2.5.1.9 - Tritium by Distillation

Introduction

The reaction of cosmic radiation with air components of the atmosphere, e.g. ¹⁴N(n,T)¹²C and ¹⁸O(n,T)¹⁴N, leads to the formation of the low energy β-emitter Tritium (E_bmax = 18 keV, T_1/2 = 12.4 a). Nuclear weapon tests, however, are still the main cause for the presence of Tritium. Thus, a natural Tritium content of 0.1 mBq/L is found in natural water samples.

The European regulations and the renovelled ordinance for drinking water in Germany limit the Tritium content in drinking and mineral water to a maximum value of 100 Bq/L.

Due to the extremely low energetic β-radiation liquid scintillation is the only measuring technique with satisfying counting efficiencies from 40 to 70 %.

After sampling, the water sample has to be well sealed for the transport (H-T-exchange!). The sample is purified by distillation and then measured as gel by liquid scintillation. The counting time has to be adapted to the activity concentration and the qualification of the instrumentation, especially with respect to background in the luminescence area.

The distillation method can be applied in the same way for Tritium in waste water, milk or urine samples. Water samples that are less than pH 5 must be neutralized before distillation. Only the Tritium present as HTO will be detected by distillation of aqueous samples.

Materials and Equipment

• Gelating LS cocktail (e.g. AquaLight+ or Insta-Gel)

• Na₂SO₃

• 0.1 M KI-carrier, 0.1 M AgNO₃

Procedure

(s. a. [BMU 2000], H-H-3-AWASS-01-01 and [Fachverband 2004], Blatt 3.1.12.2)

• 100 mL of the unfiltered water sample are distilled to dryness (Claisen on top for drop removal!).

• 8 mL of the distillate are mixed with 12 mL gelating scintillator cocktail and are measured for 4 hours in the liquid scintillation counter after decay of the luminescence (one hour storage in dark, or alternatively applying TDCR).

• Quenching has to be corrected (see chapter 1.3.4.) and taken into account for the evaluation.

• If ¹³¹I might be present, 100 mg Na₂SO₃ (reducing agent) and 1 mL each 0.1 M KI-carrier and 0.1 M AgNO₃ for precipitation have to be added before distillation.

Evaluation

The Tritium activity concentration is calculated according to the general scheme by taking into account the quench corrected measuring efficiency. Triathler data refer to an instrumentation equipped with internal and external Lead shielding, a background of 40 cpm, an effective measuring efficiency of 30 % and 4 hours counting time.

Detection Limit (MDA): 5 Bq/L (LL β-counter) and 40 Bq/L (Triathler)

Total analysis uncertainty: 4.5 %

The detection limit can be improved comprehensively by electrolytic Tritium enrichment and/or TDCR technique.

BMU 2000: Messanleitungen für die Überwachung der Radioaktivität in der Umwelt und zur Erfassung radioaktiver Emissionen aus kerntechnischen Anlagen; Der Minister für Umwelt, Naturschutz und Reaktorsicherheit, Urban und Fischer, München, H-α-Gesamt-AWASS-01

BMU 2000: Messanleitungen Umweltradioaktivität H-Rn-222-TWASS-01, 4.1 Kalibrierung; see also Sr-analysis and others

Fachverband 2004: Empfehlungen zur Überwachung der Umweltradioaktivität FS-78-15- AKU, Fachverband für Strahlenschutz Dezember 1979/März 2004, Blatt 3.1.12.7