Hello, Dr. Z – I’m working on our radioactive materials license application and got to the part about telling how I’m going to take care of our radioactive waste. I’ve got to admit I’m having trouble figuring out how it all works and when I’m allowed to, day, store things for decay, dispose into the sanitary sewer system, ship it for processing, transfer for disposal, and so forth. Can you shed some light on this for me?
Ahhh – radioactive waste – how’s that poem go? “A loaf of bread, a jug of wine, a few barrels of radioactive waste, and Thou.” No – wait – that’s not it, is it? Anyhow, since radioactive waste disposal is fairly unpoetic, I guess we can drop that line of inquiry. So here goes!
Since you didn’t mention what sort of facility you’ve got so I can’t give you a precise answer – what I’ll do instead is go over the basic types of waste disposal, how they work, and who they apply to. And I need to say, too, that this is general information – depending on the specifics of your own operation (do you generate mixed waste?) some of the details might be different.
I’ll start with what you need to do for all of your waste, regardless of how you dispose of it – in particular, you’ll need to:
- Keep track of each nuclide transfer into each waste container (nuclide, activity, and date)
- Perform periodic checks of your waste storage area to confirm that each container is physically in good shape (no rot, rust, etc.)
- Perform (and document!) periodic radiation and contamination surveys of your waste storage area
- Maintain your waste storage area to minimize water infiltration (i.e. leaks, groundwater), ensure the floor is sealed (if you have liquid waste), and has a functioning sprinkler system
- Make sure your waste storage facility is able to keep the waste safe and secure for at least a few years (longer if you’re going to use decay-in-storage). This doing whatever is needed to keep the waste safe for as long as it will be stored.
And in addition to attending to these, here’s how you handle various types of waste:
Solid, gas, or liquid waste, short half-life (decay in storage)
If the nuclide(s) you’re using have a half-life of 90 days or less then you can store it on-site until it decays through 10 half-lives AND you can’t distinguish it from background radiation with your instruments. To decay out a waste package (barrel, bag, bottle, or box) you’ll need to note the date you seal the package and wait for 10 half-lives from that date; on that date you can perform a survey on the package (documenting background radiation dose rates and the highest dose rate you measure) and, as long as radiation levels from the waste are “indistinguishable from background” then you can dispose of the waste without regards to radioactivity. That means that infectious waste (if you work at a medical facility) can be incinerated or autoclaved, hazardous waste (if you have hazardous materials that were contaminated) can be treated for the non-radiological hazard(s) they contain, biological waste can be incinerated, liquids can be disposed of into the sanitary sewer system (if they’re non-hazardous), and so forth.
With regards to the 10 half-lives rule – decaying for 10 half-lives reduces the amount of radioactivity by a factor of 1024. If you’re starting off with, say, 100 microCuries of activity then, after 10 half-lives you’ll be down to about 100 nanoCuries, which you likely won’t be able to detect. But if you start with 1 curie then after 10 half-lives you’ll still have 1 mCi of activity left, and that’s certainly enough to pick up (and it’s enough to still require licensing).
Solid waste, long half-life (transfer to licensed disposal facility)
For longer-lived materials you’ll have to ship them for disposal at a licensed waste facility. This means packing it in metal drums or another suitable container, scheduling a pickup with your radioactive waste broker, filling out the paperwork, loading it on the truck, and waving a fond farewell. The caveat here is that you never lose the title to your waste, which means that if the waste broker you select is storing it in a barn instead of trucking it down to Texas (which has happened)…at some point you’ll get a call telling you it’s time to pay to have the waste shipped back to you or to have it sent to a waste disposal facility. Or if the disposal facility it was sent to is ill-designed and is closed down (which happened to me once)…same thing. Speaking from personal experience – it sucks to have to pay to dispose the same waste twice. So check out your waste broker – and whoever they transfer the waste to – to make sure they’re on the up-and-up before you sign a contract and trust them with your waste.
As far as the waste manifest and other paperwork goes – that’s fairly simple now. Your waste broker will probably send you a manifest in the form of a spreadsheet – fill it in with each package you’re giving them and the nuclide inventory of each, print it out, and sign it when the driver arrives and you should be good to go. Oh – and if you’ve got a good waste broker, be nice to them! I worked with a waste broker once who, even told that we had a budget of a million dollars to clean up our accumulated waste, still worked to save us every dollar possible. They walked away from an easy few hundred thousand dollars of extra profit. I respect their business ethics so much that I’ve used them every chance I get for the last 30 years.
