Fiber optic pressure measurement during an electromagnetic pulse

The fiber-optic Hopkinson bar technique was modified significantly to measure the development of pressure inside a vessel during the burst from a fast burst reactor. The pressure measurement required an observation of the actual pressure buildup during a burst. The experiments are being performed at the Los Alamos National Laboratory Godiva IV fast burst reactor. Godiva IV has a 30-σs burst width and produces 1.8 MJ/burst (6 X 1016 fissions). The pressure vessel is a graphite fiber composite vessel. The fiber optic material is a high purity single-mode fused 5-μm core fiber. The fiber is expoxied around the circumference of the graphite vessel. The stress induced in the vessel walls causes a slight stretching of the optical fiber, causing constructive and destructive interference which produces fringes when a He-Ne laser light is passed through the fibers and allowed to recombine via an interferometer. A measurement of these fringes allows one to deduce the pressure history inside the vessel