Hi, Dr. Zoomie – I just took over as RSO and found out that I have to do an inventory and leak testing of my sources. Can you tell me how often I have to do these and the best way to go about it? Thanks!
Wow – I can sense your excitement with the topic! And, I have to say, I can sympathize – inventory is one of those things that we all have to do, but that nobody really gets excited about. But it’s got to be done – and done correctly – even if only because they’re regulatory requirements. Not to mention the fact that leak testing your sources will let you know if one might be starting to give you problems. Let’s take them one at a time.
Inventory
Most of the time you’re going to have to inventory your sources every six months – this is the default periodicity written into the regs. But you need to check this in your license; sometimes regulators will require a quarterly inventory. Your inventory has got to account for every source and it has got to be documented in writing, including your initials or signature confirming that you performed and vouch for the inventory.
There’s more to an inventory, though, than just accounting for the sources – you also have to account for the activity of radioactive materials in your possession. This means that, when you bring in new radioactive sources, you need to add them to your inventory and, as radioactive materials leave your facility, you again need to update your inventory accordingly.
Remember, too, that there are a number of ways that radioactive materials can leave your inventory, and you should account for all of these. For example:
- Your inventory should include corrections due to radioactive decay so that it shows (at least every 6 months) the decayed activity rather than the strength of the source as it was when you first bought it.
- If you’re using radioactive materials in experiments, medical procedures, industrial processes, and so forth remember to reflect what is used up and leaves your facility (for example, radiopharmaecuticals in your patients).
- As you dispose of radioactive waste, your inventory also has to be updated.
- Devices with radioactive sources (electron capture devices inside of gas chromatographs are one example) should not only be on your inventory, but should be updated if you sell or dispose of the devices.
These are only a few example – again, make sure that you’re updating your inventory so that it reflects not only the total number of sources (and tracks each individual source), but also the decay-corrected activity of each one.
Something else to keep in mind is that there are a number of ways you can perform an inventory.
- Best, of course, is to visually sight each source – to lay eyes on them so you can say “I saw source #12345 on this date in this location.”
- You might not be able to do this for sources that are installed inside of devices (the Ni-63 source inside of an electron capture device for example). One option in this case is to confirm that the device operates. If an electron capture device only operates when the source is installed, then showing that it operates properly will also show that the source is present.
- Alternately, you can check radiation levels on the outside of a device to confirm the source is present. Say (for example) you have an irradiator that normally reads 2 mR/hr with the source inside. As long as you get a reading of about 2 mR/hr then you have a high degree of confidence that the source is still there. But if readings drop to, say, 20 µR/hr (0.02 mR/hr) then you’ve got to wonder what happened to the source.
- If you have a source (or several sources) in storage then you’re allowed to inventory the sources by confirming that the storage location is sealed and has remained sealed since your last inventory.
- And if you can’t figure out how to inventory your source(s), call the manufacturer and ask for a copy of their inventory procedure – they should be happy to send you a copy that you can use.
Finally, if you find out that you can’t account for a source, you have to tell your regulators. According to 10 CFR 20.2201, you have to make an immediate telephone report to your regulators if the source is more than 1000 times as active as the amount requiring licensing – but only if the circumstances were such that a member of the public might be exposed to radiation from the source. The next paragraph of the regs also notes that you have to make a written report to your regulators within 30 days of losing any source that’s more than 10 times the licensable quantity.
As an example – according to 10 CFR 30.71, Schedule B the limit for exempt sources of Cs-137 is 10 µCi (you’ll also find this in Appendix C to 10 CFR 20). This means that any Cs-137 source that’s smaller than 10 µCi is exempt from regulations. If you lose a source that’s less than 100 µCi (10 times the exemption limit) you don’t have to report anything to your regulators. If you lose a source that’s more than 100 µCi you need to make a written report within a month and, if you find that a source greater than 10 mCi has been lost, stolen, or gone missing then you need to call your regulators to inform them of this as soon as you realize what’s happened.
