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In situ borehole heater test at the KAERI Underground Research Tunnel in granite
Annals of Nuclear Energy 62 (2013) 526–535
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Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Technical note
In situ borehole heater test at the KAERI Underground Research Tunnel in granite Sangki Kwon a,⇑, Changsoo Lee b, Chan-Hoon Yoon b, Won-Jin Cho b a b
Department of Energy Resources Engineering, Inha University, Republic of Korea Korea Atomic Energy Research Institute, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 July 2012 Received in revised form 11 July 2013 Accepted 14 July 2013 Available online 13 September 2013 Keywords: Borehole heater test High-level waste KURT Thermal–mechanical coupling Rock temperature
a b s t r a c t An in situ borehole heater test was carried out in an underground research tunnel at a shallow depth in granite. During the test, the heater temperature was increased to 90 °C to simulate the thermo-mechanical behavior of crystalline rock under normal underground high-level radioactive waste repository conditions. The air, wall and rock temperatures were measured over a period of about four years. At the end of the test, the heater temperature was increased to 118 °C to simulate abnormal overheating conditions. The peak temperatures at the observation holes located at 0.3 m and 0.6 m from the heater hole were approximately 50 °C and 37 °C, respectively. The temperature measurements allowed observations of the effects of rock joints and heat convection through the tunnel wall on the rock temperature distribution. When the power was shut down, the rock temperatures and stress returned rapidly to the original rock temperature. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In Korea, more than 11,000 tons of spent fuel have already been accumulated from the operation of nuclear power plants (NPP) since 1978. The 22 currently operating NPPs, which generate approximately 35% of the total electricity, produce approximately 700 tons of spent fuel every year. An additional six NPPs are currently under construction. The safe and reliable management of spent fuel, which is stored temporarily at the reactor sites, is a critical issue for the sustainable utilization of nuclear energy in Korea. Based on a R&D program supported by the government since 1997, a reference disposal system, KRS (Korea Reference Disposal System), has been suggested for the permanent disposal of spent fuel in an underground repository in a crystalline rock mass (Lee et al., 2006). In 2006, an underground research tunnel, KURT (KAREI Underground Research Tunnel), was constructed in KAERI (Korea Atomic Energy Research Institute) to validate the disposal system. During and after the construction, several in situ tests including a borehole heater test (BHT), EDZ (Excavation Disturbed Zone) test, tests for examining groundwater flow, geochemistry, and the migration of colloids through fractures, were carried out (Kwon et al., 2009; Ji et al., 2011; Ryu et al., 2012; Kim et al., 2009). One of the unique characteristics of high-level radioactive waste (HLW) is decay heat, which can affect the performance of natural and engineered barriers in the near-field of an ⇑ Corresponding authors. E-mail address: [email protected] (S. Kwon). 0306-4549/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2013.07.017
underground repository by affecting the thermal-hydro-mechanical-chemical (THMC) coupling behavior of the barriers. An accurate prediction of the THMC coupling behavior in the near-field is essential for the safe design of a disposal system as well as for providing a reliable performance assessment of underground radioactive waste repository, around which severe radiation, heat and pressure play an active role. To understand the complex coupling phenomenon in repository conditions, many in situ and laboratory tests have been carried out in many countries considering HLW disposal in geological formations. Several assessments have been made over the past few decades. These include in situ heater tests at Stripa mine and Aspo in Sweden, TSI at the Waste Isolation Pilot Plant (WIPP, a full scale heater at Hanford), heater tests with different scales at Yucca mountain in USA, an in situ THM test at Kamaishi mine in Japan, FEBEX at Grimsel and a heater experiment at Mont Terri in Switzerland, THE at URL in Canada, etc. (Schneefub et al., 1989; Hocking et al., 1990; Yow and Hunt, 2002; Jockwer et al., 2007; Berchenko et al., 2004; Andersson, 2007). Table 1 lists the characteristics of the in situ heater tests in different rock types, heating conditions and scales. Because the geological condition, rock type, waste type and characteristics, and disposal system differ around the world, it was recommended that an in situ heater test be carried out in Korea. The BHT in KURT was designed to understand the thermal and mechanical behavior in a disturbed rock zone, in which heat convection through the tunnel wall can affect the temperature distribution in the near-field rock mass. Compared to previous heater tests in other countries, the BHT in KURT has the following characteristics:
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(a) The influence of joint and anisotropic rock properties on the temperature distribution can be investigated. (b) The EDZ effect on the rock temperature distribution can be analyzed. (c) The influence of air flow by natural and forced ventilation on temperature variation can be assessed. In this paper, the status of the BHT and some results from the test are discussed.
deep in a crystalline rock mass, such as a granite body (Lee et al., 2006). The PWR spent fuel with a burn up of 45,000 MWd/tHM is considered a reference type because it comprises approximately 64%. For CANDU, spent fuel with a burn up of 7500 MWd/tHM is the reference type. For the reference PWR and CANDU spent fuel, the decay heat can be calculated using the following equations (Choi et al., 1997): 1
PðtÞ ¼ 852:34e0:2642þ0:130889t
t < 30 years for PWR
ðWatt=tonÞ
ð1Þ PðtÞ ¼ 14546 t 0:76204
2. Korea Reference Disposal System (KRS) According to the KRS, 36,000 tons of Pressurized Water Reactor (PWR) and CANada Deuterium Uranium reactor (CANDU) spent fuel generated from the 28 NPPs during their life time will be disposed of in an underground repository several hundred meters
t > 30 years for PWR
ðWatt=tonÞ
ð2Þ PðtÞ ¼ 1456:7 t 0:67667
ðWatt=tonÞ for CANDU
where t is the years elapsed after discharge from a reactor.
Table 1 In situ experiments for examining THMC coupling in rock. Test name (site, country)
Rock type
Depth (m)
Scale (size)
Coupling
Year
Heating condition
Crystalline rock Electric heater test (Stripa mine, Sweden) Buffer mass test (Stripa mine, Sweden)
Granite Granite
330 340
Borehole (U = 324 mm) Test pit (U = 750 mm)
THM THM
Casks with 5 kw and 3.6 kw 1.8 kW (6 holes)
Engineered barrier experiment (Kamaishi mine, Japan) FEBEX (Grimsel, Switzerland) THE (URL, Canada)
Granodiorite
260
Test pit (U = 1.7 m)
THM
Granite Granite
450 420
Drift scale (U = 2.27 m) Borehole (U = 96 mm)
THM TH
Heated Failure test (URL, Canada)
Granite
420
Borehole
TM
Buffer/container experiment (URL, Canada)
Granite
240
Test pit (U = 1.24 m)
THM
APSE (Aspo, Sweden)
Granite
450
Test pit (U = 1.75 m)
TM
TBT (Aspo, Sweden) SFT (Nevada test site, USA)
420 420 46
Test pit (U = 1.8) Drift scale (6.1m 4.6 m) Test pit (U = 900 mm)
TM TM
In site heater test (NSTF, USA)
Granite Quartz monzonite Basalt
Heater test (Grimsel, Switzerland)
Granite
450
Borehole (U = 300 mm)
TM
1978 1980– 1985 1988– 1998 19941995– 1998 1993– 1996 1991– 1993 2002– 2006 20031980– 1982 1980– 1983 1986– 1987
Tuff SHT (Yucca Mountain, USA)
Tuff
250
Borehole (U = 75 mm)
THMC
DST (Yucca Mountain, USA)
Tuff
250
Drift scale (5 m drift)
THMC
LBT (Yucca Mountain, USA)
Tuff
0
THMC
PEBSFT (G-tunnel, USA)
Tuff
420
Large block (3 3 4.5 m) Borehole (U = 30.5 cm)
Salt TSI (WIPP, USA)
Rock salt
650
Drift scale (5.5 m)
TM
TSDE (Asse salt mine, Germany)
Rock salt
800
Drift scale (3.5 m 4.5m)
THM
Clay HE-D test (Mont Terri, Switzerland)
Clay
300
Borehole (U = 300 mm)
THM
TER (Meuse, France),
Claystone
490
Borehole (U = 101 mm)
THM
BACCHUS (Hades URF/Swiszerland)
Clay
223
Borehole (U = 60 mm)
TM
ATLAS (Hades URF/Swiszerland)
Clay
223
Borehole (U = 190 mm)
THM
CERBERUS (Hades URF/Swiszerland)
Clay
223
Borehole
THMR
CACTUS (Hades URF/Swiszerland)
Clay
223
Borehole (U = 500 mm)
THM
PRACLAY(Hades URF/Swiszerland)
Clay
223
Drift scale (U = 2.5 m)
THMC
THM
THM
130 w/m2 2 of U = 0.9 m, 2 kW heater 0.5 kW 85 °C 1.2 kW 4 heaters (0–400 w each) 1.5 kW Total 20kw (2 kW 6 + 0.4 kW 20) 9 kW 4 kW/m (2 heaters)
1996– 1998 1997– 2005 1997– 1998 1988– 1989
5 m long 4 kW heater
1984– 1988 1990– 1999
18 W/m2
2004– 2005 2005– 2006 1988– 1990 1990– 1997 1989– 1995 1990– 1995 2009– 2019
210 kW from 50 wing heaters) 2.44 m long 5 heater holes. Total 2.25 kW 3 m long 3.3 kW
3 casks with 6.4 kW each
1.95 kW 0.57 kW 0.5 kW 0.9 kW 60
Co source
3.8 m long 4 kW 95 °C
ð3Þ
