Material Selection and Corrosion Studies for a High Energy Physics Experiment: The Neutron Time of Flight Facility at CERN

The neutron Time-Of-Flight (n_TOF) facility at CERN is a source of high flux of neutrons obtained by the spallation process of 20 GeV/c protons onto a high purity (99.99 %) lead target. In 2008 the facility, that suffered severe corrosion after few years of operation, was completely upgraded. It includes now an aluminum alloy vessel and a cooling circuit with monitored oxygen content, pH and conductivity of the demineralized water used as coolant. Oxygen produced by water radiolysis is enforced at the lowest levels in order to minimize otherwise severe galvanic corrosion effects between the lead target and the thin entrance and exit windows of the experiment, optimized for maximum transparency to proton and neutron fluxes. The paper discusses the material selection that had to be based on criteria of corrosion resistance in a galvanic and highly radioactive environment, ready weldability of the alloy for the vessel and compliance with pressure vessel codes. This selection was supported by a large campaign of corrosion tests including through a specially developed and monitored test loop using demineralized water as process fluid. The experience gained through the operation of the new facility, running smoothly since its commissioning in November 2008, will be reported.