Dear Dr. Z – I have a room and some equipment that was used for isotopic work that I want to close out. My regulators said something about decommissioning and meeting “free release” criteria. What in the world does this mean? And can you tell me how I can reach this exalted state? Thanks!
Boy – you’re about to have a lot of fun. Or maybe not…but let’s be optimistic! The big picture is that you have to be able to demonstrate to your regulators that you’ve moved all of the radioactive materials out of the room you’re decommissioning – including any radioactive contamination that might be present – and that you’ve cleaned up all of the equipment that was used to work with radioactive materials. That’s the “decommissioning” side of things; “free release” is another way of saying “release for unrestricted use,” which means that it’s been cleaned up to the point where you can do whatever you want with it. “Free release” means that you can turn the room into a lunch room without worrying that they’ll be harmed from the radioactivity, or that you can throw the equipment into the trash or give to your dog to play with – again, without risk to Fido. So that’s the big picture – now let’s get a little into the nitty-gritty.
Let’s start with the decommissioning process – what do your regulators expect you to do to show that you’ve properly closed out a room?
First, you’ve got to be able to show that you’ve moved all of your radioactive materials out of the room. If this is the only room where you use or store radioactive materials then you have to be able to show that you’ve got rid of the stuff – the only acceptable ways to do this are by transferring it to a licensed radioactive waste disposal facility or by transferring it to another radioactive materials licensee. Whichever of these you choose, you’ve got to be able to document the transfer or disposal – you will have copies of shipping papers and the manifest if you ship materials for disposal, or a letter (and possibly shipping papers) from whomever you transfer your radioactive materials to. But be careful! First, you can only transfer your radioactive materials to someone whose radioactive materials license will permit them to possess the source(s) you’re transferring to them – and you need to see a copy of the license before you ship the materials to them (whether the recipient is a waste disposal site or another licensed facility). And second, you have to be able to document the transfer – this is what the shipping papers and (if possible) a letter or email from the recipient confirming what they received from you.
Second, you’ll also need to show that you’ve cleaned up any and all contamination in the room so that it can be released for unrestricted use – this gets into the same ground as releasing equipment for unrestricted use. How to do the decontamination is beyond the scope of this posting – although soap and water or most commercial cleaning products will normally do the trick unless contamination has soaked into a porous surface or has chemically attached itself to whatever it is that you’re trying to clean up. But what we’ll go over here are the cleanup limits, regardless as to what it is that you’re cleaning up. Here, the fundamental document you’ll be referring to (unless your regulators have other requirements) is the Nuclear Regulatory Commission’s Regulatory Guide 1.86 (Termination of Operating Licenses for Nuclear Reactors). And the key part of this document is the table on Page 5. Here’s the table, along with a discussion of how to use it.
|Maximum (b, d)||Removable (b, e)|
|Uranium (natural), U-235, U-238, and associated decay products||5000 (α)||15,000 (α)||1000 (α)|
|Transuranics, Ra-226, Ra-228, Th-230, Th-228, Pa-231, Ac-227, I-125, I-129||100||300||20|
|Thorium (natural), Th-232, Sr-90, Ra-223, Ra-224, U-232, I-126, I-131, I-133||1000||3000||200|
|Beta-gamma emitters (nuclides with decay modes other than alpha emission or spontaneous fission) other than Sr-90 and others noted above)||5000||15,000||1000|
- (a) Where surface contamination by both alpha- and beta-gamma-emitting nuclides exists, the limits established for alpha- and beta-gamma-emitting nuclides should apply independently
- (b) As used in this table, dpm (disintegrations per minute) means the rate of emission by radioactive material as determined by correcting the counts per minute observed by an appropriate detector for background, efficiency, and geometric factors associated with the instrumentation
- (c) Measurements of average contaminant should not be averaged over more than 1 square meter. For objects of less surface area, the average should be derived for each such object.
- (d) The maximum contamination level applies to an area of not more than 100 cm2
- (e) The amount of removable radioactive material per 100 cm2 of surface area should be determined by wiping that area with dry filter or soft absorbent paper, applying moderate pressure, and assessing the amount of radioactive material on the wipe with an appropriate instrument of known efficiency. When removable contamination on objects of less surface area is determined, the pertinent levels should be reduced proportionally and the entire surface should be wiped.