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The spent fuel will be inserted into corrosion resistant metal canisters and transported to the underground disposal area. The canisters will be placed in deposition holes with compacted bentonite. Fig. 1 shows the disposal concept with vertical deposition holes. The space from the tunnel floor to the top of the canister is approximately 2–2.5 m and filled with compacted bentonite blocks. The spacing of the disposal tunnel and deposition hole, buffer thickness and position of the canister in the deposition hole needs to be determined carefully considering the characteristics of the waste and the barriers to satisfy the mechanical and thermal criteria. One of the most critical criteria is that the peak temperature of the bentonite buffer should be less than 100 °C. For the development of a reference disposal system, threedimensional computer simulations were performed using the reference properties of the buffer, backfill, rock and canister. From the simulations, the tunnel and deposition hole spacings were 40 m and 6 m, respectively. Sensitivity analysis was carried out to examine the relative influence of the following seven design parameters on the peak rock and buffer temperatures: (a) hole spacing, (b) tunnel spacing, (c) thermal conductivity of the buffer, (d) thermal conductivity of backfill, (e) cooling period of spent fuel, (f) thermal conductivity of an EDZ, which was assumed to be 1 m around the tunnel, and (g) thermal conductivity of rock (Kwon and Cho, 2009). The required adjustment for each parameter to reduce the peak rock and buffer temperatures by 1 °C could be determined, as listed in Table 2. In the case of the cooling period of spent fuel, approximately 1 year is needed for a 1 °C decrease in the peak buffer temperature. The thermal conductivity is affected by fracture, which can decrease the thermal conductivity under dry conditions. Under saturated conditions, which is expected in deep underground areas, the influence of fractures on the thermal conductivity would be different. When the thermal conductivity of an EDZ is assumed to be 1 order of magnitude higher than that of intact rock, the peak buffer temperature can be decreased by 3 °C compared to the case without considering an EDZ (Kwon and Cho, 2008). Because the heat generating canister will be placed near the tunnel surface, the effect of the disturbed rock zone around the tunnel and the convectional heat flow through the tunnel surface will affect the temperature distribution. This means
that an understanding of the thermal behavior of the near field rock with a disturbance is important for making an accurate prediction of the THM behavior.
3. KAERI Underground Research Tunnel (KURT) The KAERI Underground Research Tunnel, KURT, is a small scale underground research laboratory for validating the reference HLW disposal system, KRS. KURT is located at a granite body, which is believed to be the most promising host rock for the permanent disposal of HLW in Korea. The 6 m 6 m 255 m tunnel was constructed in 2005–2006 by controlled blasting. The maximum depth of KURT with a 180 m long access tunnel and two research modules at the end of the access tunnel is approximately 90 m. Extensive site characterization was accomplished to select the KURT design (Kwon et al., 2006). Fig. 2 shows the layout of KURT and the BHT area in research module I. For the easy installation and management of the instruments and sensors, the heater hole was drilled in the tunnel wall at the BHT area. The BHT area was chosen for the following reasons: (a) The BHT at KURT focuses on the mechanical and thermal behavior. Research module I is relatively dry. Hence, it is good for focusing on the mechanical and thermal behavior of a rock mass. In contrast, research module II, which is closer to a major water conducting fracture zone, as shown in Fig. 2, can be used to examine situations where groundwater is involved. (b) Because the rock at the BHT area contains countable joints, it would be possible to examine the effect of a single fracture or simple fracture network on the thermal-and mechanical behavior. (c) The test area is located at the end of the research module to minimize the effects of the ventilation effect on the temperature distribution. With a fabricated wall, a 12 m long, 6.3 m high and 6.9 m wide test area can be isolated from the ventilation system when the ventilation ducts connected to the area are closed.
Fig. 1. Disposal concept for the high-level radioactive waste.
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S. Kwon et al. / Annals of Nuclear Energy 62 (2013) 526–535 Table 2 Change required to decrease the buffer and rock temperatures by 1 °C. Parameter Unit
Hole spacing M
Tunnel spacing M
Cooling time Year
Rock T.C. w/m K
Buffer T.C. w/m K
Backfill T.C. w/m K
EDZ T.C. w/m K
Reference value Buffer temperature Rock temperature
6 0.15 0.14
40 2.28 1.30
40 1.09 2.26
2.5 0.10 0.10
1.0 0.05 2.82
2 1.20 1.97
3.5 0.71 1.20
Fig. 2. Layout of the KURT and location of the BHT area.
4. BHT at KURT 4.1. Rock properties in the BHT area The properties of rock and rock mass were measured from in situ and laboratory tests using the rock cores from boreholes drilled in KURT. The thermal properties, such as thermal conductivity, thermal expansion coefficient and specific heat capacity of the KURT rock, were measured under a range of conditions. For example, Cho and Kwon (2010) reported the change in thermal conductivity according to the porosity and saturation. From extensive tests, the depth effect on the rock properties could be observed. Because of the weathering effect near the surface, the mechanical, hydraulic and thermal properties showed different values compared to those in deeper rock. The thermal conductivity was predicted to increase with depth and stabilize at 3.2 W/m K below 50 m (Kwon et al., 2011). Table 3 lists the rock properties measured at a BHT area. Because the BHT area is located at the end of the access tunnel, the rock properties, such as UCS, tensile strength and Young’s modulus at the area, are relatively higher than the average of KURT rock. The RMR in the test area was determined to be 65 during tunnel construction. The rock deformation modulus was estimated to be 30 GPa from the following empirical equation (Bieniawski, 1978):
Em ¼ 2 RMR 100
ð4Þ
The deformation rock modulus was approximately 60% of the elastic modulus of intact rock in the test area. The joint distribution was measured at three open rock surfaces in the vicinity of the test area. Fig. 3 shows the joints visible on the rock surface of the BHT
wall, dead end of research module I, and the right wall. Steep joints (70°) were dominant. In the case of the BHT wall, the major joint set had a dip of 76 and a dip direction of 236°. The change in the mechanical properties in an EDZ was measured using KURT rocks. From the laboratory tests for measuring the elastic modulus, rock strengths, and P and S wave velocities, the EDZ size was estimated to be approximately 1.1–1.5 m, whereas the Goodman jack test showed that the deformation modules had been disturbed in the 1.5–2 m deep zone. According to the RQDs of the rock cores from the EDZ study area, the mean RQD in the 0–2 m range, where the blasting impact was significant, was 67, whereas it was 81 at a deeper range. The elastic modulus in the EDZ was decreased by approximately 50% (Kwon et al., 2009). 4.2. BHT procedure The preparation of BHT was started immediately after the completion of the KURT construction in November, 2006. A 3.2 m long horizontal heater hole was drilled with a diameter of 110 mm at approximately 1.5 m above the floor. A 2 m long heater with a maximum power of 5 kW was designed and manufactured with aluminum. The heater was installed tightly at the end of the heater hole. The remaining part of heater hole was left as an open space. The influence of open space on the rock temperature variation was minimized by installing a plug at the mouth of the hole. The shallow installation of the heater was to allow an effective examination of the EDZ and ventilation effect. Fifteen observations holes, 38 mm in diameter, surrounding the heater hole were drilled, as
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Table 3 Mechanical and thermal properties of the rock in the heater test area. Properties
Mean
Min
Max
Comments
Mechanical properties Specific gravity Porosity (%) UCS (kg/cm2) Tensile strength (kg/cm2) E (105 kg/cm2) Poisson’s ratio P wave (m/s) S wave (m/s)
2.649 0.778 957.0 102.0 5.294 0.204 4673 2444
2.55 0.07 580 70 2.93 0.14 4120 2200
2.74 1.87 1580 140 10.08 0.35 5230 2730
100% of KURT average 88% of KURT average 134% of KURT average 105% of KURT average 120% of KURT average Same as KURT average 108% of KURT average 98% of KURT average
10.36 12.08 14.51 17.37
KURT average KURT average 20–50 °C 50–100 °C 100–150 °C 150–200 °C Natural convection Forced convection
Thermal properties Thermal conductivity (W/m K) Thermal expansion (microstrain/°C)
Heat convection factor (W/m2 K)
2.9 in dry condition 3.1 in saturated condition 6.770 8.452 9.979 11.757 4.73 7.46
2.03 3.83 5.94 8.43
Fig. 3. Rock joints visible at BHT area.
