrified and disposed underground at depths greater than 300 m [1]. For the immobilization of radionulides in the geologial disposal system, it is important to restrit the groundwater flow as low as possible beause deep underground environments in Japan are generally filled with groundwater [1]. Therefore, for the onstrution of the repository, lay materials, mainly Na-type bentonite, are used as tunnel bakfilling materials and buffer materials. Bakfilling materials are prepared by mixing the exavated muk with Na-type bentonite [1, 2]. Bentonite is a layered siliate mineral ontaining Corresponding author: Tsuyoshi Sasagawa, Ph.D. student, researh fields: reation kinetis and mass transfer. exhangeable ations (e.g., sodium, alium, and potassium ions) in interlayers. In partiular, Na-type bentonite shows various exellent properties for the retardation of radionulide migration due to its layered struture, suh as high swelling, low permeability, and adsorption of radionulides. Conversely, when the Na ions in the bentonite interlayers are exhanged with Ca ions from the groundwater, the swelling property of the bentonite may be degraded [3]. Therefore, when the repository is groundwater saturated, the degradation of the swelling property may result in the formation of flow paths in the bakfilling parts, whih will inrease the permeability of the groundwater. That is, the alteration to Ca-type bentonite in the bakfilling parts may aelerate the migration of radionulides. 1.2 Clogging of Flow Paths due to Silii Aid Deposition Now, we fous on the ph gradient in/around the

2 560 Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite repository, whih is aused by the large amount of ementitious materials used for the onstrution of a geologial disposal system. As shown in Fig. 1, in the long term, alkaline speies (Na, K, and Ca ions) leahing from ementitious materials inreases groundwater alkalinity (ph 13) [4]. As this highly alkaline groundwater is mixed with natural groundwater away from ementitious materials, ph gradually dereases to ph 8 [2] ( alkaline front ). In the alkaline front, the dynami behavior of silii aid, whih is the main omponent of many underground minerals, beomes ompliated. Silii aid is eluted from siliate minerals ontaining host roks and engineered barriers under high alkalinity, where the silii aid solubility is very high. Then, silii aid deposits on the surfae of the solid phase, as the liquid phase beomes supersaturated in silii aid with dereasing solubility due to the dilution of the highly alkaline groundwater. In addition, the silii aid exess may polymerize partially to form olloids. That is, hanges in silii aid solubility with hanging ph lead to dissolution, deposition, and polymerization of silii aid. Therefore, the dynami behavior of silii aid, i.e., the redistribution of silii aid, may delay the migration of radionulides due to the flow paths logging with the deposition of silii aid on the solid surfae suh as Ca-type bentonite in the bakfilled tunnel. In the present study, we disuss the relationship between the silii aid redistribution and the barrier performane in the bakfilled tunnel. The deposition of silii aid is generally based on the following reation formula in Fig. 2 [5]. Here, M in Fig. 2 orresponds to the omponent atoms (Si, Al, and so on) ontained in the solid phase. As shown in Fig. 2, the deposition reation of silii aid is dehydration ondensation. Thus, in this study, the deposition of silii aid does not inlude a preipitation proess, suh as sedimentation of olloidal silii aid. 1.3 Previous Studies So far, various researh studies have been performed on the underground redistribution of silii aid in the field of geothermal siene (e.g., [6]). In addition, some researh results have been reported in the field of waste management. For instane, Chigira [7-9] examined the silii aid deposition from supersaturated groundwater in the fratures of host roks with a flow reator. In addition, Shikazono [10] analyzed the sequential distribution of silii-aid onentration leahed from vitrified waste in/around the repository. Furthermore, Sun and Rimstidt [11] indiated that the deposition of silii aid aused by the heating of the radioative waste resulted in the formation of the silia ap in the Yua mountain test site, and Matyskiela [12] observed the logging of fratures by the deposition of silii aid around the same site. Based on these studies, Sasagawa et al. [13, 14] estimated the apparent deposition-rate onstants of supersaturated silii aid solutions for amorphous silia experimentally, and the numerial analyses using these rate-onstants showed that the deposition of silii aid logs the fratures of the host rok for hundreds of years. Moreover, Sasagawa et al. [15] examined the deposition behavior of silii aid on Ca-type bentonite at room temperature (298 K). However, Fig. 1 Dynami behavior of silii aid. Fig. 2 Deposition reation of silii aid.

