A new wipe test methodology

RADIOACTIVE WASTE, AS IT IS produced in nuclear installations, consists mostly of activated material (like iron or steel components) or loose material mixed with radioactive isotopes.

The loose material is normally collected in packages like drums or containers. The packages avoid uncontrolled spread of radioactive material to any surfaces and part of the radiation (the alpha and beta radiation), is completely absorbed by the package.

During the filling or while storing the packages there is a possibility that radioactive material may be deposited on the outside of the package. This surface contamination is a potential danger for workers and the environment. The main danger stems not from emitted radiation, but from the possibility that surface contamination can be absorbed through body openings or skin.

To distinguish between fixed and removable contaminations is not very helpful, because the contamination may be in either state at different times.

Another source of danger lies in the enclosed radioactive sources, if the enclosure is not sealed hermetically.

For this reason, checking for surface contamination is an important procedure for handling and storage of radioactive waste or sources.

Direct measurements

Surface contamination can be measured with an alpha/beta measurement device, which is placed just in front of the surface. The sensitive area of the measurement device is placed very close to the object so it does not underestimate activity because of absorption of radiation by the air, especially in the case of alpha emitters. If the surfaces are uneven this may be very difficult. Another problem is the radiation from the object (mostly gamma radiation from the waste contained in the drum or container) or background radiation, which may limit the precision of the measurement and increase measurement errors.

For this reason, wipe tests were introduced, in which a piece of filter paper is run over the surface with low pressure and a rotating movement, thus collecting the contamination on the surface of the pad.

In principle wipes may be used with fluid for a better wiping effect. But the fluid may cover the emission of alpha or (partly) of beta particles, so dry wiping is generally preferred.

The surface contamination is calculated from the activity of the contamination collected on the pad, measured in a radiochemical laboratory, where the activity is chemically separated, concentrated and measured.

In daily practice another procedure is used: the pad is placed below an alpha/beta counter with shielding against background radiation around the device. In this method the pad does not need to be transported to a laboratory and the results are available much faster.

The activity collected at the pad surface is measured with an alpha/beta counter. The background count rate must be subtracted from the measured count rate.

The real activity analysis can be evaluated as follows: 

The parameters of Equation 1 have uncertainties, so ISO 11929 recommends data analysis on the basis of Bayesian theory. Many articles describing the calculations needed for the data analysis are available including a comprehensive article by Rolf Michel1. So, we present here only particularities of the analysis.

First the wiping factor must be determined, relating the measured activity to the surface contamination. In the past, detailed investigations have been done for different surface materials like steel, glass, plastic and wood and for different paper surfaces of the pads with different surface roughness. The investigations have shown that the reproducibility of the wiping process depends on the pressure of pads against the surface and the wiping technique. It has been also demonstrated that the use of double sided adhesive tapes has better reproducibility compared with wiping pads, because no smearing (slow wiping using pressure and rotation) is required.

The manual procedure for wiping, as described above, can be performed without any problems when:

• surface areas are small and flat,
• there is no radiation from the surface material itself or from waste contained in the package,
• the expected surface contamination is high enough to be a danger for the operator.

If large containers filled with high active waste have to be measured, then the wiping process has to be performed automatically. The following two procedures have been used in the past for this task.

In one, the wipe pad is fixed with double-sided adhesive tape to a pad holder (Figure 1), which presses the pads by spring force against the container, which is moved by roller conveyor or rotating by a turn table. After wiping, the holder head together with the pad is placed in a closed box and transported to the alpha/beta measurement station.

Alternatively, instead of a wipe pad a roller is used with double sided adhesive tape on the outer side. The roller is moved over the container surface, collecting contaminations with the adhesive tape. The roller is then transported in a closed box to the alpha/beta measurement station (Figure 2). During the rotation of the roller the alpha and beta emission is measured. This procedure simplifies the contamination sampling and gives a better reproducibility than the wiping pads. 

The wiping factor can be measured by contamination of a surface material piece with the same properties as the container or drum surfaces to be measured, by alpha and beta emitters inside a glove box. After this the contamination measurements are performed with pads or tapes and the results are compared with a direct measurement of contamination with an alpha/beta measurement device. The wiping factor is the ratio of the measurement results.

Another problem arises from the different device sensitivities for different isotopes. In alpha emitters, the energy of emitted particles is very similar, but the energies of emitted beta particles are different. Therefore, the result of the measured beta contamination depends very strongly on the expected isotope. As a consequence, the measurement for an alpha emitter has to be made separately to the measurement of beta emitters, and the data analysis of beta measurement has to be made separately for each isotope expected. In case of doubt, the result of the worst case has to be assumed.

The data analysis has to take into account errors in the different parameters. This includes stochastic errors as well as calibration errors. Earlier this was done by using a Frequentistic theory for error estimation. The problem was that the calculation with stochastic errors and systematic errors was not consistent. Nowadays the Bayesian theory is recommended for data analysis because this theory is consistent for different error types and different probability distribution functions (PDF) for the individual parameters.

If significant surface contamination is found, the surface has to be decontaminated.

This can be done by: washing or hosing with water under high pressure; rubbing off the surface; chemical treatment; electro-polishing; or fixing the contamination with an adhesive material or with decontamination paint.

After decontamination the measurements have to be repeated.

Conclusion

Some changes in respect to wipe tests procedures have been newly introduced. The use of double-sided tape rollers for wiping has been successfully introduced in the Nukem waste treatment centres in Ukraine and China to check containers and drums for possible surface contamination. The use of double sided adhesive tape instead of wipe pads has increased the reproducibility of the measurement and when used with rollers there is better handling of the wipe test for large objects like containers.

For data analysis the Bayesian theory has been introduced instead of a Frequentistic theory, which is more consistent in combining errors of different types and gives more detailed error information.

In daily practice the results of the measurement process are only semi-quantitative because of the uncertainties in respect to the wiping process and in respect to the knowledge of which isotopes have contributed to the contamination.  

Author information: Marina Sokcic-Kostic, Principal engineer Radiation Monitoring Systems at Nukem Technologies Engineering Services; Frank Scheuermann, Head of Safety Engineering & Assessment at Nukem Technologies Engineering Services; Christoph Klein, Senior engineer Radiation Monitoring Systems at Nukem Technologies Engineering Services