This is the output of the Metrology for Decommissioning and Site Clearance Workshop that was held on Thursday 30 June 2005.

The work programmes of NPL are shaped by the needs of end-users, and the aim of the workshop was to clarify the requirements for metrological support for nuclear site decommissioning and clearance. NPL would be seeking funding from its existing DTI programme or other sources to address these needs.

Bulk monitoring group summary

Ian Adsley (RWE Nukem) summarised the findings of this group.

The main points of the discussion were as follows:

• NPL had proposed various matrices such as concrete and steel, and the group were asked if other materials were higher priority. One delegate suggested matrices with low (or varying) density or Z (especially for measurements of ²⁴¹Am). Others were interested in a concrete matrix.
• Regarding sample sizes, NPL had proposed a 200-litre drum, and the group were asked if it would be useful to prepare such a drum in sections, or if other sample sizes would be preferable (e.g. to validate efficiencies from computer modelling). There was interest in 1m³ and 10m³ bags as well as 200-litre drums, and in the preparation of standardised 'sections' that could be stacked to form the larger geometries. The idea of defining a 'reference package' for waste in decommissioning was mooted. Bespoke standard packages developed for specific users were raised as another possibility, but these would probably be prohibitively expensive for the users. One delegate suggested that a calibration based on one geometry could be extrapolated by computer modelling to larger geometries.
• Regarding the radionuclides required, one option was for NPL to measure the natural series radionuclides in a NORM sample. Alternatively, NPL could artificially spike a 'clean' matrix with either single nuclides or with mixtures such as the NPL mixed (γ) radionuclide standard. Natural series radionuclides might be present in such a matrix and these would be measured also. Many in the group felt that ¹³⁷Cs (in concrete) would be very useful. A 'decommissioning mixed γ standard' (perhaps consisting of ²⁴¹Am, ⁶⁰Co and ¹³⁷Cs) was suggested, and ²³⁸U was also proposed.
• Turning to activity concentrations, there seemed to be a preference for < 0.4 Bq g^-1. One consideration here was transport costs. One delegate raised the problem of measuring packages containing 'hot particles'. Although this was recognised as being an important issue, it was felt that homogeneous materials were required initially, and that the issue of particles should be deferred to a future forum.
• The delegates would be asked to vote on whether certificated standards or test samples were preferred. NPL could provide both if necessary.

In the formal voting, the top priorities were:

• Matrices: Concrete, soft waste and brick
• Radionuclides: ¹³⁷Cs, ²⁴¹Am and ⁶⁰Co
• There was a strong preference for a sample volume of 200 litres and an activity concentration of < 0.4 Bq g^-1.
• Certificated standards (as opposed to test samples) were preferred.

Also, many users voted for standards in 'sections'.

 View Ian Adsley's presentation (PDF 634 KB)

 View Bulk Monitoring/Voting Summary (PDF 151 KB)

 

Surface monitoring group summary

Peter Burgess (UKAEA) summarised the findings of this group.

The main points of the discussion were as follows:

• It was felt that a type-testing service was needed for surface contamination monitors, in which the monitors would be characterised against a range of 'realistic' contaminated substrates. These might include concrete, fibreglass, glass, lead, linoleum, painted surfaces, Perspex, pipe work, PVC, steel and wood. Regular checks on the condition of the sources would need to be carried out and potential ageing problems (e.g. activity gradually soaking into a porous surface) would need to be considered. A solid-state β-spectrometer would be useful here. Transport of such sources would present significant traceability problems so monitors would be tested at NPL.
• NPL should endeavour to get such a type-test service adopted as an industry standard.
• The issue of 'wipe-off factors' for different surfaces was again raised and this is an area where further guidance is needed.
• NPL has run workshops on basic surface contamination monitoring in the past, and it was felt that similar events were needed, including some 'realistic' contaminated surfaces (such as those listed above) to clearly demonstrate to users the effects of changing the substrate.

In the formal voting, the top priorities were:

• Substrates: Concrete/masonry/bricks, steel and Formica
• Radionuclides: ²³⁸U, ¹³⁷Cs and Americium nuclides
• Dual-phosphor probes, 'pancake' GM tubes and α-probes were the most commonly-used monitors.

 View Surface Monitoring Voting Summary (PDF 152 KB)

 

Radiochemical analysis group summary

Dave Wickenden (AEAT Waste Management Technology) summarised the findings of this group.

The main points of the discussion were as follows:

• The top priorities would be concrete and steel. The matrices could be spiked but they could be more 'realistic' if they were activated by neutrons. There was also an interest in soil samples. It was felt that graphite would be much more difficult to analyse and would be a lower priority. Soft wastes would be very difficult to specify with so many different possible types, and again these would probably be a lower priority.
• Regarding radionuclides, the main interest was in ⁵⁵Fe, ⁶⁰Co, and ⁶³Ni. It was suggested that one could pulverise 'real' activated concrete, in which NPL could analyse and certify three or four radionuclides initially. More radionuclides could be certified over a period of time so that the material became better and better characterised. It was noted that NIREX had a list of 112 nuclides that they needed to know in ILW; however, the discussion focussed on the free release/LLW boundary.
• Activity concentrations should be below free release levels for concrete but above these levels for steel, where 1 -10 Bq/g would be more appropriate.
• A unit sample size of 500 g was strongly preferred.
• One delegate was interested in standard methods for the determination of gross α and β activities.

In the formal voting, the priorities were:

• Matrices: Concrete, steel and soil
• Radionuclides: Tritium, ¹⁴C and actinides
• Certificated standards were preferred.

 View Radiochemical Analysis Voting Summary (PDF 153 KB)