[en] The capsule has been designed to allow the irradiation of fatigue crack propagation specimens with the 0.5T compact tension (0.5T CT) geometry at elevated temperatures in the high-flux region of the ORR core. Temperature control will be achieved by a balance between gamma heat generation in the specimens and heat loss to the reactor cooling water through an externally controlled variable gas gap. The design of the ORR capsule has been derived from NRL capsules used to irradiate reactor pressure vessel steels in a number of research reactors, with modifications to the geometry to allow for the high gamma heating rate encountered in the core of ORR. The first assembly of this design will be irradiated as experiment MFE-3, scheduled for insertion in ORR by the end of FY 78