Investigation into the Mechanical Cause of Failure of Neutron Guide NG-2 at the NIST Research Reactor

This study investigates the fractures that occurred in a glass structure used to carry neutrons from a cold source in the NIST reactor into an experimental hall that contains neutron-scattering instrumentation used to perform experiments in chemistry, materials science, physics, and biology. These guides are typically made of rectangular borosilicate glass tubes, coated on the inside with a neutron-reflecting coating. The guide tubes used at the NIST Center for Neutron Research (NCNR) have internal cross sections of dimensions 150 × 60 mm, with lengths extending over as great as 60 m, with gaps for insertion of the instruments used to evaluate materials. On August 23, 2011, a 5.8 magnitude earthquake occurred in Mineral, Virginia, which resulted in significant ground motion over 150 km away at the NCNR. An initial inspection and vacuum test revealed no significant damage to the seven neutron beam lines. After a few weeks, neutron guide 2 (NG-2) that was located in the Guide Hall near the reactor building wall cracked and broke while being evacuated. The cause of fracture was identified by observation of the glass fragments and analysis of the stress distributions in the guide. The delayed fracture was caused by damage introduced during the earthquake.