LMFBR type reactor having reactivity control means of a plurality of systems independent of each other

The invention has a feature of having two indenpendent systems of reactivity control means with reduced number of control rods and without distorting the power distribution during operation and of high reactivity controlling effect. That is, reactivity of a nuclear reactor is controlled by dissolving neutron absobers in coolants and controlling the concentration of the neutron absorbers. With such a constitution, reactivity can be controlled by the means, other than the reactivity control due to control rods, but based on the evaporization, condensation or dilution utilizing the difference between the boiling points of liquid metal coolant and neutron absorber. Accordingly, completely independent two systems of control means can be obtained. In addition, the number of the control rods can be reduced as compared with usual cases. Further, since there is no requirement for inserting the control rods during operation, the power distribution is not distorted and the life time of the control rod is increased. Furthermore, there is no problem for the charged reactivity upon erroneous withdwaring of the control rod. (K.M.)