One other thing to mention is that your costs can be reduced by using various treatment options (e.g. incineration, super-compaction, metal melt, solidification, and so forth). In some cases, segregating your waste into incinerable and non-incinerable can also reduce your costs considerably.
Liquid waste, long half-life (dispose into sanitary sewer system)
For liquids with long half-life radionuclides, you can dispose of them into the sanitary sewer system, provided they’re in a chemical form that’s non-hazardous. Something in aqueous solution, as long as it’s not hazardous, can be disposed of this way. Here’s the kicker.
You’re allowed to discharge of liquid wastes into the sanitary sewer system, but you need to make sure you don’t exceed your average effluent discharge concentration (more on this in a moment), nor can you exceed an annual maximum activity discharged over the course of a year. This last part means that, at the end of the year, you calculate the total amount of activity – nuclide by nuclide – you discharged this way. In practice this means keeping track of every microcurie you discharge and dividing that by the total amount of water flowing through your sewer main to determine the average annual effluent concentration. As long as that’s less than what the regs allow, you’re golden!
What we did at one university I worked at was to take a sample of our liquid waste disposal after consolidating all of the liquids into a single 55-gallon drum. That gave us the nuclide concentration per ml, which we converted to total activity of each nuclide per drum. Summing this over the course of a year and dividing by the flow through our sewers during the year gave us the average annual activity concentration.
Biological waste or scintillation cocktail, H-3 and C-14 only, less than 0.05 µCi/gm of average activity (de minimis waste)
Both universities I worked at produced biological waste in the form or rat (or rabbit or cat or dog) carcasses that accompanied the science that was being done. If you know the amount of activity in the carcass and its weight then calculating the activity per gram is fairly straightforward; if that activity concentration falls below the threshold for de minimis waste and if that waste is scintillation cocktail or biological materials then it can be disposed of without regards to its radioactivity. This means that, if you’re at a research facility that uses tritium and carbon-14 for research on animals, as long as the activity concentration is low enough you don’t need to dispose of these animals as radioactive waste. You still need to treat them as biological waste, but at least you don’t need to worry about the radioactivity.
Mixed waste
This is a potential problem – mixed waste is waste that’s both hazardous and radioactive, and it can be difficult and expensive to dispose. This is where a good radioactive waste broker can help out; it’s also where having a good regulator can be helpful as well. One good thing, though, is that the typical 90-day storage clock is not ticking as long as the waste is radioactive – so you can store it until it decays out (if it’s short-lived).
For the former, a good radioactive waste broker will know the regulations and can tell you (for example) if the mixed waste can be solidified, incinerated, or treated in some other way that’s less expensive than other options. And for the latter, if your regulator has confidence in you, they might let you do minor treatment (e.g. neutralizing acids or bases, distilling off the non-hazardous fraction) to help reduce or eliminate the amount of mixed waste you have to dispose of. As one example of the latter, at a university I used to work at we had 20 liters of acetonitrile contaminated with tritium. We were allowed to distill off the aqueous fraction (non-hazardous) and show that it contained all of the tritium – this meant that the hazardous fraction wasn’t radioactive, and the radioactive fraction was non-hazardous. It saved us over $20,000 – and that was 20 years ago.
Final thoughts
When you’re writing your license application, try not to be too specific about how you expect to deal with your waste streams. As one example, at one place I worked our license application committed us to pack biological waste in lime, double-bag it with absorbent materials between the bags, pack it in 33-gallon drums, and freeze it before it was shipped. This was the process required of the waste disposal facility we sent our waste to. But when they changed their acceptance criteria we had to be out of compliance with our license in order to dispose of this waste. As a result we got a number of license violations. Had we left out the specifics and simply said that we would “package biological waste in accordance with the waste acceptance criteria of the licensed disposal facility to which it will be sent” we’d have been OK.
The other thing to consider is that you’re not allowed to store longer-lived nuclides (half-life greater than 90 days) for decay unless it’s permitted by your license. If you’re just not sending waste for disposal, your regulators might conclude you’re trying to “backdoor” your way into a de factor decay-in-storage. To avoid that, you should schedule a waste pickup at least once annually.
Good luck!