Editor’s Note – This is the first in a series of articles concerning topics that can or should be addressed in a licensee’s initial and annual refresher training. These articles will be designed such that a licensee can build a training library if he/she saves and stores these articles.
Loose surface contamination is radioactive material in a form that is easily spread around and is in a place where we don’t want it to be (or don’t know that It is). If people walk through an area that contains loose contamination, some of the radioactive material will be picked up on their shoes and spread around as they go about their business. Loose surface contamination (once discovered) can be cleaned up using conventional janitorial methods. (The rags and other cleaning materials must be controlled as radioactive material.)
There are three potential problems with loose contamination. First, is the possibility that it might be inadvertently ingested if not quickly discovered and removed. Second, it might be spread beyond the boundaries of the licensed facility and cause undue stress (though usually little dose) to families and friends of those workers involved. It can also become a public relations nightmare. Third, loose surface contamination might become airborne and be made available for inhalation.
If loose contamination is absorbed into or worked into surfaces, it becomes more difficult to remove. We would then call it “fixed contamination.” Sometimes, though, what was fixed contamination last week becomes loose contamination due to grinding, welding or burning operations, or just from the radioactive material leaching out of the contaminated surface.
The most common method of testing for loose contamination is through the performance of a wipe survey. These may also be referred to as swipe or smear surveys. When performing a wipe survey, a cloth or paper circle, usually about 1.5 inches in diameter, is rubbed over a surface area of approximately 100 cm2.
Performing a wipe survey over an area of 100 cm2 is standard Industry practice; it’s also required for Department of Energy surveys and for licensed facility decommissioning. This is the best surface area over which to take a wipe because most regulatory guideline levels and/or limits are specified in units of activity per 100 cm2. So, performing wipes over this surface area makes area conversion calculations unnecessary.
The area of 100 cm2, other than being an easy number to work with, was chosen because It is representative of a surface that would be brushed as a person walked through a given area. This representative area is equivalent to the area of a square, four inches on the side. However, the preferred method of performing this survey is to wipe an area in an S-shaped pattern over a distance of about 12-14 inches.
Sometimes, however, there isn’t 100 cm2 of surface area available for wiping. This would be true of small components or tools that are being surveyed. In this case, the activity should be reported per the total surface area surveyed.
In some situations, standard wipe circles won’t work because the location to be swiped is too small. In this case, cotton swabs may be used. Again, be sure to record the approximate area over which the sample was taken.
Most wipe surveys are performed with dry wipes. This is the method specified in US NRC Regulatory Guides and is representative of contamination that would most likely come off a surface if it was brushed up against. Some facilities, however, specify that the swipe should be pre-wetted with alcohol or a mixture of water and EDTA. This is fine, as it will pick up more activity than a dry wipe. ‘As long as the wipes are thoroughly dried before being counted. Otherwise, water or alcohol will shield alpha and beta particles, especially those of low energy.
When performing a wipe survey, it is important that a system be established whereby the surveyor is able to keep track of the locations where wipes were taken. You don’t want to go into an area, take a number of wipes, count the wipes, and determine that one of them contains excessive radioactive material and not know exactly where that one wipe was taken. Pre-numbered wipes are available. Some identifying information can also be written on each wipe, wipe folder or envelope prior to the start of the survey. The survey locations should also be annotated on a survey map or drawing of the area.
Surveys for loose and fixed contamination should be per-formed in various representative locations such as normal personnel traffic routes and at entrance and egress locations. Surfaces such as floors, ledges, corners. ventilation ducting, piping runs, lighting fixtures, sinks, drain covers and other areas where dust and moisture tend to collect should be smeared to obtain information on general area contamination levels. If the survey results are to be used for establishing radiological work controls, smears should be taken on representative equipment surfaces in the work area.
,Protective gloves should be worn while taking wipes. Remember, once a wipe is taken it must be considered radioactive and handled accordingly. Don’t take wipes and place them In your pocket as this might contaminate your clothing. Don’t take wipes and piece them together In a container because any “hot’ wipes might cross-contaminate other wipes. Wipes should be kept separated and contained to ensure accurate survey results and prevent the spread of contamination.
Sometimes wipes are stuck onto a rectangular piece of paper that when folded in half prevents any material on the wipe from contaminating other wipes, and prevents other wipes from contaminating it. Wipes can also be placed into small coin envelopes or baggies to prevent cross-contamination. Whichever method Is used, cross-contamination of wipe samples must be avoided.
Once the wipe samples are obtained they must be analyzed to determine their activity. Wipes must be counted with monitoring equipment that is reliable and capable of measuring the radiations of concern. The Instrument and counting technique should be able to measure a level of radioactivity, i.e., have a Minimum Detectable Activity (MDA), which is less than 25% of the guideline value. Also, the instrument must be calibrated for the radiations and energies of interest.
Usually, low background, high efficiency laboratory equipment is used to count wipes; however, portable equipment can also be used if it can be demonstrated that this equipment and counting procedures are able to meet the above-mentioned activity levels. Wipe samples for tritium (3H) contamination are usually counted in a liquid scintillation detector, due to the low energy of the tritium beta particles (so low that they are unable to penetrate the entry window present on standard detector tubes). There are wipes available that become translucent in the scintillation fluid, or that can be easily dissolved prior to counting. This maximizes the counting efficiency of the scintillation detector.
The preferred method of surveying for loose contamination is to perform the wipe in an “S Shaped” pattern over a distance of 12 to 14 inches.
by David J. Durkee