A sound way to inspect RCCA degradation30 July 1998
A novel ultrasonic technique for inspecting rod cluster control assemblies (RCCAs) uses low frequency signals to accurately measure remaining wall thickness and outer diameter of component rods, allowing the detailed evaluation of RCCA degradation. Not only are the quantity of information increased and quality improved compared with other techniques currently used, the overall inspection times are greatly reduced.
One of the key control and safety components of a PWR is the rod cluster control assembly, an array of control rods composed of absorber material sheathed with stainless steel cladding tube (Figure 1). During plant operation, the 4-metre long RCCAs are almost fully withdrawn from the fuel assembly and held in the RCCA guide tube.
The integrity of the rods is gradually degraded over the life of the assembly. Degradation occurs in two distinct ways. The first is due to the high radiation exposure, which leads to the swelling of the absorber and to the cracking of the tubes. The second degradation mechanism is the fretting wear of the cladding tubes caused by the interaction between the rods and the guide cards composing the RCCA guide tube (Figure 2). On-site RCCA inspections have been carried out in order to monitor the degradation and establish the replacement criteria.
DESIGN REQUIREMENTS AND CONSTRAINTS
Most RCCA inspection systems, whether they use strain gauges or the Eddy Current technique, measure only a variation of the outer diameter (OD) of RCCA rods and wear of cladding tubes is inferred from this OD measurement. This approach is not effective to estimate real wear. Additionally, most systems have low inspection speed and insufficient data sampling resolution.
The new system was designed to meet the following objectives:
• Accurate measurement of wear of cladding tubes and of the outer diameter of rods.
• High inspection speed and high resolution.
• Easy to transport and setup, and to calibrate.
Techniques relying exclusively on the variation of the outer diameter cannot be used for the accurate evaluation of wear of cladding tubes. From the outset of the project, it was clear that to reach the inspection objectives, we had to accurately measure the remaining wall thickness of the cladding tube as well as the outer diameter of the rod. Ultrasound can provide the information for both objectives.
The thin wall material and the required accuracy would normally dictate the use of high frequency transducers. The distance between the probes and the processing equipment made this impractical; much of the signal processing equipment would have to be placed near the probes, under water in a highly radioactive area. This is obviously a very costly tradeoff. If on the other hand, a relatively low frequency ultrasound transducer is used, the ultrasound surface echo hides the echo from the back-wall of the cladding tube. In such conditions, accurate and reliable wall thickness measurement becomes difficult.
To overcome this difficulty and still allow the use of low frequency transducers, the new system applies spectral analysis of the RF signal to extract wall thickness information. The repetitive back-wall echoes, while not easily discerned by the human eye, introduce a frequency component that is inversely proportional to the wall thickness.
The inspection system acquires the full radio frequency waveform at high resolution. While the ultrasound surface echo, which provides information about the outer diameter, is clearly visible, the back-wall echo is nearly impossible to distinguish from the surface echo. The time displacement display also clearly shows the position of the rod surface and the pattern of wear (Figure 3). Analysis in the frequency domain shows a strong signal at a frequency proportional to the inverse of the remaining wall thickness (Figure 4). The analysis software processes this information in the frequency domain and plots a true profile of the remaining wall thickness. Using this technique, an accuracy of ±30 µm is achieved on the measurement of wall thicknesses.
The inspection tool assembly is composed of 24 rotating ultrasound probes, one for each rod of the RCCA assembly. The probes are linked together with a gear mechanism and are rotated using a watertight motor assembly (Figure 6). An extra probe is used to measure a reference thickness and compensate OD and thickness measurements for errors due to temperature variations.
The motorised assembly is installed onto the spent fuel rack in the fuel pool during inspection outage. The RCCA is lowered into the probe array at the normal crane speed of 50 mm per second (Figure 7). Probes rotate synchronously at 10 rps firing ultrasound pulses every 3 degrees (data sampling resolution is 3° x 5 mm). One full inspection cycle, consisting of the lowering and lifting of the RCCA, takes less than 5 minutes.
To achieve the high inspection resolution and speed, the system must collect over 15 600 waveforms per second producing 3.9 megabytes of data per second. This enormous quantity of information requires an extremely fast ultrasound recording and processing system. R/D Tech’s Tomoscan/SV ultrasound system was selected because of its speed (20 kHz pulsing rate and 5 megabytes per second data rate), high bandwidth (30 MHz), and 12-bit, 60 MHz A/D (Figure 8).
The data acquisition workstation is a dual Pentium Pro PC with 128 megabytes of RAM, very fast hard disk drives, and a high-capacity magneto optical disk drive. The workstation is connected to the Tomoscan /SV using a standard LAN connection. The system setup, calibration, and data acquisition are all performed from that workstation.
Each system component is very compact and lightweight and the complete system only takes half a day to set up under field conditions. The system calibration is carried out at the beginning of the inspection using a dummy RCCA and takes less than 30 minutes.
The RCCA inspection systems relying exclusively on the measurement of the rod’s outer diameter cannot provide accurate evaluation of cladding tube degradation. The new inspection system described here measures the remaining wall thickness as well as the outer diameter using ultrasonic testing techniques, and provides accurate and reliable information for evaluation of both degradation modes while improving the inspection speed compared with previous techniques.
This system has reached all the design goals set and demonstrated excellent performance. The verified specifications of the system are as follows:
• Remaining wall measurement accuracy: ±30µm.
• Inspection speed for all rods of cluster: Less than 5 minutes.
• Inspection resolution: 3° x 5 mm.
• System calibration time: Less than 30 minutes.
• System weight (probe array, ultrasonic testing system, workstation, motor drive unit, cables, etc.): Less than 150 kg.