JPH05240997

PURPOSE:To maintain a proper water level without causing excess response in a reactor water supply system even in the case of core void variation by calculating net reactor water level from the variation of coolant inventory of the reactor with a reactor water level calculation means and outputting a reactor water level signal. CONSTITUTION:A reactor water level calculation circuit calculates a net reactor water level signal (corresponding to total coolant inventory) from the mismatch of an inputting supply water flow signal Wf and a main steam flowrate signal Ws and outputs to a comparator 1. The comparator 1 outputs a water level deviation signal (a) between a reference water level signal set in advance and the net reactor water level signal. An adder 6 adds the mismatch flowrate signal (c) and the water level signal (a) and outputs the added signal (d) to a PI controller 7. Where the signal (c) is obtained by multiplying the mismatch gain Km to a difference signal (b) output by a subtracter 5. The controller 7 outputs the value integrating the signal (d) to a water supply system 4 as a supply water flow demand signal (e) and thus the reactor water level is maintained within a certain range.