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Transactions, SMiRT-24 BEXCO, Busan, Korea - August 20-25, 2017 Division I

CONCRETE STRENGTH EVALUATION OF MASSIVE CONCRETE STRUCTURES BASED ON THE DATA OBTAINED FROM DECOMMISSIONING NUCLEAR POWER PLANT Kazuhiro Yokokura1, Hiroyuki Wada2, Osamu Kontani3, Ippei Maruyama4 1

Manager, Nuclear Safety Research & Development Center, Chubu Electric Power Co., Inc., Japan Senior Manager, Nuclear Civil & Architectural Engineering, Chubu Electric Power Co., Inc., Japan 3 Senior Manager, Nuclear Power Department, Kajima Corporation, Japan 4 Professor, Dept. of Environmental Engineering and Architecture, Nagoya University, Japan 2

ABSTRACT The soundness evaluation of aging concrete structures in nuclear power plants (NPPs) is basically conducted based on the strength of concrete cores taken from these structures. Since the extraction of cores from various structural members can damage the concrete structure, a rational soundness evaluation method based on minimum core information is required in order to minimize the impact of coring on the concrete structure's performance. The decommissioning of Hamaoka Nuclear Power Station (NPS) Units 1 and 2, which were operated for nearly 40 years, is underway. A research project on the aging concrete of Hamaoka NPS Unit 1 was started in FY2015. In this project, a large amount of data on the physical properties of the aging concrete was obtained and the concrete strength evaluation method combining non-destructive techniques and numerical analysis is studied. This paper describes results of concrete core tests conducted for one of the inner walls of Hamaoka NPS Unit 1. INTRODUCTION In Japan, plant life management (PLM) is conducted to ensure the safe and long-term operation of NPPs. PLM is required by government to determine whether NPPs are sound against aging after 30 years of operation. After the Fukushima-1 NPP accident, electric utilities have been required to conduct special inspections (SIs) and PLM, as well as apply to the Nuclear Regulatory Authority (NRA) for permission to extend operation periods beyond 40 years. In SIs, electric utilities are required to obtain core samples from various primary structures under different environmental conditions to check for any aging-related effects on strength, shielding performance, alkali silica reaction, neutralization, and chloride penetration of concrete. The NRA does not provide evaluation indexes and criterions, therefore, electric utilities are seeking the right way to evaluate the soundness of concrete structures. For the maintenance of NPPs, not only concrete strength but also the effects of different types of concrete deterioration need to be checked. The deterioration of concrete structures often emerges as cracks and changes on the surface; thus, it is very important and efficient to check concrete surfaces by visual inspections. Concrete strength, however, cannot be determined through visual inspections, and is usually measured by destructive methods, i.e., core sampling. Since destructive methods cause damage to concrete structures at sampling points, there is a need to minimize the amount of core samples collected. It would be efficient if we could check the status of concrete strength mainly using non-destructive evaluation (NDE) rather than destructive methods. In this project, a large amount of data on the physical properties of the aging concrete was obtained and a concrete strength evaluation method that combines NDE and numerical analysis is studied.

24th Conference on Structural Mechanics in Reactor Technology BEXCO, Busan, Korea - August 20-25, 2017 Division I

PROJECT OVERVIEW Hamaoka NPS Unit 1 Table 1 shows the specifications for Hamaoka NPS Unit 1–5. The construction of the Hamaoka NPS Unit 1 began in 1971, so nearly 46 years has passed until the present, 2017. Its operating period was approximately 16 years and half, except for the periods of periodic inspections. It is currently being decommissioned, a process that began in 2009 and will last for 28 years. Data on the concrete properties of actual NPPs in Japan, which were often obtained from periodic inspections, is limited. If huge amounts of data were obtained and accumulated on concrete structures undergoing decommissioning, like Hamaoka NPS Unit 1, they would be very useful to verify the NDE method/numerical analysis method and develop a new soundness evaluation method for concrete structures. Data on the properties of concrete exposed to various environmental conditions can be obtained from decommissioned plants. In particular, it is possible to examine the effects of long-term exposure to heating (16.5 years) on concrete compared to experimental studies. Core sampling of important parts such as concrete under irradiated conditions, which cannot be sampled during operating periods, can be performed.

