Index > 3 Characterisation of radioactively contaminated sites >

3.9.5 Radiochemical analysis

Contents
3.9.5.1 Soil analysis for radionuclide determination
3.9.5.2 Water analysis for radionuclide determination
3.9.5.3 Tritium

When samples contain radioisotopes other than pure gamma emitters, some radiochemical preparation will usually be required [ASTM-1995], [EPA-1980], [EPA-1982], [EPA-1984a]. This may be a simple distillation to separate tritium or a long and complex assay to separate actinides or strontium. For environmental samples, even if gamma emitters are being quantified, there may be advantages in terms of improved sensitivity and reduction of counting time if sample preparation can concentrate the radioactivity. The ashing of biological samples, for instance, will reduce the volume enabling a more sensitive counting geometry to be used. For complex preparations, the addition of yield tracers may be useful to quantify how much nuclide has been lost in the procedure.

Radiochemistry will generally be useful when it is necessary to separate a ‘hard to measure’ or ‘difficult to measure’ radionuclide for analysis by alpha spectrometry or liquid scintillation counting (LSC), when an isotope is present at such low levels that it needs to be concentrated before counting is possible, or when it is necessary to separate an interfering radionuclide or one which overwhelms assay of another radionuclide of interest.

‘Fingerprinting’ techniques are often useful in reducing the number of samples which require complex chemical separations. ‘Fingerprinting’ involves using measurements of easy to measure radionuclides (usually gamma emitters) to quantify harder to measure nuclides. This could involve measurements of 241Am to determine actinide inventories, or use of measurements of 137Cs to indicate 90Sr levels.

is formed as a decay product of 241Pu. 241Pu itself (and other plutonium isotopes) is difficult to measure, but 241Am is a low energy (59.5 keV) gamma emitter.

Americium and plutonium (as well as other actinides) often behave in a similar manner in the environment (perhaps as insoluble particulates). Thus, if the ratio of 241Am to other actinides can be established, a measurement of americium will serve to quantify the other actinides.

At least one measurement of the hard-to-measure radionuclide will usually be required, and it may be necessary to invest considerably more effort in order to establish the validity of the fingerprint.

Fingerprints may be used in several ways. First, a firm correlation based on readily justifiable scientific grounds may be established between the easy to measure radionuclide and the hard-to-measure isotopes (such as may be the case in the relationship between 241Am and 241Pu, or between 137Cs and 135Cs).

Second, an empirical correlation may be made between two isotopes which may not be necessarily expected to behave in an identical manner: for instance, it may be established that 90Sr levels can be linked to 137Cs levels.

Third, it may be possible to establish a bounding relationship between two isotopes. As a hypothetical example, if a correlation between 90Sr and 137Cs can be found at the surface of a site, and it can be shown that 137Cs is less mobile than 90Sr, measurements of 137Cs in core samples could give an indication of the amount of’ 90Sr in the subsoil.

3.9.5.1 Soil analysis for radionuclide determination

In Figure 3.11 a suggestion is presented for the radionuclide determination in soil.

Figure 3.11 Soil analyses for radionuclide determination
Figure 3.11 Soil analyses for radionuclide determination

3.9.5.2 Water analysis for radionuclide determination

In Figure 3.12 a suggestion is presented for the radionuclide determination in water.

Figure 3.12 Water analyses for radionuclide determination
Figure 3.12 Water analyses for radionuclide determination

3.9.5.3 Tritium

Tritium is a common contaminant on nuclear-licensed sites. It is present as tritiated water, which behaves in a chemically identical way to naturally occurring water (H2O). As a consequence, it is highly mobile and commonly migrates from the near-surface environment into groundwater. The extent of migration is limited by the short half-life of tritium (12.3 years). Special precautions are needed when sampling and analyzing for tritium, to prevent evaporation of the sample and/or isotopic exchange with naturally occurring water. It is possible to analyse for tritium both in soil samples and in waters; in both cases, tritium is present in the aqueous phase. Quantification of tritium contamination in the unsaturated zone generally involves analysis of soil samples. The tritium activity can be expressed either as Bq/g of soil or as Bq/l of soil porewater. The latter is more informative, because it can be directly compared with the activity concentration of tritium in the underlying groundwater.

However, the moisture content of the soil sample must be measured to derive the porewater activity. In the saturated zone (i.e., below the water table), either soil samples or water samples can be collected and analysed. In practice, determination of tritium activities in soil or rock from beneath the groundwater table would only be undertaken if the samples were cohesive and fine-grained (i.e., porewater did not freely drain from the samples on collection).

It is preferable to determine tritium activity concentration directly in the groundwater although, in principle, the tritium activity concentration in the soil porewater and in the groundwater should be identical if the porewater and the mobile groundwater that is sampled by pump testing are in close contact. Analysis of tritium in soils may be appropriate at an early stage of a characterisation program, in order to evaluate whether a potential problem exists. If tritium contamination below the groundwater table is detected, it is best practice to install groundwater monitoring boreholes and to obtain groundwater samples for further analysis.