Pebble bed reactor vessel is closed by keyed blocks - with monitoring of second redundant key to detect failure of first

摘要

A redundant connection between two adjacent blocks of the top closure of a prestressed pressure vessel comprises two or more keys or pins; it is so designed that one key (or pin) only transmits the forces between the blocks under normal conditions, the second key constituting a reserve which comes into action only if the first key fails. The upper face of the upper, outer key is separated by a gap from the corresponding face of both the blocks; if the inner key fails, then the resultant closure of this gap is observed through inspection openings. Used esp. for a gas-cooled pebble-bed reactor, where the vessel closure comprises cast iron or concrete blocks. The observation of a change in the state of the outer key enables failure of the inner key - the first line of defence - to be detected.