Table 1. Specifications of Hamaoka NPS Unit 1 Reactor type Thermal output (MWt) Containment type Power output (MWe)

Unit 2

BWR-4 (1593)

Unit 4

BWR-5

(2436)

Mark-1 (540)

Unit 3

3293

ABWR 3293

Advanced Mark-1 (840)

1100

Unit 5

1137

3926 RCCV 1380

Total power output (MWe)

(under decommissioned)

Start of construction Start of commercial operation

Mar. 1971

Mar. 1974

Nov. 1982 Feb. 1989 Mar. 1999

Mar. 1976

Nov. 1978

Aug. 1987 Sep. 1993

3617

Jan. 2005


Project Work Scope In order to formulate a rational soundness evaluation method for concrete structures with minimum core sampling, the following steps will be conducted: 1. Development of a concrete database: We will analyze the conditions of reactor buildings by obtaining concrete cores and creating a concrete database on aging-related degradation. The database will be very useful for the PLM of structures in operation. 2. Experimental verification of the NDE: The NDE will be carried out in locations where the cores were sampled, and the results will be verified compared with the database. 3. Experimental verification of numerical analysis methods: The applicability of the numerical analysis method will be verified using the database.

DEVELOPMENT OF A CONCRETE DATABASE Planning The concrete structures of nuclear facilities are exposed to various environmental conditions during service. In particular, the nuclear reactor building has a special environment wherein the interior of the containment vessel is exposed to high temperature and radiation. In addition, the thickness of the structural member exceeds 1 m owing to seismic performance requirements. In the reactor building of Hamaoka NPS Unit 1, core samples were obtained from locations with different environmental conditions and member thickness. The images of core sampling points are shown in Figure 1. Test specimens measuring 100 mm in diameter and 200 mm high (φ100 × 200 mm) were prepared from the collected cores, and various properties (compressive strength, Young’s modulus, moisture content, bound water ratio, etc.) of concrete were acquired. As Hamaoka NPS Unit 1 is a plant with a large amount of concrete quality control records left over from the time of construction, by comparing the initial data and clarifying the mechanism of concrete hydration reaction, a soundness assessment method for long-term operation of the reactor will be proposed. Table 2 shows specifications of the ready-mixed concrete used for Hamaoka NPS Unit 1. 【Sampling Points】 ① PCV Side Wall ② RPV Pedestal ③ Main Steam Tunnel ④ Spent Fuel Pool ⑤ Inner Wall of R/B ⑥ Outer Wall of R/B ⑦ Base Mat

Figure 1. Core sampling points


Table 2. Specifications of ready-mixed concrete for Hamaoka NPS Unit 1 (Ref. [1]) Designed No. strength Cement* [kg/cm2]

Maximum aggregate size [mm]

Slump [cm]

Air content [%]

W/C [%]

Fine aggregate ratio [%]

