The Corrosion of Materials in Spallation Neutron Sources

During the past 30 years numerous particle accelerators have been developed around the world: the Fermi proton/anti-proton ring in Illinois, the KEK proton synchrotron in Japan, the CERN proton synchrotron in France, and the ISIS pulsed neutron spallation source in Oxford, to name a few. The world’s highest current spallation neutron source is the Los Alamos Neutron Science Center (LANSCE) at Los Alamos National Laboratory. At LANSCE, 800 MeV protons at currents as high as 1 mA strike a tungsten target, producing high-energy neutrons. Circulating water loops are used to cool the target and moderate the energy of the neutrons. This source of moderated neutrons is then used for neutron scattering and neutron diffraction studies. Spallation neutrons have also been proposed for the production of tritium. This has led to the Accelerator Production of Tritium (APT) project headed by the Los Alamos National Laboratory in collaboration with Westinghouse Savannah River Company, Brookhaven National Laboratory, Lawrence Livermore National Laboratory, and Sandia National Laboratories. In the APT project, high-energy protons would be accelerated to 1.7 GeV by a linac operating at currents above 1 mA. As at LANSCE, high-energy neutrons would be produced when the proton beam leaves ultra-high vacuum and strikes a tungsten target. A circulating water loop would provide neutron moderation and target cooling. Tritium would be produced by capturing the moderated neutrons in a 3He gas blanket. To study the feasibility of building such a facility, the APT project has been divided into three separate thrust areas: accelerator technology (including RF quadropole design, high-energy linac, etc.); target/blanket technology (which includes neutron production and materials verification); and tritium separation technology (such as 3He blanket, isotope separation, etc.) The materials verification studies for the target blanket loop include examining the effects of proton/neutron radiation on materials strength and ductility as well as materials corrosion during high-energy proton irradiation and in radiolyzed water. As such, this article presents a summary of current Efforts to measure the real-time corrosion rates of alloy 718 during 800 MeV proton radiation at currents up to 1 mA are reported. Specially designed corrosion probes, which incorporate ceramic seals, were mounted in a water manifold that allowed samples to be directly exposed to the proton beam at the Los Alamos Neutron Science Center. The water system that supplied the manifold provided a means for controlling water chemistry, measuring dissolved hydrogen concentration, and measuring the effects of water radiolysis and water quality on corrosion rate. Real-time corrosion rate measurements during proton irradiation showed an exponential increase in corrosion rate with proton-beam current. These results are discussed within the context of water radiolysis at the diffusion boundary layer/beam-spot interface. However, additional factors that may influence these parameters, such as oxide spallation and charge build-up in the passive film, are not ruled out.