Patent Number: 
Section: claims

1. A burn-up profile measuring method comprising:setting plural detectors, which detect radiation, in an axial direction of a fuel assembly at a predetermined interval to face at least one side of the fuel assembly on which neutrons are irradiated in a nuclear reactor;measuring distributions of radiation signals with the detectors while relatively moving the fuel assembly and the detectors along the axial direction of the fuel assembly;determining, in every measurement, soundness of radiation signals measured by the detectors by comparing radiation signal distributions obtained by measuring a same portion in the axial direction of the fuel assembly in a multiplexed manner with these plural detectors; andmeasuring a burn-up profile by calculating relative burn-up based on the measured radiation signals after the measured radiation signals soundness determination step. 2. The burn-up profile measuring method according to claim 1, wherein the determination of soundness of radiation signals measured by the detectors is carried out by normalizing radiation signals measured by the detectors in a multiplexed manner and comparing a maximum of differences among normalized values of these radiation signals with a determination value. 3. The burn-up profile measuring method according to claim 1, wherein the determination of soundness of radiation signals measured by the detectors is carried out by normalizing radiation signals measured by the detectors in a multiplexed manner and comparing a maximum and a minimum of ratios of normalized values of these radiation signals with respective determination values. 4. The burn-up profile measuring method according to any one of claims 1 to 3, wherein the detectors are two detectors arranged close to each other at a distance in a range of 5 to 50 cm, which is relatively short compared with length in the axial direction of the fuel assembly. 5. A burn-up profile measuring method comprising:setting plural detectors, which detect radiation, along an axial direction of a fuel assembly at an interval substantially equal to a distance obtained by dividing length of a fuel effective section of the fuel assembly by a number of the detectors to face at least one side of the fuel assembly on which neutrons are irradiated in a nuclear reactor;arranging, at a lower end of the fuel effective section, the detector in a bottom position and measuring distributions of radiation signals with the detectors while relatively moving the fuel assembly and the detectors along the axial direction of the fuel assembly in a distance longer than the detector interval;partially redundantly measuring a same portion of the fuel assembly with the detectors adjacent to each other in the axial direction of the fuel assembly and determining, in every measurement, soundness of radiation signals measured by the detectors by comparing a relation of radiation signals in the redundant portion with a reference value including a measurement result of a last time;calculating a radiation signal distribution over entire length in the axial direction of the fuel assembly by superimposing, on radiation signals measured by the plural detectors, of which the soundness are determined, radiation signals in a portion where the radiation signals are redundantly measured to combine the radiation signals; andmeasuring a burn-up profile by calculating relative burn-up based on the radiation signal distribution over the entire length in the axial direction. 6. The burn-up profile measuring method according to claim 5, wherein the determination of soundness of radiation signals measured by the detectors is carried out by calculating ratios of radiation signals concerning a portion redundantly measured by the detectors and comparing an average, a maximum, and a minimum of these ratios with reference values including an average of ratios of radiation signals obtained in a same manner in last measurement, and wherein the calculation of a radiation signal distribution over the entire length in the axial direction of the fuel assembly is carried out by, after correcting the radiation signals by the detectors to a same sensitivity level, combining these radiation signals. 7. The burn-up profile measuring method according to claim 5, wherein the detectors are two detectors arranged spaced apart by a distance substantially a half of the length of the fuel effective section in the fuel assembly, and distributions of radiation signals are measured while the fuel assembly and the detectors are relatively moved along the axial direction of the fuel assembly in a distance substantially longer than the half of the length of the fuel effective section. 8. The burn-up profile measuring method according to claim 5, wherein the detectors are three to six detectors arranged at an equal interval in the axial direction of the fuel assembly, and distributions of radiation signals are measured while the fuel assembly and the detectors are relatively moved along the axial direction of the fuel assembly in a distance longer than the arrangement interval of the detectors. 