Hydrogen Vessel Weld Porosity Stress Analysis Using CT Scan Data

The Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory produces the most intense, pulsed neutron beams in the world for various scientific research experiments and industrial development activities. The neutrons spalled from a mercury target are moderated and directed into instrument channels by the Inner Reflector Plug (IRP), which is designed for the purpose of moderating, reflecting, and shielding these neutrons. The SNS lower IRP is the bottom-most segment of the inner reflector plug components. The lower IRP is a complex structure consisting of an aluminum vessel, beryllium and stainless-steel reflectors, water and cryogenic hydrogen moderators, neutron poisons, and various channels for neutron transport. The lower IRP must be replaced regularly due to consumption of the neutron poisons and embrittlement of the structural materials from radiation. The hydrogen moderators are constructed from 6061-T6 aluminum. Each hydrogen moderator has a vessel with concentric tubing for supply and return of 18K hydrogen. The hydrogen moderator vessel is welded to a connecting tube using gas tungsten arc welding (GTAW). Radiographic Testing (RT) of a moderator vessel during fabrication indicated porosity within the weld but did not define the location or shape of the porosity. For a better definition of the weld defects, the computed tomography (CT) scan technique was used to produce an accurate 3D image of the weld and weld defects. This paper will describe the procedure to incorporate the CT scan porosity data into the finite element model of a hydrogen moderator with a GTAW weld to determine fitness-for-service. The stress analysis was performed to evaluate the weld defects and to qualify the GTAW weld via the ASME BPVC design criteria.