Fig. 4. Layout of the boreholes drilled for the BHT.
shown in Fig. 4a and b shows the configuration of the heater hole. Table 4 lists the dimensions and direction of the boreholes and the number of sensors in each hole. The RQD of each observation
hole was measured from rock cores and were varied from 68 to 96, as listed in Table 4. In the ‘A’ type observation holes, which were located at approximately 0.6 m from the heater hole, 9 temperature sensors were installed at 0.5–1 m intervals. In the ‘B’ type
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S. Kwon et al. / Annals of Nuclear Energy 62 (2013) 526–535 Table 4 List of the test holes and sensors installed in the observation holes. Type
Hole name
Length (m)
Size (mm)
Direction
Number of sensors
Average RQD
Heater Temperature (vibration wire type)
H A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 RS1 RS2 RS3 RS4
3.2 5.1 5.1 5.1 5.1 5.1 5.1 5.0 5.1 5.7 5 5.1 5.1 5.1 5.1 5.1
110 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38
Horizontal Horizontal Horizontal Horizontal Horizontal Horizontal Horizontal Horizontal Horizontal 9° down 26° inside 12° up 26° inside Horizontal Horizontal Horizontal Horizontal Horizontal
6 thermocouples 9 9 9 9 12 12 12 12 12 3 8 5 5 5 5
91.6 87.2 86 95.7 89.8 91.4 89.4 95.8 91.4 67.8 89.4 90.4 93.9 87.8 96.1
Stressmeter (vibration wire type)
Table 5 List of the items measured in the heater test.
Table 6 List of the history of the BHT.
Properties
Items
Comments
Condition (duration)
Date
Heater temperature (°C)
Temperatures
Air temperature in the BHT room Air temperature outside of the BHT room Air temperature outside of the tunnel Wall temperature in the access tunnel Wall temperature around the heater Wall temperature in the opposite wall Heater temperature Rock temperature
1 point 1 point 1 point 18 points 13 points 1 point 6 points 117 points
Installation of heater Preliminary test (12 days)
Humidity in the BHT room Humidity outside of the BHT room Heater power Rock stress
1 point 1 point
2006-11 2007-08-30 2007-08-30 2007-08-31 2007-09-11 2007-9–2007-12 2007-12-03 2007-12-07 2007-12-11 2007-12-18 2007-12-26 2008-01-03 2008-01-11 2008-01-18 2008-01-22 2008-02-01 2008-02-12 2008-02-28 2008-03-28 2008-05-02 2008-06-11 2010-09-28 2010-12-16 2011-01-13 2011-02-10 2011-02-28 2011-08-08 2011-12-1
40 30 63 70 Power off Power off 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 Power off End of test
Humidity Power Stress
System upgrade (3 months) Main test (34 months)
20 points
holes, 12 temperature sensors were installed 0.3 m from the heater hole at 0.3–1 m intervals. Two other temperature observation boreholes, C1 and C2, were drilled outside of the test area with an upward and downward slope, respectively. The two holes had a 26° inclination angle to pass over the center of the heater. Four holes, RS1-RS4, for installing the stress meters were located at approximately 0.5 m from the heater hole. In each RS hole, 5 stress meters were installed at 1 m intervals. The vibrating wire type temperature and stress sensors were chosen for the test because durable sensor operation is preferred for a long term in situ test. After installing the heater and sensors, a preliminary test was run to check the performance of the heater and data logging system for two weeks in September, 2007. In the test, the heater temperature was increased rapidly to 70 °C. The main test was started in December, 2007 after allowing 3 months for the heated rock to return to its original condition. In the main test, the heater temperature was increased steadily to the target temperature of 90 °C in June, 2008. A target temperature of 90 °C was chosen using the thermal criteria of the buffer, which should be maintained under 100 °C. The heater power was controlled to maintain the heater temperature until September, 2010. In September, 2010, the heater temperature was increased to 118 °C to examine the response of the rock under abnormal overheating conditions. In August, 2011, the heater power was shut down and a cooling test was performed for approximately 4 months. During the tests, different items listed in Table 5 were measured. The measurements were recorded using an automatic data logging system as well as manual instruments in May and June, 2008, November, 2008, August, 2009, July, 2010, and May, 2011. Table 6 lists the duration of the test and the change in heater temperature during the BHT.
Overheating test (10 months)
Cooling test (4 months)
Fig. 5. Air and wall temperature change with time.
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tunnel. When heating started, the air and wall temperatures increased steadily and were essentially stabilized from 2009. Although the BHT area was blocked with the fabricated wall in July, 2007, it was still possible to observe slight seasonal temperature variations. The peak air and wall temperatures were 20.8 °C and 23.2 °C, respectively. A close relationship between the air and wall temperatures was observed. As shown in Fig. 6, the rock surface temperatures measured on the BHT wall and opposite wall increased linearly with increasing air temperature. Owing to heat generation from the heater, the difference between the BHT wall temperature and opposite wall temperature increased with increasing air temperature. The air temperature appeared to be affected by the BHT wall temperature. The opposite wall temperature, which was always lower than the air temperature, was affected by the change in air temperature. Fig. 6. Relationship between the air temperature and rock surface temperature measured on the BHT wall and opposite wall.
4.3. Measurements from the testa 4.3.1. Air and tunnel wall temperature outside the BHT area In the region where KURT is located, the annual average air temperature over the last 3 years was 13.2 °C. The monthly average temperature was high in August and low in January. The tunnel wall temperature variation was recorded periodically along the tunnel outside of the BHT area using a non-contact infrared laser thermometer with a 0.1 °C resolution. The significant fluctuations of the annual wall temperature near the tunnel entrance were reduced significantly to approximately ±2 °C from an average of approximately 14 °C (Kwon et al., 2011). 4.3.2. Air and tunnel wall temperature inside of BHT area Fig. 5 shows the air temperatures inside the BHT area and outside KURT, as well as the tunnel wall temperature measured around the heater hole. The outside air temperature change was estimated from the measured data in 2006 and 2007. The inside air and wall temperatures were measured from November, 2006. As a reference, the expected air temperatures for the case without heating was estimated from previous measurements before the test and are plotted together. From the air temperature measurements before the heater test, there was a time interval of a few months between the peak temperatures inside and outside the
4.3.3. Rock temperature With increasing heater temperature, the internal rock temperatures changed with time. Figs. 7 and 8 show the temperature variation at B1 and A1 holes located at 0.3 m and 0.6 m, respectively, above from the heater hole. The rock temperatures increased to a maximum of approximately 25–30 °C during the preliminary test in September, 2007, and returned rapidly to the original rock temperatures. A continuous increase in rock temperature was
Fig. 8. Temperature measurements at the observation hole, A1, 0.6 m from the heater hole.
