My short bio: Hi Reddit, I am a nuclear engineering Ph.D. that primarily does research in nuclear waste disposal in the United States! I did a similar AMA about a year ago, but I'm back to see if there are more questions about radioactive waste, waste disposal, reprocessing, policy, and legal matters. I'll try to answer your questions to the best of my ability. Ask me anything!

My Proof:

Edit: This has been a ton of fun! I may go to sleep soon, but I will try my best to keep answering questions for a while longer and when I wake up tomorrow. Big thanks to everyone who has asked questions already!

Comments: 210 • Responses: 17  • Date: 

Buhroocykins35 karma

Ive never heard of your line of work. What does a nuclear waste engineer do? How do you get rid of nuvlear waste?

Repo_co73 karma

Thanks for your question! When nuclear waste is removed from a reactor, it is highly radioactive, making it also very physically hot. You first need to actively cool the waste in a pool of water (water is a rather effective radiation shield as well), but after a couple of decades, it can be transferred to so called "dry cask storage" until it can be reprocessed or disposed. Reprocessing (or recycle) is a chemical process which removes useful components from used fuel and allowing the remaining non-useful, highly radioactive waste to be disposed. Disposal can be achieved through a number of ways, but the method chosen by most developed countries at this point is in underground repositories. These are deep caverns bored into favorable geologic media which can store the waste for the entire time that it will take for the radioactive materials to decay. This can take hundreds of thousands of years, so careful consideration must be taken to study the geology, geochemistry, volcanology, and hydrology of a region. Engineers in my field study in all of these areas to optimize the process.

dkschrute7913 karma

Hey there! Thanks for doing this AMA. Here are a few questions that come to mind:

1) What are your thoughts on reprocessing used commercial nuclear fuel from the cooling ponds? Seems like a huge opportunity for future fuel sources, but would generate waste.

2) What are your thoughts about molten salt reactors from a perspective of waste, nonproliferation, and flex fuel options?

3) What is your prediction of when (and where?) the US might get a HLW repository? I’m interested in your speculation from your experience.

Repo_co17 karma

Thanks for your great questions! I'll try and answer them individually.

I'm a big proponent of fuel reprocessing from a theoretical standpoint! Used reactor fuel in the United States is typically somewhere between 90% - 95% unreacted uranium, which is ripe for recovery to reduce waste volume, and has potentially many other valuable resources which could be recovered (whether higher actinides that could be used as fuel or stable fission products which could be hugely beneficial upon recovery). Unfortunately, several things may prevent its commercial development. The first being cost, in that construction and operation of a commercial facility for used fuel reprocessing would be very high (billions to tens of billions of dollars), and the price of natural gas and other fuel sources are so low that a return on investment for this type of facility would be difficult. Other aspects such as waste disposal uncertainty, non-proliferation concerns, high chemical waste burden, and lack of a modern licensing pathway would also make things tough.

I'm not an expert in molten salt reactor technology, but to the best of my knowledge, they mostly advertise using thorium fuel. This could alleviate some of the steps toward recovery of uranium necessary for refueling the reactor, making the thorium pathway more enticing. Any sort of isolation of a fissile material will present non-proliferation risk however, so that will need to be addressed somehow. I'd also be concerned that the NRC has never licensed a commercial reactor of this type either, and will need experts to review the license application, which could lead to cost and schedule increases.

I would imagine that the United States will eventually license the Yucca Mountain facility. The NRC has been dutifully pursuing the license review with whatever little money they have allocated to it, and have been conferring with the DOE to establish environmental impact statement requirements and things of that nature. It will, however, likely take some Congressional action to push the project forward however. This could push the schedule out to the 2050 type time frame before YM starts taking waste. That would be the over/under I would set, but I would be happy to be wrong if it takes less time!

brookz11 karma

Is it safe?

Repo_co35 karma

Nuclear power generation is very safe! In the 60+ years of commercial nuclear power generation in the United States, there have not been any public fatalities due to their operation and only one major incident. The industry is highly regulated by the NRC and are subject to serious safety standards across all aspects of plant construction and operations. This makes for one of the safest industries in the history of the United States.

xtingu11 karma


If you had a magic wand and could do anything you wanted, what would you do with nuclear waste? Where would you put it?

