The reactor vessel was manufactured in 1957-58 from a dome-shaped part welded to a cylinder made of four curved plates. These five pieces were made of 60 mm thick boiler steel with a 5 mm lining of roll-bonded austenitic stainless steel. The reactor was started in 1959 and has been in operation since. The reactor vessel is certified for operation at 33.6 bar/240°C. The welds of the vessel were subject to 100 % radiographic examination before and after being covered with stainless steel.
The integrity of the vessel was during the first years of operation ensured by neutron embrittlement measurements and assessment, internal visual examination and pressure tests. Since 1976 this has been supplemented by ultrasonic testing every third year, in accordance with a procedure recommended by Det Norske Veritas, and based on the requirements of Norwegian boiler authorities and ASME XI. This examination covers the regions of the welds receiving 90-100% of the neutron irradiation. Straight beam probes are employed for detection of reduced vessel wall thickness, while the longitudinal welds and the upper part of the lower circumferential weld are covered by 45° shear wave probes. Since 1994 70° transmitter-receiver longitudinal (TRL) probes are used in order to increase the probability for detecting defects in or close to the inner cladding, replacing previously employed 60° probes. Initially immersion probes were used, but since 1979 contact probes.
As it is not possible to remove the reactor core nor the vessel lid, ultrasonic examination is done by means of a 10 m long manipulator operated through 77 mm dia. fuel element / control rod holes in the reactor lid. The ultrasonic probes are subject to at a high radiation level, and a temperature of 70 °C.
The investigations with the 45° probes have confirmed findings from previous in-service inspections with indications in accordance with these. The 70° TRL probes have given no indications signifying the presence of underclad cracks or other inner wall near surface defects. The straight beam probe examination has given no evidence of thickness reductions.
The NDE results, and the supplementary investigations, indicate that there has been no defect development since the first in-service inspection, and the integrity of the vessel seems thus well maintained after the almost 40 years of reactor operation.
Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00
Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:
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