Answer to Question #11228 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

My question concerns effective radioactive contamination control. No matter how good a program may be, some "clean-area" contaminations occur when radioactive material spreads into clean areas outside of posted contamination areas.

I have read that one of the characteristics of an effective contamination control program is very few clean-area contaminations. Can you quantify "very few"?

Also, can you refer me to publications summarizing performance of effective contamination control programs (with data on personnel contaminations, clean area contaminations, size of posted contamination areas, etc.), similar to NUREG-0713 (which deals with occupational dose but not necessarily contamination events)?

I am interested in all types of radioactive contamination, including handling of plutonium solutions inside glove boxes. In this case, the glove box is posted as a contamination area but there is no posting outside the glove box (this area is a radiological buffer area, or RBA, which is considered clean). Over the last three years, less than 0.3% of over 300,000 swipes taken outside glove boxes have shown contamination above Department of Energy (DOE) limits.

A

Your question covers a lot of territory. I will attempt to provide some information, but there certainly will remain aspects of contamination control for which some subjectivity prevails and for which there are legitimate arguments as to what is appropriate.

Regarding your observation that you have read that an effective contamination control program is characterized by resulting in "very few clean area contaminations," to my knowledge there has not been a formal recommendation by any responsible group or agency as to what constitutes "very few" in regard to these "clean area contaminations." Based on my own experience and observations I would judge that a reasonable limit to define "very few" would be less than 1 in 100 (1%) and preferably less than 5 in 1,000 (0.5%). The actual acceptable value may vary appreciably, depending on such factors as what administrative contamination level has been established as significant (i.e., requiring remediation or specific control), the potential dose significance of exposure in these very few instances, the particular radionuclides contributing to the contamination, the practical limits to uncertainties in evaluation of contamination values, the background levels, the instrument sensitivity, etc.

Normal statistical variations place restrictions on our ability to make firm decisions. For example, if we apply normal statistics and use an assumption that any value outside of ± 2 sigma of the expected reading, when the expected reading is representative of the background (clean area), then we might conclude that we would expect to make false positive conclusions about the presence of contamination about 5% of the time even when none exists. If we use ± 3 sigma as the decision level for contamination, then we might make false positive decisions about 0.3% of the time. When we are making measurements to assess beta/gamma-emitting contamination, the radiation backgrounds of many commonly used instruments are often appreciable, frequently tens of counts per minute or higher. Such backgrounds will result in elevated minimum detectable surface concentrations compared to many cases involving alpha contamination for which many instrument backgrounds will be quite low, often less than one count per minute. Often, the higher the background, the more likely it is that some false negative results will be obtained. The general requirement when performing contamination assessments is that the instrumentation and methods of assessment used must be sufficient to ensure that when no net contamination is detected in a false negative conclusion, the possible doses to potentially exposed individuals are below legal and administrative limits. This applies whether one is concerned with the occupational environment or whether exposure of members of the public is the concern.

There has been considerable work done, especially in relationship to decommissioning activities, to develop appropriate statistical analyses to demonstrate that contamination levels at a given site will not result in doses that are in excess of acceptable values to subsequent occupiers of the site. The approach is well documented in the well-known and heavily used Multi-Agency Survey and Site Investigation Manual (MARSSIM). While the MARSSIM report is intended for use in decommissioning of nuclear facilities, some of the techniques used for making statistically valid decisions about contamination levels could be applied to other situations, such as the occupational work environment. If you aren't familiar with the document, I believe you might find it interesting and useful.

There are a number of professional groups and other sources that have presented analyses and associated recommendations of acceptable contamination limits for different circumstances. The overriding concern with personal exposure to radioactive contamination is the potential for external and/or internal doses to exposed individuals. In most cases the dose limits for radiation workers are well defined, and assessment and control measures for dealing with radioactive contamination in the workplace are specified—e.g., in U.S. Nuclear Regulatory Commission (NRC) regulations 10 CFR 20 and in associated documents, such as various regulatory guides issued by the NRC. Limits have also been established for exposure of members of the public from operation of nuclear facilities, and these include specific recommendations. The overriding approach these days is to limit contamination levels to ensure that the exposure to the individual(s), typically expressed in terms of dose limits, is acceptable; the exposure is based on assumptions associated with a conservative exposure scenario, often requiring determination of dose to a critically exposed individual. There are many recommendations and associated documents that deal with restriction of contamination associated with the release of sites for unrestricted use or, in some cases, for limited use. I think these considerations are probably outside of your concern, but I will provide one link here to a summary page available through Phil Rutherford that gives a list of numerous pertinent documents.

Regarding workplace radioactive contamination, there are generally fewer sources of recommendations, and in many instances, licensees working within regulatory requirements will set their own administrative contamination limits for different areas in their facilities. The DOE has provided some specific recommendations within its main document for occupational radiation protection, 10 CFR 835. In Appendix D of that document there are surface contamination levels that represent values above which areas require posting as either contamination areas or high contamination areas with regard to both removable and total (fixed ± removable) radioactivity recommendations. Values range from a low of 20 dpm in 100 cm2 for specified removable transuranic radionuclides to a maximum of 10,000 dpm in 100 cm2 for removable tritium. The DOE Standard 1128-98, Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities, contains additional information particularly regarding plutonium contamination control and is probably worth reviewing if you have not already done so. It is worth noting that the U.S. NRC has stated in IE Circular No. 81-07, Control of Radioactively Contaminated Material, that it is generally not possible to detect (with a typical thin-window portable detector) fixed beta/gamma-emitting discrete particulate radionuclides (typical of nuclear reactor environments) at activities below 5,000 dpm. They also concluded that surface contamination limits (beta/gamma activity) should not be set below 5,000 dpm in 100 cm2 for total radioactivity and 1,000 dpm in 100 cm2 for removable contamination. The dose implications of such contamination levels were very small, less than 50 μSv y-1. (The document is referring to contamination of discrete objects.) If you are concerned about contamination of various materials and objects and subsequent releases of these from controlled areas, I would recommend that you consult the American National Standards Institute/Health Physics Society (ANSI/HPS) Standard 13.12, Surface and Volume Radioactivity Standards for Clearance (2013). If you are a member of the Society, you can obtain a free copy of the standard through the HPS website (you will have to follow the procedure to download the standard from an affiliated organization, IHS).

Others have studied the dose implications of certain residual radioactive surface contamination. For example, a document from Argonne National Laboratory (ANL/EVS/TM/11-3) analyzed the dose consequences of residual radioactive contamination in selected occupational environments (an office and a warehouse) for a variety of different radionuclides.

I realize that I have not provided specific answers to some of your questions, but as you review some of the available information (and there is much more available) I hope you will find satisfactory approaches to your concerns.

George Chabot, PhD

Answer posted on 11 May 2015. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.