Patent Number: 
Section: claims

1. A reactor water-level measurement system comprising:a core bottom water-level measuring device that includes a heating element, a heat insulating element installed by surrounding part of the heating element in a height direction of the heating element, and a temperature difference measuring element that measures a temperature difference between an insulated portion of the heating element surrounded by the heat insulating element and a non-insulated portion not surrounded by the heat insulating element;a water-level evaluation device that evaluates a water level of a reactor based on the temperature difference,wherein the core bottom water-level measuring device measures at least a water level from a lower end of a reactor core contained in a reactor pressure vessel to a bottom of the reactor pressure vessel, andwherein the reactor pressure vessel includes a neutron detector that monitors neutron fluxes in the reactor core and a protective tube that contains the neutron detector, and wherein the core bottom water-level measuring device is contained in the protective tube together with the neutron detector. 2. The reactor water-level measurement system according to claim 1, further comprising an in-core water-level measuring device that includes a heating element, a heat insulating element installed by surrounding part of the heating element in a height direction of the heating element, and a temperature difference measuring element that measures a temperature difference between an insulated portion of the heating element surrounded by the heat insulating element and a non-insulated portion not surrounded by the heat insulating element, and measures a water level within a vertical range of the reactor core. 3. The reactor water-level measurement system according to claim 2, wherein the in-core water-level measuring device is contained in the same protective tube together with the neutron detector. 4. The reactor water-level measurement system according to claim 1, wherein the core bottom water-level measuring device is arranged in an outer peripheral portion of the reactor core. 5. The reactor water-level measurement system according to claim 1, wherein a plurality of the core bottom water-level measuring devices are arranged in different sections obtained by dividing an area inside the reactor core into monitoring zones. 6. The reactor water-level measurement system according to claim 2, wherein one of both the a plurality of the core bottom water-level measuring devices and a plurality of the in-core water-level measuring devices are installed, and wherein one of both the temperature difference measuring elements of the core bottom water-level measuring devices and the temperature difference measuring elements of the in-core water-level measuring devices are respectively installed in a vertical direction at locations differing among the water-level measuring devices. 7. The reactor water-level measurement system according to claim 2, wherein one of both the core bottom water-level measuring device and the in-core water-level measuring device further includes a temperature measuring device that measures temperatures. 8. The reactor water-level measurement system according to claim 7, wherein the temperature measuring device measures temperatures at a plurality of locations along a vertical direction in the reactor. 9. The reactor water-level measurement system according to claim 7, wherein the temperature measuring device is arranged in a horizontal central portion of the reactor core.