PURPOSE: To suppress a steep increase of a nuclear reactor output, by providing a nuclear reactor surveillance/controlling device to calculate a concentration change of a Xe and a temperature change of reactor water, and an output controlling device to put out an inhibiting signal against control rod withdrawal or an withdrawing signal of selective control rods.

CONSTITUTION: At a nuclear reactor surveillance/control device 9, a period signal is put out to an output controlling device 14 with a condition that there exists an input of a control rod withdrawal signal 2. And a concentration of Xe is obtained by solving a dynamic characteristics equation of Xe induced by a neutron flux 5 and a changing rate signal of Xe concentration 12 is calculated from a difference between a present value and the previous one and then is put out to the output controlling device 14. And from a difference between a present value and the previous value of a reactor water temperature 6, a changing rate signal of reactor water temperature 13 is calculated and is put out to the output controlling device 14. And when the changing rate signal of reactor output 11 reaches a predetermined setting value, a changing rate signal of reactor output 15 which shortens calculational intervals between each repeated calculations of both the changing rate signal of Xe concentration 12 and that of reactor water temperature 13 is put out to the nuclear reactor surveillance/ controlling device 9.