Patent Number: 041815696
Section: claims

1. A nuclear reactor having a reactivity control system comprising: at least one control rod of neutron absorbing material arranged for vertical displacement between a first fixed operable position wherein the neutron absorbing material is disposed above the reactor core and a variable second operable position wherein the neutron absorbing material is disposed within the core, means for monitoring an operating parameter of the core and initiating a trip signal in response to a predetermined condition, a fixed first latching device for securing the control rod in the first operable position, the device being responsive to the monitoring means whereby on initiation of a trip signal the device can release the control rod, a second latching device for variably arresting fall of the control rod in the variable second operable position, and means for adjusting the variable second operable position relative to the reactor core while said rod is held by either of said first and second latching devices, the adjusting means being responsive to reactivity within the reactor core, whereby the initial position of arrest of a released rod and subsequent controlled insertion in the reactor core can be varied. 2. A nuclear reactor according to claim 1, wherein the means for adjusting the second operable position of the control rod is optionally capable of operation by alternative control means whereby the control rod can be returned to the first operable position. 3. A nuclear reactor according to claim 2 wherein the latching devices are of the electro-magnetic kind releasable by interruption of the power supply to the latching devices. 4. A nuclear reactor according to claim 3 having a plurality of second latching devices arranged in series, each second latching device being responsive to a discrete reactor parameter.