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Ultrasonic Measurement of Calandria Tube Sags in a Korean PHWR'sTae Ryong Kim
Seok Man Sohn
Senior Research Engineer
Periodic Safety Review Group
Korea Electric Power Research Institute.
Calandria tube in a pressurized heavy water reactor is known to sag due to irradiation creep and growth during plant operation. When the sag of calandria tube is big enough, the calandria tube possibly comes in contact with liquid injection shutdown system tube crossing beneath the calandria tube, which subsequently may prevent the safe operation of the plant. It is therefore necessary to check the gap between the two tubes in order to confirm no contacts using a proper measure periodically during the plant life. An ultrasonic gap measuring probe assembly which can be inserted in calandria via viewing ports of the calandria was developed and utilized to measure the sags of both tubes in a pressurized heavy water reactor in Korea. It was found that the centerlines of calandria tubes and liquid injection shutdown system tubes can be precisely detected by ultrasonic wave. The sags of calandria tubes and liquid injection shutdown system tubes were obtained by comparison of the present centerlines with the initial elevations at the beginning of plant operation. The gaps between two tubes were also easily obtained from the relative distance of the measured centerline elevations of the tubes. Based on the irradiation creep equation and the measurement data, a computer program to calculate the sags was also developed. With the computer program, the sag at the end of plant life was predicted.
Keywords: Ultrasonic Measurement, Calandria Tube Sag, LISS Nozzle,Irradiation Creep, PHWR
Pressurized heavy water reactor(PHWR) consists of a big cylinder(called "calandria") with many fuel channels installed horizontally. Each fuel channel consists of a calandria tube(CT hereafter) and a pressure tube(PT hereafter) which is supported by 4 spacers as shown in Fig. 1. 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. 2. Since the calandria is filled with heavy water as moderator, CT's and LISS tubes (LIN hereafter) are immersed in heavy water.
|Fig 1: Fuel channel configuration of PHWR .|
|Fig 2: Schematic diagram of a PHWR (700MW) calandria.|
CT and LIN of PHWR made of zirconium alloy are known to sag during the plant operation due to irradiation creep and growth [Fidleris, 1988]. Of course the CT can sag due to the loads of tube/fuel weight and the counterforce of buoyancy, but it is known to be negligible comparing to the irradiation effects when the plant becomes older. When the sag of CT is big enough, the CT can possibly contact the LIN crossing beneath the CT, which subsequently may prevent the safe operation of the plant. It is therefore necessary to measure the gap between the two tubes for the safe operation.
|Tube||Number||Length (mm)||Inner Diameter (mm)||Thickness (mm)||Material|
|Table 1: Material and dimension of CT and LIN.|
There are several ports to insert a measuring device into the calandria, that is, through LIN itself [Abucay, 1995] or horizontal flux detector guide tube [Goszczynski, 1996] or vertical flux detector guide tube or viewing port (VP hereafter). Direct insertion method through LIN has an advantage that the gaps between CT and LIN can be directly measured, but requires that the LIN be straight and be equipped with a flange for easy insertion of the measuring device.
After reviewing the prerequisites, the licensing matters, inserting procedures, site work volume, cost, personal exposure to radiation, experience, etc., VP was selected as a most suitable port for the insertion of a measuring device. Accordingly a new ultrasonic probe for measurement of gaps between CT and LIN which can be vertically inserted into calandria through VP was developed [Park, 2000] and applied for the first time in the world to a PHWR in Korea [Kim, 2000]. Similar measurement method through VP using a mechanical tool was also developed by AECL and applied to Point Lepreau [Eijsemian, 2001] after the successful measurement in Korea.
Ultrasonic technology was thought to be most suitable measurement because of two advantages. First, calandria of PHWR is filled with heavy water that is a proper medium for the ultrasound propagation. Secondly, the performance of the ultrasonic probe is hardly affected by the high radiation levels in the calandria.
A new ultrasonic probe for measurement of gaps between CT and LIN to be vertically inserted into calandria through VP was developed. The principle of the ultrasonic measurement is that ultrasound perpendicularly incident to a tube is reflected back to the transducer and the strongest reflection is detected when ultrasonic beam is directed at the centerline of the tube.
2.1 Ultrasonic probe assembly and mechanical driving mechanism
Measurement system consists of ultrasonic probe assembly, mechanical driving mechanism, guide tube and data acquisition & analysis system as shown in Fig. 3. The ultrasonic probes used were of immersion type (2.25 MHz). The ultrasonic probe assembly is driven by an AC servomotor (controlled by either a joystick or main computer) along with a ball screw inside the guide tube. An encoder mounted on the ball screw emits 720 counts per revolution and encoder/ ultrasonic data are transmitted to the computer for the evaluation of the actual elevations of CT and LIN. A radiation proof camera equipped with light source was also mounted to see the actual status of tubes in the calandria for a supplementary inspection tool.
|Fig 3: The diagram of ultrasonic sag and gap measuring systems.|
2.2 Data acquisition hardware and software
The data acquisition system consists of an ultrasonic pulser/receiver and analog to digital converter. The main program for data acquisition uses X/Motif window and provides graphics window interface. Users can select an appropriate window(s) and control system motion. Main program uses pull-down menu that consists of "ultrasonic calibration", "ultrasonic examination", "signal analysis", "sag analysis" and "system utility". It includes multitasking and real time operation software by using Lynx operating system. In addition, TCP/IP LAN protocol is used, which enables 10Mbps data transfer rate from data acquisition computer to data analysis computer. Data acquisition software transfers ultrasonic data in real time and stores them to optical and/or hard disk. Concurrently, ultrasonic signals and probe positions are displayed on CRT. Some features of the software for data acquisition and data analysis are as follows:
2.3 Mock-up test
In order to confirm the performance of the ultrasonic measurement system, a partial scaled-down mock-up of calandria was designed and constructed. Ten CT's of the actual diameter but short length were horizontally placed with the actual pitch of 285.75mm. A port was prepared on the top of the calandria mock-up as a VP for the insertion of the ultrasonic measuring probe. Four LIN's of the actual size were placed at the distance of 40cm, 60cm, 80cm and 100cm from the measuring probe when inserted. The CT and LIN position data obtained by ultrasonic measurement system was compared with the position data pre-measured by calipers. From the performance test results at the mock-up, the positioning errors were found to be within ± 1mm that was thought to be the sufficient accuracy for the measurement of sag of CT/LIN and gap between two tubes.