3 Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite 561 in general, an underground environment undergoes a geothermal gradient, whih rises by approximately 3 K per depth of 100 m [16]. That is, the temperature around a repository at a depth of 500-1,000 m is estimated in the range of K (ground level temperature is assumed to be 288 K). For understanding the redistribution of silii aid oexisting with Ca-type bentonite in underground, it is neessary to omprehend the influene of temperature on the deposition behavior of silii aid. 1.4 Objetive of This Study The objetive of this study is to determine whether or not the redistribution of silii aid in the repository affets the barrier performane in the bakfilled tunnel after the alteration to Ca-type bentonite. Thus, our objetive is to estimate the logging effet with silii aid deposition for flow paths formed in the bakfilled tunnel onsidering temperature. First, we estimate the apparent deposition-rate onstants by examining the dynami behavior of silii aid in supersaturated groundwater at temperatures up to 323 K. Next, we disuss the logging effet for flow paths by estimating the Damköhler number based on the apparent deposition-rate onstants obtained from the deposition experiments. 2. Experimental Method 2.1 Bentonite Sample Ca-type bentonite was prepared by exhanging Na ions in the interlayer of Na-type bentonite for Ca ions. The preparation proedure for Ca-type bentonite followed the authors previous study [17]. First, 10 g Na-type bentonite (Kunigel-VI, purhased from Kunimine Industries Co., Ltd) were stirred in 1 L 1 M CaCl 2 solution for 24 h. Then, the sample solution was left to stand for 24 h without stirring, and the supernatant solution was removed. This deantation proess was repeated four times. After the ion-exhange proedure, the exess Cl, Na, and Ca ions ontained in wet bentonite sample were removed using a dialysis tube and ultra-pure water. The ion-exhanged bentonite was dried at 378 K. Finally, the Ca-type bentonite was sieved in order to retain only the partiles with diameters 75 μm and less. Exhanging Na ions with Ca ions in the bentonite interlayer was onfirmed by the following measurement of the ation exhange apaity (interlayer ations) [19]. A Ca-type bentonite sample of 1 g weight was immersed in 200 ml of a solution of 0.05 M CH 3 COONH 4 and M SrCl 2. After stirring the solution with the bentonite for 1 h, the onentrations of ations (sodium, alium, potassium, and magnesium) in the liquid phase were measured by AAS (atomi adsorption spetrometry, ice3300, Thermo Fisher Sientifi K.K., MA, USA) and ICP-AES (indutively oupled plasma atomi emission spetrometry, SPS7800, Seiko Instruments In., Chiba, Japan). As a result, 91.8% of the total exhangeable ations were oupied by Ca ions. Additionally, in order to estimate the deposition-rates onstants, it was important to measure the surfae area ontributing to the deposition of silii aid. However, we ould not onfirm if silii aid was deposited only on the surfae of the bentonite partiles, or it was also interalated in the bentonite interlayer. Therefore, for omparison, we used two kinds of speifi surfae areas of the Ca-type bentonite for the estimation of the apparent deposition-rate onstants. One is the overall speifi surfae area inluding the interlayer of Ca-type bentonite, whih was estimated at m 2 /g by the EGME (ethylene glyol mono-ethyl ether) method [18]. The other is the BET (Brunauer-Emmett-Teller) speifi surfae area exluding the interlayer, whose value was estimated at m 2 /g using nitrogen gas. 2.2 Experimental Method for the Silii Aid Deposition The experiments for the deposition of silii aid were performed by a bath reator shown in Fig. 3, sine it was important to appropriately estimate the