Here’s how to interpret Reg Guide 1.86:
- Notice that there are two types of contamination – removable and fixed. And with the fixed contamination, there is both the maximum and average levels. Removable contamination (according to Note e) is contamination that will come off when you wipe the surface with a piece of dry filter paper; fixed contamination is whatever can’t be wiped off.
- We look at contamination in terms of disintegrations per 100 square centimeters. So what we do is to survey (or wipe) an area of 100 square cm (about 4”x4”) to see how much contamination we is there.
- What you’re looking for is the amount of contamination, but you have to remember that there’s background radiation also and you don’t want to find yourself cleaning up contamination that isn’t actually there. Before you start to survey (or count a wipe) you’ll want to turn on your meter when you’re a distance away from any contamination to see what it reads. With a 1”x1” sodium detector, for example, background is likely to be several hundred cpm. So if you’re reading, say, 1900 cpm with your detector and the background count rate is 500 cpm then the net counts (the amount of contamination) is 1900-500=1400 cpm.
- Here, we have to convert from the counts per minute (cpm) shown on your instrument to disintegrations per minute (dpm) required by regulations. The way to do this is by knowing the counting efficiency of your meter for the radionuclide you’re surveying for. For example, say you’re surveying for Cs-137 with a 1”x1” sodium iodide detector. When you send it for calibration you can ask the calibration laboratory determine your counting efficiency, which will probably be in the neighborhood of 5-10%. A 10% counting efficiency means that, for every 10 gammas that are given off by the Cs-137, only one is counted. So to turn cpm into dpm, you have to divide the meter reading (cpm) by the counting efficiency. In other words, dpm = cpm/efficiency. So if you’re reading, say, 1400 cpm with your detector, you have 1400/10% = 1400/0.1 = 14,000 dpm.
- OK – now that all of that’s out of the way, here’s how you use the table.
- There are different limits for a variety of nuclides. If you have, for example, I-131 then you’re only allowed to have 200 dpm/100 square cm of removable contamination, average contamination levels of up to 1000 dpm/100 square cm (averaged over 1 square meter), and no single location can be higher than 3000 dpm/100 square cm.
- The first three rows list the limits for a number of specific radionuclides. And if what you’ve got isn’t one of the ones that’s specifically listed then you go to the final row. This is the most commonly used set of cleanup standards and you’ll frequently hear health physicists talk about 1000 dpm/100 square cm or removable (or smearable) contamination.
- So what you have to do is to first know what radionuclide(s) you’ve got, then determine the appropriate cleanup limits for each. After this, you survey with a survey meter to measure total contamination over each square meter and, finally, take a number of smear wipes to see how much of this contamination is removable versus fixed. Finally, if you exceed these limits, you’ll need to decontaminate until the limits are met.
Third, there will need to be a formal closeout survey. This is where your regulators or (more likely) a contractor will come in to check to make sure that you’ve done everything properly, including your decontamination. This will include a contamination (and likely a radiation) survey to confirm your own readings. Once the survey confirms that your lab is cleared up (and cleared out) then you can go on to the next step.
Finally, we need to talk about documentation. I already mentioned the need to show what you did with your radioactive materials – the shipping papers and whatever transfer documents you might have. You’ll also need to keep copies of your own survey maps, showing the radiation and contamination levels you measured while you were closing out and cleaning up. Or if you have a consultant or contractor do this work, make sure that they document it properly. Incidentally, there’s also a requirement for you to keep records of any spills or areas of contamination with long-lived nuclides – your regulators might want to check these locations to make sure that you got them properly cleaned up.
When you’ve done all of your work, you’ll need to write to your regulators, requesting to terminate your license. You’ll have to include all of your documentation so that your regulators will know that you’ve properly disposed of your radioactive materials and cleaned up your lab, then they might want to pay you a visit to verify your work. When they’re satisfied that everything is OK they’ll go ahead and let you close out your lab and terminate your license.