Elemental boron, boron carbide, and zirconium diboride were irradiated at reactor ambient temperature (150 deg ) and elevated temperatures (500 to 750 deg F) to burnups ranging from 20 to 95 per cent of the boron-10 isotope. Zirconium diboride containing both natural and enriched amounts of boron-10 was evaluated. The amount of helium released during irradiation was measured, and x ray diffraction, metallographic, and chemical examinations were performed before and after irradiation. Zirconium diboride containing natural boron appeared to be the most promising material from a helium-retention standpoint. None of the materials exhibited excessive particle fragmentation or dimensional changes during the course of the experiments even though there was evidence of temperatures much higher than design conditions. (auth)