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I. Introduction
Since 1992, 300 concrete silo dry storage systems have been used to store 162,000 bundles of CANDU spent nuclear fuel on the Wolsong NPP site (see Fig. 1).
The concrete silo system consists of a concrete structure, a steel liner plate in the inner space and a fuel basket as shown in Fig. 2 [1]. In the design of radiation shielding for spent fuel handling and storage facilities, such as concrete structures for silo storage systems, the radiation dose rate limits at the Controlled Area Boundary are applied in accordance with 10 CFR 72.104(a) and Articles 6 and 16 of the Nuclear Safety and Security Commission Notice No. 2019-10, “Standards for Radiation Protection, etc”. For design basis accidents, the individual dose limits within and outside the controlled area are applied according to 10 CFR 72.106(b) and Article 5 of the Nuclear Safety and Security Commission Notice No. 2015-19, “Detailed Standards for Structures and Equipment of Interim Storage Facilities for Spent Nuclear Fuel”. To achieve this, it is necessary to maintain structural integrity and ensure adequate radiation shielding performance against the high radioactivity of spent fuel stored within the system [2-8]. However, the concrete structure is directly exposed to the external climatic environment in the storage facility and can be expected to deteriorate during long-term storage due to the heat of spent nuclear fuel, and there is a possibility of deterioration in durability and external damage, especially cracks in the concrete structure (see Fig. 3). These cracks may reduce the radiation shielding performance of concrete structure and may further exceed the allowable radiation dose rate limits of silo system [9-11]. The characteristics of concrete cracks are uncertain and variable depending on the size and formation pattern of crack cross-section, making it difficult to evaluate them analytically [12]. Here, the effect of cracks in concrete structure on radiation shielding performance was investigated by forcibly creating cracks in specimens with the same composition and properties as the concrete structure of silo storage system [13] and then irradiating them to measure the changes in radiation dose rate.
Table 1
Classification of the concrete
| Name | Grade | Design strength (kg·cm−2) | Slump (cm) | Slump tolerance (cm) | Max. aggregate size (mm) | Concrete type |
|---|---|---|---|---|---|---|
| Specimen 1 | B | 300 | 7.5 | ±1.5 | 20 | Type-I |
Table 2
Mix design table of the concrete
| Gmax. (mm) | Strength (kg·cm−2) | Slump (cm) | Air content (%) | w/c (%) | s/A (%) | Unit material usage (kg·m−3) | |||
|---|---|---|---|---|---|---|---|---|---|
| Water | Cement | Fine aggregate | Coarse aggregate | ||||||
| 20 | 300 | 7.5 | 5.0 ± 1.0 | 44.0 | 42.0 | 165 | 375 | 731 | 1,020 |
(Note) Gmax: max. size of coarse aggregate
Slump: test value of consistency
w/c: ratio of water and cement
s/A: fine aggregate modulus
2. Irradiation Test for Concrete Crack
The test was performed by forcibly creating cracks of different depths and sizes on the specimens with the same concrete composition and properties as the structure of silo storage system, and then performing irradiation tests to measure the change in radiation dose rate.
2.1 Test Specimens
The test specimens were prepared by applying the concrete mixing design used in the construction of the silo system. The classification and mixing of cement were performed as shown in the tables.
The test specimens were divided into a reference specimen and a cracked specimen to obtain the radiation dose rate value under the crack-free condition. The dimensions of specimens are 200 m × 200 mm in area and 35 mm in height (see Fig. 4). The height of specimen was set to 35 mm, reflecting the crack depth of the silo structure (10.4– 34.1 mm) measured using an ultrasonic measuring device (see Fig. 5).
The crack in the cracked specimen was notched to facilitate crack initiation, and the specimen was separated after applying a hydraulic load, and the separated specimen was cut (see Fig. 6).
Generally, the progressiveness of cracks in a silo can be determined by variations in crack width. Base on this, an evaluation of crack progressiveness in the silo was conducted using a crack width variation measuring device (DEMEC 5364 Gauge). The results showed changes ranging from 0.004 mm to 0.052 mm, indicating minimal changes in crack width. Therefore, it was concluded that the cracks were not progressive (see Fig. 7). Base on this, the specimen was assumed to have progressive cracks, considering a crack width 0.02 mm as the standard for progressive cracking, with a maximum crack width of 0.05 mm: (1) 0 mm for the reference specimen (no crack), (2) 0.2 mm for the maximum actual measurement, (3) 0.5 mm for the assumed progressive crack, and (4) 1.0 mm for the assumed progressive crack.