4 562 Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite behavior of silii aid in o-presene of a solid phase. The deposition experiments were performed by the following method based on the authors previous study [15]. At the beginning, the silii aid solution was prepared by diluting water glass (Na 2 SiO 3 solution) in a polypropylene vessel. Next, Ca-type bentonite as the solid phase was poured into the silii aid solution (ph > 10). Then, the deposition experiment started at the time when the ph of the solution was set to ph = 8 by adding a ph buffer and a HNO 3 solution. The total volume of the sample solution in these experiments was finally adjusted to 250 ml. The ph buffer was prepared by mixing THAM (tris (hydroxymethyl) aminomethane) and MES (2-morpholinoethanesulfoni aid, monohydrate). Table 1 summarizes the parameters of the deposition experiments. As shown in Fig. 3, the supply of nitrogen gas was ontinued to maintain a nitrogen atmosphere for the experiments, and nitrogen gas was one moistened by a gas-washing bottle for preventing evaporation from the silii aid solution. The deposition experiments lasted 6 h, until the deposition reation reahed approximately an apparent equilibrium state. Eah aliquot solution olleted at given time-intervals was separated into a liquid phase and a solid phase by entrifuging at 7,500 rpm for 2 min and filtrating with a 0.45-μm membrane filter. This timing of the filtration was defined as the time when the deposition reation ended. The supersaturation onentrations of silii aid in Table 1 were defined by the differene between the onentration of silii aid at the beginning of the deposition experiments and the solubility of amorphous silia. Considering the deposited material as amorphous silia, the solubility of silii aid at ph = 8 was 1.62 mm at 288 K and 3.49 mm at 323 K [20, 21]. Thus, for example, when the total onentration of silii aid at the beginning of the experiment was 5.62 mm at 288 K, the initial supersaturated onentration of silii aid was onsidered 4 mm. 2.3 Measurements and Analysis of Silii Aid We defined the state of silii aid as soluble silii aid (monomeri and oligomeri), olloidal silii aid, or deposited silii aid, as shown in Fig. 4. The respetive quantifiation methods were as follows. The onentration of soluble silii aid was quantified by the siliomolybdenum-yellow method. The onentration of olloidal silii aid was alulated by subtrating the onentration of soluble silii aid from the total onentration of silii aid in the sample solution, measured with ICP-AES. The silii aid deposited on Ca-type bentonite was alulated by subtrating the onentration of silii Fig. 3 Apparatus for deposition experiment. Table 1 Experimental onditions. Temperature (K) 288, 323 Initial supersaturated onentrations of silii aid (mm) 4, 6, 8, 10 Addition amount of solid sample (g) 2.5, 5.0, 10.0 ph 8 Pore size of membrane filter (μm) 0.45 Rate of stirring (rpm) 300 Fig. 4 Definition of silii aid speies.

5 Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite 563 aid in the sample solution (ICP-AES) from the initial onentration of silii aid (ICP-AES) at the beginning of the experiment. Finally, the apparent deposition-rate onstants k (m/s) were estimated from the above results and the speifi surfae areas (m 2 /g) alulated with the two methods (EGME and BET), the weight amount (g) of Ca-type bentonite, and the solution volume (250 ml = m 3 ). In addition, the partile size and the zeta potential of olloidal silii aid in the liquid phase were measured by the dynami light sattering method and the laser Doppler method (ELSZ-2 Plus, Otsuka Eletronis Co., Ltd.), respetively. The measurements were performed as quikly as possible after the olletion of the sample solution (within approximately 20 min), maintaining the temperature of the solution by a heat-retaining funtion attahed to this instrument. The total number of measurements for the partile size was 70, and that for the zeta potential was Results and Disussion 3.1 Dynami Behavior of Silii Aid in the Presene of Ca-Type Bentonite Figs. 5-7 show the results of the deposition experiments at 288, 298 or 323 K; for 2.5, 5, or 10 g of Ca-type bentonite; and for initial supersaturation onentrations of 4, 6, 8, and 10 mm. The horizontal axis is time (min), and the vertial axis f is the frations of soluble silii aid, olloidal silii aid, and deposition on Ca-type bentonite to the initial onentration of silii aid. As shown in these figures, (a) (b) () Fig. 5 Effets of amount of Ca-type bentonite on behavior of silii aid at 323 K (initial supersaturated onentration: 8 mm, amount of solid phase: (a) 2.5 g, (b) 5.0 g, () 10.0 g). (a) (b) () Fig. 6 Effets of initial supersaturated onentration on behavior of silii aid at 323 K (amount of Ca-type bentonite: 5.0 g, initial supersaturated onentration: (a) 4 mm, (b) 6 mm, and () 10 mm).