Fig. 7. Temperature measurements at the observation hole, B1, 0.3 m from the heater hole.
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Fig. 9. Comparison of the variations in joint spacing and the mean rock temperature along the ‘A’ type observation holes with the distance from the wall on March, 27, 2011.
observed, even though the heater temperature was maintained at 90 °C after June, 2008. The slight but continuous increase in rock temperature suggests that the influenced rock zone expanded continuously with time. The rock temperature measured from the in situ test in a jointed rock mass showed non-uniform variations depending on the distance from the tunnel wall, the distance from the heater, and geological conditions. In the case of the B1 hole, the peak temperature was recorded at approximately 50 °C during the overheating phase, whereas it was approximately 37 °C in the A1 hole. The rock temperatures at 4 m, 5 m and 2.1 m were relatively higher than at 3 m, 1.5 m, 1 m and 0.4 m in the B1 hole. The lower temperatures at
533
0.4 m to 1.5 m can be explained by the heat loss through the tunnel wall and the distance from the heater center. In contrast, the temperature variation observed at the A1 hole showed an extremely low temperature at 3 m. This was attributed to the effect of the rock joint crossing near the measurement location. If a water conducting rock joints exist near a temperature sensor, its influence would be significant because it would supply flowing groundwater with the virgin rock temperature (14 °C) (Kwon et al., 2011). Interestingly, the rock temperatures at different measuring points and holes converged rapidly to a temperature below 20 °C in approximately 3 weeks after the heater power was shut down in August, 2011. The rate of the decrease to the original rock temperature increased with increasing peak rock temperature. To check the effect of the rock condition on the temperature distribution in the rock mass, the mean joint spacing calculated from the rock cores was plotted as a function of the mean rock temperatures measured in the four ‘A’ type observation holes in Fig. 9. The mean rock temperatures and average joint spacing changed similarly with the distance from the wall, particularly in a deeper location. The lower rock temperature measured at approximately 3 m appears to be closely related to the influence of rock joints. A similar pattern could be found in the other temperature sensors. Fig. 10 shows the close relationship between the joint spacing and rock temperature on a section, which crosses vertically the observation holes, A1, A3, B1 and B3. The joint spacing was calculated by counting the number of joints observed from the rock cores in 0.5 m intervals. The lower rock temperatures at a 3– 3.5 m depth can be explained by the small joint spacing at that range. The relatively short joint spacing near the tunnel wall might be due to the impact of blasting. The rock near the tunnel wall showed a lower temperature than the deeper rock temperatures. This might be due to the influence of the joints, which can disturb significantly the thermal properties of the rock as well as the effect of heat convection through the tunnel wall.
Fig. 10. Comparison of the joint spacing and rock temperature distribution during the heating phase.
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Fig. 11. Distribution of the peak rock temperatures at each measuring point.
carried out at the KAERI Underground Research Tunnel (KURT) in Daejeon, Korea. During the test, the heater temperature was increased to 90 °C to simulate the normal condition, and the air, wall and rock temperatures were measured. In September, 28, 2010, the heater temperature was increased to 118 °C to simulate an abnormal heating condition. From the borehole heater test, the following conclusions could be drawn:
Fig. 12. Change in thermal stress with time.
Fig. 11 shows the contour of the peak rock temperature. Higher temperatures were recorded around the heater installed 1.2–3.2 m from the tunnel wall. The peak rock temperature decreased with increasing distance from the heater. Back-analysis of the measured temperatures to determine the influence of the parameters including heat convection through the tunnel wall, distance from the heater, and discontinuities, will be carried out in a future study. 4.3.4. Stress The change in thermal stress due to a temperature change was recorded using stress meters installed at the RS holes located at approximately 0.5 m from the heater hole. Fig. 12 shows the change in the thermally-induced stress along the radial direction from the heater hole with time. When the power was off in August, 2011, the thermal stress decreased almost immediately. Interestingly, the thermal stress increased almost linearly, even though the heater temperature had been maintained at 90 °C. This might be due to the continuous increase in the heated area with time. As expected, relatively higher thermal stresses developed at a 2– 3 m depth, where the rock temperature was high, than at 1 m and 5 m. The maximum increase in stress was approximately 5 MPa. 5. Conclusions According to the reference disposal concept for the permanent disposal of spent fuel in a deep underground repository, a heat generating canister containing spent fuel will be placed in deposition holes with compacted bentonite blocks. Because the canister will be located at 2–2.5 m from the tunnel surface, the influence of the disturbed rock zone around the tunnel and the convectional heat flow through the tunnel surface can affect the temperature distribution. In this study, an in situ borehole heater test was
(a) When the main test was started in December, 2007, the air and wall temperatures measured in the BHT area increased immediately. When the power was shut down suddenly in August, 2011, the rock temperatures returned rapidly to a temperature below 20 °C within approximately 3 weeks. The slope of the temperature drop during the cooling phase increased with increasing peak temperature. (b) During the normal heating phase, the heater temperature increased step by step to 90 °C and was maintained at that temperature for approximately 28 months from June, 2008 to September, 2010. During that period, the internal rock temperatures increased slightly but steadily with time. Under abnormal overheating conditions, the heater temperature was increased to 118 °C. The peak temperatures at the observation holes located at 0.3 m and 0.6 m away from the heater hole were approximately 50 °C and 37 °C, respectively. (c) A plot of the change in temperature with time and the average joint spacing revealed the influence of the joints on the temperature distribution. The lower temperature at around 3–3.5 m from the tunnel wall could be explained by the effect of groundwater flowing through the joints located frequently around the depth. The contour of the joint spacing could also show the development of a disturbed zone, which can affect the temperature distribution. (d) The relatively low temperatures around the tunnel could be explained by the possible effect of heat convection through the tunnel wall, the distance from the heater, and the influence of the excavation damage zone. (e) The change in thermal stress due to the temperature change was recorded by stress meters installed at approximately 0.5 m from the heater hole. The thermal stress increased almost linearly and a relatively larger increase in stress was observed at a 2–3 m depth than at 1 m and 5 m. The maximum increase in stress was approximately 5 MPa. In future studies, the measured data from the in situ heater test will be compared with three- dimensional computer simulations to understand the thermo-mechanical behavior of the rock mass during the heating and cooling phases. In addition, a future study will perform back-analysis of the measured temperatures to examine the influence of various parameters including heat convection through the tunnel wall, distance from the heater and discontinuities.
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References Andersson, J.C., 2007. Äspö Pillar Stability Experiment. Final Report. SKB TR-07-01. Berchenko, I., Detournay, E., Chandler, N., Martino, J., 2004. An in-situ thermohydraulic experiment in a saturated granit I: Design and results. Int. J. Rock Mech. Min. Sci. 41, 1377–1394. Bieniawski, Z.T., 1978. Determining rock mass deformability-experience from histories. Int. J. Rock Mech. Min. Sci. 15, 237–247. Cho, W.J., Kwon, S., 2010. Estimation of the thermal properties for partially saturated granite. Eng. Geol. 115, 132–138. Choi, J.W., Ko, W.I., Kim, S.G., et al., 1997. Reference spent fuel and its characteristics for the concept development of a deep geological disposal system. KAERI/TR914/97. Hocking, G., Williams, J.R., Mustoe, G.G.W., 1990. Post-test assessment of simulations for in situ heater tests in basalt – Part I. Heater test description and rock mass properties. Int. J. Rock Mech. Min. Sci. 27, 143–159. Ji, S., Lee, M., Koh, Y., Choi, J.W., 2011. Influence of the pressure on a fracture aperture controlling a fracture transmissivity, American Geophysical Union. Fall Meeting, abstract #H21B-1083. Jockwer, N., Wieczorek, K., Fernández, A.M., 2007. Measurements of gas generation, water content and change in the water distribution in a heater experiment in the underground laboratory Mont Terri. Physics and Chemistry of the Earth 32, 530–537. Lee, J.Y., Cho, D.K., Kim, S.G., Choi, H.J., Choi, J.W., Hahn, P.S., 2006. Development of the Korean Reference Vertical Disposal System Concept for Spent fuels. Waste Management, 06, Tucson, AZ.
