Patent ID: 8401141

Claim:
A method for evaluating an axial void fraction distribution of a fuel irradiated in a nuclear reactor, the method comprising: measuring a first intensity A z of radioactivity originated from a first group at an axial position of the fuel, the first group consisting of neutron-emitting nuclides generated by a neutron capture reaction of a heavy nuclide; measuring a second intensity B z of radioactivity originated from a second group at the axial position of the fuel, the second group consisting of gamma-emitting radioactive fission product nuclides that is not generated by a neutron capture reaction and of which generated amount is substantially proportional to an exposure of the fuel in a certain range of the exposure; measuring a first reference intensity A 0 of radioactivity of the first group at an axial reference position of the fuel at which a void fraction of the fuel can be evaluated; measuring a second reference intensity B 0 of radioactivity rays of the second group at the axial reference position; calculating an exponent constant a used in an expression of A z =a z ×E α and A 0 =a 0 ×E α where E is an exposure of the fuel, a z and a 0 are proportionality constants; evaluating a value of (a z /a 0 ) by an equation of (a z /a 0 )=(A z /A 0 )(B 0 /B z ) α (b z /−b 0 ) α , where b z is a value used in an expression of B z =b z ×E as a proportionality constant, b 0 is a value used in an expression of B 0 =b 0 ×E as a proportionality constant; evaluating a correlation curve of (a z /a 0 ) and a void fraction; and evaluating the axial void fraction distribution based on the value of (a z /a 0 ) and the correlation curve.