X-RAY SPECTROMETRY X-Ray Spectrom. 2002; 31: 167–172 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/xrs.540 Total reflection by synchrotron radiation: trace determination in nuclear materials † S. M. Simabuco, 1∗ C. V ´ azquez, 2,3 S. Boeykens 3 and R. C. Barroso 4 1 Campinas State University, Civil Engineering Faculty, P.O. Box 6021, 13083-970 Campinas, S ˜ ao Paulo, Brazil 2 Comisi ´ on Nacional de Energ´ ıa At ´ omica, Av. del Libertador 8250, 1429 Buenos Aires, Argentina 3 Universidad de Buenos Aires, Facultad de Ingenier´ ıa, P. Col ´ on 850, 1063 Buenos Aires, Argentina 4 State University of Rio de Janeiro, Physics Institute, 20550-900 Rio de Janeiro, RJ, Brazil Received 7 March 2001; Accepted 29 August 2001 The chemical control of impurities in nuclear materials is indispensable in order to ensure efficient operation of the reactors. The maximum concentration admitted depends on the elements and in most cases is in the parts per billion range. Conventional analytical methods require a preconcentration treatment of the sample and a previous separation of the matrix (uranium). In this work, we investigated the use of total reflection x-ray fluorescence with synchrotron radiation excitation as an alternative methodology for the determination of impurities in nuclear materials, namely K, Ca, Ti, Cr, Mn, Fe, Ni, Cu and As. The detection limits obtained were in the range 0.1 – 20 ng ml -1 for a 1000 s counting time. Copyright  2002 John Wiley & Sons, Ltd. INTRODUCTION Heavy water reactors (HWRs) utilize uranium dioxide as fuel, which must have a high degree of purity in order to guarantee safe operation. This is based on the fact that some of these impurities have high neutron capture cross-sections and they will modify the fission yield. Several techniques have been developed for the deter- mination of trace elements in nuclear materials. Trace concentrations are usually determined by neutron activa- tion analysis (NAA), optical emission spectrometry, atomic absorption spectrometry, spark source mass spectrometry and inductively coupled plasma methods. These techniques are very sensitive but can show some limitations depending on the elements to be determined. Total reflection x-ray flu- orescence with synchrotron radiation (SR-TXRF) excitation compared with the mentioned techniques has the following advantages: (1) the opportunity to determine simultaneously a wide range of elements, (2) high sensitivity down to the 1 – 2 ng g 1 level and (3) in the particular case of NAA a lack of fission products from U. These fission products cause additional errors during elemental determinations. In this work, SR-TXRF was investigated in order to prove its effectiveness for determining impurities in nuclear fuel, namely K, Ca, Ti, Cr, Mn, Fe, Ni, Cu and As. The TRXRF technique was introduced in 1971 by Yoneda and Horiuchi 1 and developed by Aiginger and Wobrauschek. 2,3 This method is based on the reflection of an x-ray beam at a small angle on the flat surface of a reflecting L Correspondence to: S. M. Simabuco, Campinas State University, Civil Engineering Faculty, P.O. Box 6021, 13083-970 Campinas, S˜ ao Paulo, Brazil. E-mail: silvana@fec.unicamp.br † Presented at SARX-2000, 7th Latin-American Conference on Analysis by X-ray Techniques, S˜ ao Pedro, Brazil, 19–24 November 2000. Contract/grant sponsor: FAPESP. support on which the sample to be analysed is deposited. In this condition, the scattering effect is minimized and thus a better peak-to-background ratio is obtained, improving the detection limits. Another advantage of this technique is the small volumes required for liquid sample analysis (microliters) or small masses (micrograms) for solid samples after chemical digestion. Quantitative analysis can be performed according to Eqn (1), considering the sample as a thin film. In this way, absorption and enhancement effects can be neglected. The thin film formed does not have a regular geometry and therefore the x-ray intensities depend of the position of the thin film. The relative intensity for each element in relation to an internal standard (Ga) added to every sample and standard allows the correction of errors caused by excitation/detection geometry. R i D g i C i ⊲1⊳ where R l is the counting rate (counts per second) of the characteristic x-ray of element i, C i is its mass concentration ⊲μg ml 1 ⊳ and g i is the system sensitivity for this element. As an internal standard is added in the samples, the relative counting rate can be calculated by the expressions R i R Ga D g i g Ga C i C Ga ⊲2⊳ R i R Ga C Ga D g i g Ga C i ⊲3⊳ Taking Q i D R i R Ga C Ga ⊲4⊳ and S i D g i g Ga ⊲5⊳ Copyright  2002 John Wiley & Sons, Ltd.