WIMS Library Update Project

The fission product yields were retrieved from the corresponding evaluated nuclear data files using the AVRFPY code. They are usually given on a very coarse energy grid (no more than three points), therefore it is not necessary to use an elaborate weighting function for averaging. A four points averaging function was defined in the AVRFPY code according to the following conditions:

1. The lower boundary of the thermal group is at 0.00001 eV or at the first energy point at which the yields are given (whichever is higher).The upper boundary of the thermal group is at 0.55 eV, which is approximately the cadmium cutoff energy. The upper boundary of the epithermal group is at 1 keV. The upper boundary of the fast group is 10 MeV.

2. The integral of the thermal spectrum is defined such that the spectral ratio (i.e. ratio of the thermal flux to the sum of the epithermal and fast flux) is 5.785.The integral of the fast spectrum is defined such that the ratio of the fast to epithermal flux is 0.9279.

3. The fission cross section of the parent fissile nuclide is defined in the same group structure from the average thermal cross section, the resonance integral and the fission spectrum averaged cross section. The fission reaction rate in three groups is calculated to define the weighting function.

4. The spectrum at 0.55 eV is defined assuming 1/E shape in the epithermal range. The fission cross section at this energy is defined assuming a 1/v cross section behavior in the thermal group, where v is the neutron velocity. The reaction rate weighting function is the product of the spectrum and the fission cross section. At other energies it is defined to conserve the integrals, assuming log-log interpolation.

The three-group averaging flux is typical of light water reactors and corresponds to the EPRI-CELL LWR spectrum available for cross section averaging as option IWT=5 in the NJOY nuclear data processing system.