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Kim, S.S., Baik, M.H., Kang, K.C., 2009. Solubility of neptunium oxide in the KURT (KAERI Underground Research Tunnel) groundwater. J. Radioanal. Nucl. Chem. 280, 577–583. Kwon, S., Cho, W.J., 2008. The influence of an excavation damaged zone on the thermal- mechanical and hydro-mechanical behavior of an underground excavation. Eng. Geol. 101, 110–123. Kwon, S., Cho, W.J., 2009. A sensitivity analysis of design parameters of an underground radioactive waste repository using a backpropagation neural network. Tunn. Undergr. Space 19, 203–212. Kwon, S., Cho, W.J., Han, P.S., 2006. Concept development of an underground research tunnel for validating the Korean reference HLW disposal system. Tunn. Undergr. Space Technol. 21, 203–217. Kwon, S., Lee, C.S., Cho, S.J., Jeon, S.W., Cho, W.J., 2009. Investigation of excavation damaged zone at KAERI underground research tunnel. Tunn. Undergr. Space Technol. 24, 1–13. Kwon, S., Cho, W.J., Choi, J.W., 2011. Initial thermal conditions around an underground research tunnel at shallow depth. Int. J. Rock Mech. Min. Sci. 48, 86–94. Ryu, J.H, Koh, Y.K., Park, S.W., Kim, G.Y., Choi, J.W., 2012. Geochemical characterization of deep groundwater in KURT using geochemical modeling. J. Environ. Eng. 137, 351–359. Schneefub, J., Gommlich, G., Noell, U., 1989. The heater test at the grimsel rock laboratory: the thermo-mechanical response of the rock mass to a heat flux. Nucl. Eng. Des. 116, 25–31. Yow, J., Hunt, J.R., 2002. Coupled processes in rock mass performance with emphasis on nuclear waste isolation. Int. J. Rock Mech. Min. Sci. 39, 143–150.
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Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Technical note
In situ borehole heater test at the KAERI Underground Research Tunnel in granite Sangki Kwon a,⇑, Changsoo Lee b, Chan-Hoon Yoon b, Won-Jin Cho b a b
Department of Energy Resources Engineering, Inha University, Republic of Korea Korea Atomic Energy Research Institute, Republic of Korea
a r t i c l e
i n f o
Article history: Received 31 July 2012 Received in revised form 11 July 2013 Accepted 14 July 2013 Available online 13 September 2013 Keywords: Borehole heater test High-level waste KURT Thermal–mechanical coupling Rock temperature
a b s t r a c t An in situ borehole heater test was carried out in an underground research tunnel at a shallow depth in granite. During the test, the heater temperature was increased to 90 °C to simulate the thermo-mechanical behavior of crystalline rock under normal underground high-level radioactive waste repository conditions. The air, wall and rock temperatures were measured over a period of about four years. At the end of the test, the heater temperature was increased to 118 °C to simulate abnormal overheating conditions. The peak temperatures at the observation holes located at 0.3 m and 0.6 m from the heater hole were approximately 50 °C and 37 °C, respectively. The temperature measurements allowed observations of the effects of rock joints and heat convection through the tunnel wall on the rock temperature distribution. When the power was shut down, the rock temperatures and stress returned rapidly to the original rock temperature. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In Korea, more than 11,000 tons of spent fuel have already been accumulated from the operation of nuclear power plants (NPP) since 1978. The 22 currently operating NPPs, which generate approximately 35% of the total electricity, produce approximately 700 tons of spent fuel every year. An additional six NPPs are currently under construction. The safe and reliable management of spent fuel, which is stored temporarily at the reactor sites, is a critical issue for the sustainable utilization of nuclear energy in Korea. Based on a R&D program supported by the government since 1997, a reference disposal system, KRS (Korea Reference Disposal System), has been suggested for the permanent disposal of spent fuel in an underground repository in a crystalline rock mass (Lee et al., 2006). In 2006, an underground research tunnel, KURT (KAREI Underground Research Tunnel), was constructed in KAERI (Korea Atomic Energy Research Institute) to validate the disposal system. During and after the construction, several in situ tests including a borehole heater test (BHT), EDZ (Excavation Disturbed Zone) test, tests for examining groundwater flow, geochemistry, and the migration of colloids through fractures, were carried out (Kwon et al., 2009; Ji et al., 2011; Ryu et al., 2012; Kim et al., 2009). One of the unique characteristics of high-level radioactive waste (HLW) is decay heat, which can affect the performance of natural and engineered barriers in the near-field of an ⇑ Corresponding authors. E-mail address: [email protected] (S. Kwon). 0306-4549/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2013.07.017
underground repository by affecting the thermal-hydro-mechanical-chemical (THMC) coupling behavior of the barriers. An accurate prediction of the THMC coupling behavior in the near-field is essential for the safe design of a disposal system as well as for providing a reliable performance assessment of underground radioactive waste repository, around which severe radiation, heat and pressure play an active role. To understand the complex coupling phenomenon in repository conditions, many in situ and laboratory tests have been carried out in many countries considering HLW disposal in geological formations. Several assessments have been made over the past few decades. These include in situ heater tests at Stripa mine and Aspo in Sweden, TSI at the Waste Isolation Pilot Plant (WIPP, a full scale heater at Hanford), heater tests with different scales at Yucca mountain in USA, an in situ THM test at Kamaishi mine in Japan, FEBEX at Grimsel and a heater experiment at Mont Terri in Switzerland, THE at URL in Canada, etc. (Schneefub et al., 1989; Hocking et al., 1990; Yow and Hunt, 2002; Jockwer et al., 2007; Berchenko et al., 2004; Andersson, 2007). Table 1 lists the characteristics of the in situ heater tests in different rock types, heating conditions and scales. Because the geological condition, rock type, waste type and characteristics, and disposal system differ around the world, it was recommended that an in situ heater test be carried out in Korea. The BHT in KURT was designed to understand the thermal and mechanical behavior in a disturbed rock zone, in which heat convection through the tunnel wall can affect the temperature distribution in the near-field rock mass. Compared to previous heater tests in other countries, the BHT in KURT has the following characteristics:
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(a) The influence of joint and anisotropic rock properties on the temperature distribution can be investigated. (b) The EDZ effect on the rock temperature distribution can be analyzed. (c) The influence of air flow by natural and forced ventilation on temperature variation can be assessed. In this paper, the status of the BHT and some results from the test are discussed.
deep in a crystalline rock mass, such as a granite body (Lee et al., 2006). The PWR spent fuel with a burn up of 45,000 MWd/tHM is considered a reference type because it comprises approximately 64%. For CANDU, spent fuel with a burn up of 7500 MWd/tHM is the reference type. For the reference PWR and CANDU spent fuel, the decay heat can be calculated using the following equations (Choi et al., 1997): 1
PðtÞ ¼ 852:34e0:2642þ0:130889t
t < 30 years for PWR
ðWatt=tonÞ
ð1Þ PðtÞ ¼ 14546 t 0:76204
2. Korea Reference Disposal System (KRS) According to the KRS, 36,000 tons of Pressurized Water Reactor (PWR) and CANada Deuterium Uranium reactor (CANDU) spent fuel generated from the 28 NPPs during their life time will be disposed of in an underground repository several hundred meters
t > 30 years for PWR
ðWatt=tonÞ
ð2Þ PðtÞ ¼ 1456:7 t 0:67667
ðWatt=tonÞ for CANDU
where t is the years elapsed after discharge from a reactor.