6 564 Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite (a) (b) Fig. 7 Effets of temperature on behavior of silii aid (amount of Ca-type bentonite: 5.0 g, initial supersaturated onentration: 8 mm, temperature: (a) 288 K and (b) 298 K). the deposition of silii aid on Ca-type bentonite inreased with the onentration of soluble silii aid under all experimental onditions. Espeially, drasti deposition of silii aid was observed within the first 20 min of the experiments. In addition, the amounts of silii aid deposited inreased with the amount of Ca-type bentonite, as shown in Fig. 5. Therefore, probably, hemially ative sites on the surfae of Ca-type bentonite mainly ontribute to the deposition of silii aid [22]. Conversely, signifiant amounts of olloidal silii aid were formed when the amount of Ca-type bentonite was low, and the initial onentration of silii aid was high, as shown in Figs. 5 and 6. However, onsidering that the underground ratio of solid phase to liquid phase is very large, the predominant reation in the bakfilled tunnels of the repository will not be based on the formation of olloidal silii aid, but on the deposition of soluble silii aid on the solid surfae. Besides, regarding the influene of temperature, the amount of silii aid deposited at 323 K was apparently larger than that at 288 K and 298 K, as shown in Figs. 5b and 7. Fig. 8 shows the partile size of olloidal silii aid in the liquid phase (8 mm of initial supersaturated onentration, and 5 g of Ca-type bentonite, 20 minutes later from the beginning of the experiments). The plots ( and ) in Fig. 8 show the mode of the partile size, and the error bars show the size range, where 90% or more of the partiles in the liquid phase are present. As shown in Fig. 8, the olloidal silii aid observed in Figs. 5-7 was around 100 nm in size. Moreover, the partile size was not affeted by the formation and depolymerization of olloidal silii aid at K, and within the time of these experiments. Fig. 9 shows the zeta potential of olloidal silii aid under the same onditions as Fig. 8, whih also shows similar values for 288 K and 323 K. That is, the partile size and the zeta potential of olloidal silii aid in the experiments were not affeted by temperature in the range of K. In general, the aggregation and preipitation of olloidal speies progress with absolute values lower than the zeta potential Fig. 8 Partile size (supersaturated silii aid onentration 8 mm, weight of solid phase 5.0 g).

