2.7.3.2 - Radiocarbon

Introduction

The long-lived ¹⁴C (T_1/2 = 5730 a) with a maximum energy of E_βmax = 156 keV results from both natural formation and in nuclear reactors by the reaction of Nitrogen with neutrons (¹⁴N(n,p)¹⁴C). From the latter, it is released through the gas phase as ¹⁴CO₂, ¹⁴CO or as methane.

In Germany, a balance accounting of the emitted Radiocarbon in off-gas of nuclear power plants is mandatory for the operator, according to KTA 1503.

For the enrichment and separation of ¹⁴CO₂ gas, absorption in 5 M NaOH or on molecular sieves according to the procedure described above is used (fig. 43). The saturated ¹⁴CO₂ cartridge is heated or treated with HCl. In all cases the exhaled CO₂ is absorbed in NaOH before further treatment.

Details of the emanation process of CO₂ with HCl and absorption in NaOH can be found in [BMU 2000] and [Möbius 1988].

Radiocarbon (14C) as CO₂ takes part in the assimilation process and is therefore transferred into the biotic cycle. LSC is the most effective counting method for Radiocarbon in biological materials and for age dating; for the analysis in biological sample see chapter 2.2.2.1.

Materials and Equipment

• I^-/Cs⁺ carrier solution

• Hydrazine hydrate

• NH₄Cl solution

• 1 M BaCl₂ solution

• Glass filter apparatus

• Toluene Scintillator and 1 g Cab-O-Sil

• Ultrasonic bath

• Internal ¹⁴C-standard

Procedure

1. The analyte solution is mixed with 1 mL I^-/Cs⁺ carrier solution.

2. After adding 10 ml hydrazine hydrate and 100 ml NH₄Cl solution, the sample is heated to boil.

3. Barium carbonate is precipitated with 20 mL 1 M BaCl₂ solution and filtered hot through a glass filter apparatus (weight has to be determined in advance!).

4. The filter is washed with hot distilled water, some alcohol and acetone and then further dried for 2 h at 120 °C.

5. The filtering device is weighed (BaCO₃ yield). 1 g of the fine dispersed precipitate is mixed with 20 mL Toluene Scintillator and 1 g Cab-O-Sil as gelating agent.

6. After vigorous shaking till homogeneity (ultrasonic bath), the sample is measured by LSC in heterogeneous phase.

7. Quench correction has to be done by adding an internal standard.

Evaluation

The evaluation of the measurements is done in analogy to Tritium; however, the amount of carbonate has to be considered.

The annually emitted ¹⁴C-activity A_E is calculated from:

Activity of the cartridge is

A_P = Activity of the cartridge (Bq)

D_K = Stack throughput (m³/h)

D_B = Bypass throughput (m³/h)

R_N = Net rate (counts/min)

ε = Measuring efficiency

_η = Chemical yield (99 %)

G_G = BaCO₃ yield (g)

G_T = Weight of measuring sample (g)

t = Sampling time in years

Lower Limit of Detection LLD: 0.2 Bq per batch

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

Möbius S. 1988: Experiments for training in nuclear and radiochemistry; Report KfK 3920, Kernforschungszentrum Karlsruhe, Karlsruhe 1988