Influence of sulfuric acid concentration in the simultaneous voltammetric determination of uranium and plutonium in nuclear fuels

Abstract

Interfacial coupled chemical reaction between U(IV) (formed at the electrode surface) and Pu(IV) (diffuses from the bulk towards the electrode) regenerates U(VI) at the electrode-solution interface and causes enhancement in the U(VI) reduction current, thus creating problems in the simultaneous voltammetric determination of U and Pu. Despite such interference between U(IV) and Pu(IV), the simultaneous voltammetric determination of U and Pu in FBTR Mark-1 fuel samples in sulfuric acid (1 M H₂SO₄) on a poly(3,4-ethylenedioxythiophene) (PEDOT)-poly(styrenesulfonate) (PSS)-modified glassy-carbon (GC) electrode (PEDOT-PSS/GC) has been reported. However, the reported method is applicable only for FBTR mark-1 fuel samples, in which the ratio [Pu]/[U] > 2 is always maintained. For nuclear samples having [Pu]/[U] < 2 (e.g., PFBR fuel), the simultaneous voltammetric determination of U and Pu is extremely challenging. Herein, we report a modified version of the earlier method for the simultaneous determination of U and Pu in nuclear samples ((U, Pu)C and (U, Pu)O₂), irrespective of the [Pu]/[U] ratio. The effect of acidity (H₂SO₄ conc.) on the coupled chemical reaction between U(IV) and Pu(IV) was examined. It was observed that an increase in the acidity of H₂SO₄ minimized the coupled chemical reaction, and at 5 M H₂SO₄, change in the Pu(IV) concentration did not have any effect on the U(VI) reduction current. The coupled chemical reaction between U(IV) and Pu(IV) ceased at 5 M H₂SO₄ and hence, the simultaneous voltammetric determination of U and Pu was possible on PEDOT-PSS/GC, irrespective of the [Pu]/[U] ratio in 5 M H₂SO₄. The method was applied for both (U, Pu)O₂ (PFBR) and (U, Pu)C (FBTR) samples and was compared with the well-established biamperometric method. The present method shows accuracy and precision comparable to biamperometry and did not show any interference from the commonly encountered impurities in nuclear samples. Thus, both FBTR and PFBR nuclear fuels having different [Pu]/[U] ratios can be analyzed by the present approach and it is a strong competitor to replace the well-established biamperometric method for routine sample analysis.