Patent Number: 041815696
Section: summary

BACKGROUND OF THE INVENTION This invention relates to the control of nuclear reactors. In a nuclear reactor of the kind wherein fluid coolant is circulated through the reactor core and heat exchange means by a plurality of circulators there is a risk that in the event of a reduction in the rate of coolant flow, caused by, for example, loss of a coolant circulator, the nuclear fuel will overheat. It has been the normal practice therefore to arrange for a trip signal to be initiated in response to a significant reduction in the rate or pressure of the coolant flow whereby neutron absorbing material in the form of control rods is dropped or injected into the core to effect immediate and total shut-down of the reactor. Where the reactor is used in an electricity generating station this solution to the problem is not entirely satisfactory because it means a total loss of output, requires the introduction of auxiliary services for maintaining the reactor in a safe condition and, to recommence operation of the station, involves a complex start-up procedure. Clearly, immediate and total shut-down of a station imposes a heavy financial burden on the operators. It is an object of the invention to provide for a nuclear reactor control means which will achieve a rapid reduction in power by change of reactivity of appropriate magnitude to meet the requirements for continued safe operation following a partial reduction in coolant flow or pressure. SUMMARY OF THE INVENTION According to the invention there is provided 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 operable position wherein the neutron absorbing material is disposed above the reactor core and a second operable position wherein the neutron absorbing material is disposed within the core, means for monitoring an operating parameter of the core, a 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 arresting fall of the control rod in the second operable position, and means for adjusting the second operable position relative to the reactor core, the means being responsive to reactivity within the reactor core. Means for effecting insertion of the said control rod in two successive stages may comprise a first latching device for suspending the control rod above the reactor core the device being releasably sensitive to the trip signal, a second latching device for arresting free fall or uncontrolled injection of the control rod and means responsive to control signals for adjusting the position of the second latching device whereby the vertical position of arrest of the control rod and subsequent controlled insertion in the reactor core can be varied. The invention is applicable to liquid metal cooled fast breeder nuclear reactors and it provides that in the event of an emergency condition wherein the rate of coolant flow is reduced, a rapid power set-back of appropriate magnitude is achieved by causing limited injection of control rods into the reactor, the insertion being sufficient to reduce reactivity to a safe level, and, subsequently, fine control at the reduced power output is achieved by automatic control.