·Table of Contents
·Methods and Instrumentation
Ultrasonic Measurement of Saggings of Calandria Tubes and Liquid Injection Shutdown System Tubes Using Newly Developed Device
Tae Ryong Kim, Seok Man Sohn, Jun Shin Lee
Korea Electric Power Research Institute
Jong Po Lee, Chul Hoon Park, Keum Chong Joo, Byung Guk Um
Research Institute of Korea Advanced Inspection Technology
Sagging of calandria tubes and liquid injection shutdown system tubes in pressurized heavy water reactor are known to occur due to irradiation creep and growth with plant operation time. When the sagging of calandria tube is big enough, the calandria tube can possibly contact liquid injection shutdown system tube crossing beneath the calandria tube, which subsequently may results in difficulties on the safe operation. It is therefore necessary to check the gap for the confirmation of no contacts between the two tubes with a proper measure during the life of plant. An ultrasonic measuring device which can be fed through viewing port installed on clalandria was developed and utilized to measure the saggings of both tubes in a pressurized heavy water reactor in Korea. Ultrasonic wave was found to precisely detect the centerlines of calandria tubes and liquid injection shutdown system tubes. The saggings of calandria tubes and liquid injection shutdown system tubes were obtained from the measured elevations of the centerlines and the initial elevations at the beginning of plant operation. The gaps between calandria tubes and liquid injection shutdown system tubes were calculated based on the ultrasonic measurement data and mathematical modeling for the deflection curves of both tubes by using the classical beam theory.
Pressurized heavy water reactor(PHWR) consists of a big cylinder(called "calandria") with many calandria tubes(CT hereafter) installed horizontally, and each CT wraps one pressure tube(PT hereafter) which contains several nuclear fuel assemblies. Heavy water coolant is flowing through PT for the transportation of nuclear energy to steam generator. Annulus between PT and CT is filled with a gas for thermal insulation. In addition to CT, there are some other tubes horizontally or vertically installed inside of the calandria for the purpose of safe operation. Among them 6 tubes of liquid injection shutdown system(LISS) are installed crossing beneath CT at two elevations as shown in Fig. 1. Since the calandria is filled with heavy water as moderator, CT's and LISS tubes (LIN hereafter) are immersed in heavy water. Materials and dimensions of the tubes installed inside calandria are summarized in Table 1.
Fig 1: Schematic diagram of a PHWR 600MW calandria
|Table 1: Material and dimension of CT and LIN
(*) Refer to Fig. 4 for the positions "a", "b" and "c"
Sagging of calandria tubes and liquid injection shutdown system tubes in pressurized heavy water reactor are known to occur due to irradiation creep and growth with plant operation time[1,2]. When the sagging of CT is big enough, the CT can possibly contact with LIN crossing beneath the CT, which subsequently may results in difficulties on the safe operation. It is therefore necessary to check the gap between the two tubes and to confirm no contacts during plant lifetime with a proper measure. Ultrasonic technology can be applied to the measurement of saggings and gaps, since calandria of PHWR is filled with heavy water..
at "a" and "b"
||48.1 / 44.1
||55.4 / 50.8
||22.9 / 26.9
||16.9 / 18.6
||72.2 / -
||79.1 / -
|CT "R20/ -"
||45.6 / -
||52.5 / -
|Table 2: Summary of saggings and gap (unit:mm)|
Bruce-4(900MW PHWR) in Canada had an experience to directly measure the gaps between CT's and a LIN feeding an ultrasonic device into the LIN in 1993. As a result, the gaps were found to have enough margin than expected from computer code. At Pt. Lepreau(600MW PHWR) in Canada, the same ultrasonic device was used to measure the gaps between CT's and a horizontal flux detector guide tube(HFD hereafter) by feeding the device into the HFD. In this paper, an ultrasonic wave technology was also adopted for the measurement of gaps between CT's and LIN. For this purpose an ultrasonic device was newly developed for a PHWR(600 MW, started commercial operation in 1983) in Korea and introduced in this Conference. Several ports for the insertion of ultrasonic device into calandria, LIN, HFD, vertical flux detector guide tube(VFD hereafter) and viewing port(VP hereafter) were reviewed. Considering the measurement accuracy, site work volume, cost, personal exposure to radiation, experience etc., VP was selected.
