USE OF NUCLEAR SYSTEMATICS TO ESTIMATE GAMMA-SPECTRA

Empirical data concerning fission product gamma spectra after reactor shutdown were examined. Data were ample for times long after shutdown and for long irradiation times, when long-lived nuclides predominate. A simplified method of estimating gamma spectra was used, which involved sorting nuclides with known gamma spectra into ″boxes″ on the basis of simple nuclear systematics. A normalized spectrum for each nuclide was created from the relative intensity of each gamma energy. Composite spectra were formulated from the individually sorted spectra by weighting the average relative intensities. Final spectra representative of a specific reactor model were obtained by applying composite spectra to those fission products that contribute significant gamma activity after shutdown. For a fixed time after shutdown, final spectra for long irradiation times were more smooth than for short irradiation times. The most notable feature was a decisive shift toward softer, but less smooth, spectra as time after shutdown increased.