High fluxes of neutrons can be generated by high- energy and current (power) proton beams via spall­ation reactions that fragment the atomic nuclei heavily in a heavy metal target (like W, Pb, and Hg). At 500 MeV, these reactions produce «10 neu­trons per proton. Applications of spallation sources include neutron scattering, nuclear waste transmu­tation, and driving subcritical fission reactors. A key challenge to developing advanced high-power, long- lived spallation source targets is the ability of structural alloys to withstand severe radiation dam­age, corrosive fluids, and mechanical loading. Most notably, radiation damage in spallation source irradia­tions, produced by the neutrons and protons, results in both high dpa and concentrations of transmutation products, including He and H (see Table 1). As a consequence, there have been and continue to be international programs on radiation effects in SPNI environments, beginning with a large program in Los Alamos Neutron Science Center (LANSCE) in 1996 and 1997,55 followed by a continuing SINQ (Swiss Spallation Neutron Source) Target

Irradiation Program — STIP, started in 1998, that con­tinues to this day, at the Paul Sherrer Institute, in Switzerland, involving an international collaboration of ten institutions in China, Europe, Japan, and the United States.56,57

Because of the accelerator production of tritium tar­get application, the irradiation temperature LANSCE experiment was up to «164 °C. The highest damage levels, mostly produced by protons, were «12 dpa and «180 appm He/dpa.4 About 20 materials were irradiated in a variety of specimen configurations in this study.

The maximum damage levels in the STIP-I to — IV irradiations56,57 were « 25 dpa and 2000 appm He. The corresponding temperatures ranged from 80 to 800 °C, but most specimens were nominally irra­diated between 100 and 500 °C. The temperatures directly depend on the high nuclear heating rates in the target, and both varied by ±15% during the 2-year irradiation; and, in the case of STIP-I and-IV, some capsules experienced a significant overtem­perature transient. The high heating rates also result in fairly large uncertainties in the temperatures of individual specimens. Note that the temperature control in the most recent STIP-V experiment was significantly better than that in previous studies. Over 60 elemental metals and alloys, ceramics, and composites have been irradiated in the STIP-I to — V, in the form ofminiaturized specimens for both micro­structural studies and mechanical testing, including tensile, fatigue, fracture toughness, and Charpy V-notch (CVN) measures of the DBTT. Some speci­mens were irradiated in contact with stagnant liquid Hg, PbBi eutectic, and Pb. The STIP database is discussed in Section 1.06.4