Precise Permeability Measurement for High Strength and Ultra Low Permeability Concrete under Controlled Temperature

Abstract

High strength and ultra low permeability concrete (HSULPC) is being considered as a material used to package transuranic (TRU) waste for disposal in geological repositories. Therefore, information on the permeability of HSULPC is essential. Permeability tests need to be highly accurate to determine the hydraulic conductivity of HSULPC because of its ultralow permeability. In our study, we measured the permeability of HSULPC samples using the transient pulse method. The temperature of the concrete was finely controlled and held constant. The hydraulic conductivities were determined from the measurements to be around 10⁻¹³ to 10⁻¹² m/s for confining pressures between 2 and 10 MPa. The pore pressure was a constant 1 MPa. The results further showed that the permeability of HSULPC had a hysteretic dependence on the effective confining pressure. We found that the hydraulic conductivity of HSULPC is comparable to or less than that of intact Toki granite obtained from Gifu Prefecture in central Japan. It was also considered that the hydraulic conductivity of HSULPC stabilized at around 10⁻¹³ m/s after being buried and stressed. The high density and impermeability of HSULPC would enable it to effectively confine ¹⁴C radionuclides found in TRU waste.

 

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 69 (2020) 263–268. Acknowledgement is added.

1. Introduction

An important aspect of geological disposal of radioactive waste is the confining ability of the disposal system. For engineered or natural rock mass barriers, this ability mainly consists of retardation of migration and adsorption of radionuclides by clay (specifically bentonite).^1,2) Consequently, the material properties of clay and rock are essential. However, transuranic (TRU) waste often contains radionuclides, which are difficult for engineered or natural barriers to absorb such as ¹²⁹I and ¹⁴C. To ensure that the confining ability of the disposal system works well, an alternative disposal technique has been suggested for ¹²⁹I and ¹⁴C, which are the main radionuclides in the safety evaluation of radioactive waste disposal.³⁾ Long-term confinement of radioactive waste has been proposed as an alternative technique for ¹⁴C.⁴⁾

Figure 1 schematically illustrates one option for the long-term confinement of radionuclides using a cementitious material. This concept proposes using High Strength and Ultra Low Permeability Concrete (HSULPC) to retard ground water migration. In this concept, radioactive waste (TRU waste) is stored in a canister inside a metal box. HSULPC is then used to cover the metal box containing the canister with TRU waste. Due to the high confining ability of HSULPC, groundwater cannot penetrate the waste over the long term (i.e., 60,000 years), which corresponds to ten-times of the ¹⁴C half-life.⁴⁾ In this alternative concept, the permeability and time-dependent properties of crack propagation and closure should be investigated.

Fig. 1

Schematic illustration of alternative concept using HSULPC to radioactive waste packaging.⁵⁾

Nara et al.^5,6) studied the influence of the surrounding environment on the crack velocity by measuring subcritical crack growth in HSULPC. The crack velocity increased significantly in water.⁵⁾ Additionally, neither the temperature nor the humidity had a negligible influence on the crack velocity in HSULPC in air.⁶⁾ Fukuda et al.^7–9) examined crack closure in HSULPC kept in artificial sea water. Crack closures occurred significantly at the end of the sample according to X-ray CT observations.⁷⁾ Moreover, crack closure was easier for a smaller crack aperture.⁸⁾ Scanning electron photomicrographs revealed that the precipitation of calcium carbonates strongly affected the crack closure of HSULPC in water.⁹⁾

Various studies have investigated crack propagation and closure in HSULPC. The permeability properties of HSULPC have also been studied,^10,11) but permeability measurements are difficult using conventional methods.¹²⁾ Large scattering occurs because HSULPC is dense and has an extremely low permeability.^13,14) Therefore, HSULPC requires precise permeability measurements via a method applicable to low-permeability materials.

This study investigates the applicability of permeability measurements for HSULPC using the transient pulse method.¹⁵⁾ In particular, we conduct permeability measurements with the transient pulse method by minimizing the change in the temperature of the surrounding environment and evaluate the permeability precisely. Considering the influence of the external load applied on radioactive waste after backfilling, the influence of the pressure on the permeability of HSULPC is also investigated. In addition, the permeability of HSULPC is compared to that of granite, which is a typical low-permeability rock material, to understand the low-permeability property of HSULPC.