Note

Base mat

1

225

M

40

10

3.5±1

48.2

38.5

2

225

O

25

12

3.5±1

48.3

38.5

3

225

O

25

12

3.5±1

48.9

38.5

4

225

M

25

12

3.5±1

49.3

40.0

5

225

M

25

12

3.5±1

48.0

39.7

6

225

M

25

15

3.5±1

49.0

42.0

7

225

O

25

15

3.5±1

48.3

40.0

8

225

O

25

18

3.5±1

49.0

42.0

9

225

O

25

18

3.5±1

50.0

40.0

18

3.5±1

51.0

43.0

10

225 O 25 *M---- Moderate-heat Portland cement *O---- Ordinary Portland cement

PCV side wall mat

Primary Core Sampling We started collecting samples from the inner walls, which are 1.5 m thick without surface coating, for the preliminary test in Feb. 2016 using the core sampling machine TS-402 Shibuya with a diamond saw and coring speed of approximately 10–20 mm/min. Figure 2 shows the locations of the core sampling sites, and Table 3 shows the core sampling specifications. Since it was difficult to extract the penetrating core from an extremely thick member with a thickness of 1.5 m, it was decided that cores would be collected from both sides of the wall, as shown in Figure 3. This also made it possible to avoid the problem that the core would penetrate the walls constituting the boundary. In addition, two types of core sampling methods ―wet coring and dry coring―were employed in order to evaluate the influence of the sampling method on the physical properties of concrete. Dry core sampling uses air to cool the end of the saw, whose temperature was elevated by the friction with concrete during coring. The advantage of this method is that it keeps the sample away from water, which may affect concrete strength and water content measurement. However, because of the moderate cooling performance, the specimen might be affected by elevated temperatures. Wet core sampling uses water; the cooling performance of water is greater than that of air, but the immersion of concrete specimens in water might affect the obtained results. In particular, water content measurements are not representative of values under actual conditions. Therefore, in the case of sampling on B2F, both of these methods were employed and the wet and dry core samples were compared to determine their differences. All penetration cores taken from the inner walls were assigned an identification number. Table 4 lists the penetration core samples. After a radiation survey, these penetration cores were moved to the Taiheiyo Consultant laboratory for testing.


1W1-1F

1W1-B1F

1F Plan

B1F Plan

1W1-B2F

B2F Plan Figure 2. Locations of core sampling sites

850mm＋α

②

③

④

Φ100×850mm core 100mm 650mm＋α ⑤

⑥

Φ100×650mm core

⑦

Controlled area

Non-controlled area

①

1500mm Φ100×200mm specimen

Figure 3. Penetration core-sampling from inner walls


Table 3. Core sampling specifications

Name

Floor

Level [mm]

Member thickness [mm]

Sampling method

1W1-1F

1F

1FL±0

1,500

wet

1W1-B1F

B1F

1FL-5,200

1,500

wet

1W1-B2F

B2F

1FL-12,700

1,500

wet dry

Table 4. Penetration core samples Core ID of penetration core

Length [mm]

Date of cored [m/d]

760,950

2/9

780,970

2/9

1W1-1F-W-#3

970,960

2/11

1W1-B1F-W-#1

960,960

2/12

970,950

2/12

1W1-B1F-W-#1

970,950

2/13

1W1-B2F-W-#1

1200,1200

2/10

1200,1200

2/10

950,990

2/11

950,950

2/15

870,960

2/15~16

970,940

2/16~17

Name

Sampling method

1W1-1F-W-#1 1W1-1F-W-#2

1W1-B1F-W-#1

1W1-1F

1W1-B1F

1W1-B2F-W-#2

wet

wet

wet

1W1-B2F-W-#3 1W1-B2F 1W1-B2F-D-#1 1W1-B2F-D-#2 1W1-B2F-D-#3

dry

Compressive Strength Specimens Tests Test specimens of φ100 × 200 mm were prepared from the extracted cores. The compressive strength, Young’s modulus, apparent density, ultrasonic wave velocity, and dynamic elastic modulus of the concrete were measured. The results of distributions of the compressive strength, Young’s modulus, and apparent density of the samples from inner wall obtained by wet core sampling are shown in Figure 4. The compressive strength was greatest toward the center of the wall. There was a difference of 15–25 MPa between the surface and center of the wall on the 1F, B1F, and B2F. The patterns in the Young’s modulus were similar to that of compressive strength, while the distribution of the apparent density was almost constant.


Both the compressive strength and Young’s modulus had a convex-shaped trend, which can be explained by the water transfer and resultant stagnation of cement hydration in the wall. However, the fact that the depth of moisture loss from the surface reached approximately 500 mm beyond 40 years seems larger than the values estimated by previous studies on moisture transfer. To understand these trends, the moisture movement, Calcium silicate hydrate behavior under drying, sorption isotherms of concrete, and stagnation of hydration due to drying should be investigated.

