Effects of Neutron Capture Transformations on the Decay Power of Fission Products

Neutron capture effects on the decay power of fission products have been examined by varying the fissile nuclide, neutron spectrum, neutron flux, and irradiation and cooling times. Neutron capture transformations of fission products usually increase the decay power. However, at short cooling times, i.e., <C10⁴ s, the capture effects are small, especially in a thermal reactor, where the negative contribution of135Xe offsets the positive contributions of other nuclides. The capture effect exhibits peaks at cooling times of 10⁶ and 10⁸ s and becomes negligible at 10⁹ s. The former peak results mainly from the increases in the activities of¹⁰³Ru, 134Cs, ¹³⁶ Cs, 148Pm, ¹⁴⁸Pm, and ¹⁵⁶Eu, and the latter by activities of ¹³⁴ Cs and ¹⁵⁴Eu. The capture effect increases with increase of the flux level or irradiation time, and it is approximately proportional to the integrated flux at long cooling times. There is only a slight difference between the capture effect of two thermal reactors with epithermal indices of 0.1 and 0.2. In fast reactors, the effect is smaller than in thermal reactors at cooling times over 10⁵ s, and depends only a little on the fissile nuclide. The decay power in fast reactors depends on the cross-section library selected to less than ∼ 1%.