A. Gaston-Bellegarde1, 2, A. Habibi1, P. Fichet3

1, 2 Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PSE-ENV/SAME/LERCA, Sorbonne University, 3Alternative Energies and Atomic Energy Commission (CEA)

* amayes.gaston-bellegarde@irsn.fr (corresponding author)

36Cl is a pure-beta emitter radioisotope with a long half-life of 3.01 x 105 years. The production of this radionuclide is mainly due to the nuclear industry. Its presence in the environment is at ultra-trace level with an isotopic ratio 36Cl/Clstable of 10-13g.g-1. 36Cl is produced by neutron activation of 35Cl present as impurities, in the nuclear fuel and reactor building materials. Due to chlorine high volatility and mobility, the reported environmental soil activities in 36Cl are 0.026 – 1.201mBq.kg-1 close to fuel reprocessing plants [1] and 0.0021 – 0.54Bq.L-1 close to nuclear waste storage facilities [2]. Liquid Scintillation (LS) and Accelerator Mass Spectrometry (AMS) are the traditional technics for 36Cl quantification with limits of detection (LOD) in the range of 10-12g.g-1 and 10-15g.g-1 (36Cl/35Cl) [3] respectively.

Even if the AMS is the only environmentally compatible technic for 36Cl quantification in terms of LOD, this device is not easily available for many laboratories due to its cost and size. Previous works highlighted that a LOD of 10-8g.g-1 could be reached for 36Cl by tandem inductively plasma mass spectrometry (ICP-MS/MS) [4]. The aim of this research is to develop a new method, initially applied to aqueous samples and based on a purification protocol using the CL resin (Triskem International) [5] and ICP-MS/MS quantification, providing 36Cl LOD lower than the one obtained by SL and compatible with 36Cl levels encountered in reactor building materials and some environmental samples activities, like downstream of discharge point of nuclear facilities.

The main difficulties when quantifying 36Cl+ by ICP-MS/MS are ionisation efficiency (less than 1% due to its high first ionisation energy 12.967eV), isobaric (e.g., 36S+ and 36Ar+) and polyatomic (e.g., 18O21H+) interferences. It was demonstrated that the use of H2 in the ICP-MS/MS’s collision reaction cell (CRC) lead to the formation of 36ClH2+ and 36ArH2 while no reaction occuring with 18O21H+ [6]. Focus was made on H2 flow rate, integration time and the stabilisation time optimization leading to a complete charge transfer between H2 and Ar+. 36SH2+ formation in the CRC (due to 36S impurities in reagents, solvents…) is nonetheless observed from a concentration of 5.10-9g.g-1 showing then the importance of the sulfur removal during the purification step. Indeed, the purification step using CL resin was studied and chlorine eluent (e.g., NH4SCN 0.1M) was replaced by Na2CO3 0.05M to minimize sulfur presence. The developed protocol led to excellent figure of merits in terms of turnaround time and LOD. Complementary tests to concentrate the sample, to completely remove sulfur while maximizing chlorine signal have to be continued.

References

[1] S. Le Duzès. Journal of Environmental Radioactivity, 2019, 196, 82-90

[2] D. Calmet, N. Coreau, P. Germain, F. Goutelard, P. Letessier, C. Fréchou, D. Maro. Chlorine-36 measurment in the near-field environmentof a spent nuclear fuel reprocessing plant

[3] C.Bouchez, J.Pupier, L.Benedetti, P.Deschamps, V.Guillou, K.Keddadouche, G. Aumaître, M.Arnold, D.Boulès. Chemical Geology, 2015, 404, 62-70.

[4] B. Russel, S.L. Goddard, H. Mohamud, O. Pearson, Y. Zhang, H. Thompkins, R.J.C. Brown. Journal of Analytical Atomic Spectrometry, 2021, 36, 2704-2714.

[5] I. Llopart-Barbot, M.Vasile, A. Dobney, S. Boden, M. Leermakers, J. Qiao, P.Warwick. Journal of Radioanalytical and Nuclear Chemistry, 2022, 331, 3313-3326

[6] A. Zulauf. Journal of Radioanalytical and Nuclear Chemistry, 2010, 286, 539-546