Answer to Question #1233 Submitted to "Ask the Experts"

Q

Is there a threshold dose associated with ALARA? I have observed that some sites spend thousands of dollars to reduce a 50 mrem TEDE (total effective dose equivalent). This appears odd, given that sites are typically not required to monitor if individual personnel doses are expected to be below 100 mrem (which is also the allowable dose to individual members of the public). Does a professional consensus on this topic exist?

A

No, there is no threshold dose for ALARA, in the sense that dose should be reduced whenever it can reasonably be reduced. However, ensuring that doses are ALARA involves a wide range of activities, from formal calculations, staff meetings, and pre-job briefings to informal common sense actions. Clearly, the higher the doses involved, the more complex and formal this activity becomes. Many sites set job collective dose limits of 1-5 manrem as the cut-off point for formal ALARA assessments. Formal in this case means that certain questions are asked and meetings and briefings held to ensure that the job is being conducted in a manner that minimizes doses. Below the cutoff, no formal ALARA assessments are done, but it is still expected that all doses be minimized, mainly by ensuring that personnel observe common sense precautions to avoid unnecessary doses, such as not to stand in high-dose-rate areas when they don't need to, to get to the job location through low-dose-rate routes if possible and not too inconvenient, to wait for tools and parts in low-dose areas or outside radiation areas, to plan and make sure the job is done properly the first time, to use vial and syringe shields, and so on. Formal cost-benefit assessments are done mainly for high-dose, high-cost projects and those that involve installation or modification of facilities or equipment, such as the installation of a permanent or temporary shield, system decontamination, and so on, and in these cases, the dollar value of a man-rem is formally set at $2,000, although many sites go much higher, up to $10,000 or more. It is clear, therefore, that ALARA measures can range from the very expensive to those activities that do not cost more than a slight inconvenience, if that.



The key words in all of this are reasonably achievable. As for the monitoring threshold that is mentioned in the question, the threshold as set by the US Nuclear Regulatory Commission (NRC) rules is 10% of the annual limits. These annual limits include 5 rem on total effective dose equivalent (TEDE) as well as the limits on the skin and the extremities, individual organs and tissues, and the lens of the eye. For TEDE, this threshold is then 500 mrem. It should be noted, however, that the decision on monitoring must be made before the start of the monitoring period.



Sami Sherbini

US Nuclear Regulatory Commission