Patent Number: 
Section: claims

1. A steam turbine control device in a nuclear power plant comprising: a main steam system connected to lead steam generated in a nuclear reactor into a steam turbine comprising a main steam line, a main steam isolation valve, and a main steam control valve;  a turbine by-pass system connected to by-pass the steam turbine, branched from the main steam system, and connected to a condenser, the turbine by-pass system comprising a turbine by-pass valve;  a main steam pressure detector in the main steam system;  a main steam pressure control calculating means, for outputting a main steam pressure control signal dependant on the signal from the main steam pressure detector;  a reactor dome pressure detector in the nuclear reactor;  a reactor dome pressure control calculating means for outputting a reactor dome pressure control signal dependent on the signal from the reactor dome pressure detector;  a main steam isolation valve fully closed position detector for outputting a pressure control change trigger signal when the main steam isolation valve is detected to be fully closed; and  a pressure control signal calculating means for outputting a pressure control signal to control the position of the main steam control valve and/or the turbine by-pass valve comprising a pressure control signal changeover means to change over a pressure control signal from the reactor dome pressure control signal to the main steam pressure control signal when the main steam isolation valve is detected to be closed by the main steam isolation valve fully closed position detector. 2. The steam turbine control device according to  claim 1 , wherein the pressure control signal calculating means outputs; the pressure control signal to close the main steam control valve and/or the turbine by-pass valve when the main steam isolation valve is detected to be closed. claim 1 3. The steam turbine control device according to  claim 1 , further comprising; claim 1 a display means for displaying the signal changeover state depending on the signal of the pressure control signal changeover means. 4. In a nuclear power plant including a main steam isolation valve in a main steam system between a nuclear reactor and a steam turbine, a control device for controlling at least one of a first valve and a second valve, the first valve being in the main steam system and the second valve being in a steam turbine by-pass system branched from the main steam system, the control device comprising: a first pressure monitor connected to the nuclear reactor;  a main steam isolation valve position monitor connected to the main steam isolation valve;  a second pressure monitor connected to the main steam system between the first valve and the main steam isolation valve;  a control means for controlling the position of the first valve or the second valve based on a first pressure signal from the first pressure monitor during normal operation of the nuclear reactor, and based on a second pressure signal from the second pressure monitor when the main steam isolation valve position monitor detects that the main steam isolation valve is closed. 5. The control device according to  claim 4 , wherein the controlling means fully closes the first and the second valves to stop steam from flowing into the steam turbine when the main steam isolation valve position monitor detects that the main steam isolation valve is closed. claim 4