Quantitative determination of 137Cs and 90Sr in dissolver solutions without pre-separation by isotope dilution thermal ionization mass spectrometry

Abstract

It is very essential to determine the amount of ⁹⁰Sr and ¹³⁷Cs present in high-level liquid waste (HLLW) due to their radiological nature. Conventionally, such determination requires prior separation of these elements in pure fractions for both radiometric and mass spectrometric methods. The present study demonstrates, for the first time, the potential use of isotope dilution thermal ionization mass spectrometry (ID-TIMS) for the quantitative estimation of ⁹⁰Sr and ¹³⁷Cs without prior chemical separation. By controlled heating of the sample filaments, it is possible to optimise preferential evaporation conditions (Cs at a filament current of 1.3 A/1000 K and Sr at 2.6 A/1450 K), thereby eliminating the isobaric interference in situ and overcoming the shortcomings of conventional techniques. The success of this method is initially demonstrated using a non-radioactive sample mixture of Sr, Rb, Zr, Cs and Ba with natural isotopic abundances. Subsequently, ⁹⁰Sr and ¹³⁷Cs present in dissolver solutions of spent fuels from fast and thermal reactors are determined. The burn-up and cooling periods undergone by fuels of fast and thermal reactors are 108 GWd per ton and 11 years; 8100 MWd per ton and 20 years, respectively. The amounts of ¹³⁷Cs and ⁹⁰Sr measured in the given fast reactor fuel dissolver solution are 1729 ± 1 μg g⁻¹ and 969 ± 0.6 μg g⁻¹ of solution, respectively; similar values obtained for the thermal reactor dissolver solution are 68 ± 0.4 μg g⁻¹ and 33 ± 0.1 μg g⁻¹, respectively.