Mechanism for displacing and securing the rod of a nuclear reactor control bar, comprising an enclosure coaxial with the rod and forming a hydraulic cylinder in which the rod slides while defining a decompression chamber, a device for mechanically securing the rod in its high position, and a positive displacement reciprocating pump inserted between the decompression chamber and the inside of the tank, whose piston is actuated electromagnetically means and causes at each backward and forward movement lifting of given amplitude of the rod and the device for securing the rod in the high position disengagable also electromagnetically.

A redundant reactivity control system, operating in conjunction with other systems in a nuclear power plant, to mitigate the potential consequences of an anticipated transient without scram event, is described. The invention interfaces to a boiling water nuclear reactor with the reactor's neutron monitoring system, standby liquid control system, control rod drive system, reactor recirculation system, reactor water clean-up feed water control system, and control room and local panels. Two divisionally separated control panels include associated detection and actuation logic and the necessary interface logic to other systems to perform the specific functions in response to an anticipated transient without scram event. Each separate division includes two identical channels. Actuation of any associated reactor system by the invention requires the agreement of the two channels within a division. A one channel indication actuates appropriate annunciators and alarm equipment to notify plant personnel of a possible system fault. Each divisional panel receives independent analog signals from high dome pressure transmitters, reactor low water level transmitters, and average power range monitor down-scale trip units. These inputs are combined and processed by solid state logic circuits to produce output actuation signals for interfacing systems. The present invention actuates a stand-by liquid control system to inject sodium pentaborate into a reactor core if an anticipated transient without scram event is sensed. Such insertion is made only after all other means of mitigation have not been able to reduce radioactivity, i.e. control rod insertion, recirculation pump trip, etc. The present invention provides a high degree of reliability and adds further levels of nuclear plant safety, while avoiding the possiblity of inadvertent stand-by liquid control actuation.

The electromagnetic device is suitable for use in actuating a pressurized water reactor control bar. It comprises a shaft and a fluid-tight casing in which are movable longitudinally a first and a second sets of grippers for gripping the shaft, staggered in the longitudinal direction. The first set of grippers is associated with a movable pole co-operating with a holding coil, carried by the casing and movable by energization and de-energization of the coil between a position in which the first set of grippers holds the shaft and a position in which it releases it. The second set of grippers is associated with a movable plunger co-operating with a transfer coil and movable by energization and de-energization of the transfer coil to and from a position in abutment against an other pole. The other pole is movable between two positions spaced apart by a given step. The movable pole belongs to a mechanism for cutting off a path coolant fluid constituting a thermosiphon causing convection flow between the inside of the vessel and the space defined by the casing.

A first assembly for low-speed translational displacement and dropping of a control rod comprises a leak-tight casing tube which communicates with the reactor vessel, a screw and nut system coupled to the control rod by means of the nut and a screw-retaining device which is capable of releasing the screw. In the event of accidental failure of the casing tube, upward displacement of the control rod and of the broken portion of the casing tube is limited by a second assembly comprising means for rotational locking of the screw drive rotor and calibrated elastic means for producing action on the rotor in opposition to the vertical thrust produced by failure of the casing tube.

Coupler for absorber rods being suspended during operation of nuclear reactors including a plurality of actuating elements being movable for individually and jointly releasing the coupler, the movement of each of the actuating elements for releasing the coupler being independently controllable.