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At the Laboratory for Low-level radioactivities (LLR) of the Ruđer Bošković Institute (RBI), Zagreb Croatia, the natural activities of 14C and 3H have been measured for the past five decades. For almost 30 years 14C and 3H activities were measured by gas proportional counter. For the last 15 years 14C is measured by liquid scintillation counter (Quantulus 1220) and in 2007 accelerator mass spectrometry was introduced as additional technique. In 2008 electrolysis enrichment system was implemented for 3H activities measurement by LSC technique in environmental water samples. Croatia and Slovenia share a nuclear power plant NPP Krško located in Slovenia near the Croatian border. LLR conducts monitoring of the 14C activity in vegetation surrounding the NPP Krško. Also, RBI is a part of the DONES project for testing of future fusion power plant construction materials. Recently, LLR started with analyses of organically bound tritium (OBT) and it is plan to analyze the samples from the vicinity of the NPP and also to contribute in the DONES project with OBT analyses. The system for extraction of combustion water for OBT analyses is the same as one used for CO2 production from samples for 14C LSC analyses. The only difference is that the combustion water, which is usually discarded during 14C analyses, is now collected for OBT analyses. The combustion water often contains organic molecules which are yellowish in color, so first Na2O2 is added to pH>12 and KMnO4 for oxidation of organic molecules until dark-wine color appears. The samples are then distilled. The scintillation cocktail is prepared by mixing 8 ml of combustion water to 12 ml of Ultima Gold LLT. The 3H activity of samples is measured with overall counting time of 500 minutes in Quantulus 1220. Here we show the results of the 4th and 5th OBT inter-comparison exercises. The 4th, which was our first, showed a good agreement with the consensus values, but had inadequate precision which was due to experimenting in procedure and yielding too small quantities of combustion water, resulting in large dilution factors. After adaptation of the procedure, the 5th inter-comparison yielded a good agreement with the consensus value and satisfactory precision. Our results of OBT activity in combustion water from the sample distributed in the 6th OBT inter-comparison will also be presented. |
