Patent Number: 
Section: claims

1. A system, comprising:a reactor core;a γ-ray thermometer comprising:a container,a γ-ray heat generation detection section,a thermocouple thermometer, anda calibration heater,the γ-ray heat generation detection section contained in the container and located in the reactor core, the γ-ray heat generation detection section comprising:a cylindrical structural member contained in the container, the structural member provided with a heat insulator in the reactor core, part of the structural member that is located in the reactor core generating heat due to γ-rays, anda differential thermocouple contained in the structural member, the differential thermocouple including a hot junction located at a same vertical level as the heat insulator in the reactor core and a cold junction located at a vertical level lower than the heat insulator in the reactor core,the thermocouple thermometer contained in the structural member and located lower than a bottom of the reactor core,wherein the thermocouple thermometer measures the temperature of coolant flowing in the reactor core, andthe calibration heater being installed inside the structural member, extending vertically at least between the γ-ray heat generation detection section and the thermocouple thermometer, and reaching the thermocouple thermometer; andan output signal processor receiving a signal output from the thermocouple thermometer and the differential thermocouple and computationally determining a local temperature of the coolant at the thermocouple thermometer. 2. The system of claim 1, wherein a plurality of thermocouple thermometers is provided and divided into groups according to their positions, and the output signal processor is configured to computationally determine an average temperature of coolant of each of the groups. 3. The system of claim 2, wherein the output signal processor is configured to diagnose a thermocouple thermometer as out of order if a deviation of the local temperature of coolant measured by the thermocouple thermometer from the average temperature of coolant exceeds a predefined criteria. 4. The system of claim 3, wherein the output signal processor is adapted to regard a local temperature of coolant, at a position symmetrical relative to the thermocouple thermometer diagnosed as out of order, as a local temperature of coolant at a position of the thermocouple thermometer diagnosed as out of order. 5. A system, comprising:a reactor core;γ-ray thermometers comprising:respective containers, categorized into groups according to their positions, andrespective γ-ray heat generation detection sections contained in the respective containers and located in the reactor core, the γ-ray heat generation detection sections each comprising:a cylindrical structural member contained in the container, the structural member provided with a heat insulator in the reactor core, part of the structural member that is located in the reactor core generating heat due to γ-rays, anda differential thermocouple contained in the structural member, the differential thermocouple including a hot junction located at a same vertical level as the heat insulator in the reactor core and a cold junction located at a vertical level lower than the heat insulator in the reactor core,respective thermocouple thermometers contained in each respective structural member and located lower than a bottom of the reactor core,wherein the thermocouple thermometers measure the temperature of coolant flowing in the reactor core, andrespective calibration heaters installed inside each respective structural member, extending vertically at least between the respective γ-ray heat generation detection sections and the respective thermocouple thermometers, and reaching the respective thermocouple thermometers;an output signal processor receiving respective signal outputs from the thermocouple thermometers and the differential thermocouples and determining computationally local temperatures of coolant at positions of the thermocouple thermometers and an average temperature of each of the groups; anda diagnosing device diagnosing a nuclear reactor as being in an abnormal condition if at least one of the average temperature, a change per unit time of any of the local temperatures and a deviation of any of the local temperatures from the average temperature exceeds a corresponding criteria. 6. The system of claim 5, wherein the abnormal condition is loss of feed water heating.