The calandria of PHWR under investigation has 380 CT's with lattice of 11.25" and 6 LIN installed between CT row "F" and "G" at upper elevation, and row "Q" and "R" at lower elevation as shown in Fig. 1. Two viewing ports, of no use at present, were equipped for the observation of inside of calandria during construction and for the insertion of start-up unit at the beginning of operation. VP-1 is located at around center of the calandria axis between CT column "20" and "21" as shown in Fig. 1, while VP-2 between CT column "6" and "7".
In the gap measurement method proposed here, those gaps and the saggings of CT/LIN are directly measured by using a newly developed ultrasonic device inserted into the calandria through VP. With the measured data of CT and LIN at the position of VP, the gaps between a LIN and its upper /lower CT's along the axis of the LIN which were estimated.
2. ULTRASONIC MEASUREMENT OF CT AND LIN SAGGINGS
The ultrasonic device was inserted into the calandria via VP-1 and VP-2 to measure the saggings of CT and LIN in the PHWR under investigation. The objects to be measured were 24 CT's (E20~S20, G21~Q21) and LIN #1, #2, #4, #5 via VP-1, and 40 CT's (B06~V06, B07~V07) and LIN #2, #3, #5, #6 via VP-2. The principle of the ultrasonic measurement is that the strongest reflection is detected at the centerline of the tube as shown in Fig. 2. Ultrasonic probe was travelling along the VP sending ultrasound at the interval of 0.2mm and receiving the reflected wave on the surface of the CT and LIN. The peak amplitude of the reflected wave at the centerline of CT and LIN was found to be quite clear as shown in Fig. 3 as expected. Therefore the present
elevations of the CT and LIN were easily obtained from the travelling distance detected by an encoder attached on the drive mechanism of ultrasonic probe. To get the saggings of CT and LIN, the present elevations measured should be compared with the initial elevations installed at the beginning of plant operation. The gap between CT and LIN can be easily obtained by subtracting the radii of two tubes from the elevation difference between two tubes. Table 1 summarized some data for the saggings of CT and LIN, and the gap between two tubes. Strictly speaking, however, the measured gap is not the value at the position of "c" as shown in Fig. 4, but the value of difference between the CT elevation at the position of "a" and the LIN elevation at the position of "b". Therefore it is necessary to mathematically model CT and LIN for the calculation of the gap at the position of "c".
Fig 3: Typical C-scan and A-scan display in measurement
Fig 2: Ultrasonic measuring method of tube centerline
Fig 4: Measuring locations of CT/LIN saggings and gap to be estimated
3. MATHEMATICAL MODELING OF CT AND LIN SAGGING
CT can be mathematically modeled as a beam the deflection curve of which should be fitted to the deformed shape of CT. Since CT is fixed at both ends by the calandria tube sheets, it can be modeled as a beam with both ends clamped. Then the deflection curves of can be expressed by 4th order equation. With boundary conditions for both ends clamped and measured sagging data at one point, the deformed shape of CT can be theoretically determined.
LIN can be also mathematically modeled as a beam but with different boundary conditions from CT, since LIN is fixed at one end but supported by a bracket at the other end as shown in Fig. 5. The actual boundary condition at the end supported by bracket seemed to be between "clamped" and "simply supported". The measured data showed the boundary condition at this end was near to "simply supported". With the same mathematical procedure as used in CT, the deformed shape of LIN was obtained.
With the obtained deflection curves of CT and LIN, the gap between CT and LIN at the position of "c" can be calculated.
Fig 5: LIN configuration as installed
Ultrasonic technology was successfully applied to measure the saggings of calandria tubes and liquid injection shutdown system tubes in a PHWR in Korea. Ultrasonic wave generated from ultrasonic probe which was inserted into calandria through viewing port was found to precisely detect the centerlines of calandria tubes and liquid injection shutdown system tubes. The saggings of calandria tubes and liquid injection shutdown system tubes were obtained from the present elevations of the centerlines measured and the initial elevations at the beginning of plant operation. The gap between calandria tubes and liquid injection shutdown system tubes were obtained based on the ultrasonic measurement data (the difference in elevation between two tubes) and mathematical modeling for the deflection curves of both tubes by using the classical beam theory. The results for saggings and gaps were found to be reasonable
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