Determination of the Transuranic Elements Inventory in High Burnup Pwr Spent Fuel Samples by Alpha Spectrometry

Determination of transuranic elements such as U, Pu, Np, Am and Cm in spent nuclear fuels is of importance for a fuel characterization, evaluation of a burnup credit, and burnup determination of spent nuclear fuels. Additionally, the exact amount of the actinide elements, as the major elements of a source term, of the spent nuclear fuels is also used for a code verification that predicts the amount of the nuclides produced or decayed during a neutron irradiation. For the determination of the elemental components in spent nuclear fuel samples using a chemical method, a tracer is generally used depending on the elements and determination method. Accordingly, a special precaution is required in the selection of an appropriate tracer, as spent nuclear fuel samples contain a variety of elements and various levels of activities compared to environmental samples. For example, Np emits relatively low alpha activity due to its long half life (2.14x10 y) while the isotopes of Am, Cm and Pu emit relatively high activities. Consequently, a simultaneous determination of these transuranic nuclides by alpha spectrometry with a single run taking only one sample is limited due to the different activity ratios between each nuclide, as mentioned above. In this work, high-burnup PWR spent fuel samples (36.97 ~ 62.85 GWD/MTU) were analyzed. The contents of transuranic elements were compared with those calculated by the ORIGEN-2 code. Alpha spectrometry was applied for the determination of the elements using a tracer or a spike. The isotopes determined in this work were Pu, Pu, Pu, Np, Am, Cm and Cm. In particular, for Np, a spike addition method as an isotope dilution technique [1] was applied using Np as a spike after verification of the method because the spent nuclear fuel samples contain a considerable amount of Np. Other nuclides were determined by alpha spectrometry using Pu and Am as the tracers. Generally, the recommended tracers [2] are Am for Am, Cm for Cm and Cm, Pu for Pu and Pu, and Np [3], Np(t1/2, 5000y) [4], Np(t1/2, 396.1d) [5], Pu [6] for Np. The tracers, Cm and Np or Np, The contents of transuranic elements in high-burnup spent fuel samples were determined. The activity amounts of Pu, Pu, Pu, Am, Cm and Cm were measured by alpha spectrometry using Pu and Am as tracers, respectively. A spike addition method for Np was established by an alpha and gamma spectrometry using Np as a spike after the optimum conditions for the measurements of Np and Np, respectively, were obtained. A separation system using anion exchange chromatography and diethylhexylphosphoric acid extraction chromatography was applied for the separation of these elements. This method was applied to high-burnup spent nuclear fuel samples (40~60GWD/MTU). The contents of the transuranic elements were compared with those by ORIGEN-2 code. Measurements and the calculations of the contents of the plutonium isotopes Pu, Pu and Pu agreed to within 10% on average. The contents of Np agreed to within approximately 5% except for one instance of a calculation, while those of Am, Cm and Cm showed higher values by approximately 19%, 35% and 14% on average, respectively, compared to the calculations according to the burnup.