Recent measurements supporting N reactor graphite stack stored energy analysis

During the accident at the Chernobyl reactor in the Soviet Union last April, many speculated that Wigner stored energy in the graphite stack contributed to the burning of the moderator. This speculation prompted a reevaluation of the stored-energy question in a graphite-moderated reactor such as the Hanford N Reactor. This reactor is operated by UNC Nuclear Industries, Inc., the for US Dept. of Energy. In N Reactor, the core region operates at an average graphite temperature of ∼ 5100C (well below the graphite ignition temperature), such that continuous annealing of the stored energy occurs during operation. However, the operating temperature of the reflector graphite is much lower. Experimental measurements were performed recently to determine the operating temperature and epithermal flux level inside the reflector graphite. Subsequent analyses using the experimental measurements demonstrated that the release of the stored energy from the reflector graphite would not raise the maximum reflector graphite above the core graphite temperature. This conclusion is based on the following facts: (1) the amount of graphite affected by stored energy is relatively small compared to the full core-reflector graphite volume. (2) The epithermal flux and reflector graphite temperature result in a small amount of stored energy in the regions affected. (3) The stored energy release is a gradual rather than an instantaneous process