1W1-1F- W-#1–#3

1W1-B1F- W-#1–#3

1W1-B2F-W-#1–#3 Figure 4. Test results of wet core sampling (1F, B1F, and B2F)


Compressive strength comparison

Young’s modulus comparison

Apparent density comparison Figure 5. Test results comparison between dry and wet core sampling on B2F (Left Figure: dry core sampling, Right Figure: wet core sampling)


In Figure 5, the test results of dry and wet core sampling are compared. Dry and wet coring resulted in a large difference in compressive strength. For the samples obtained by dry core sampling, the maximum strength was approximately 10 MPa lower than the samples obtained by wet core sampling, while the minimum strength observed near the wall surface was almost the same. In the case of the Young’s modulus, it also has a convex-shaped trend. The Young’s modulus of the dry coring sample was slightly smaller than that of the wet one. There was a large difference in the apparent density between these samples. The apparent density of dry coring samples was greatest at the center of the wall, while that of wet coring samples was almost constant throughout the wall. This suggests that the wet coring samples absorbed water, while the water content of dry coring samples remained unchanged or decreased slightly as a result of evaporation from the sample surface caused by the elevated temperature. Figure 6 shows the results of ultrasonic pulse velocity measurements of coring samples obtained using the wet and dry process, respectively. The colored lines indicate the ultrasonic pulse velocity of penetration cores. These lines have different lengths because of the coring conditions, and the widths of the colored lines also differed as a result. The results obtained for the penetration cores were within the range of data obtained for the samples measuring φ100 × 200 mm, except for #2S of dry coring samples. They were almost the average values of the data for φ100 × 200 mm specimens. The mechanism causing this pattern should be investigated.

Distance from member surface (mm)


Figure 6. Ultrasonic pulse velocity measurements between dry and wet core sampling on B2F (Left Figure: dry core sampling, Right Figure: wet core sampling)

CURRENT ACTIVITIES AND THE NEXT STEP A computational cement-based material model (CCBM Ref. [2]) was developed at Nagoya University. Figure 7 shows the calculated compressive strength of a 1.5-m-thick concrete wall by CCMB. This is only the result of one study, but the convex-shaped trend appeared after a few years. We are going to calibrate the CCBM calculation based on the construction records. Details will be described in another paper (Ref. [3]).

Compressive strength (N/mm2)



Figure 7. Calculated concrete wall compressive strength CONCLUSIONS A summary of the obtained results is presented below.   

Both the compressive strength and Young’s modulus showed a convex-shaped trend, indicating that the center of the cross-section was larger than the surface. Since the distribution of the apparent density of the cores sampled by dry coring method also showed a convex-shaped trend, the strength of surface concrete was presumably affected by drying during the operating period. The behavior of the distribution of the compressive strength and Young’s modulus in the crosssection may be caused by the effects of drying from the surface, while the moisture condition of the concrete is may be a key factor in the strength development of concrete.

ACKNOWLEDGMENTS This project is supported by “R&D of the safety improvement of nuclear facilities” project Ministry of Economy, Trade and Industry (METI) in Japan. The authors acknowledge the helpful feedback provided by Associate Professor Noriyuki TAKAHASHI of Tohoku University, Associate Professor Hitoshi HAMASAKI of Shibaura Institute of Technology, Associate Professor Yo HIBIMNO of Hiroshima University, and Assistant Professor Atushi TERAMOTO of Hiroshima University. REFERENCES [1] [2]

[3]

Horiuchi, M., Sugihara, K. and Iwasawa, J. (1975). “Concrete works of Hamaoka NPS Unit-1,” Concrete Journal, 13, 8, pp. 11-20. Maruyama, I. and Igarashi, G. (2015). “Numerical approach towards aging management of concrete structures: Material strength evaluation in a massive concrete structure under one-sided heating,” Journal of Advanced Concrete Technology, 13, 11, pp. 500-527. Maruyama, I. et al., (2017). “Soundness evaluation method for concrete structures based on the data obtained from decommissioning Hamaoka nuclear power plant Part 4: Combination of numerical calculation and non-destructive method for evaluating property of concrete member,” Summaries of technical papers of annual meeting Architectural Institute of Japan.

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