Leak testing
Something else you’ll have to do periodically is to check your sealed sources for leakage (you don’t have to check unsealed sources such as vials of stock solution used for experiments, or syringes filled with radiopharmaceuticals). In addition to unsealed sources there are other sources you don’t have to leak test:
- Beta- and gamma-emitting radionuclides don’t need to be leak tested as long as the activity is less than 100 µCi
- Alpha-emitting sources don’t need to be leak tested as long as the activity is less than 10 µCi
- Sources filled with radioactive gases (such as Kr-85) need not be leak-tested
Specific leak testing requirements are found in a number of places in the regulations:
- 10 CFR 32.59 (for companies that manufacture and sell items that contain sources)
- 10 CFR 34.27 (industrial radiography) and 10 CFR 34.67 (for depleted uranium shielding)
- 10 CFR 35.67 (medical sources)
- 10 CFR 39.35 (for well logging sources)
There may be others; these are the ones that showed up in a search of the NRC website.
The purpose of leak testing is fairly obvious – to check to see if your radioactive sources are still sealed tightly or if they’re leaking. A leaking source can be a big deal – a single leaking source at a university I used to work at was responsible for contaminating an entire laboratory and adjacent rooms and a different leaking source contaminated an entire warehouse.
Performing leak testing on lower-activity sources can be fairly straight-forward; simply wipe the entire source with a piece of filter paper or a cotton-tipped swab. But you might not be able to do this – the source might be locked away inside a piece of equipment, for example. If this is the case, what you need to do is to wipe the outside of the source holder or an access panel. You’re not going to be permitted to disassemble a device to get at the source, so you should try to get as close as you can without doing so. One specific example would be a self-shielded irradiator (such as a blood bank irradiator) – this is a device with a drawer or a sample holder that’s exposed to the radiation, even though it’s heavily shielded to prevent exposing you to radiation. In this case, wiping the sample holder will suffice for a leak test since, if the source is leaking, the sample holder will be contaminated.
You’ll want to be careful when you’re leak testing high-activity sources. First, you should make a habit of doing all of your leak testing with a cotton-tipped applicator and not with a wipe you’re holding in your hands. And for any source that’s more than about 1 mCi in activity you should also be holding the source (if at all) with tweezers or tongs to avoid a high dose to your fingers. And with higher-activity sources even this might not be enough to ensure your safety. For example, industrial radiography sources often contain several tens – even up to a couple of hundred – curies of activity; forget about dose to your hands, this much activity could push you over an annual dose limit fairly quickly. For sources like this you want to survey the guide tube that the source travels through, or the shutter or exit port that the source is extended through – the thinking is to survey something that the source passes through or comes in contact with under the assumption that, if the source is leaking, these are the things that will become contaminated.
There are still more ways to check for leakage. I once had to perform leak tests on a 50,000 Ci source of Cs-137 that was stored in a large pool of water. First, the only way to wipe the source directly would have required scuba gear, which I’m not qualified to use. But more importantly, there’s no way in the world I’m going to go anywhere close to a source of that activity – such a source is an immediate danger to health and safety. On the other hand, cesium is soluble and any leakage would quickly dissolve into the water of the pool. Our approach was to take a liter of the water, evaporate it away, and count the residue in our gamma counter; if the source were leaking then this would show in the sample.
As with various methods for inventorying your sources, if you can’t figure out the best way to safely perform a leak test, ask the vendor. They’re certain to have developed a procedure as a part of their radiation safety program and they should be happy to share it with you.
Taking the sample is a start, but you also have to count the sample to show whether or not the source is leaking. A lot of licensees will send their wipes to a consultant or a laboratory for counting, but this isn’t always necessary as long as you can demonstrate that your counting methodology is capable of detecting leakage. Here’s how you can do that. And keep in mind – this is an example only – you’ll have to do this for the equipment you actually use at your facility!
- To start – the standard you have to be able to detect is 185 Bq (0.005 µCi). This is 185 radioactive decays per second, or 11,100 decays per minute.
- So – say you’re using a 1”x1” sodium iodide detector for the counting and the source contains Cs-137. The counting efficiency of this detector for Cs-137 is about 10% (you can look this up, but it’s better to have this determined when you send your meter out for calibration).
- In addition, normal background count rates with this detector will be about 500-1000 counts per minute (cpm).
- If you’re using a 1”x1” sodium iodide detector to count a wipe that holds 11,100 dpm of activity, you’ll get a count rate of 1110 cpm.
- For the final step, you can point out that the count rate you’d see from 185 Bq of contamination with your detector is at least double the normal background count rate you see with this same detector.
- This is “visible” to your detector; thus, your analysis methodology is capable of detecting a leaking source.
You might still want to have a consultant do your leak test counting for you – that’s your choice. But if you’d rather count your own leak testing, this is how you can get started.