Passive shut-down mechanism

In order to achieve passive safety with reactive material, the radius of the reactor tube is designed to be small to avoid any thermal explosion inside the tube. Using the Frank-Kamenetskii approach (see Chapter 13), the radius remains below the critical radius. Thus, even assuming a purely conductive heat transfer mechanism, corresponding to a worst case, no instable temperature profile can develop inside the reaction mass. The reactor can be shut down and restarted safely. 

The broad functions of the safety systems are common to most reactors. In the event of an abnormal condition they should shut down the reactor, isure a suffici t supply of coolant for the fuel, and contain any fission products which might escape from the fuel elemrats. Such safety features can be active (requiring some action from a control system, involving mechanical devices, and relying on an external power source in order to operate) or passive (built-in physical fail-safe features whose operation is not dq)end t on any control system, mechanical device or external power source). 

Further enhanced reliability of the shutdown, achieved by providing a passive shutdown device operated by steam pressure for injection of poison in case of extremely low probability case of failure of both mechanical shutoff rods and liquid poison shut down system,... 

Figure 15.19 (a) Control and safety rod drive mechanism and (b) diverse shut-down rod drive mechanism with passive safety features proposed for FBR-600. 

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