Table 1 In situ experiments for examining THMC coupling in rock. Test name (site, country)
Rock type
Depth (m)
Scale (size)
Coupling
Year
Heating condition
Crystalline rock Electric heater test (Stripa mine, Sweden) Buffer mass test (Stripa mine, Sweden)
Granite Granite
330 340
Borehole (U = 324 mm) Test pit (U = 750 mm)
THM THM
Casks with 5 kw and 3.6 kw 1.8 kW (6 holes)
Engineered barrier experiment (Kamaishi mine, Japan) FEBEX (Grimsel, Switzerland) THE (URL, Canada)
Granodiorite
260
Test pit (U = 1.7 m)
THM
Granite Granite
450 420
Drift scale (U = 2.27 m) Borehole (U = 96 mm)
THM TH
Heated Failure test (URL, Canada)
Granite
420
Borehole
TM
Buffer/container experiment (URL, Canada)
Granite
240
Test pit (U = 1.24 m)
THM
APSE (Aspo, Sweden)
Granite
450
Test pit (U = 1.75 m)
TM
TBT (Aspo, Sweden) SFT (Nevada test site, USA)
420 420 46
Test pit (U = 1.8) Drift scale (6.1m 4.6 m) Test pit (U = 900 mm)
TM TM
In site heater test (NSTF, USA)
Granite Quartz monzonite Basalt
Heater test (Grimsel, Switzerland)
Granite
450
Borehole (U = 300 mm)
TM
1978 1980– 1985 1988– 1998 19941995– 1998 1993– 1996 1991– 1993 2002– 2006 20031980– 1982 1980– 1983 1986– 1987
Tuff SHT (Yucca Mountain, USA)
Tuff
250
Borehole (U = 75 mm)
THMC
DST (Yucca Mountain, USA)
Tuff
250
Drift scale (5 m drift)
THMC
LBT (Yucca Mountain, USA)
Tuff
0
THMC
PEBSFT (G-tunnel, USA)
Tuff
420
Large block (3 3 4.5 m) Borehole (U = 30.5 cm)
Salt TSI (WIPP, USA)
Rock salt
650
Drift scale (5.5 m)
TM
TSDE (Asse salt mine, Germany)
Rock salt
800
Drift scale (3.5 m 4.5m)
THM
Clay HE-D test (Mont Terri, Switzerland)
Clay
300
Borehole (U = 300 mm)
THM
TER (Meuse, France),
Claystone
490
Borehole (U = 101 mm)
THM
BACCHUS (Hades URF/Swiszerland)
Clay
223
Borehole (U = 60 mm)
TM
ATLAS (Hades URF/Swiszerland)
Clay
223
Borehole (U = 190 mm)
THM
CERBERUS (Hades URF/Swiszerland)
Clay
223
Borehole
THMR
CACTUS (Hades URF/Swiszerland)
Clay
223
Borehole (U = 500 mm)
THM
PRACLAY(Hades URF/Swiszerland)
Clay
223
Drift scale (U = 2.5 m)
THMC
THM
THM
130 w/m2 2 of U = 0.9 m, 2 kW heater 0.5 kW 85 °C 1.2 kW 4 heaters (0–400 w each) 1.5 kW Total 20kw (2 kW 6 + 0.4 kW 20) 9 kW 4 kW/m (2 heaters)
1996– 1998 1997– 2005 1997– 1998 1988– 1989
5 m long 4 kW heater
1984– 1988 1990– 1999
18 W/m2
2004– 2005 2005– 2006 1988– 1990 1990– 1997 1989– 1995 1990– 1995 2009– 2019
210 kW from 50 wing heaters) 2.44 m long 5 heater holes. Total 2.25 kW 3 m long 3.3 kW
3 casks with 6.4 kW each
1.95 kW 0.57 kW 0.5 kW 0.9 kW 60
Co source
3.8 m long 4 kW 95 °C
ð3Þ
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The spent fuel will be inserted into corrosion resistant metal canisters and transported to the underground disposal area. The canisters will be placed in deposition holes with compacted bentonite. Fig. 1 shows the disposal concept with vertical deposition holes. The space from the tunnel floor to the top of the canister is approximately 2–2.5 m and filled with compacted bentonite blocks. The spacing of the disposal tunnel and deposition hole, buffer thickness and position of the canister in the deposition hole needs to be determined carefully considering the characteristics of the waste and the barriers to satisfy the mechanical and thermal criteria. One of the most critical criteria is that the peak temperature of the bentonite buffer should be less than 100 °C. For the development of a reference disposal system, threedimensional computer simulations were performed using the reference properties of the buffer, backfill, rock and canister. From the simulations, the tunnel and deposition hole spacings were 40 m and 6 m, respectively. Sensitivity analysis was carried out to examine the relative influence of the following seven design parameters on the peak rock and buffer temperatures: (a) hole spacing, (b) tunnel spacing, (c) thermal conductivity of the buffer, (d) thermal conductivity of backfill, (e) cooling period of spent fuel, (f) thermal conductivity of an EDZ, which was assumed to be 1 m around the tunnel, and (g) thermal conductivity of rock (Kwon and Cho, 2009). The required adjustment for each parameter to reduce the peak rock and buffer temperatures by 1 °C could be determined, as listed in Table 2. In the case of the cooling period of spent fuel, approximately 1 year is needed for a 1 °C decrease in the peak buffer temperature. The thermal conductivity is affected by fracture, which can decrease the thermal conductivity under dry conditions. Under saturated conditions, which is expected in deep underground areas, the influence of fractures on the thermal conductivity would be different. When the thermal conductivity of an EDZ is assumed to be 1 order of magnitude higher than that of intact rock, the peak buffer temperature can be decreased by 3 °C compared to the case without considering an EDZ (Kwon and Cho, 2008). Because the heat generating canister will be placed near the tunnel surface, the effect of the disturbed rock zone around the tunnel and the convectional heat flow through the tunnel surface will affect the temperature distribution. This means
that an understanding of the thermal behavior of the near field rock with a disturbance is important for making an accurate prediction of the THM behavior.
3. KAERI Underground Research Tunnel (KURT) The KAERI Underground Research Tunnel, KURT, is a small scale underground research laboratory for validating the reference HLW disposal system, KRS. KURT is located at a granite body, which is believed to be the most promising host rock for the permanent disposal of HLW in Korea. The 6 m 6 m 255 m tunnel was constructed in 2005–2006 by controlled blasting. The maximum depth of KURT with a 180 m long access tunnel and two research modules at the end of the access tunnel is approximately 90 m. Extensive site characterization was accomplished to select the KURT design (Kwon et al., 2006). Fig. 2 shows the layout of KURT and the BHT area in research module I. For the easy installation and management of the instruments and sensors, the heater hole was drilled in the tunnel wall at the BHT area. The BHT area was chosen for the following reasons: (a) The BHT at KURT focuses on the mechanical and thermal behavior. Research module I is relatively dry. Hence, it is good for focusing on the mechanical and thermal behavior of a rock mass. In contrast, research module II, which is closer to a major water conducting fracture zone, as shown in Fig. 2, can be used to examine situations where groundwater is involved. (b) Because the rock at the BHT area contains countable joints, it would be possible to examine the effect of a single fracture or simple fracture network on the thermal-and mechanical behavior. (c) The test area is located at the end of the research module to minimize the effects of the ventilation effect on the temperature distribution. With a fabricated wall, a 12 m long, 6.3 m high and 6.9 m wide test area can be isolated from the ventilation system when the ventilation ducts connected to the area are closed.
Fig. 1. Disposal concept for the high-level radioactive waste.
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S. Kwon et al. / Annals of Nuclear Energy 62 (2013) 526–535 Table 2 Change required to decrease the buffer and rock temperatures by 1 °C. Parameter Unit
Hole spacing M
Tunnel spacing M
Cooling time Year
Rock T.C. w/m K
Buffer T.C. w/m K
Backfill T.C. w/m K
EDZ T.C. w/m K
Reference value Buffer temperature Rock temperature
6 0.15 0.14
40 2.28 1.30
40 1.09 2.26
2.5 0.10 0.10
1.0 0.05 2.82
2 1.20 1.97
3.5 0.71 1.20
Fig. 2. Layout of the KURT and location of the BHT area.
