Radwaste management | Assays
The mock drum24 August 2010
The UK National Physical Laboratory has developed voluntary test procedures for laboratories assaying radioactive waste. By Julian Dean
Many of the nuclear facilities built in the second half of the last century are reaching the end of their lives and are at various stages of being decommissioned. A large part of the industry is now concerned with the decommissioning process and the disposal of resulting nuclear waste. The correct identification and measurement of contaminated (or activated) materials is a vital aspect of this process and poses a major challenge to the industry. Also, correctly classifying waste as ‘exempt’ as opposed to low level waste (LLW) can reduce such costs by a factor of 50 typically.
Measurements of this type can be hampered by a lack of suitable standards and reference materials. Waste on a site can range widely, both in activity level and physical form, and it is usually impractical for the user to prepare a range of standard sources which match all the samples which might arise. To deal with this, nuclear sites often use modelling software to predict detector responses (e.g. for measurements of gamma-emitters in bulk waste), although some sites do prepare their own in-house standards.
Difficulties with defining uncertainties for measurements of this type can have major cost implications. For example, when the activity level is close to the border between two radioactive waste categories, companies will always err on the side of caution; this may result in material being incorrectly assigned to the higher category. If, however, measurements were more accurate, less waste would be incorrectly assigned, saving money for both the government and the companies involved.
NPL surveyed the UK nuclear industry to establish the exact measurement needs of the user base. The results showed a wide range of needs with no particular sample type being dominant. A subsequent meeting found that a core requirement was reference materials for (gamma) non-destructive assay of concrete and laboratory ‘soft waste.’ This material is a poorly defined waste form that comprises materials such as plastics, paper, ion-exchange resins and vermiculite; while its density can vary, its uncompacted form it is typically around 300kg/m3. The preferred format for gamma reference materials was a large container (such as a 200 litre drum) containing material with an activity concentration of just under 0.4Bq/g, the current UK limit for exempt waste.
In response, NPL developed a mock radioactive waste drum traceable to national standards so nuclear decommissioning teams could test their procedures for characterising potentially radioactive waste. The low density was achieved by loading the drum with plastic bottles, each partially loaded with ion-exchange resin. The resin in each bottle had been previously spiked with a mixture of Am-241, Cs-137 and Co-60, all traceable to national standards. The drum would be used primarily as the basis of a comparison exercise, but feedback on the usefulness of this format for calibration standards would also be sought.
The monitors used by the exercise participants were mostly commercial gamma-spectrometry systems designed to accommodate waste drums. Some groups measured the drum on more than one monitor and some used more than one detection efficiency calibration. Many of the groups used mathematical modelling to derive their efficiencies.
NPL sent the drum as a blind sample to 16 nuclear sites across the UK for 18 measurement teams to provide a traceable comparison sample to validate their systems. The teams used the drum to test their ability to measure gamma-emitters in soft waste at the borderline between LLW and exempt waste. The results were discussed at a follow-up workshop.
In total, 88 results were submitted. A total of 51 results were in agreement with the NPL values; of the remaining results, 24 were explained by the participants concerned (or had been revised to provide supplementary values), but the other 13 results were either clearly discrepant or questionable.
The exercise demonstrated differences between laboratories which had used the same modelling software for their efficiency calibrations, and indeed facilitated an exchange of models between these laboratories. The importance of an accurate knowledge of the form and structure of the sample in efficiency modelling was clearly demonstrated. Uncertainty budgets were of variable quality. Some participants quoted minimum detectable activity (MDA) values for one or more of the radionuclides, sometimes because the detector’s sensitivity had been set too high. The exercise led to some of the companies concerned re-examining their results or procedures.
The work from the first NPL comparison has brought added confidence to companies measuring exempt waste, and low level waste (activity concentration up to 12kBq/g for beta- and gamma-emitters), and also intermediate levels of nuclear waste (with a higher-still activity concentration, but not so high as to be heat-generating). It has helped users to validate their existing efficiency models and should help reassure industry, the regulators and the public that radioactive waste is being dealt with safely and cost-effectively.
“The inter-comparison exercise has enabled me to demonstrate the validity of Nuvia’s approach to our regulators, clients and the public,” said senior radiation protection adviser Ian Pearman. “It has also enabled me to meet with other participants and develop good working relationships. All in all these inter-comparisons have enabled us to develop and improve the quality of our gamma spectrometry product in a demonstrable manner. Continual improvement of the product has to be good for all our stakeholders.”
Exercises such as this comparison help improve accuracy for the measurement of the radioactivity content of waste materials; moreover, they are voluntary, confidential and independent of the nuclear industry.
We have almost completed a second comparison of drums with heterogeneous activity distributions, which are more difficult to measure. Those results will be published in a 2010 NPL report. A third, yet to be determined comparison exercise is planned for 2011.
Julian Dean, National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW