Before I launch into this, let me preface. I do not speak for Kaiser-Hill, the integrating contractor at RFETS. But my comments do reflect the opinion of my employer, The Alpha Group & Associates, LLC. An important, and well documented, component of the Rocky Flats Environmental Technology Site (RFETS) low level radioactive waste program is field characterization of surface contaminated objects (SCO). Kaiser-Hill has shipped tens of thousands of cubic meters of SCO waste from RFETS and will probably ship ¼ million cubic meters before the closure project is completed. This has been a key factor in the timely D&D and closure of Rocky Flats. It has resulted in huge cost avoidance, hazard reduction, and personnel dose avoidance when compared to the baseline technology that was based on size reduction of components. (see Bracken and Morris, “SCO Shipments from Rocky Flats - Experience and Current Practice” in proceedings of Waste Management Symposium 2001; and Morris, “Overview of Surface Contaminated Object Characterization for Waste Disposal at Rocky Flats Environmental Technology Site” in proceedings of the HPS 2002 Midyear Meeting).
Wiping efficiency is not normally used by HP’s in the US. Our traditional release standard for potentially contaminated items prescribes a wiping method, and there is an implicitly assumed wiping efficiency built into the numerical release limits. No wiping efficiency factor is included in the calculation of removable activity. One simply swipes an area, typically 100 square cm, counts the swipe, divides by counting efficiency and compares to the release limit. That’s all there is to a release survey, so don’t make more complicated that it needs to be. This is the same method (except for the area swiped) that DOT applies to the outside of the shipping container. But characterization of the surface contaminated objects inside the shipping container is a different story.
Characterization of SCO material for DOT-regulated transit requires use of a wiping efficiency factor for calculation of non-fixed (in other words, removable) contamination. Wiping efficiency can be defaulted to 10%, but the guidance also allows for direct measurement of the value. One acceptable technique for determining wiping efficiency is defined in ISO 7503-1 (1988). ISO calls this the removal factor. To calculate the removal factor, repeated wipes on exactly the same surface are taken and counted. Divide the activity on the first wipe by the sum of activity on all wipes as shown in the following equation.
where
W1 is the net activity removed in the first wipe, and
WT is the net activity removed all subsequent wipes, including the first.
In my opinion, a statistically negligible difference for this sequence of wipes would be reached when a subsequent wipe showed only 5% to 10% of the activity collected on the first wipe. For example, if the true removal factor was 20% and the sequence of wipes was stopped after a subsequent wipe contained only 10% of the activity found in the first wipe, the removal factor would be estimated to be 22%. This difference from the true value of 20% which practically insignificant for the use described here.
In some cases when operating near instrument detection limits, the wiping efficiency-corrected removable level exceeds the fixed plus removable contamination level measured prior taking to the wipe at that location. This illogical outcome stems from the error propagated by multiplying two poorly defined values. When this occurs, the fixed plus non-fixed contamination value (i.e., the direct measurement of total contamination) should be substituted for the non-fixed contamination value. In other words, if the removable contamination level exceeds the total contamination level, the removable contamination value is discarded and replaced with the total value and then all of that contamination is assumed to be removable.
I’ll be happy to follow up on this discussion. But please note that The Alpha Group is in the process of relocating away from Rocky Flats to our new commercial calibration facility, so my e-mail and phone are in flux (my new phone will soon to be (303) 531-8880 and my e-mail will be Robert.Morris@AlphaGroup.us). So please be patient if I am a little tardy in response.
Robert Morris, CHP
The Alpha Group & Associates, LLC
Building 790
Rocky Flats Environmental Technology Site
please page me at (303) 212-3265