Posiva testing copper canister inspection technology at Olkiluoto
The copper cast iron canister is the most significant technical release barrier of the final disposal concept. It is required to remain intact for tens of thousands of years, which is why the quality of the canister must be fully verified prior to final disposal.
NDT methods enable quality assurance without changing the characteristics of the actual final disposal product. In addition to ultrasound and X-ray inspection, the high quality of the final disposal canister is ensured by utilising eddy current and visual inspection methods. In the eddy current method, a magnetic field is induced in copper, which conducts electricity. With eddy current sensors, it is possible to identify changes in the magnetic field. The changes are caused by material faults, i.e. changes in the material characteristics. Because the material characteristic changes in components are minor, the signals (changes) identified with sensors are usually caused by faults in the material. The ultrasound method is an acoustic method requiring an acoustic contact between the sensor and the item being tested. Water is the usual contact substance in mechanised inspections. Should the tested canister component have defects, they are reflected back to the ultrasound sensor in the same way as in using sonar to find shoals of fish.
Posiva has been testing different NDT methods in disposal canister inspection at the SKB canister laboratory in Oskarshamn for a long time already. The same methods have also been used in defect identification in the quality assurance of welds created in sealing the canister cover.
This year, the copper overpack NDT inspection method has been tested in Olkiluoto. The main focus has been on the joint operation of the Posiva-obtained canister component rotation equipment and eddy current and ultrasound equipment. The equipment has been tested in the inspection of the final disposal canister's full-sized copper overpack (T84).
Why are copper overpack NDT inspections being performed in Olkiluoto right now?
"The rotation device (manipulator) was completed in the autumn of last year and the first copper cast mould inspection was performed with it in the autumn of 2013", says development engineer Jorma Pitkänen. We are now continuing with more detailed testing that provides information about the joint operation of the manipulator and the inspection equipment."
How does the equipment functionality seem? Have there been any surprises?
"The manipulator has been robust and efficient enough. However, there have been some deficiencies in the speed control of the rotation device in relation to the eddy current device. The deficiencies will be taken care of in future. We have also noticed that the copper pipe moves, as expected, to some extent in the longitudinal direction when it is rotated. This will be prevented in future inspections by taking relatively small structural steps.
What experiences have been obtained from the ultrasound inspection?
"Changes are needed in the speed control of the rotation device engine. In addition, the water through which the ultrasound signal travels from the sensor to the copper pipe does not spread evenly enough on the active contact surface of the sensor, which means that the sound barrier does not properly penetrate the component being tested. This can be fixed by making minor changes to the structure of the ultrasound sensor clamp."
How small a defect is eddy current technology capable of identifying?
"Regarding surface defects, eddy current inspection is used, and its impact extends to a depth of about 10 mm. At this depth range, eddy current technology is capable of reliably identifying defects of an approximate depth of 2 mm. The requirement is that at least 35 mm of the copper overpack thickness is intact, flawless material."
What is the case for using ultrasound technology?
"With ultrasound technology, the copper overpack can be inspected at a depth of 5–50 mm, that is, throughout the overpack thickness. This results in a fault size of up to 1 mm, but in the powerfully dampening copper, the fault size identified increases to about 2–2.5 mm according to initial estimates."
How long does the copper overpack inspection take?
"Now that the equipment is being introduced, the inspection will take about one and a half weeks. From now on, testing can be done within a week, and during the final disposal plant's operation stage, it should be possible to inspect one copper canister in one day."
How will the inspection equipment package be developed further?
"The sensor and clamp geometry needs further improvement, and the sensor technology IT and application software need to be developed so that the entire copper overpack is covered in a single inspection in regard to ultrasound. Different kinds of technology can also be combined in the same measuring process so that the inspection time is shorter."