Patent Number: 
Section: claims

1. A method for evaluating an axial void fraction distribution of a fuel irradiated in a nuclear reactor, the method comprising:measuring a first intensity Az 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 Bz 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 A0 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 B0 of radioactivity rays of the second group at the axial reference position;calculating an exponent constant a used in an expression of Az=az×Eα and A0=a0×Eα where E is an exposure of the fuel, az and a0 are proportionality constants;evaluating a value of (az/a0) by an equation of (az/a0)=(Az/A0)(B0/Bz)α(bz/−b0)α, where bz is a value used in an expression of Bz=bz×E as a proportionality constant, b0 is a value used in an expression of B0=b0×E as a proportionality constant;evaluating a correlation curve of (az/a0) and a void fraction; andevaluating the axial void fraction distribution based on the value of (az/a0) and the correlation curve. 2. The method of claim 1, wherein the first intensity and the first reference intensity are neutron emission rates, and the second intensity and the second reference intensity are gamma ray intensities of a fission product that is proportional to the exposure within a certain exposure range. 3. The method of claim 2, wherein the neutron emission rates include rates of neutron emission of nuclides except curium 242, and the gamma ray intensities include a gamma ray intensity of cesium 137. 4. The method of claim 2, wherein the neutron emission rates include rates of neutrons emission of curium 244, and the gamma ray intensities include a gamma ray intensity of cesium 137 or cerium 144. 5. The method of claim 2, wherein the neutron emission rates include rates of neutrons emission of curium 242, and the gamma ray intensities include a gamma ray intensity of cesium 137 or cerium 144. 6. The method of claim 2, wherein the neutron emission rates are neutron emission rates except for curium 242 and curium 244, and the gamma ray intensities include a gamma ray intensity of cesium 137 or cerium 144. 7. The method of claim 2, further comprising:evaluating a neutron multiplication factor kz at a plurality of axial position of the fuel assembly, and a neutron multiplication factor k0 at the reference position;wherein the step of evaluating a value of (az/a0) includes:measuring a neutron flux or a neutron counting rate φz;measuring a neutron flux or a neutron counting rate φ0 at the reference position;calculating a ratio of the neutron emission rate to that at the reference position, Sz/S0, by an equation of Sz/S0=(φz/φ0)/(1−kz)/(1−k0); andassuming Az/A0 equal to Sz/S0. 8. The method of claim 2, further comprising:calculating an axial exposure distribution based on an assembly-averaged fuel exposure and an axial distribution of an intensity of gamma ray, by assuming the axial exposure distribution equal to the axial distribution of an intensity of gamma ray; and,evaluating a conversion factor from an infinite multiplication factor to a neutron multiplication factor by a neutron transport diffusion calculation according to a condition of the measurement;wherein the step of evaluating the axial void fraction distribution includes evaluating the axial void fraction distribution by repeating following (i) to (vii) steps until a convergence of the axial void fraction distribution, assuming an infinite multiplication factor can be expressed as a quadratic function of the exposure where parameters of the function depends on the void fraction:(i) assuming an axial void fraction distribution as appropriate one for an initial calculation, or an axial void fraction distribution calculated at the step of (vii) of a previous loop for a calculation except the initial calculation;(ii) calculating an infinite multiplication factor based on the assumed axial void fraction distribution;(iii) calculating an neutron multiplication factor kz and k0 by using the conversion factor and the multiplication factor;(iv) calculating a ratio of neutron emission rate to that at the reference position Sz/S0 based on the neutron multiplication factor kz and k0;(v) calculating az/a0 based on Sz/S0;(vi) evaluating the axial void fraction distribution based on az/a0 and the correlation curve; and(vii) terminating if a convergence of the axial void fraction distribution is achieved, or returning to the step of (i), if not. 9. The method of claim 8, further comprising:evaluating an exposure at the reference position by a neutron emission rate technique. 10. The method of claim 1, wherein the first group includes gamma ray emitting nuclides transmuted by neutron capture reactions after generation by fission, and the second group includes gamma ray emitting nuclides that is not subjected to a neutron reaction and emit gamma ray proportional to the exposure within a definite range. 11. The method of claim 10, wherein the first group includes cesium 134 or europium 154, and the second group includes cesium 137 or cerium 144.