2. Sample Material

The sample material was HSULPC, which was prepared according to the guidelines published by the Concrete Committee of Japan Society of Civil Engineers.¹⁶⁾ Table 1 summarizes the composition. HSULPC used in this study was made by placing concrete in a mold, steam curing at a temperature of 363 K for 2 days, and subsequently holding for 2 days in air at a temperature of 293 K. The relative humidity during steam curing was 99%.

Table 1 Composition of HSULPC.⁵⁾

For the measurements, we used the HSULPC sample after holding under ambient air conditions. It had P-wave velocities in the three orthogonal directions of 4.98, 5.06, and 5.08 km/s. The uniaxial compressive strength and the tensile strength determined by the diametral compression test were 203 MPa and 10.9 MPa, respectively.⁵⁾

Figure 2 shows the cylindrical specimen used in the permeability measurements. Its diameter and length were 50 mm and 25 mm, respectively. Prior to the permeability measurements, the specimen was saturated by placing in distilled water under vacuum conditions for several days.

Fig. 2

Photo of cylindrical specimen of HSULPC for permeability test. The diameter and height of the specimen are 50 and 25 mm, respectively.

3. Experimental Method

3.1 Permeability test system

Figure 3 shows the permeability test system. It consisted of a temperature-controlled chamber with triple thermal insulation walls and an air conditioner set in the outermost place of the temperature-controlled chamber.^17,18) The pressure vessel, including the specimen, was set in the innermost chamber, which lacked heat and light sources. Figure 4 shows a photo of the inside of the temperature-controlled chamber. The temperature change around the pressure vessel was around 0.1°C during the permeability measurements.

Fig. 3

Schematics of permeability test system. (IC: Triple insulated chamber, AC: Air conditioner, RT1-3: Resistance thermometer 1 to 3, BR: Barometer, PT: Pressure transducer, DPT: Differential pressure transducer, PV: Pressure vessel, ER: Extra reservoir, UL: Upstream line, DL: Downstream line, SV: Separation valve, PV: Pressure pulse valve, EP: Evacuating port, AP: Air discharge port, WP: Water supply port, SC: Specimen, EC: End caps, HT: Heat shrinkable tube, DP: Double plunger pump, SP: Syringe pump, CU: Controlling unit for syringe pumps, LG: Data logger, PC: Laptop computer)

Fig. 4

Photo of triple insulated chamber.

3.2 Experimental procedure

Although the permeability test system could support different testing methods for the measurements, this study adopted the transient pulse method because HSULPC is a low-permeability material. A previous study demonstrated that the transient pulse method is suitable for measurements of low-permeability materials.¹⁵⁾

The experimental procedure of the transient pulse method in this study involved the following steps:

1. (1)    The specimen saturated by distilled water was set in the pressure vessel between two end pieces, which provided the flow of the pore fluid, and covered it with a jacket. Here, the jacket was a heat shrink tube. This jacket prevented the flow of the pressurizing medium into the specimen when a confining pressure was applied. Then the specimen placed between the end pieces was set in the specimen closure connected with the line of the pore fluid.
2. (2)    All the lines of the water flow and the extra reservoir were filled with distilled water. A vacuum pump removed the air, and a syringe pump applied water to the lines of water flow while maintaining a constant pressure.
3. (3)    During a measurement, the temperature around the pressure vessel in the innermost chamber was constant. Additionally, we checked for leaks by monitoring the amount of water flow from the syringe pump.
4. (4)    After confirming that the system was not leaking, the confining and pore pressures were set. The temperature fluctuation was less than 0.1°C. The confining pressure was set in the range of 2–10 MPa. The pore pressure was 1 MPa. After setting the confining pressure and pore pressure to the predetermined values for each measurement, we waited for several hours until the pressure values settled and remained constant. In this study, the confining pressure was initially increased from 2 MPa to 10 MPa. Then it was decreased from 10 MPa to 2 MPa to investigate the hysteresis of the results of the permeability measurements.
5. (5)    The separation valve was closed to divide the upstream and downstream lines of water flow. Then the fluctuation in the pore pressure difference of the upstream and downstream was monitored to detect water leakage. If leaks were detected, we returned to step (3). After closing the valve, several hours were necessary for the water head distribution in the specimen to become uniform.
6. (6)    If there was not an observable fluctuation in the upstream and downstream pore pressure difference, a pore pressure pulse (25–100 kPa) was applied to the upstream side of the specimen. The pore pressure difference decreased with elapsed time after applying the pore pressure pulse. The measurement was finished once the pore pressure difference approached the value of the difference before applying the pore pressure pulse. For confining pressures of 2, 3, and 4 MPa, each measurement using the transient pulse method was repeated twice. For other confining pressures, each measurement was conducted once.
7. (7)    As suggested in Refs. 15), 19), the nonlinear least square method by applying the solution of the transient pulse method was used to evaluate the hydraulic conductivity. The temporal change of the pore pressure difference was obtained in the measurement according to   
   \begin{equation} \frac{\Delta h(t)}{H} = \exp \left\{- \frac{KAt}{l} \left(\frac{1}{S_{u}} + \frac{1}{S_{d}} \right) \right\} \end{equation} (1)
   • where t [s] is the time after applying the pore pressure pulse, Δh(t) [m] is the difference of the upstream and downstream water heads, H [m] is the water head pulse (the initial difference between the upstream and downstream water heads), l [m] is the specimen length, A [m²] is the cross-sectional area of the specimen, K [m/s] is the hydraulic conductivity, S_u [m²] is the compressional storage of the upstream reservoir, and S_d [m²] is the compressional storage of the downstream reservoir. S_u and S_d were determined by the method proposed by Zhang et al.²⁰⁾ In this study, both S_u and S_d were 8.5 × 10⁻¹⁰ m².

4. Results

For the transient pulse permeability test, the hydraulic conductivity was evaluated using the temporal change of the pore pressure difference between the upstream side and the downstream side of the specimen. The data from the differential pressure indicated a change in the temperature of the surrounding environment. In addition, the difference of the thermal expansion or contraction in the reservoirs and water flow lines between the upstream and downstream also influenced the pore pressure difference data. Consequently, the data of the temporal change indicated disturbances in the pore pressure difference. Figure 5 shows an example of the temporal change of the pore pressure difference, where the confining pressure, pore pressure, and pore pressure pulse are 2 MPa, 1 MPa, and 28 kPa, respectively.

Fig. 5

Decline curve of differential pressure during transient pulse permeability test.

Figure 6 shows the temperature changes in the outermost and the innermost places in the temperature-controlled chamber during a measurement. For the temperature in the outermost place (measured with the resistance thermometer “RT1”), the influence of the air conditioner appeared as a periodic change in the temperature. On the other hand, the change of the temperature in the innermost place (measured with the resistance thermometer “RT3”) was around 0.1°C, indicating highly accurate permeability measurements and evaluation of the hydraulic conductivity in this study.¹⁷⁾

Fig. 6

Temperature variations in air-conditioned room during permeability test corresponding to Fig. 3.

Figure 7 shows the hydraulic conductivity of HSULPC evaluated under various effective confining pressures. The hydraulic conductivity of Toki granite is shown for comparison. HSULPC has a hydraulic conductivity value on the order of 10⁻¹³–10⁻¹² m/s. The hydraulic conductivity of HSULPC decreased as the effective confining pressure increased during the loading confining pressure procedure. The hydraulic conductivity measured for HSULPC showed a hysteresis because the procedure to unload the confining pressure was lower than that of the loading confining pressure.

Fig. 7

Hydraulic conductivities of HSULPC comparing to those of intact Toki granite.

