1.1 ,A sample of spent nuclear fuel is analyzed to determine the quantity and atomic ratios of uranium and plutonium isotopes, neodymium isotopes, and selected gamma-emitting nuclides (¹³⁷Cs, ¹³⁴Cs, ¹⁵⁴Eu, ¹⁰⁶Ru, and ²⁴¹Am). Fuel burnup is calculated from the ¹⁴⁸Nd-to-fuel ratio as described in this method, which uses an effective ¹⁴⁸Nd fission yield calculated from the fission yields of ¹⁴⁸Nd for each of the fissioning isotopes weighted according to their contribution to fission as obtained from this method. The burnup value determined in this way requires that values be assumed for certain reactor-dependent properties called for in the calculations (1, 2).2