7 Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite 565 Fig. 9 Zeta potential (supersaturated silii aid onentration 8 mm, weight of solid phase 5.0 g). shown in Fig. 9 (e.g., [5]). Therefore, olloidal silii aid left in the liquid remained stable at least for 6 hr in this study. However, the aggregation and preipitation of olloidal silii aid might ontribute to the logging effet of flow-paths for the long term at higher temperatures in the deep underground, even if olloidal silii aid temporarily forms in the flow paths. 3.2 Estimation of Apparent Deposition-Rate Constant Based on the results of the deposition experiments, we estimate the apparent deposition-rate onstant k (m/s). Here, as shown in Eqs. (1)-(4), the onentrations of soluble silii aid, olloidal silii aid, and deposited silii aid are expressed as frations of the initial onentration of silii aid, as shown below: f f (1) f ini (2) ini (3) d fd 1 f f ini f e e (4) ini where f,, and d are the onentrations of soluble silii aid, olloidal silii aid, deposition of silii aid (mm), respetively. e is the solubility of amorphous silia (mm); ini is the initial onentration of silii aid (mm); and f, f, f d, and f e are the frations of the onentration of soluble silii aid, olloidal silii aid, deposited silii aid, and solubility to initial onentration of silii aid. Then, the hanges in the onentrations of soluble silii aid and olloidal silii aid per unit time are desribed by Eqs. (5)-(7). df ( e) ( e) n ka f f k f f k m f (5) dt am A (6) V df ( e) n k f f k m f (7) dt where t is time (s), k is the apparent deposition-rate onstant (m/s), and A is the surfae area (1/m) based on the amount of Ca-type bentonite. Then, a is the speifi surfae area (m 2 /g), M is the amount of Ca-type bentonite, V is the liquid phase volume (m 3 ), + - k and k are the polymerization and the depolymerization rate-onstants of olloidal silii aid (1/s), and n and m refer to the reation order. The hange in the deposition of silii aid per unit time is summarized as Eq. (8) by using Eqs. (1), (5), and (7). dfd d(1 f f) df df ka( f fe ) (8) dt dt dt dt Finally, Eq. (9) is obtained by substituting f = 1 at t = 0 in Eq. (8). df (1 ) (9) d rini ka fe dt ini where r ini is the initial deposition rate (1/s), whih is obtained from the gradient (at t = 0) of a quadrati urve based on initial three plots of deposition data. Moreover, r ini is alulated through two methods in order to onsider the initial drasti hange in the deposition of silii aid. Hereinafter, the two methods are referred to as Case (A) and Case (B). Case (A) inludes the point f d = 0 at t = 0 for the alulation of r ini, and Case (B) exludes the point f d = 0 at t = 0.

8 566 Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite These r ini values are alulated onsidering the surfae area of the solid phase, as shown in Eq. (9). In this study, after the values of r ini are alulated for 2.5, 5, and 10 g Ca-type bentonite, the apparent deposition-rate onstants k (m/s) based on Eq. (9) are estimated from the orrelation of r ini and A defined by Eq. (6). For the speifi surfae area a, the EGME method gives m 2 /g, and the BET method give m 2 /g. Table 2 shows the apparent deposition-rate onstants k alulated from the experimental results and Eq. (9). The values of k for 298 K are alulated by referring to the results of author s previous study [15]. As shown in Table 2, the k values for m 2 /g speifi surfae area are one order of magnitude or more larger than those for m 2 /g. Conerning the differene between Case (A) and Case (B), most of the k values estimated for the ondition of Case (A) exeed those of Case (B), and the range of k for Case (B) is apparently wide in omparison with those for Case (A). As shown in the results of the deposition experiments (Figs. 5-7), the gradients of the deposition amount in the time range from 0 to 40 minutes for Case (A) seriously exeed those for Case (B) in the time range from 20 to 60 minutes. This means that the derease of the surfae area is aused by the deposition of silii aid relatively early in the experiments. In this present study, the apparent deposition-rate onstants are evaluated in onsideration of the surfae areas measured by the EGME method and the BET (N 2 gas) method. In the results, almost the same apparent deposition-rate onstants are obtained in the range of m/s for m 2 /g and m/s for m 2 /g. In partiular, these values of k for Case (A) onverge within the range of one order of magnitude. Besides, the initial supersaturation onentration of silii aid sarely affets the apparent deposition-rate onstants. Note that the rate-onstant in Eq. (9) is essentially independent of the initial