The test specimens were could be easily cracked, the crack width could be maintained and the surface dose rate could be easily measured so that the dose difference before and after cracking could be examined.
2.2 Irradiation Test
The tests were conducted by a KOLAS internationally accredited institution using a 137Cs gamma-emitting radionuclide. The reference dose rate was selected by reflecting the applicable minimum dose rate of the specimen (1.0 mSv·h−1) and the testable dose rate (5.0 mSv·h−1, 2.5 mSv·h−1 and 1.0 mSv·h−1) of the equipment. The test was conducted in a room equipped with source equipment, specimen support, ranging laser, instruments and CCTV (see Fig. 8).
The 35 mm reference specimen without cracks was first irradiated, and then the cracked specimens were irradiated with crack widths of 0.0 mm, 0.2 mm, 0.5 mm and 1.0 mm.
The radiation dose rate through the crack was selected as the average value of measured results after 10 irradiation tests for the same specimen and test conditions. The radiation dose rate through the crack was recorded by a pre-installed instrument on the opposite side of the test specimen and read out using CCTV (see Fig. 9).
According to the test results, it was confirmed that as the crack width increases, the surface radiation dose rate also increases. The detailed results are shown in Table 3 and Fig. 10 below.
Table 3
Dose rates according to crack width by reference values
| [Unit: mSv·h−1] | ||||
|---|---|---|---|---|
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| Crack depth: 35 mm | ||||
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| Ref. dose (mSv·h−1) | Crack width (mm) | |||
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| 0.0 | 0.2 | 0.5 | 1.0 | |
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| 5.0 | 3.989 | 4.036 | 4.220 | 4.382 |
| 2.5 | 1.979 | 1.962 | 2.089 | 2.171 |
| 1.0 | 0.783 | 0.788 | 0.827 | 0.865 |
3. Dose Rate Evaluation for Cracks
The maximum radiation dose rate on the surface of silo system obtained by radiation shielding analysis using a computational analysis program was found to be 0.015 mSv·h−1, which is the case where there are no cracks in the silo system (see Fig. 11).
The crack of concrete was assumed to be a progressive crack, and the radiation dose rate of silo structure was evaluated in accordance with the progress of crack. Table 4 shows the radiation dose rate according to the size of crack width by reflecting the maximum radiation rate of silo system obtained by the analysis in the increase of radiation rate for the crack width relative to the reference dose rates.
Table 4
Dose rates according to crack width based on the silo’s max. dose rate (unit: mSv·h−1)
| Crack width (mm) | 0 | 0.2 | 0.5 | 1.0 |
|---|---|---|---|---|
| Maximum dose rate (mSv·h−1) | 0.0151) | 0.015 | 0.016 | 0.017 |
1) max. dose rate on the surface of silo system obtained by the analysis
From this result, in Fig. 10 and Table 4, the relationship between dose rate increase and crack width was almost linear, so linear regression was applied. Linear regression analysis was used to predict the crack width that satisfies the allowable radiation rate of 0.025 mSv·h−1 on the surface of silo system, and is shown in Fig. 12.
As a result of predicting the crack width of silo system by reflecting the result of irradiation test according to the crack width of concrete, the maximum crack width reaching the allowable dose rate of silo system was found to be 4.8 mm for the maximum crack depth of 35 mm in the actual silo structure inspection. In other words, if the crack width is 4.8 mm or more, the allowable dose rate of silo system is exceeded and the concrete structure must be repaired.
4. Conclusion
Irradiation tests were carried out by forcibly cracking specimens with the same characteristics as the concrete applied to the silo storage system, and the effect of concrete cracking on the radiation dose rate was examined. The change in radiation dose rate according to the size of the crack width was evaluated through crack simulation and irradiation tests. For constant dose rates, the dose rate increased with the progress of crack width. The crack width exceeding the allowable dose rate of the silo system was calculated by reflecting the maximum radiation rate of silo system obtained by analysis in the increase of radiation dose rate according to the size of crack width. It was found that if the crack width was 4.8 mm or more for the maximum crack depth of 35 mm obtained from the inspection of silo system, it exceeded the allowable dose rate of silo system. In the actual inspection, the maximum crack width of concrete was 0.05 mm, so the silo system maintains radiation shielding performance in the current crack condition. However, it is important to continuously monitor the radiation shielding performance in preparation for the progress of concrete cracks due to long-term operation of the silo system.