5. Discussion

Various studies have investigated crack propagation and closure using HSULPC.^5–9) Since the crack propagation in HSULPC accelerates in water, the formation of water flow pathways by the crack propagation must be avoided if the radioactive waste repository is submerged by ground water. In contrast, since crack sealing by mineral precipitations has been observed in aqueous conditions, HSULPC is a suitable material to confine radioactive waste. Therefore, the permeability measurement of HSULPC is meaningful.

In general, a permeability test using a low permeability material does not provide accurate measurement results due to the long measurement time and significant influence of the temperature change in the surrounding environment. The transient pulse method typically has a shorter measurement time than other methods, minimizing the impact of the temperature change. Using the transient pulse method to conduct permeability measurements under conditions where the temperature change is small, the hydraulic conductivity can be evaluated with a high accuracy. The change of the temperature at the innermost place in the temperature-controlled chamber is around 0.1°C (Fig. 6). By conducting the permeability test with the transient pulse method with a small temperature change, the hydraulic conductivity of HSULPC is measured successfully.

Figure 7 shows the hydraulic conductivities of granite as well as those of HSULPC. The hydraulic conductivities of granite are those previously reported for an intact sample of Toki granite by Nara et al.²¹⁾ The hydraulic conductivity for HSULPC is similar or less than that for intact Toki granite, indicating that HSULPC is a dense and low permeability material, which can effectively confine radionuclides.

The existence of cracks and pores remarkably affects the permeability²²⁾ because they provide a pathway for water flow. Introducing an open crack in a dense material increases the hydraulic conductivity significantly,^23–25) whereas closing a crack by pressure or mineral precipitation decreases the hydraulic conductivity.^26–30) Although the hydraulic conductivity of HSULPC is quite low, it is important to investigate the water flow pathway in HSULPC.

We conducted microscopic observations of a thin section of HSULPC. HSULPC contains a few microcracks and isolated pores. Figure 8 shows a photomicrograph with a microcrack in the central part. The aperture and aspect ratio of this microcrack are 1 µm and 10⁻³, respectively.

Fig. 8

Photomicrograph of HSULPC showing microcracks.

Generally, a higher pressure is necessary to close cracks and pores mechanically as the aspect ratio becomes higher.³¹⁾ However, materials containing numerous pores and a high aspect ratio do not exhibit a detectable change in the permeability even as the applied pressure is increased.^32,33) In this study, HSULPC has a decreased hydraulic conductivity due to the closure of the microcrack with a low aspect ratio (Fig. 8).

HSULPC shows a similar hysteresis as that for rock materials.³⁴⁾ After placing a TRU waste package using HSULPC, backfilling an underground repository of radioactive waste was conducted using buffer and backfilled materials. In this case, the following factors should be considered: the pore pressure increases due to the increased groundwater level, the overburden pressure due to backfilling, the swelling pressure generated from buffer materials and the backfill materials. To assess these factors an external pressure was applied on the TRU waste package using HSULPC place in the underground repository for radioactive waste. The hydraulic conductivity of HSULPC is on the order of 10⁻¹³ m/s due to the effect of the external pressure.

6. Conclusion

The transient pulse method using HSULPC realizes a highly precise permeability test when the confining pressure is 2–10 MPa and the pore pressure is 1 MPa. The hydraulic conductivity of HSULPC is on the order of 10⁻¹³–10⁻¹² m/s. This value is similar or lower than that of intact granite. The hydraulic conductivity measured in HSULPC shows hysteresis. The hydraulic conductivity decreases as the confining pressure increases (Fig. 7), although it is a low value even though the confining pressure decreases.

HSULPC is suitable for the long-term confinement of ¹⁴C due to its density and low permeability. After placing a TRU waste package using HSULPC in the repository, backfilling of the underground repository of radioactive waste was conducted using buffer and backfilled materials. Assuming that the pore pressure increases with the groundwater level, the overburden pressure is due to backfilling, the swelling pressure is generated from buffer materials, and the backfill materials are known, applying an external pressure on the TRU waste package using HSULPC results in the hydraulic conductivity of HSULPC around the order of 10⁻¹³ m/s.

Acknowledgment

We appreciate the support from Supporting Program for Interaction-based Initiative Team Studies (SPIRITS) of Kyoto University for English proofreading.

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