4. BHT at KURT 4.1. Rock properties in the BHT area The properties of rock and rock mass were measured from in situ and laboratory tests using the rock cores from boreholes drilled in KURT. The thermal properties, such as thermal conductivity, thermal expansion coefficient and specific heat capacity of the KURT rock, were measured under a range of conditions. For example, Cho and Kwon (2010) reported the change in thermal conductivity according to the porosity and saturation. From extensive tests, the depth effect on the rock properties could be observed. Because of the weathering effect near the surface, the mechanical, hydraulic and thermal properties showed different values compared to those in deeper rock. The thermal conductivity was predicted to increase with depth and stabilize at 3.2 W/m K below 50 m (Kwon et al., 2011). Table 3 lists the rock properties measured at a BHT area. Because the BHT area is located at the end of the access tunnel, the rock properties, such as UCS, tensile strength and Young’s modulus at the area, are relatively higher than the average of KURT rock. The RMR in the test area was determined to be 65 during tunnel construction. The rock deformation modulus was estimated to be 30 GPa from the following empirical equation (Bieniawski, 1978):
Em ¼ 2 RMR 100
ð4Þ
The deformation rock modulus was approximately 60% of the elastic modulus of intact rock in the test area. The joint distribution was measured at three open rock surfaces in the vicinity of the test area. Fig. 3 shows the joints visible on the rock surface of the BHT
wall, dead end of research module I, and the right wall. Steep joints (70°) were dominant. In the case of the BHT wall, the major joint set had a dip of 76 and a dip direction of 236°. The change in the mechanical properties in an EDZ was measured using KURT rocks. From the laboratory tests for measuring the elastic modulus, rock strengths, and P and S wave velocities, the EDZ size was estimated to be approximately 1.1–1.5 m, whereas the Goodman jack test showed that the deformation modules had been disturbed in the 1.5–2 m deep zone. According to the RQDs of the rock cores from the EDZ study area, the mean RQD in the 0–2 m range, where the blasting impact was significant, was 67, whereas it was 81 at a deeper range. The elastic modulus in the EDZ was decreased by approximately 50% (Kwon et al., 2009). 4.2. BHT procedure The preparation of BHT was started immediately after the completion of the KURT construction in November, 2006. A 3.2 m long horizontal heater hole was drilled with a diameter of 110 mm at approximately 1.5 m above the floor. A 2 m long heater with a maximum power of 5 kW was designed and manufactured with aluminum. The heater was installed tightly at the end of the heater hole. The remaining part of heater hole was left as an open space. The influence of open space on the rock temperature variation was minimized by installing a plug at the mouth of the hole. The shallow installation of the heater was to allow an effective examination of the EDZ and ventilation effect. Fifteen observations holes, 38 mm in diameter, surrounding the heater hole were drilled, as
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Table 3 Mechanical and thermal properties of the rock in the heater test area. Properties
Mean
Min
Max
Comments
Mechanical properties Specific gravity Porosity (%) UCS (kg/cm2) Tensile strength (kg/cm2) E (105 kg/cm2) Poisson’s ratio P wave (m/s) S wave (m/s)
2.649 0.778 957.0 102.0 5.294 0.204 4673 2444
2.55 0.07 580 70 2.93 0.14 4120 2200
2.74 1.87 1580 140 10.08 0.35 5230 2730
100% of KURT average 88% of KURT average 134% of KURT average 105% of KURT average 120% of KURT average Same as KURT average 108% of KURT average 98% of KURT average
10.36 12.08 14.51 17.37
KURT average KURT average 20–50 °C 50–100 °C 100–150 °C 150–200 °C Natural convection Forced convection
Thermal properties Thermal conductivity (W/m K) Thermal expansion (microstrain/°C)
Heat convection factor (W/m2 K)
2.9 in dry condition 3.1 in saturated condition 6.770 8.452 9.979 11.757 4.73 7.46
2.03 3.83 5.94 8.43
Fig. 3. Rock joints visible at BHT area.
Fig. 4. Layout of the boreholes drilled for the BHT.
shown in Fig. 4a and b shows the configuration of the heater hole. Table 4 lists the dimensions and direction of the boreholes and the number of sensors in each hole. The RQD of each observation
hole was measured from rock cores and were varied from 68 to 96, as listed in Table 4. In the ‘A’ type observation holes, which were located at approximately 0.6 m from the heater hole, 9 temperature sensors were installed at 0.5–1 m intervals. In the ‘B’ type
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S. Kwon et al. / Annals of Nuclear Energy 62 (2013) 526–535 Table 4 List of the test holes and sensors installed in the observation holes. Type
Hole name
Length (m)
Size (mm)
Direction
Number of sensors
Average RQD
Heater Temperature (vibration wire type)
H A1 A2 A3 A4 B1 B2 B3 B4 C1 C2 C3 RS1 RS2 RS3 RS4
3.2 5.1 5.1 5.1 5.1 5.1 5.1 5.0 5.1 5.7 5 5.1 5.1 5.1 5.1 5.1
110 38 38 38 38 38 38 38 38 38 38 38 38 38 38 38
Horizontal Horizontal Horizontal Horizontal Horizontal Horizontal Horizontal Horizontal Horizontal 9° down 26° inside 12° up 26° inside Horizontal Horizontal Horizontal Horizontal Horizontal
6 thermocouples 9 9 9 9 12 12 12 12 12 3 8 5 5 5 5
91.6 87.2 86 95.7 89.8 91.4 89.4 95.8 91.4 67.8 89.4 90.4 93.9 87.8 96.1
Stressmeter (vibration wire type)
Table 5 List of the items measured in the heater test.
Table 6 List of the history of the BHT.
Properties
Items
Comments
Condition (duration)
Date
Heater temperature (°C)
Temperatures
Air temperature in the BHT room Air temperature outside of the BHT room Air temperature outside of the tunnel Wall temperature in the access tunnel Wall temperature around the heater Wall temperature in the opposite wall Heater temperature Rock temperature
1 point 1 point 1 point 18 points 13 points 1 point 6 points 117 points
Installation of heater Preliminary test (12 days)
Humidity in the BHT room Humidity outside of the BHT room Heater power Rock stress
1 point 1 point
2006-11 2007-08-30 2007-08-30 2007-08-31 2007-09-11 2007-9–2007-12 2007-12-03 2007-12-07 2007-12-11 2007-12-18 2007-12-26 2008-01-03 2008-01-11 2008-01-18 2008-01-22 2008-02-01 2008-02-12 2008-02-28 2008-03-28 2008-05-02 2008-06-11 2010-09-28 2010-12-16 2011-01-13 2011-02-10 2011-02-28 2011-08-08 2011-12-1
40 30 63 70 Power off Power off 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 Power off End of test
Humidity Power Stress
System upgrade (3 months) Main test (34 months)
20 points
holes, 12 temperature sensors were installed 0.3 m from the heater hole at 0.3–1 m intervals. Two other temperature observation boreholes, C1 and C2, were drilled outside of the test area with an upward and downward slope, respectively. The two holes had a 26° inclination angle to pass over the center of the heater. Four holes, RS1-RS4, for installing the stress meters were located at approximately 0.5 m from the heater hole. In each RS hole, 5 stress meters were installed at 1 m intervals. The vibrating wire type temperature and stress sensors were chosen for the test because durable sensor operation is preferred for a long term in situ test. After installing the heater and sensors, a preliminary test was run to check the performance of the heater and data logging system for two weeks in September, 2007. In the test, the heater temperature was increased rapidly to 70 °C. The main test was started in December, 2007 after allowing 3 months for the heated rock to return to its original condition. In the main test, the heater temperature was increased steadily to the target temperature of 90 °C in June, 2008. A target temperature of 90 °C was chosen using the thermal criteria of the buffer, which should be maintained under 100 °C. The heater power was controlled to maintain the heater temperature until September, 2010. In September, 2010, the heater temperature was increased to 118 °C to examine the response of the rock under abnormal overheating conditions. In August, 2011, the heater power was shut down and a cooling test was performed for approximately 4 months. During the tests, different items listed in Table 5 were measured. The measurements were recorded using an automatic data logging system as well as manual instruments in May and June, 2008, November, 2008, August, 2009, July, 2010, and May, 2011. Table 6 lists the duration of the test and the change in heater temperature during the BHT.
Overheating test (10 months)
Cooling test (4 months)
Fig. 5. Air and wall temperature change with time.
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tunnel. When heating started, the air and wall temperatures increased steadily and were essentially stabilized from 2009. Although the BHT area was blocked with the fabricated wall in July, 2007, it was still possible to observe slight seasonal temperature variations. The peak air and wall temperatures were 20.8 °C and 23.2 °C, respectively. A close relationship between the air and wall temperatures was observed. As shown in Fig. 6, the rock surface temperatures measured on the BHT wall and opposite wall increased linearly with increasing air temperature. Owing to heat generation from the heater, the difference between the BHT wall temperature and opposite wall temperature increased with increasing air temperature. The air temperature appeared to be affected by the BHT wall temperature. The opposite wall temperature, which was always lower than the air temperature, was affected by the change in air temperature. Fig. 6. Relationship between the air temperature and rock surface temperature measured on the BHT wall and opposite wall.
