Materials research and development for the spallation neutron source mercury target

Abstract

In the Spallation Neutron Source target, the structural material will be exposed to intense pulsed fluxes of high-energy protons and neutrons, which produce radiation damage. These pulsed fluxes also lead to pressure pulses created by beam heating. In turn, the pressure pulses give rise to fluctuating stresses in the 316 LN austenitic stainless steel target vessel, and to cavitation in the liquid mercury spallation target. Corrosion reactions and related changes in mechanical properties also may occur through contact with flowing mercury. We describe the materials research and development program for the spallation target. The program covers the areas of cavitation erosion, radiation effects, and compatibility. Cavitation erosion work includes pressure wave tests at the LANSCE proton accelerator, as well as laboratory tests that simulate aspects of the actual in-beam exposures. Materials irradiations are being carried out in spallation environments at high-energy and high-power proton accelerators. Other experiments are conducted at irradiation facilities that simulate aspects of spallation conditions. Extensive radiation damage and transmutation calculations supplement these experiments. Compatibility work includes both thermal convection and pumped flow loop tests to examine temperature gradient mass transfer, as well as fatigue and tensile tests in contact with Hg. Based on the information developed for radiation effects and compatibility with mercury, our analysis indicates that the target will meet its intended service requirements. In the past year and one half the new issue of cavitation erosion has been included in the program. Both in-beam and laboratory experiments indicate that cavitation erosion may occur in the target. The highest priority activity is now to determine whether cavitation erosion will limit target lifetime to a level below the lifetime limit set by radiation effects.