Answer to Question #13280 Submitted to "Ask the Experts"

Category: Industrial Radiation

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

Q

What type of safety measures need to be considered for an electron beam sterilization facility? I am unsure which type of radiation survey meter would be best suited for checking the radiation level of such a facility. Would the workers be required to wear a dosimeter when working?

A

The answer to this question will vary depending on many factors, so I will make some assumptions and you can apply them according to your situation.

The biggest radiation safety measure at an irradiator (electron beam sterilization facility) is good engineering design. A person should not be able to access the source (the x-ray tube) during operation. That said the boxes or items you irradiate may be large enough and similar enough to a human size that it can be "tricked" and a person may "ride the conveyor belt" or whatever system you use to carry the product to the beam. For this reason, you need some kind of control of people in your work area. Only those who are trained, rational, and aware can have unrestricted access. Testing, refresher training, buddy systems, cameras, and observers can all be considered as part of a multi-element method of protection. Audits, inspections, and interviews all have their place. Your primary concern will be x rays and electrons during energized operations. Neutron activation (high-energy photons making structural components radioactive) is largely eliminated by design. It is possible over time that the area around the electron beam may become radioactive and this must be measured and accounted for during periods of worker entry.

Before operation a final radiation survey or commissioning survey is critical for radiation safety personnel to read and understand. Survey instrumentation need only be a device that can detect photons. A standard device such as a Ludlum Model 3 with a Geiger-Mueller probe is sufficient. There is always a scientific argument to be made about energy and efficiency but that does not drive this working situation. 

If the shielding design is done well then dosimetry is not likely to be needed; but if there is no personnel dosimetry history, it is reasonable to deploy it for at least six months of operation at a known average workload. If doses are at expected and acceptable levels then dosimetry can be terminated. If workload changes or the design or location of the source or shielding changes then it should be reconsidered. This analysis should be performed periodically and at least annually to evaluate the whole of the radiation program. Regulatory guidance may dictate a more expansive program.

Radiation safety issues may radically change during maintenance or "shutdown" periods. It is imperative to understand the design and function of the x-ray source. Even if it is "off" it can produce lethal radiation levels if the cathode is subject to current flow (dark current). The radiation safety officer should be familiar with the Proceedings of the International Conference Nuclear Energy for New Europe, Bled, Slovenia, September 11-14-17 which reviews the analyses of industrial irradiator accidents. 

X rays and electrons are not the only safety issue at your facility. High voltage is a deadly hazard. Proper knowledge, access, and understanding of systems of lock-out/tag-out, proper insulation, flash resistant clothing, and other safeguards should be fully researched to mitigate the hazard.

Douglas A. Johnson, MS

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