9. A burn-up profile measuring method comprising:setting plural detector units along an axial direction of the fuel assembly at an interval substantially equal to a distance obtained by dividing length of a fuel effective section of the fuel assembly by a number of the detector units to face at least one side of a fuel assembly on which neutrons are irradiated in a nuclear reactor, the detector units including plural detectors that are set at a predetermined interval in the axial direction of the fuel assembly and detect radiation;arranging, at a lower end of the fuel effective section, the detector in a top position in the detector unit in a bottom position and measuring distributions of radiation signals with the detectors of the detector units while relatively moving the fuel assembly and the detector units along the axial direction of the fuel assembly in a distance longer than the interval of the detector units;partially redundantly measuring a same portion of the fuel assembly with the detector units adjacent to each other in the axial direction of the fuel assembly and measuring a same portion in the axial direction of the fuel assembly in a multiplexed manner with the plural detectors in the detector units;determining, in every measurement, soundness of radiation signals measured by the detectors of the detector units by comparing radiation signal distributions measured in a multiplexed manner by the plural detectors in the detector units;calculating a radiation signal distribution over entire length in the axial direction of the fuel assembly by superimposing, on radiation signals measured by the plural detectors of the plural detector units, of which the soundness are determined, radiation signals in a portion redundantly measured by the detector units adjacent to each other in the axial direction of the fuel assembly to combine the radiation signals; andmeasuring a burn-up profile by calculating relative burn-up based on the radiation signal distribution over the entire length in the axial direction. 10. The burn-up profile measuring method according to claim 9, wherein the determination of soundness of radiation signals measured by the detectors of the detector units is carried out by calculating ratios of radiation signals measured in a multiplexed manner by the plural detectors of the detector units and comparing an average, a maximum, and a minimum of these ratios with reference values including an average of ratios of radiation signals obtained in a same manner in last measurement, and wherein the calculation of a radiation signal distribution over the entire length in the axial direction of the fuel assembly is carried out by correcting averages of radiation signals measured by the respective detectors of the detector units to a same sensitivity level and combining the average values. 11. The burn-up profile measuring method according to claim 9 or 10,wherein the detector units are an upper detector unit and a lower detector unit arranged spaced apart by a distance substantially a half of the length of the fuel effective section in the fuel assembly, each of the detector units including two detectors arranged close to each other at a distance in a range of 5 to 50 cm, which is relatively short compared with the length in the axial direction of the fuel assembly,wherein the detectors of the upper detector units and the detectors of the lower detector unit respectively measure a radiation signal distribution in a multiplexed manner concerning an upper half portion of the fuel effective section in the fuel assembly and concerning a lower half portion of the fuel effective section in the fuel assembly, andwherein the each detectors of the upper detector unit and the lower detector unit measure a radiation signal distribution in a multiplexed manner concerning a center portion of the fuel effective section in the fuel assembly. 12. The burn-up profile measuring method according to any one of claims 1-3 and 5-10, wherein the detectors are ionization chambers that measure a gamma ray as a gamma ray energy gross amount without discriminating the gamma ray by energy, a relational expression between radiation signals detected by the detectors and specific burn-up is calculated in advance, and relative burn-up is calculated from the radiation signals by utilizing the relational expression. 13. A non-transitory computer-readable medium storing computer executable instructions for causing a computer to execute burn-up profile measurement processing operations for a fuel assembly, and at a time of setting plural detectors, which detect radiation, in an axial direction of the fuel assembly at a predetermined interval to face at least one side of a fuel assembly on which neutrons are irradiated in a nuclear reactor and measuring a burn-up profile utilizing distributions of radiation signals measured by the detectors while relatively moving the fuel assembly and the detectors along the axial direction of the fuel assembly, the burn-up profile measuring processing operations comprising:determining, in every measurement, soundness of radiation signals measured by the detectors by comparing radiation signal distributions obtained by measuring a same portion in the axial direction of the fuel assembly in a multiplexed manner with these plural detectors; andcalculating relative burn-up utilizing the measured radiation signals to measure a burn-up profile after the determination step. 14. A non-transitory computer-readable medium storing computer executable instructions for causing a computer to execute burn-up profile measurement processing operations for a fuel assembly, and at a time of setting plural detectors, which detect radiation, along an axial direction of the fuel assembly at an interval