4.3. Measurements from the testa 4.3.1. Air and tunnel wall temperature outside the BHT area In the region where KURT is located, the annual average air temperature over the last 3 years was 13.2 °C. The monthly average temperature was high in August and low in January. The tunnel wall temperature variation was recorded periodically along the tunnel outside of the BHT area using a non-contact infrared laser thermometer with a 0.1 °C resolution. The significant fluctuations of the annual wall temperature near the tunnel entrance were reduced significantly to approximately ±2 °C from an average of approximately 14 °C (Kwon et al., 2011). 4.3.2. Air and tunnel wall temperature inside of BHT area Fig. 5 shows the air temperatures inside the BHT area and outside KURT, as well as the tunnel wall temperature measured around the heater hole. The outside air temperature change was estimated from the measured data in 2006 and 2007. The inside air and wall temperatures were measured from November, 2006. As a reference, the expected air temperatures for the case without heating was estimated from previous measurements before the test and are plotted together. From the air temperature measurements before the heater test, there was a time interval of a few months between the peak temperatures inside and outside the
4.3.3. Rock temperature With increasing heater temperature, the internal rock temperatures changed with time. Figs. 7 and 8 show the temperature variation at B1 and A1 holes located at 0.3 m and 0.6 m, respectively, above from the heater hole. The rock temperatures increased to a maximum of approximately 25–30 °C during the preliminary test in September, 2007, and returned rapidly to the original rock temperatures. A continuous increase in rock temperature was
Fig. 8. Temperature measurements at the observation hole, A1, 0.6 m from the heater hole.
Fig. 7. Temperature measurements at the observation hole, B1, 0.3 m from the heater hole.
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Fig. 9. Comparison of the variations in joint spacing and the mean rock temperature along the ‘A’ type observation holes with the distance from the wall on March, 27, 2011.
observed, even though the heater temperature was maintained at 90 °C after June, 2008. The slight but continuous increase in rock temperature suggests that the influenced rock zone expanded continuously with time. The rock temperature measured from the in situ test in a jointed rock mass showed non-uniform variations depending on the distance from the tunnel wall, the distance from the heater, and geological conditions. In the case of the B1 hole, the peak temperature was recorded at approximately 50 °C during the overheating phase, whereas it was approximately 37 °C in the A1 hole. The rock temperatures at 4 m, 5 m and 2.1 m were relatively higher than at 3 m, 1.5 m, 1 m and 0.4 m in the B1 hole. The lower temperatures at
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0.4 m to 1.5 m can be explained by the heat loss through the tunnel wall and the distance from the heater center. In contrast, the temperature variation observed at the A1 hole showed an extremely low temperature at 3 m. This was attributed to the effect of the rock joint crossing near the measurement location. If a water conducting rock joints exist near a temperature sensor, its influence would be significant because it would supply flowing groundwater with the virgin rock temperature (14 °C) (Kwon et al., 2011). Interestingly, the rock temperatures at different measuring points and holes converged rapidly to a temperature below 20 °C in approximately 3 weeks after the heater power was shut down in August, 2011. The rate of the decrease to the original rock temperature increased with increasing peak rock temperature. To check the effect of the rock condition on the temperature distribution in the rock mass, the mean joint spacing calculated from the rock cores was plotted as a function of the mean rock temperatures measured in the four ‘A’ type observation holes in Fig. 9. The mean rock temperatures and average joint spacing changed similarly with the distance from the wall, particularly in a deeper location. The lower rock temperature measured at approximately 3 m appears to be closely related to the influence of rock joints. A similar pattern could be found in the other temperature sensors. Fig. 10 shows the close relationship between the joint spacing and rock temperature on a section, which crosses vertically the observation holes, A1, A3, B1 and B3. The joint spacing was calculated by counting the number of joints observed from the rock cores in 0.5 m intervals. The lower rock temperatures at a 3– 3.5 m depth can be explained by the small joint spacing at that range. The relatively short joint spacing near the tunnel wall might be due to the impact of blasting. The rock near the tunnel wall showed a lower temperature than the deeper rock temperatures. This might be due to the influence of the joints, which can disturb significantly the thermal properties of the rock as well as the effect of heat convection through the tunnel wall.
Fig. 10. Comparison of the joint spacing and rock temperature distribution during the heating phase.
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Fig. 11. Distribution of the peak rock temperatures at each measuring point.
carried out at the KAERI Underground Research Tunnel (KURT) in Daejeon, Korea. During the test, the heater temperature was increased to 90 °C to simulate the normal condition, and the air, wall and rock temperatures were measured. In September, 28, 2010, the heater temperature was increased to 118 °C to simulate an abnormal heating condition. From the borehole heater test, the following conclusions could be drawn:
Fig. 12. Change in thermal stress with time.
Fig. 11 shows the contour of the peak rock temperature. Higher temperatures were recorded around the heater installed 1.2–3.2 m from the tunnel wall. The peak rock temperature decreased with increasing distance from the heater. Back-analysis of the measured temperatures to determine the influence of the parameters including heat convection through the tunnel wall, distance from the heater, and discontinuities, will be carried out in a future study. 4.3.4. Stress The change in thermal stress due to a temperature change was recorded using stress meters installed at the RS holes located at approximately 0.5 m from the heater hole. Fig. 12 shows the change in the thermally-induced stress along the radial direction from the heater hole with time. When the power was off in August, 2011, the thermal stress decreased almost immediately. Interestingly, the thermal stress increased almost linearly, even though the heater temperature had been maintained at 90 °C. This might be due to the continuous increase in the heated area with time. As expected, relatively higher thermal stresses developed at a 2– 3 m depth, where the rock temperature was high, than at 1 m and 5 m. The maximum increase in stress was approximately 5 MPa. 5. Conclusions According to the reference disposal concept for the permanent disposal of spent fuel in a deep underground repository, a heat generating canister containing spent fuel will be placed in deposition holes with compacted bentonite blocks. Because the canister will be located at 2–2.5 m from the tunnel surface, the influence of the disturbed rock zone around the tunnel and the convectional heat flow through the tunnel surface can affect the temperature distribution. In this study, an in situ borehole heater test was
(a) When the main test was started in December, 2007, the air and wall temperatures measured in the BHT area increased immediately. When the power was shut down suddenly in August, 2011, the rock temperatures returned rapidly to a temperature below 20 °C within approximately 3 weeks. The slope of the temperature drop during the cooling phase increased with increasing peak temperature. (b) During the normal heating phase, the heater temperature increased step by step to 90 °C and was maintained at that temperature for approximately 28 months from June, 2008 to September, 2010. During that period, the internal rock temperatures increased slightly but steadily with time. Under abnormal overheating conditions, the heater temperature was increased to 118 °C. The peak temperatures at the observation holes located at 0.3 m and 0.6 m away from the heater hole were approximately 50 °C and 37 °C, respectively. (c) A plot of the change in temperature with time and the average joint spacing revealed the influence of the joints on the temperature distribution. The lower temperature at around 3–3.5 m from the tunnel wall could be explained by the effect of groundwater flowing through the joints located frequently around the depth. The contour of the joint spacing could also show the development of a disturbed zone, which can affect the temperature distribution. (d) The relatively low temperatures around the tunnel could be explained by the possible effect of heat convection through the tunnel wall, the distance from the heater, and the influence of the excavation damage zone. (e) The change in thermal stress due to the temperature change was recorded by stress meters installed at approximately 0.5 m from the heater hole. The thermal stress increased almost linearly and a relatively larger increase in stress was observed at a 2–3 m depth than at 1 m and 5 m. The maximum increase in stress was approximately 5 MPa. In future studies, the measured data from the in situ heater test will be compared with three- dimensional computer simulations to understand the thermo-mechanical behavior of the rock mass during the heating and cooling phases. In addition, a future study will perform back-analysis of the measured temperatures to examine the influence of various parameters including heat convection through the tunnel wall, distance from the heater and discontinuities.
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