Nuclear Science User Facilities 98 synchrotron X-ray diffraction To study the irradiation embrittlement on F/M alloys, in situ synchrotron X-ray diffraction (XRD) measure- ments of the tensile deformation of irradiated samples were conducted. Because the samples were irradiated by neutron and activated with various radioactive isotopes, special confine- ment techniques were required to prevent radioactive contamination of the Advanced Photon Source (APS) beamline areas.Two types of radiation- confinement methods were developed for the in situ APS experiment. In APS beamline 1-ID, we employed double- contained encapsulation for irradi- ated samples on the universal tensile machine. For the APS beamline 10-ID experiment, glovebox type confine- ment system was designed to enclose the samples and all experimental stages. An in situ tensile test with high-energy X-ray provides more profound infor- mation over a conventional tensile test, including elastic and plastic behaviors with connections to specific crystal planes. The lattice strains generated under applied stresses were mainly due to the elastic properties with hkl planes and plastic deformation with crystal slip planes and slip directions. Dislocation loops and the α' precipitation are the major contributors of irradiation hard- enings.The theoretical contributions to irradiation hardening by dislocation Figure 11. Cu-rich precipitates found in T91 irradiated around 600°C to 3.23 dpa