Tritium (denoted by ³H or T) is a radioactive isotope of hydrogen that can originate from both natural and anthropogenic sources. Monitoring its concentrations, as the predominant radionuclide in releases from nuclear facilities, is important to ensure that tritium levels are kept within safe limits (consistent with regulations in place to protect human health and the environment). Work on optimising methods in the study of low concentrations of tritium activity in surface waters is being carried out at NCBJ by scientists of the Radiation Protection Measurements Laboratory.

The main natural source of tritium on Earth is the atmosphere. Tritium is formed in the upper layers of the atmosphere as a result of nuclear reactions of cosmic ray neutrons with atoms of oxygen or nitrogen. As an isotope of hydrogen, tritium exhibits its chemical properties as well as its high mobility in the environment. Like stable hydrogen, it can exist in the gaseous state in the form of HT or T₂ diatomic molecules, while it is more commonly found in the form of the tritiated water molecule HTO, which is subject to the natural water cycle in nature.

This isotope is also formed as a result of human activity, e.g. in connection with the operation of nuclear reactors and spent fuel reprocessing plants. As a result of thermonuclear weapons tests carried out in the 1950s and 1960s, significant quantities of tritium were released into the atmosphere (approximately 200 times its natural reserves ). Since the signing of the Convention on the Prohibition of Atmospheric Nuclear Weapons Testing in 1963, the amount of tritium released into the environment due to nuclear testing has gradually decreased, now reaching values close to background (pre-nuclear test) levels.

Currently, human-related sources of tritium in the environment include the operation of nuclear power plants, nuclear fuel reprocessing plants, industrial plants producing tritium-containing products (e.g. photoluminescent escape signs), and hospitals using tritium for diagnostic or therapeutic purposes.

Tritium is one of the primary isotopes targeted for local environmental monitoring around facilities such as nuclear power plants and nuclear fuel reprocessing plants, as most tritium is released from these facilities into the environment in gaseous or liquid form due to the technical difficulties associated with tritium disposal.

The use of sensitive methods in environmental tritium surveys allows background levels to be identified and any abnormal changes to be detected. Although 'background' radiation levels of tritium are decreasing, measurements of tritium in various environmental components, including river systems and other surface waters, continue to be of interest to many researchers, including hydrologists. Accurate data on tritium activity concentrations are also important for exposure assessment of critical groups and populations. As tritium activity declines in the environment, methods are needed to measure lower and lower activity concentrations. For this reason, research has been conducted for years in laboratories around the world to improve tritium determination methods or to develop new methods that are even more sensitive.

At the NCBJ, such work is carried out at the Radiation Protection Measurements Laboratory (LPD). Recently, LPD staff carried out a study to test and compare two potential tritium determination methods that could be applied to the measurement of low concentrations of tritium activity in waters. The first method tested was a modification of the standard method used in this laboratory (modifications concerned the choice of measurement conditions). The second method involved the use of electrolytic enrichment. Both methods used liquid scintillation spectrometry as the measurement technique. Using these, the scientists examined samples of seawater taken in the Baltic coastal zone, as well as river water from various regions of Poland, including samples taken near the National Centre for Nuclear Research, which operates Poland's only nuclear research reactor, MARIA.

The first method studied assumes that a properly prepared sample (often subjected to prior filtration, distillation to separate interfering isotopes and so-called quenching agents) is mixed with a scintillation cocktail in a measuring vial and then placed in a counter. The beta rays (electrons) of the tritium in the sample interact with the scintillator molecules, causing flashes of light that can be detected by the detector. This method is reliable and relatively fast, but has a minimum detectable activity concentration (MDC) of a few Bq/L (its value depends on the sensitivity of the apparatus used and the measurement conditions), which may not be sufficient for tritium activity measurements in surface water samples with expected low activities.

