New electrolytic enrichment system for tritium determination in water research institute in Bratislava and IT’S first results of tritium activity in precipitation

Highlights

• The process of electrolytic enrichment of water samples was described.

• New and old electrolytic enrichment system were compared.

• The seasonal variations of tritium activities in precipitation are observed.

Abstract

The method of tritium activity measurements by electrolytic enrichment in combination with liquid scintillation counting is well known for many years. In the Water Research Institute in Bratislava was this system employed since the 60-ties of the 20th century. In 2018 the laboratory of radiochemistry of Water Research Institute obtained a new electrolytic enrichment system with higher enrichment factor (varying from 9 to 22, depending on the total ampere-hours used). The enrichment factor of the previous system was about 6. Complementary to the new system, also the new LCS counter Quantulus GCT 6220 was added. This spectrometer has active background suppression function (Guard Compensation Technology - GCT) and for the tritium measurement the background counts decreased from cca. 9 cpm (for Tricarb 2900 TR) to approximately 1 cpm. To demonstrate the capabilities of this system, we present results of the tritium concentration in precipitations during the period from May 2016 to May 2019. The seasonal variations of tritium activities in precipitation are observed, with maximum values in spring season and minimum in winter. Additionally, selected tritium activities from the extensive monitoring of river waters in Slovakia are presented.

Introduction

Tritium has a great significance in isotope hydrology in addition to the stable isotopes ²H and ¹⁸O. Tritium is a pure beta emitter (E_max = 18.6 keV) with the half-life of 12.32 years (Lucas and Unterweger, 2000). Its concentration is described in terms of tritium units (TU), where 1 TU = 1 atom of tritium per 1018 atoms of hydrogen = 0.11919 ± 0.00021 Bq.kg⁻¹ of water (Groning and Rozanski, 2003).

Natural tritium is produced in the upper layers of the atmosphere by the reaction of neutrons produced by the cosmic rays and the nitrogen atoms. After formation, ³H is oxidized to water (HTO) and removed from the troposphere to the ground via precipitation. The latitudinal dependence of the natural production rate and the transport of the tritium from the stratosphere to the troposphere at the high latitudes should lead to non-uniform distribution of tritium in precipitation. Usually, it is assumed that tritium content is in the range between 10 and 20 TU in the northern hemisphere and less than 10 TU in the intertropical belt and the southern hemisphere, except Antarctica (Von Buttlar and Libby, 1955).

Tritium is also produced in nuclear reactors, entering the environment by periodical releases from the nuclear power plants. But the major source of tritium pollution was the hydrogen bomb testing in 1960′s. The amount of tritium released to the atmosphere by thermonuclear tests was enormous, exceeding the natural production by the two to three orders of magnitude over many years. From 1952 to 1962, approximately 600 kg of tritium was released to the environment (Rozanski et al., 1991). For comparison, the natural background mass of tritium in the atmosphere from cosmic ray spallation is estimated at 4 kg (Michel et al., 2015).

Since the Nuclear Test Ban Treaty in 1963, the level of bomb tritium measured in precipitation has decreasing trend due to its radioactive decay and dilution in the world oceans. Recently the concentration of tritium in precipitation and river waters has almost returned to the pre-nuclear test level (about 5–20 TU) (Guetat al., 2011). However, some studies suggest that even 55 years after the 1963 maximum of nuclear atmospheric tests the tritium did not yet reached the equilibrium value (Duliu et al., 2018).

In the north hemisphere activity concentrations of cosmogenic tritium in precipitation follow characteristic seasonal variations. The spring/summer maximum, referred to as “Spring Leak”, is explained by the exchange of tropospheric and stratospheric air masses occurring mainly during late winter and spring. At this time is the border layer between the stratosphere and troposphere lowered due to the heating of the continents (Zahn et al., 1998; Clark and Fritz, 1997; Gat et al., 2001) and this decline is followed by a release of tritium from the stratosphere into the troposphere and thus increases the flux of tritiated water to the ground by precipitation.

As tritium concentration in environment is 8.5 TU or lower and especially lower than 4.2 TU in seawater, it is difficult to measure this concentration even using a low background liquid scintillation counting system (Muranaka et al., 2011). To solve the problem, the tritium in water samples must be concentrated through a process of electrolytic enrichment (electrolysis) to attain a measurable level by liquid scintillation counting (Satake and Takeuchi, 1991; Saito, 2007; Plastino et al., 2007). Afterwards, the liquid scintillation counting system is used to count the released electrons by the beta-decay of ³H.

In the Water Research Institute in Bratislava was the tritium enrichment system coupled with LSC employed since the 60-ties of the 20th century. We present the results of the tritium concentration in precipitations during the period from May 2016 to May 2019. For this study we used a new electrolytic enrichment system with higher enrichment factor and also the new LCS counter QUANTULUS GCT 6220 obtained in 2018. Moreover, selected data from the extensive monitoring of river waters in Slovakia are presented.