2.7.3.1 - Tritium

Besides the formation through cosmic radiation and nuclear weapon tests, Tritium (3H) is formed during the normal operation of nuclear power plants (e.g. by ternary fission of ²³⁵U). It can therefore be found in Nuclear Power Plants and other facilities of the nuclear fuel cycle (e.g. reprocessing, decommissioning). It is emitted in controlled amounts as HT-gas and tritiated water HTO through off-gas and waste water.

For the radiation exposure of the population, the emission of Tritium as HTO is of importance as it takes part in the biotic exchange cycle.

HTO is enriched e.g. from a bypass of the stack off-gas by condensation of the humidity of air in a cryostat. The off-gas temperature and the relative humidity must be known for the calculation of the emitted Tritium activity; a quantitative condensation is not necessary.

Recent sampling methods apply the adsorption of HTO on a molecular sieve followed by water exchange, which is desribed below.

The instrumentation for a combined sampling of Tritium (3H) and Radiocarbon (14C) is shown in figure 43.

 A bypass from the off-gas stack passes the following modules:

The purification step of HTO by distillation is equal to procedure 2.2.1.9.

Materials and Equipment

• Zeolithe molecular sieve in cartridge

• Distillation apparatus

• NaI solution

• Na₂CO₃ (water free)

• Na₂SO₃ solution

• AgNO₃ solution

• Gelating cocktail

Procedure

1. The Tritium saturated molecular sieve is weighed and transferred together with 250 mL distilled water into a polyethylene bottle.

2. The bottle is shaken for 30 minutes for isotopic exchange.

3. A part of the exchanged water (ca. 60 mL) is transferred into a distillation apparatus.

4. 1 mL NaI solution (hold-back carrier for Iodine), 1 g water free Na₂CO₃ (hold-back carrier for ¹⁴C ), 1 mL Na₂SO₃-solution (reduction of higher valentic Iodine compounds) and 1 mL AgNO₃-solution (Iodide precipitation) are added.

5. The sample is quantitatively distilled whereas the first 5 mL are rejected.

6. 8 mL of the homogenized distillate are pipetted into a counting vial and are mixed with 12 mL scintillation cocktail.

7. The sample is measured and data are corrected for quenching.

Evaluation

The annually emitted Tritium activity A_E is calculated from:

Activity of the cartridge is

with

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

G_T = Weight of measuring sample (2 g)

G = Weight of water added (250 g)

ΔG_P = Increase of weight of cartridge (g)

t = Sampling time in years

Lower Limit of Detection LLD: 1 Bq per batch (cartridge)