X-ray fluorescence spectrometry: An alternative technique for analysis of waste

This study aims to develop reliable analytical methodology that is, cost-effective, and requires minimal sample quantity to quantify uranium content in nuclear waste and others. The Energy Dispersive X-ray Fluorescence Spectrometry (EDXRF) technique was used, and a rigorous comparison was made between the fundamental parameters (FP) method and the empirical (EMP) method. Statistical evaluation of results demonstrated that the FP method showed a satisfactory level of confidence for precision and limit of quantification


INTRODUCTION
Nuclear technology has been playing an important role in several science areas, such as medicine, agriculture, materials and others.The advancements in this technology are represented by the work of researchers, students and technicians at Universities, Research Institutes, and other institutions worldwide.The uranium waste can be produced in experiments or processes using uranium compounds.Despite specific standards and guidelines storage and identification [1,2], it may be required to confirm uranium content in waste.Therefore, an analytical technique efficient and efficacy for uranium content determination in waste is of great value.
The X-ray fluorescence spectrometry (XRF) is an instrumental analytical technique.It has been applied in different areas of knowledge, such as nuclear, nanotechnology, environmental, health, forensic and others [3][4][5][6][7].XRF technique compared with atomic absorption spectrometry (AAS), inductively coupled plasma optical emission spectrometry (ICP-OES) and others has been highlighted for allowing direct and non-destructive, i.e., element identification and quantification without previous sample chemical treatments [7].
Qualitative analysis is efficient because irradiated sample by X-rays generate photons with characteristic energies of the elements present in the sample [8,9].However, satisfactory quantitative results are obtained using certified reference materials (CRMs), i.e. of chemical composition similar to the unknown sample.The high cost or absence of CRMs have motivated researchers and technique users to develop methodologies to decrease the use of specific CRMs.
Fundamental Parameters Method (FP) has been most applied and current [10].This method, also known as "Standardless Quantitative Analysis" allows the emission intensity of each element's characteristic photons to be calculated using parameters such as primary spectral distribution (polychromatic source), absorption coefficients (photoelectric and mass), fluorescence yield, and others, providing a multivariate calibration curve [6].
Thus, this study aimed to develop a robust and reliable analytical methodology that is costeffective and requires minimal sample quantity to quantify uranium content in wastes by EDXRF.
The methodology is expected to assist in safeguarding and waste management area for quantifying uranium content in unknown samples.
The U3O8 concentration was converted to UTotal according to equation 1: where, Impe ≡ total mass of impurities according to the certificate [11,12]; 0,848 g ≡ gravimetric factor for converting U3O8 to UTotal.
Data acquisition was performed using a Shimadzu Co. model 720 Energy Dispersive X-ray Fluorescence (EDXRF) Spectrometer, configured with an X-ray tube and Rh anode (250W), Si (Li) semiconductor detector, cooled with liquid N2 and 3-millimeter collimator.The instrumental set up were: voltage 50 keV, auto adjustable current of max. 1 mA and vacuum atmosphere between 30-10 Pa.For each CRM, 2048 points were measured, from 0.00 to 40.96 keV, with a 0.02 keV interval, exposure time for 100s and detector "dead time" (DT) 39%.
Under established conditions acquisition of the count rate (cps μA -1 ) for U-Lα using FP method (ROI ≡ 13.6-14.2keV) were registered in EDX-720 software and obtained concentration for Utotal.
The Utotal concentration by EMP method was obtained using count rate (cps μA-1) and OriginPro8 Software.Applying Gauss Fit single peak (ROI ≡ 13.6-14.2keV), normal probability density, according to equation 2: The correlation between count rates (cps μA-1) versus UTotal concentration was calculated by least squares method for FP and EMP methods.Precision was evaluated in terms percent relative standard deviation (RSD%) using Horwitz equation as acceptability criterion.Accuracy in terms of relative error (RE%) using Z-score test [13] as acceptability criterion.The limit of quantification (LOQ) was calculated for 95% confidence level [13].Comparison between FP and EMP methods was performed using t-test, considering mean of two paired samples.
Tables 1 and 2 agreement shows in terms of precision for FP and EMP methods.With regard to accuracy showed disagreement according to the Z-score test.Thus, the methodology is precise, but not accurate.
The comparison of FP and EMP methods result by t-test (mean of two paired samples) showed the following values: tcalculated = 0.58 and tcritical = 2.45.Thus, it was verified that FP and EMP results are statistically similar.

Scapin et al. • Braz. J. Rad. Sci. • 2023
Regarding the Limit of Quantification (LOQ) it was observed that Method FP = 1.2% and EMP = 1.6%.The comparison by t-test showed the following values: tcalculated = 0.58 and tcritical = 2.45.Thus, it was verified that LOQs for FP and EMP methods are statistically similar.
This study allowed verifying that FP and EMP methods are statistically similar for uranium determination.Although accuracy to have been unsatisfactory, fact that little sample using not require chemical treatments in sample preparation and provides results fast, show applicability for semi-quantitative analyses in uranium determination.

CONCLUSIONS
The necessity for chemical analysis of nuclear materials and waste pose challenges for analytical experts.The proposed methodology proved to be an interesting alternative technique, in the way it demands minimum handling and sample amounts, as it also eliminates the need of previous chemical treatments for the identification and quantification of uranium content in unknown samples.The results provided by the FP method showed applicability for semiquantitative analyses evidencing uranium content in wastes.

Table 1 :
Parameters determined for the FP Method.
= average of measurements in triplicate; σ = standard deviation.
Scapin et al. • Braz.J. Rad.Sci.• 2023 Results showed that precision in terms of RSD% for FP method was satisfactory once it HorRat test values are lesser than < 2 [13].Regarding accuracy four CRMs were rejected (NBL 123-3,123-

Table 2 :
Parameters determined for the EMP Method.