Online Measurement of Cerium and Gadolinium in Pyroprocessing Salt via Laser-Induced Breakdown Spectroscopy

Pyroprocessing of used nuclear fuel (UNF) is being investigated worldwide as an alternative to the traditional aqueous method. Pyroprocessing technology offers many advantages including its small footprint, radiation tolerance, and proliferation resistance. In pyroprocessing, uranium from the UNF is electrochemically dissolved at the anode, transported through the molten salt electrolyte and deposited on the cathode within the electrorefiner (ER). As part of this process, special nuclear materials (SNM) are also dissolved from the UNF but are not deposited on the cathode, resulting in a buildup within the ER. This buildup of material has been monitored via inductively coupled plasma mass spectroscopy (ICP-MS); however, with this method there is substantial delay between sample acquisition and analytical results due to delays from material transfer from the hotcell and sample preparation. Recently, a laser-induced breakdown spectroscopy (LIBS) technique has been explored to directly measure the SNM concentrations within the ER. In LIBS, a small portion of the sample is ablated via a high energy pulsed laser. As the ablated material returns to ground states, characteristic light is emitted which is captured to form a spectrum of the material. From the spectrum, qualitative and quantitative information about the elemental composition of the salt can be determined. The main motivation of this work was to explore cerium (Ce) and gadolinium (Gd) in a solid LiCl-KCl salt that had been formed from the melt using a vacuum extraction process. A total of 36 samples were made with Ce and Gd concentrations ranging from 0.5 wt% to 5 wt%. For each sample, six replicate LIBS measurements were made, each comprised of 100 laser pulses. From the data, calibration curves for Ce and Gd were constructed. For Ce, the limit of detection (LOD) was calculated to be 0.099 wt% with a root mean squared error of cross validation (RMSECV) of 0.197 wt%. For Gd, the LOD was 0.027 wt% and the RMSECV was 0.295 wt%. This work demonstrated that concentrations of Ce and Gd can be quantitatively determined in solid salts drawn from the melt using an online vacuum extraction process—thus enhancing the material accountability and safeguards of pyroprocessing.