A comparison of different approaches for the analysis of 36Cl in graphite samples

The safe disposal of wastes originating from decommissioning of nuclear installations generally entails restrictions on the radioactivity concentrations of the radionuclides that are addressed by the risk analyses for the disposal site. 36Cl is one of these radionuclides considered in the Belgian legislation for the final classification of the waste (e.g. category) and route for disposal (Fréchou and Degros 2005; Poncet 2017; FANC 2001). It is a neutron activation product of natural chlorine, which is commonly present as an impurity in construction materials (such as graphite or concrete). Its high soil-plant transfer factor, volatility and long half-life (T1/2 = 3,02 • 105 year) require the follow up of this radionuclide and the need for its characterization in decommissioning waste streams from nuclear facilities (Hou, Østergaard, and Nielsen 2007). In our previous work, we reported two approaches for the determination of 36Cl in solid samples from decommissioning activities by means of liquid scintillation counting (LSC) (Llopart Babot et al. 2022; Llopart Babot 2022). Croudace et al. 2017 reported inductively coupled plasma mass spectrometry (ICP-MS) as a possible technique for the quantification of 36Cl (Croudace, Russell, and Warwick 2017). However, the isobaric interferences 36S and 36Ar can clearly limit the application of the technique. More recently, Warwick et al. 2019 described the use of tandem mass spectrometry (ICP-MS/MS) for 36Cl analysis. This spectrometric technique uses a collision cell in order to remove the isobaric interferences (i.e. 36S and 36Ar) (Warwick et al. 2019; Russell et al. 2021). Also the use of Accelerator Mass Spectrometry (AMS) for the quantification of 36Cl has been reported in the literature, however, AMS is not usually accessible to all the scientific community (i.e. expensive technique) (Hou 2013; Hou and Roos 2008; Ashton 2000; Tolmachyov et al. 2001). This work compares two different measurement techniques for the analysis of 36Cl in graphite samples: LSC using LS cocktail and plastic scintillation resin (PS resin), and ICP-MS/MS. Controlled pyrolysis was used for sample combustion, release and trapping of 36Cl in all cases. Interaction with a Cl resin was used for the radiochemical separation followed respectively by the measurement of the pure chlorine fractions by LSC and by ICP-MS/MS. For the application of LSC when using PS resin, the trapping solution was loaded onto a PS resin and the cartridge was measured directly. The advantages and disadvantages of these techniques following from their comparison will be described and discussed in detail.