Strain Scanning Measurements of OPAL Cold-Neutron-Source Vacuum-Containment Welds

Authors
 

Marty Jones, Warren Brown and Vladimir Luzin (ANSTO-Bragg Institute), Phil Bendeich and Michael Law (ANSTO-Institute of Materials Engineering)

The OPAL cold-neutron-source vacuum containment is a major structural element of the cold-source unit serving several functions, one of which is to hold cold source inside. The cold source vacuum containment is essentially a long pipe, about 3 meters long and ~330 mm in diameter, and it is made of zirconium alloy.

Because of its size, it is fabricated from several sections which are joined together using the electron-beam welding technique (manufactured by PNPI in Russia).

The safety case for continued use of the OPAL cold neutron source vacuum containment is intended to show it to be safe beyond 10 years. The validation process consists of a number of experimental activities in the frame of the Surveillance Programme, aimed at providing mechanical property data; principally the determination of the effects of radiation on mechanical properties and the measurements on representative welds to determine the level of residual stress.

The KOWARI strain scanner has been used to determine the residual stress in an electron-beam weld that was produced as part of the manufacturing process assessment and that is representative of the actual welds in the vacuum containment (Fig. 1).

This measurement is particularly difficult, due to the fact that the weld is in Zr-2.5%Nb alloy which has both a large grain size, a low neutron scattering cross section but fine required spatial resolution, which was only 1x1x1 mm3. This resolution is necessary because of the fact that the electron welding process produces very narrow welding and heat-affected zone.

The whole obstacle was overcome by the ingenuity of Institute staff (Marty Jones and Warren Brown) who constructed a special rocking cradle for measuring the entire length of the sample weld (Fig. 1). In addition, the scattering vector rocking technique was employed, and also several reflections, Zr(103), Zr(112) and Zr(201), were used to further improve the grain statistics.

The preliminary results are that the strains measured in the weld region were low (Fig. 2) and thus residual stresses will not add significantly to the design stresses in the component.