PURPOSE:To provide a proper criyical approach method by devising means to extract poison solution from poison tube. CONSTITUTION:During reactor operation, the power is detected constantly with power zone neutron instrument 13 to transmit to neutron flux-power converter 19. When the reactor power is larger than the power set with power set unit 20, poison feed valve 22 and heavy water water level adjustment valve 32 are opened by a command of posison rapid feed indicator 21 to shutdown reactor. For re-starting up, the heavy water in the heavy water tank 31 is feeding into core tank, which extracts the pison solution contained in poison tube 3 in order to enlarge neutron flux within the core. Simultaneously, neutron flux is measured with re-start zone neutron instruments 4-7. By using these measurements, the condition to extract the neutron absorption element is found around the critical approach, which is extracted slower than preceedingly extracted neutron absorption element.
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