Answer to Question #955 Submitted to "Ask the Experts"

Q

I have done a survey of our radioactive sources and find most of them to exceed the maximum permissible dose of 2.5 µSv h^-1. Most are around 4 or 5 but I have one at 50 and another at 100. We are intending to move these gauges into storage. Can my mechanics work on these for a shift without damage to health? (The gauges will not be put back into use for at least four years and there are no other sources on the job site.) Thanks.

A

First, one should be careful with the new vs old units being read on survey meters, that is, microSv h^-1 (µSv h^-1) compared with millirem h^-1 (mrem h^-1) or milliroentgen (mR h^-1), and more importantly, comparing to what might be in federal or state radiation control regulations. With most regulations, one gets into posting certain radiation caution signs on source storage rooms in the 25 µSv h^-1 (2.5 mR h^-1) to 50 µSv h^-1 (5 mR h^-1) range.

If the workers are classified as occupational radiation workers, their whole-body effective dose limit for the year is 50 mSv (5,000 mrem), if not, then the annual public dose limit of 1 mSv (100 mrem) should be used. One should consult the specific federal or state regulation for dose limits and other radiation protection requirements applicable to these sources. Links to the various federal and state agencies can be found on the Conference of Radiation Control Program Directors, Inc. Web site. Given the highest dose rate of 100 µSv h^-1 (10 mrem h^-1), and assuming that is 1 meter from the source and comparable to a whole-body exposure scenario when handling these sources, in theory one could work an eight-hour shift and only get 800 µSv (80 mrem). This is below the public dose limit. And, the very low health risk associated with such an exposure could be explained in relation to the few mSv (few hundred mrem) one gets from natural background each year.

Having said that, if this is not routine work, it would be prudent to monitor the worker's exposure with whole-body and extremity dosimeters (for example, thermoluminescent dosimeters) and be sure they've had radiation safety training and that the work is planned and a dry run is performed to keep exposures as-low-as-reasonably-achievable (ALARA). The sources should be kept in a secure location and leak tested prior to placing in storage. With most federal or state specific licenses for sealed sources, there is a contamination limit condition for leak testing of 185 becquerel (Bq) or 0.005 microcuries (µCi).

Again, it would be prudent to investigate any sealed source that is found to have any removable contamination. Lastly, the outside environs of the storage location should be surveyed and regulations consulted for posting requirements.

David Allard, CHP