The Fast Flux Test Facility being built near Richland, Washington, for materials and component testing for development of the liquid-metal fast breeder reactor will employ “gas tagging” for locating failed fuel and control elements in the Fast Test Reactor (FTR). The fuel and control pins are “gas tagged” by loading a capsule with specially blended krypton and xenon isotopes. The encapsulation of the tag gas employs a unique application of laser technology: Stainless-steel capsules within a sealed glass-covered fixture containing the tag gas are first pierced and then seal welded with a laser beam. After inspection, the capsules are loaded in pins, and the gas is released by piercing the capsule with an electro-magnetically activated internal penetrator. If the pin should develop a leak, the gas is released and the defect assembly is then located by mass spectrometric analysis of the reactor cover gas. Capsule filling yield for 40 000 capsules fabricated for 2 FTR cores was ∼95%.