Nuclear Science User Facilities 52 The NSUF University of California at Santa Barbara (UCSB)-1 Library Advanced Test Reactor (ATR) irradiation experi- ment was designed to create a new and unprecedented library of alloys and irradiation conditions to facilitate understanding of and modeling to ultimately predict and improve the behavior of structural materials used in nuclear energy systems.The UCSB-1 experiment comprised ≈1400 specimens of various types composed of 40 alloys that included tempered martensitic, nanostructured ferritic, dual phase stainless, maraging, and bainitic RPV steels.The irradiation also included Fe-Cr, Fe-Cu-Mn-Ni-Si ferritic and Cu-Nb multilayer model alloy systems.The four-cycle (145A to 146B) irradiation in the A10 position in the ATR represented 210 effective full power days.Thirty-two isothermal temperature packets contained in nine capsules were irradiated from ≈ 1.8 to 6.8 displacements per atom (dpa) at seven temperatures, ranging from ≈ 320 to 750°C.The side-by-side irradiation of so many alloys under so many conditions has provided a unique library that is enabling a campaign of wide ranging collabora- tive research studies. Project Description UCSB-1 was a drop-in experiment, and so did not have thermocouples to directly monitor temperatures. Instead, a specified packet temperature was achieved by a combination of nuclear heating and a partially insulating gas gap (comprising a mixture of helium and argon).To minimize temperature uncertainties, researchers at UCSB carried out an extensive finite- element-based thermal-design study. The packets were hollow, profiled cylinders, containing multipurpose disc specimens, or cylindrical holders for other specimens. Each packet had an individual gas-gap profile designed to yield a uniform specimen center- line target temperature. Stainless steel pins provided interpacket thermal isolation and forced the heat flow to be primarily in the radial direction, crossing a minimum number of inter- faces.This packet concept is illustrated in Figure 1a; the temperature-dpa profile is shown in Figure 1b. One capsule was removed from its center- line position after one reactor cycle when the contents reached a damage level of 1.8 dpa, and a replace- ment capsule was inserted where it remained for the rest of the irradia- tion.An identical packet to that irradi- ated at the centerline position during The NSUF UCSB-1 Library ATR Irradiation Experiment G. R. Odette – University of California, Santa Barbara – odette@engineering.ucsb.edu This experiment was designed to create a new and unprecedented library of alloys and irradiation conditions to facilitate understanding of and modeling to ultimately predict and improve the behavior of structural materials used in nuclear energy systems. the first cycle was irradiated in a lower-flux position to reach 1.8 dpa at the end of the experiment—i.e., after four cycles, providing some dpa-rate variation.The maximum damage level the experiment could reach was deter- mined by the period of irradiation. Unfortunately, this time was limited by a reactor-power increase after the fourth cycle that would have resulted in higher temperatures. Finally, a post- irradiation adjustment of the nuclear heating rates resulted in temperature estimates being somewhat higher than planned; for example, Capsule 6 ran at ≈593 K versus the planned 563 K. The alloys, including proper certifica- tions, were acquired by UCSB from various sources. UCSB also fabricated all the specimens in the irradiation and loaded them into packets under the supervision of an INL quality assurance engineer.The packets were sent to INL to be loaded into capsules. Mechanical properties of the alloys have been (or will be) variously assessed, both prior to and after irradiation by nanoindentation, microhardness, shear punch, tensile, compression, chevron wedge, and fracture toughness subsized-specimen tests. Sample specimens are shown in Figure 1c. Microstructural char-