Power plant inspection
What's the value of non-destructive testing?
By Simon Francouer
Simon Francouer of SGS explains the value of NDT in reactor safety inspections technology
During annual outages of CANDU type nuclear power plants (a Canadian designed pressurised heavy water reactor), non-destructive testing (NDT) inspectors are required to perform specialised inspections, in different locations.
An important one is the ultrasonic inspection of the primary heat transport system feeders on the reactor face (on the pipes that carry heavy water heated from the reactor to the steam generators). These have to be verified at every shutdown because past experience has shown that these pipes can present axial cracking in their first and second bend.
The cracks appear because the feeders can be affected by the intergranular stress corrosion cracking phenomena.
These feeder pipes carry the primary coolant required to cool the nuclear fuel. If one or more pipe ruptures, it would cause a real emergency and the operators would need to completely shutdown the reactor, because without enough cooling a meltdown would be possible.
For these reasons, it is important to know if there are cracks in some bends. To detect these, an ultrasonic procedure has been developed. But, because of the high level of radiation, a robotised ultrasonic inspection (BCC: bend cracking crawler) has been developed to minimise the time exposure to the NDT operators and to ensure a maximum repeatability.
Because the cracks are oriented axially, we have to inspect the pipe over the entire circumference. The thickness of the pipe wall is quite thin. So, it is possible to inspect the entire thickness of the wall (after two skips, the ultrasonic beam is so wide that it covers both the internal and the external faces). Also, each probe can scan about 250o of the pipe circumference.
Using four probes, two face-to-face targeting the extrados and two face-to-face targeting the intrados of the pipe, it is possible to scan the entire circumference in one rotation and also, because each zone is scanned with two probes, we are able to obtain a high rate of probability of detection.
This set-up provides inspection rates much faster than manual ultrasonic and more reliable for the sequence and the positioning. Sometimes with manual ultrasonic inspection, it is difficult to keep a good contact with the pipe material and maintain a good scanning position, because the access can be difficult, which is not a problem with the BCC.
There are several different steps to the inspection process. First, the specialist responsible for the data acquisition performs and validates the calibration of the BCC. Then, an operator standing in a shielded cabin (once again to protect the worker from radiation) installs the crawler on a feeder. The data acquisition’s specialist confirms the position of the BCC with the operator and starts the inspection.
While scanning the pipe, they make sure that the data is collected properly with no data missing. The operator, during that time, can protect himself in the shielded cabin to minimise his exposition to radiation. Once the scan is complete, the data acquisition specialist makes sure, one last time, that the data is good and he sends the files to two analysts. Both of them evaluate each file independently to make sure that there is no rejectable indication in the pipe. If a suspicious indication is detected in one file, further evaluation has to then be carried out.
At this particular position, a second inspection has to be performed by manual ultrasonic methods to confirm the indication. If it is still present and is considered as a crack, many other techniques will be used to characterise the indication, such as: phased array, time of flight diffraction, magnetic rubber replica, or eddy current.
SGS works alongside the nuclear industry to develop methods of inspection to maximise results and minimise the total exposure of the worker to radiation.