Repo_co43 karma

Thanks for your questions! I'd want to do a couple of different things, depending on how magical this wand is. I'll start with the most ambitious, and go to the most pressing/realistic need.

First, I would wish that the waste was about 1,000 years older than it is now. This would mean that the major, problematic short term radionuclides (like Cs-137 and Sr-90) will have decayed away. This would make the waste a LOT easier to handle. I am of the opinion that there is a ton of valuable things in used nuclear fuel that could be used for other purposes, and some of the more dangerous components were completely gone, it would be easier to recover them. If the magic wand can't do that...

...I would wish that there was a piece of land ideal for underground waste disposal. This would probably be something like a completely homogeneous salt deposit, far from any aquifers, far from civilization, and with a local government who would welcome the material due to the financial benefit that might come from hosting such a facility. If the magic wand can't do that...

...I would wish that international governmental bodies could agree to a framework for deep seabed disposal of used nuclear fuel. Disposing of waste in ocean trenches and letting plate tectonics carry it into the earth is probably the ultimate form of disposal. You would never see that waste again. It's a really ugly geopolitical problem, so if we could get everyone on the same page it would be nice. If the magic wand can't do that (and now we're talking about a pretty crappy magic wand)...

...I would wish that Congress would take meaningful action. It's a pretty hot button issue and most lawmakers don't want to touch it. Because of this inaction though, a tremendous financial liability is mounting. Thanks for your question!

kevincrazykid8 karma

Should the world build more nuclear power plants? Can nuclear waste ever be made fully safe (other than taking a really, really long time of course)?

Repo_co24 karma

I think that the world should build more nuclear power plants because it is the most readily available technology for producing large amounts of baseline electricity without carbon emissions. This makes it the best option for addressing climate change from an electricity production standpoint, in my opinion. Nuclear waste, much like any other industrial waste, will probably always have some element which is harmful to humans. With proper consideration given to waste disposal however, this danger can be almost completely eliminated. Effective, long term disposal in favorable geologic media will be what renders nuclear waste as safe.

McD-Szechuan7 karma

Pretty interesting stuff, just read through your last one (anyone interested can find it here )

What’s the political climate like for the future of nuclear waste? Probably a loaded question, but I guess I mean do you have any new insights as to where things are headed? Is the red tape incapacitating to the growth of nuclear energy? And is there any merit to the red tape or is it just more political firepower?


Repo_co16 karma

The politics can be pretty frustrating for the folks on the technical side of things. I'll try to answer all aspects of your question the best I can (disclosure: I'm not part of the nuclear power industry, so I'm not completely keen to the commercial financial aspects).

I'm not exactly sure what is going to happen in the near future on the political side of things. The most recent House of Representatives passed the Nuclear Waste Policy Amendments Act of 2018 with a vote of 340-72-16, showing pretty significant bipartisan support on the House side. Unfortunately, it was assigned to a Senate committee last May and hasn't seen the light of day since. This bill was intended to resume the Yucca Mountain license review, but had a number of other really interesting provisions (for you policy nerds out there). Among them were the increase of the administrative limit of Yucca Mountain (to 110,000 MT), changes in the benefits structure for the host community, allowances for the commercial construction of a monitored retrievable storage with DOE waste ownership (big news), and so on. Unfortunately, with any bill regarding Yucca Mountain, Nevada is going to feel like they're on the losing end. Now that they are represented by two Democrat Senators, this bill might go forward given the bipartisan House support, Senate majority, and Republican President. That's pure speculation however...

In my opinion the red tape behind waste disposal is doing less to harm the industry than a couple of other factors. The cost of construction, including QA of construction materials, mounting contractor idle times, and delays with long lead-time items are the main negative from a new reactor standpoint. Vogtle Units 3 and 4 have had a number of delays and ownership changes, and its completion is still in question. Unfortunately, it seems like a problem of scale. With a $10 M construction project, a 50% cost overrun is expensive, but with a $10 B project, this kind of overrun can cause large companies to go bankrupt. For old reactors, the cost of natural gas is probably what is causing most of the closures. Natural gas is so cheap that the economics of keeping old reactors open just doesn't add up any more.