The image obtained from the mounted camera was processed and displayed on a monitor. Unfortunately the edges of the tube were not clearly found with the camera due to the illuminating problem. That is, the light emitted from camera was reflected at the edge of the tube, but never came back to the camera.
The ultrasonic probe assembly developed in this study was used to measure sags of CT and LIN at Wolsong-1 (679MW PHWR) in Korea which has been operated for 18 years. The same ultrasonic probe assembly was again used in Wolsong-4 (700MW) which started its commercial operation in 1998 for the measurement of the CT sag in the early stage due to the irradiation creep.
The calandria of PHWR under investigation has 380 CT's with lattice of 285.75mm and 6 LIN's installed between CT row "F" and "G" at upper elevation, and row "Q" and "R" at lower elevation as shown in Fig. 2. Two viewing ports, of no use at present, were equipped for inside observation 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". 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.
|Fig 4: Typical C-scan and A-scan display.|
The ultrasonic probe traveled along the VP sending ultrasound at intervals of 0.2mm and receiving the reflected waves 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. 4 as expected. The sags of CT and LIN were easily obtained from the relative difference between their present elevations and the initial elevations when constructed. Since the initial positions of CT and LIN as installed were not measured, however, the designed position from a reference point in the calandria as per the construction drawings were used in the comparison. Therefore the installation tolerance of the CT and LIN should be considered in the measured data.
The measured sags of CT and LIN in Wolsong-1 and Wolsong-4 are summarized in Table 2. The measured CT sag was in the range of 41~48mm for Wolsong-1 and 4~11mm for Wolsong-4, while that of LIN sag in the range of 16~21mm for Wolsong-1 and -4.1~0.7mm for Wolsong-4. The negative sag in LIN is thought to happen due to the installation tolerance (± 1/16") in the construction stage. The average CT sag rate of Wolsong-4 was found to be higher than that of Wolsong-1. This is because the structural sag takes a dominant portion in CT sag of the early stage as mentioned earlier.
|CT or LIN||Wolsong-1(18 years)||Wolsong-4 (1.8 years)|
|Measured Sag (mm)||Avg. Sag Rate (mm/year)||Measured Sag (mm)||Avg. Sag Rate (mm/year)|
|Table 2: Sags of some CT/LIN measured at VP-2.|
The LIN sag was found to be much smaller than that of CT as expected, because LIN is located in the weaker irradiation environment than CT is.
The inspection plan to see the actual status of tubes in the calandria by the radiation proof camera was failed as expected. A root cause of the failure is the illuminating problem as already experienced in the mock-up test.
The gap can be easily obtained by simply subtracting the radii of two tubes from the elevation difference between the measured centerlines of two tubes. To check whether the two tubes come in contact, the gap between them at their crossing point should be found. Strictly speaking, however, the measured gap data observed at viewing port is not the value at the crossing point of CT and LIN. To get the actual gap at the crossing point, an estimation method utilizing mathematical deflection curves of two tubes based on the classical beam theory was developed and applied. Since the effect was found so small, the gap obtained by simply subtracting the radii of two tubes from the elevation difference between the measured centerlines is to be referred for the engineering purpose.
Table 3 summarizes some of the results for LIN-5 and its upper CT (Q06)/lower CT (R06) of main concerns for Wolsong-1 and Wolsong-4.
|Wolsong-1(18 years)||Wolsong-4 (1.8 years)|
|Sag (mm)||Gap (mm)||Sag (mm)||Gap (mm)|
|Table 3: Summary of sags and gaps.|
To precisely compute the irradiation creep of CT, it is necessary to build a mathematical model including CT and PT, since both tubes are coupled as mentioned earlier. The basic creep equations used for CT and PT are as follows:
Here and are the creep rates of CT and PT respectively, x is the coordinate of location to compute the creep deformation along CT and PT axis, t is time when the creep deformation is computed, Fmc and Fmp represent the maximum fast neutron flux at CT and PT respectively, l is the total length of CT and PT, T is the absolute temperature, F is the compensation coefficient.
Based on the above irradiation creep equation and the measurement data, a computer program to calculate the sags was also developed. With the computer program, the sag at the end of plant life was predicted [Kim, 2001].
Sag of calandria tubes or liquid injection shutdown system tubes in PHWR due to irradiation creep could induce a contact of two tubes, which subsequently may prevent safe plant operation. It is therefore necessary to monitor the gap between the two tubes for the preventive and predictive maintenance of the plant.
An ultrasonic system designed to insert into calandria through viewing ports on the reactor top without any plant system design change or modification was developed for the measurement of CT and LIN sag at PHWR. It was the first time in the world to insert an ultrasonic measurement system into calandria through viewing port. The measuring accuracy was found to be within ± 1mm, which was sufficient for the measurement of gap between CT and LIN.
The developed ultrasonic measurement system was successfully used to measure sags of the horizontally installed CT and LIN at operating PHWR's in Korea. To predict the sagging trend along the operation, a computer program based on the measurement data and the irradiation creep equation was also developed. With the computer program, the sag at the end of plant life was predicted.
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