Patent Number: 
Section: claims

1. Gamma scanning apparatus comprising:a gamma-ray detector;a collimator that limits a measuring range in a fuel assembly and limits entry of gamma rays from the fuel assembly into the gamma-ray detector;an absorber that controls intensity of the gamma rays entering the gamma-ray detector;a shield that shields the gamma-ray detector;a housing that contains the gamma-ray detector, the collimator, the absorber, and the shield;moving and fixing means that moves the housing to a definite position and fixing the housing thereto;rotating and moving means that moves the fuel assembly vertically in addition to rotating the assembly;a gamma-ray counting circuit that measures an output of the gamma-ray detector; anddata collecting/analyzing and controlling apparatus that analyzes data output from the gamma-ray counting circuit, in association with data relating to the rotation and movement of the fuel assembly by the rotating and moving means;wherein:the rotating and moving means, after fixing the vertical position of the fuel assembly with that of the housing, rotates the fuel assembly through 360 degrees with its height kept constant, at a fixed or variable angular velocity;during the 360-degree rotation of the fuel assembly, the gamma-ray counting circuit measures either a time average of any count values which the gamma-ray detector has detected during the rotation of the fuel assembly (i.e., an average counting rate), or an integral value of any counts detected within a fixed time by the detector; andthe data collecting/analyzing and controlling apparatus analyzes the data output from the gamma-ray counting circuit, in association with the data relating to the rotation and movement of the fuel assembly by the rotating and moving means, and thus derives a burn-up distribution and power distribution of the fuel assembly. 2. The gamma scanning apparatus according to claim 1, wherein:the gamma-ray detector is disposed in plurality in a vertical direction and the detectors are each installed at desired measuring height in the vertical direction or at an integral multiple of the desired measuring height. 3. The gamma scanning apparatus according to claim 1, wherein:during the rotation and vertical movement of the fuel assembly, the data collecting/analyzing and controlling apparatus simultaneously collects data on rotational and vertical positions of the fuel assembly existing at particular moving time of day, and data on counting rates of the gamma-ray detector existing at the particular time. 4. The gamma scanning apparatus according to claim 1, wherein:the burn-up or power distribution of the fuel assembly is derived by using an average counting rate with respect to a burn-up index nuclide or power index nuclide or using an integral value of any count values detected within a fixed time. 5. The gamma scanning apparatus according to claim 1, wherein:a germanium semiconductor detector or an LaBr3 (Ce) scintillation detector is used as the gamma-ray detector. 6. The gamma scanning apparatus according to claim 1, further comprising:collimator driving means that drives a collimator, the collimator being disposed in the gamma scanning apparatus, so as to make height of the collimator variable and to render a distance between the fuel assembly and the detector variable;wherein the collimator driving means makes variable a solid angle at which the fuel assembly is viewed from the detector. 7. The gamma scanning apparatus according to claim 1, wherein:the shield includes iron, stainless steel, or any other metallic equivalent, which is placed around the gamma-ray detector in order to shield against characteristic X-rays generated when a heavy metal is irradiated with the gamma rays. 8. Gamma scanning apparatus comprising:a gamma-ray detector;a collimator that limits a measuring range in a fuel assembly and limits entry of gamma rays from the fuel assembly into the gamma-ray detector;an absorber that controls intensity of the gamma rays entering the gamma-ray detector;a shield that shields the gamma-ray detector;a housing that contains the gamma-ray detector, the collimator, the absorber, and the shield;moving and fixing means that moves the housing to a definite position and fixing the housing thereto;rotating and moving means that moves the fuel assembly vertically in addition to rotating the assembly;a gamma-ray counting circuit that measures an output of the gamma-ray detector; anddata collecting/analyzing and controlling apparatus that analyzes data output from the gamma-ray counting circuit, in association with data relating to the rotation and movement of the fuel assembly by the rotating and moving means;wherein:after fixing the housing, the rotating and moving means simultaneously conducts the rotation and vertical movement of the fuel assembly, and during 360-degree rotation, moves the fuel assembly vertically to an appropriate position according to desired measuring height;during the 360-degree rotation, the gamma-ray counting circuit measures either the time average of the count values which have been detected during the rotation and vertical movement of the fuel assembly (i.e., the average counting rate), or the integral value of the counts detected within a fixed time by the detector, and thus derives the burn-up and power distribution of the fuel assembly; andthe data collecting/analyzing and controlling apparatus analyzes the data output from the gamma-ray counting circuit, in association with the data relating to the rotation and movement of the fuel assembly by the rotating and moving means, and thus derives a burn-up distribution and power distribution of the fuel assembly. 9. Gamma scanning apparatus comprising:a gamma-ray detector;a collimator that limits a measuring range in a fuel assembly and limits entry of gamma rays from the fuel assembly into the gamma-ray detector;an absorber that controls intensity of the gamma rays entering the gamma-ray detector;a shield that shields the gamma-ray detector;a housing that contains the gamma-ray detector, the collimator, the absorber, and the shield;moving and fixing means that moves the housing to a definite position and fixing the housing thereto;rotating and moving means that moves the fuel assembly vertically in addition to rotating the assembly;a gamma-ray counting circuit that measures an output of the gamma-ray detector; anddata collecting/analyzing and controlling apparatus that analyzes data output from the gamma-ray counting circuit, in association with data relating to the rotation and movement of the fuel assembly by the rotating and moving means;wherein:after fixing the housing, the rotating and moving means simultaneously conducts the rotation and vertical movement of the fuel assembly, and during 180-degree rotation, moves the fuel assembly vertically to an appropriate position according to desired measuring height;during the 180-degree rotation, the gamma-ray counting circuit measures either a time average of any count values which have been detected during the rotation and vertical movement of the fuel assembly (i.e., an average counting rate), or an integral value of any counts detected within a fixed time by the detector, and thus derives a burn-up and power distribution of the fuel assembly; andthe data collecting/analyzing and controlling apparatus analyzes the data output from the gamma-ray counting circuit, in association with the data relating to the rotation and movement of the fuel assembly by the rotating and moving means, and thus derives a burn-up distribution and power distribution of the fuel assembly.