substantially equal to a distance obtained by dividing length of a fuel effective section of the fuel assembly by a number of the detectors to face at least one side of a fuel assembly on which neutrons are irradiated in a nuclear reactor, arranging, at a lower end of the fuel effective section, the detector in a bottom position, and measuring a burn-up profile utilizing distributions of radiation signals measured by the detectors while relatively moving the fuel assembly and the detectors along the axial direction of the fuel assembly in a distance longer than the detector interval, the burn-up profile measuring processing operations comprising:determining, in every measurement, soundness of radiation signals measured by the detectors by comparing a relation of radiation signals in a redundant portion of the fuel assembly, which is partially redundantly measured by the detectors adjacent to each other in the axial direction of the fuel assembly, with a reference value including a measurement result of a last time;calculating a radiation signal distribution over entire length in the axial direction of the fuel assembly by superimposing, on radiation signals measured by the plural detectors, radiation signals in a portion where the radiation signals are redundantly measured to combine the radiation signals after the determination step; andmeasuring a burn-up profile by calculating relative burn-up from the radiation signal distribution over the entire length in the axial direction. 15. A non-transitory computer-readable medium storing computer executable instructions for causing a computer to execute burn-up profile measurement processing operations for a fuel assembly, and at a time of setting plural detector units along an axial direction of the fuel assembly at an interval substantially equal to a distance obtained by dividing length of a fuel effective section of the fuel assembly by a number of the detector units to face at least one side of a fuel assembly on which neutrons are irradiated in a nuclear reactor, the detector units including plural detectors that are set at a predetermined interval in the axial direction of the fuel assembly and detect radiation, measuring a burn-up profile utilizing distributions of radiation signals measured by the detectors of the detector units while relatively moving the fuel assembly and the detector units along the axial direction of the fuel assembly in a distance longer than the interval of the detector units when the detector in a top position in the detector unit in a bottom position are arranged at a lower end of the fuel effective section, the burn-up profile measuring processing operations comprising:determining, in every measurement, soundness of radiation signals measured by the detectors of the detector units by comparing radiation signal distributions measured in a multiplexed manner by the plural detectors in the detector units;calculating a radiation signal distribution over entire length in the axial direction of the fuel assembly, obtained by superimposing, on radiation signals measured by the plural detectors of the plural detector units, radiation signals in a portion redundantly measured by the detector units adjacent to each other in the axial direction of the fuel assembly by combining the radiation signals after the determining step; andmeasuring a burn-up profile by calculating relative burn-up from the radiation signal distribution over the entire length in the axial direction. 16. A radiation signal distribution measuring apparatus comprising:plural detectors which detect radiation;a detector holding unit configured to hold the detectors that face at least one side of a fuel assembly, on which neutrons are irradiated in a nuclear reactor, and are set at a predetermined interval in an axial direction of the fuel assembly;a driving unit configured to move at least one of the fuel assembly and the detectors along the axial direction of the fuel assembly;a radiation-signal-distribution acquiring unit configured to acquire a radiation signal distribution in the axial direction of the fuel assembly on the basis of radiation signals obtained by measuring a same portion in the axial direction of the fuel assembly in a multiplexed manner with the plural detectors;a determining unit configured to determine, in every measurement, soundness of radiation signals measured by the detectors on the basis of plural radiation signal distributions acquired by the radiation-signal-distribution acquiring unit; andan averaging calculating unit configured to obtain, when the determining unit determines that the radiation signals are sound, a radiation signal distribution in the axial direction of the fuel assembly on the basis of an average radiation signal value obtained by averaging plural radiation signal values measured concerning the same portion in the axial direction of the fuel assembly. 17. A burn-up profile measuring apparatus further comprising, in the radiation signal distribution measuring apparatus according to claim 16, a relative-specific-burn-up calculating unit configured to calculate relative burn-up on the basis of the radiation signal distribution measured by the radiation signal distribution measuring apparatus, and wherein the burn-up profile measuring apparatus is configured to output the relative burn-up, calculated by the relative-specific-burn-up calculating unit, as a relative burn-up measurement result.