Flux Shaping in N Reactor: A Method to Retard Graphite Stack Distortions

摘要

N Reactor, a graphite moderated reactor operated for the Department of Energy by Westinghouse Hanford Company, has experienced graphite distortion over its 24 years of operation. Graphite is a highly anisotropic material. Polycrystalline forms, as used in N Reactor, are moderately anisotropic. Prolonged exposure to high energy neutrons and elevated temperatures has resulted in dimensional distortions in graphite moderator blocks. Judicious use of fuel distributions and control rod manipulations can result in flux distributions that will retard the rate of localized distortions. The flux-flattening program at N Reactor was initiated to control flux levels in locations with historic high fluence values and shift power production to regions that have experienced low fluence. The program is implemented through axial enrichment variations in fuel charges and selective use of control rods. Complex three dimensional analyses supported by a number of test loadings and monitored by a series of incore detectors have been essential to this effort. Projections indicate that operation of the reactor in this mode will retard graphite growth equivalent to three to five years of operation. N Reactor is currently being placed in a standby mode. Should future operation of the reactor be required, flux flattening could substantially extend the duration of its mission. 1 ref., 8 figs. (ERA citation 13:051245)