Answer to Question #9871 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

I have noted that various nuclear facilities occasionally require their radiation protection (RP) staff to perform "whole body" (WB) frisks on personnel for the purpose of detecting external surface contamination. For example, (1) when a stand-in monitor is out of service, (2) when workers are leaving a contaminated work area, and (3) when RP staff is responding to a personnel contamination incident.  

I have calculated that the time to perform this WB frisk, if done properly, should be ~30 minutes.

This assumes that RP staff:

  • use a "pancake" detector (area ~15.5 cm2)
  • scan at a rate of ~2.54 cms-1
  • scan at an area rate of ~ 11.3 cm2 s-1 (i.e., the diameter of the pancake times the linear scan rate), and
  • a body surface area of ~ 1.8 m2 and
  • no overlap of scanning area during the frisk

A 30-minute WB frisk is certainly not performed by staff members in practice. Additionally, it would not be cost effective to request that they do so.  

My question is: How should a "whole body" frisk for personnel contamination be performed?

Should we simply frisk hands, feet, and suspect areas OR should we allow staff to scan faster over certain low-risk locations? The former suggestion may eventually lead to personnel with contamination on certain body locations to go completely undetected. The latter would increase the MDA (minimum detectable activity) and may lead to an increase in low-level contamination incidents being undetected.

Any guidance would be greatly appreciated.

A

Your question raises a legitimate point—namely, how much time and how thorough a job is required when doing manual whole-body frisking to detect possible contamination on individuals’ clothing, skin, and hair. At least part of the answer lies in consideration of how many people must be surveyed. If only one or two individuals are affected, it may be more acceptable to spend more time in frisking than might be the case when several individuals are involved, especially when instrumentation and/or assisting personnel are limited. Another consideration relates to the potential for significant personnel contamination—what radionuclides are involved and what levels of transferrable radioactivity are present in the work areas. In any event, there will be some subjectivity involved in setting the protocol(s) for manual frisking. It is fair to say that what is commonly referred to as “whole body” frisking using a portable detector is seldom literally “whole body” frisking—i.e., the entire surfaces of the body (clothing) are not monitored.

For typical beta/gamma emitters that often constitute the major contaminating radionuclides at nuclear power plants and many other facilities, frisking with the standard 15 cm2 pancake GM detector held within about 1.3 cm of the surfaces being measured provides a reasonably effective means of detecting significant contamination. This is especially true when dealing with radionuclides that emit moderate- to high-energy beta radiation for which the detection efficiency is quite high. When using this fairly large area probe, it is reasonable to select a short time constant (fast response time), if such choice is available on the ratemeter being used, and to use a linear scan rate of about 5 cm per second. The 2.5 cm per second that you cite does provide somewhat greater sensitivity and is likely slower than necessary if using a ratemeter with a reasonably short response time. When doing manual frisking, it is generally impractical and probably impossible to cover every square inch of surface area. As you have noted, some areas are more likely to become contaminated than others, and care should be taken to cover these rather completely—e.g., hands, face, especially around the nose and mouth, shoes—but a reasonable attempt should be made to cover a representative part of all external surfaces.

A scan of the complete surface area of the individual, even at 5 cm per second, would require more time than is usually practical—about 14 minutes for reference man. In actuality, some body areas are inaccessible by routine frisking and/or may be judged to be otherwise unimportant from a contamination standpoint, thus justifying a reduced scan time. A less comprehensive but generally acceptable frisk would move at about 5cm per second and cover the hands, head (spend a bit more time on nose and mouth) and neck, shoulders and arms, chest and abdomen, back and backside, hips and legs, and tops and bottoms of shoes or other footwear. It would cover all of the most likely to be contaminated areas and cover representative sections of areas unlikely or less likely to be contaminated. Often an areal sampling frequency of about 25 percent may be adequate for representative frisking of an area on which contamination is unlikely.

Such a scan would likely require a minimum of about three minutes and should be comfortably doable within about five minutes. The decisions as to which body parts should get the most attention will depend in part on the physical characteristics of the contaminated work site(s) and what kind of activities were being conducted by the affected workers in those areas. The immediate implication of performing a three-to-five-minute scan is that approximately 65 percent to 80 percent of the possible surface area is not being directly scanned. Because the probe is held a short distance away from the surface while scanning, some response to significant activity lying outside of the 15 cm2 defined by the active area of the probe is possible, thus increasing somewhat the effective area covered during the scan.

The bottom line is that decisions as to what constitutes an acceptable frisking procedure will vary somewhat among facilities, based on operating conditions, the potential for personnel contamination, and the history of contamination events. When numbers of affected workers and/or time constraints require reduced frisking durations, protocols must ensure that the most important body areas receive priority and that at least representative external areas of the entire body are covered when a “whole body” frisk is specified. Reducing scan times by increasing scan speed significantly beyond two inches per second is usually not a desirable alternative because of the reduced sensitivity of the detector at the higher scan rates. This is especially true when dealing with likely spot or small area contamination (as opposed to widespread area contamination) as is likely in events of personnel contamination.

I’m sorry I am not able to provide an absolutely definitive answer to your question, but I hope the discussion provides you with a basis for enhanced confidence in your decisions. Good luck.

George Chabot, PhD, CHP

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