Nuclear reactor safety device

In a nuclear reactor having a core, a coolant for contacting and cooling the core, at least one control rod movable axially relative to the core for varying the power output of the reactor, and control means for moving the control rod to regulate the power output, a safety device is described to afford additional protection against an upward thermal excursion of the core. Such safety device consists of: a laminated helical ribbon configured as a tube-like laminated helical coil having contiguous helical turns with slidably abutting edges, and means connecting such helical coil in an operative relationship between the control means and the control rod, the laminated helical ribbon having outer and inner laminae made of different materials having different thermal coefficients of expansion, the material of the outer lamina having a greater coefficient of expansion than the material of the inner lamina, the laminated helical coil being in a position to receive heat from the coolant, the helical coil being heated by an upward thermal excursion which is thereby effective to cause the helical coil to wind up to a smaller diameter, the winding up of the helical coil being operative to produce a substantial increment in the total axial length of the helical coil, whereby the increment in the length of the helical coil produces axial movement of the control rod relative to the core to reduce the power output of the reactor independently of the position of the control means