For tritium determinations in samples with low concentrations (even < 1 Bq/L), an electrolytic enrichment method is often additionally used. During the electrolysis process, H₂O molecules are decomposed into gaseous oxygen and hydrogen more efficiently than HTO molecules, due to the isotopic effect affecting the rate of chemical reactions at the electrodes. As a result, the tritium concentration in solution increases by several times or even tens of times, significantly reducing the MDC. Unfortunately, this method is time-consuming, as the electrolysis process itself usually takes several days.

A team of scientists from the NCBJ's Radiation Protection Measurements Laboratory carried out an optimisation of the method used so far, which included the selection of the correct ratio of sample volume and liquid scintillator, as well as the extension of the measurement time in the liquid scintillation counter.

The optimisation steps applied compared to the standard method allowed the researchers to reduce the detection limit from about 3.1 Bq/L to 1.8 Bq/L. In comparison, the parameters of the electrolytic method used enabled a detection limit of 0.20 Bq/L to be reached, but its use involves a much longer sample preparation time.

Both methods were then applied to water samples taken from the Baltic Sea, as well as from Polish rivers (including the Vistula, Oder, Narew, Bug, as well as the Świder river flowing near the National Centre for Nuclear Research).

"The results of tritium activity concentrations in surface water samples obtained with both tested analytical procedures were in agreement considering their expanded uncertainties," describes Małgorzata Dymecka, M.Sc., from the NCBJ Radiation Protection Measurements Laboratory. "The determined tritium activity concentrations of all tested water samples collected from Poland were above the minimum detectable activity concentration for the electrolytic enrichment method. Only five samples tested showed activity concentrations slightly above the detection limit for the modified standard method. Activity concentrations measured by the electrolytic enrichment method for the surface water samples tested ranged from 0.43 ± 0.16 to 0.94 ± 0.32 Bq/L".

The measurements showed that the lowest tritium activity concentrations were obtained for small rivers in the north-eastern region of Poland. The values obtained ranged from 0.43 ± 0.16 to 0.75 ± 0.27 Bq/L, meaning that they were too low to be determined using the modified standard method. Slightly higher activity concentrations were obtained for water samples from larger rivers, as well as for water from the Baltic Sea, but these were at levels common to this type of water. Tritium activity concentrations measured in samples taken from the Świder River above and below the Świerk Nuclear Centre were lower than the values measured for other major Polish rivers, which showed (together with data from the Annual Report of the President of the PAA) that the operation of the NCBJ facilities did not contribute to an increase in tritium activity concentrations in river water in 2022.

The authors of the study also compared the obtained values of tritium isotope concentrations with those obtained by other research groups. The results obtained are part of a downward trend in the concentration of this radioisotope in nature. Researchers from the Laboratory of Dosimetric Measurements chose the method using electrolysis for future analyses of tritium content in water samples with activities at levels currently observed in surface waters in Poland. Despite the much longer time required for sample preparation, it provides five times lower minimum detectable activity concentrations and smaller measurement uncertainties. It is therefore a more accurate and reliable method.

The researchers' full results are available in the publication: Dymecka, M., Szaciłowski, G., Rzemek, K., Ośko, J. Low-level tritium measurements in freshwater and seawater samples. J Radioanal Nucl Chem (2024). https://doi.org/10.1007/s10967-023-09295-4

Supplementary information

Radiation Protection Measurements Laboratory is involved in the assessment of the impact of the Spruce nuclear facility on the environment and in the monitoring of exposure to ionising radiation of NCBJ personnel. The LPD also provides services to external entities in the field of calibration of dosimetry apparatus, dosimetry measurements, as well as training and expertise in the field of dosimetry and radiological protection.

The Research Laboratory No. AB 567 (Contamination Measurements Division), operating within the structures of LPD, has been continuously accredited since 2005 by the Polish Centre for Accreditation in Warsaw (a signatory to multilateral agreements within international organisations active in the field of accreditation, i.e..: EA MLA, IAF MLA and ILAC MRA). The Contamination Measurement Division performs tests of environmental, biological and technological samples for the determination of activity concentrations of radioactive substances. Research work is carried out in the laboratory to develop applied analytical procedures and methodologies.