<title>Time-Resolved Coded Aperture Imaging System For Nuclear Fuel Motion Measurements</title>

Abstract

A coded aperture imaging system for the measurement of nuclear fuel motion in simulated core disruptive accidents is described. A one dimensional Fresnel zone plate type coded aperture is used to modulate the fission gamma rays emitted from the fuel pin under test. Two active imaging systems have been developed to detect the coded image and record it on film using a high speed camera. The first system is an X-ray image intensifier which converts fission gamma rays to optical photons in a thin (0.1 mm) CsI(Tl) scintillator screen on which a photocathode is deposited. The photoelectrons thus created are collected and focused by electron optics and produce an image on the output phosphor. The second system utilizes a NaI(Tl) scintil-lator crystal and a separate high gain optical image intensifier to produce the desired coded image. With a few modifications both systems appear capable of achieving the preliminary design goals of 1 mm radial resolution, 5 mm axial (along the fuel pin axis) resolution and 1 msec temporal resolution. Presently, the X-ray image intensifier system offers the best resolution, but the second system provides higher gain and a wider selection of operating conditions. The temporal response of both systems is limited to a few milliseconds by the output phosphor. Experimental performance data for each system will be presented, and the results of pre-liminary nuclear fuel pin imaging experiment performed at the Sandia Pulsed Reactor will be shown.