supersaturation onentration. Furthermore, as shown in Table 2, the hange in temperature hardly influenes the apparent deposition-rate onstants. In order to ompare those quantitatively, the apparent ativation energy (kj/mol) based on the Arrhenius plot is estimated using the k values of Case (A) in Table 2. Table 3 shows the apparent ativation energy alulated from Table 2. As a result, these values of the apparent ativation energy are very low. These results suggest that the deposition of silii aid on Ca-type bentonite is limited not only by the hemial reation, as shown in Fig. 2, but also by some diffusion proesses, e.g., the diffusion of silii aid into the interlayer of Ca-type bentonite. Table 2 Apparent deposition-rate onstant k (m/s). Speifi surfae area m 2 /g (EGME method) m 2 /g (BET method) Temperature 288 K 298 K [15] 323 K 288 K 298 K [15] 323 K Case Initial supersaturated onentration of silii aid 4 mm 6 mm 8 mm 10 mm (A) (B) (A) (B) (A) (B) (A) (B) (A) (B) (A) (B)

9 Effets of Temperature on the Deposition Rate of Supersaturated Silii Aid on Ca-type Bentonite 567 Table 3 Apparent ativation energy (kj/mol). Initial supersaturated onentration (mm) Apparent ativation energy (kj/mol) Damköhler Number As a reasonable index, we use the Damköhler number for the estimation of the deposition effet of silii aid on Ca-type bentonite; the Damköhler number is a non-dimensional number referring to hemial proesses, whih takes into onsideration for example the deposition rate of silii aid to the mass-transport rate inluding the groundwater flow, and it is alulated by Eq. (10), as follows, xak 1 Da (10) u where, D a is the Damköhler number, x is the harateristi length (m), and u is the groundwater flow rate (m/s). In this study, the parameters are set as follows: x = 5 (m) assuming the diameter of tunnels [1], u = 0.5 (m/y) [1], and k = (m/s) as obtained in this study. The speifi surfae area per unit volume, a 1, is the produt of the apparent speifi gravity 0.6 g/m 3 and the speifi surfae area per unit mass, a = m 2 /g. These values of the parameters are seleted onservatively so that, when the Damköhler number beomes smaller, the deposition of silii aid is lower. As a result, the Damköhler number values are estimated as 10 2 or more for all apparent deposition-rate onstants. This means that the silii aid deposition on Ca-type bentonite is largely determined by the groundwater flow, and the flow-paths in bakfilled tunnels are effiiently logged. 4. Conlusions We estimated the apparent deposition-rate onstant k (m/s) for Ca-type bentonite quantitatively, onsidering the effet of temperature around the repository. In addition, the logging effet with the deposition of silii aid in the flow-paths was disussed by using the Damköhler number. In the deposition experiments, the deposition amount of silii aid inreased seemingly due to the growing of olloidal silii aid at high temperature experiments (323 K) in the ase of relatively smaller in amount of Ca-type bentonite. The estimated apparent deposition-rate onstant differed by more than one order of magnitude depending on the speifi surfae area ( m 2 /g by EGME method or m 2 /g by BET method) and/or on whether f d = 0 at t = 0 or not. Conservative values from these were about m/s in the experiments of speifi surfae area m 2 /g and Case (B) (exluding f d = 0). Additionally, the alulated apparent ativation energy to estimate the effets of temperature was suffiiently small. That is, the deposition reation of silii aid on Ca-type bentonite was not signifiantly affeted by temperature in the range of K. Finally, the Damköhler number for an apparent deposition-rate onstant of m/s exeeded This means that the deposition of silii aid is suffiiently fast in omparison with the groundwater flow rate around the repository. Therefore, these results suggest that the flow paths will be logged by the deposition of silii aid on Ca-type bentonite in a high temperature environment (323 K) suh as a deep underground environment, even if the flow paths are developed in bakfilled tunnels. As a future work, the temporal and spatial redistribution of silii aid in bakfilled tunnels should be larified by the numerial mass-transport simulation using the apparent deposition-rate onstant and a flow experiment simulating an atual environment. Aknowledgements This work supported by JSPS KAKENHI, Grant-in-Aid for Young Sientists (B) No. 16K18345 and Grant-in-Aid for JSPS Fellows No. 16J Referenes [1] JNC (Japan Nulear Cyle development institute) H12 Projet to Establish the Sientifi and Tehnial Basis for HLW Disposal in Japan, Supporting Report II,

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