Patent Number: 041522079
Section: summary

1. Field of the Invention This invention relates to nuclear reactors and more particularly to a system for laterally restraining, compressing, and expanding the core of a reactor. 2. Description of the Prior Art Throughout the history of nuclear reactor design extreme care has been taken to ensure safe, reliable, and redundant shutdown and hypothetical accident controls. Typical of such controls are, for example, control rods maintained in an upper position by electromagnets which, upon loss of electrical power, drop the rods into the reactor core. Such devices have proved to be extremely reliable. They do, however, require some type of signal, or a loss of power, to position the control rods within the core. The signal can be in response to a multitude of continuously monitored plant parameters. It is particularly desirable to provide control mechanisms which respond directly to selected plant parameters. Such mechanisms can be referred to as "inherent" controls or shutdown systems. The most typical of these control mechanisms is the negative moderator coefficient experienced by many pressurized water cooled reactors upon a rising coolant temperature. This phenomenon, however, is not realized in many other reactor types. In view of the conservative design approach attendant the nuclear industry, it is desirable to provide additional inherently safe control mechanisms, applicable to a broad range of nuclear reactors. SUMMARY OF THE INVENTION This invention provides an inherently responsive control mechanism and core restraining device for fluid cooled nuclear reactors. It is based upon the nuclear characteristic that expanding the geometry of a core in response to an undesirable condition will decrease the reactivity of the core, and can place it in a subcritical configuration. The invention includes an electromagnetic circuit through ferromagnetic structure arranged to radially compress core fuel assemblies adjacent one another for normal plant operating conditions. An electromagnet can act upon the structure which compresses the core, and can also form an electromagnetically induced circuit incorporating adjacent core assemblies. Between the adjacent assemblies are desirably provided compressible Belleville-type springs which are compressed during normal operation and relaxed upon a selected signal or accident condition by decreasing the electromagnetic force. The invention further includes an inherent shutdown mechanism when a portion of the electromagnetic circuit is made of a ferromagnetic material which has a predetermined curie temperature and which is in heat transfer relation with the reactor coolant fluid exiting the core. Upon an accidental rise in the coolant temperature resulting in raising of the ferromagnetic material to a temperature at which the magnetic saturation decreases, the electromagnetic force is inherently reduced, allowing the adjacent assemblies and core to radially expand. The expanded configuration decreases the reactivity of the core, leading to a controlled shutdown.