As for the merits of the red tape, since nuclear waste disposal is such a hot button issue that doesn't really seem to be of immediate concern to most politicians, any amount of red tape is pretty tough to cut through. I'm glad that this is an anonymous account when I make a speculation like this, but I think that many people would agree with this assessment. The Obama Administration put up some pretty significant red tape to the Yucca Mountain review by attempting to pull the application and cutting NRC funding. Harry Reid was the Senate Majority Leader at the time who spent a significant portion of his career adamantly opposed to Yucca Mountain, so it was an easy decision for Obama to try and ease passage of the rest of his political agenda. This red tape is still around, and who knows how long it will take to go away. Thanks for your questions!

rabblerowener5 karma

How much nuclear waste would it take to turn your average human into a superhero?

Repo_co16 karma

It would make you a pretty crappy superhero, honestly. Who wants to be gastrointestinal and nervous system failure man? Or latent cancer woman?

MyPostsAreHalal5 karma

What’s the future of nuclear waste management?

Repo_co8 karma

Disclaimer: As an American, I can really only speak to the disposal of used nuclear fuel in the United States with an certainty.

The United States is at a bit of a standstill at the moment with regards to used nuclear fuel (UNF) and high-level radioactive waste (HLW) disposal. HLW is produced through nuclear fuel reprocessing, while UNF is unreprocessed nuclear fuel. The Nuclear Waste Policy Amendments Act of 1987 established Yucca Mountain as the legal location which must be investigated for suitability for disposal of UNF (and eventually HLW through executive action prescribed in the bill), and no act of Congress has ever lifted this requirement. The Obama Administration removed Yucca Mountain from consideration as an act of policy by removing funding and attempting to pull the NRC license application. Pulling the application was deemed illegal in courts, but additional funds were not spent on continuing the project. At the moment, the license application of Yucca Mountain (deeming it suitable or unsuitable) must be completed before any other location is considered, but money has not been allocated to continue the review. There was a bill passed in the House of Representatives last year to resume allocation of funds, among other changes, which died in the Senate, so we'll see what the next Congress does.

It's really a political hot button issue. Most Americans don't think too much about radioactive waste in their day-to-day lives, so a politician spending a lot of time on dealing with the issue can make for some pretty bad optics. Unfortunately, since utilities have been paying into a waste disposal fund since the 1980s under the promise that the federal government would take ownership of waste by 1998 (they still haven't), there is a tremendous financial liability present (to a tune of about $35B). Utilities companies have begun to sue, and win judgement against the federal government for it's inaction. The government can't just give back the money collected however, as they still have the legal requirement of handling the waste, so the money just comes from the federal judgement fund. At this point, Congress needs to do something to get the program back on track.

Sadaijin4 karma

Do you mostly work around commercial generation, research or military reactors? Also, how do you feel about the trend toward big, efficient, but twitchy reactors as opposed to clusters of smaller, less efficient, but relatively more stable reactors?

Repo_co6 karma

I actually don't have very much experience with reactors. I mostly work on the waste side of things, so I don't care where it comes from, just as long as it exists. Currently, each of those types of waste is legally classified for disposal in the same location, so it's all the same to me!

In terms of small modular reactors (SMR) as opposed to large scale reactors, I think SMRs have some significant benefits. The allow for scalability based on demand, have nice safety features due to their lower heat loads, and can provide a nice economic middle ground, reducing corporate risk to an extent.

SatanMaster3 karma

I’ve heard that we’re getting to the point where there isn’t even anymore waste to dispose of because of new methods. Is this true? I worry that only right wing traitor lunatics are in favor of this technology.

Repo_co6 karma

Regardless of the technology developed for recovery of useful components from nuclear waste, there will always be some components produced of which will need to be disposed. The global consensus is that deep, underground repositories are the best method for isolating these waste from human contact for the amount of time it takes for them to decay away.

Julioscoundrel3 karma

What’s the best way to handle the radiation issues at Fukushima?

Repo_co8 karma

I honestly wish that I could answer this question, but I have no idea what the current status is of the cleanup efforts at Fukushima, and I don't know the extent to which the damage affected plant systems. Any answer that I had would be purely speculative. Sorry to duck your question!

ALSX33 karma

What are your thoughts about Thorium as a replacement fuel to Uranium, or as a supplement to Uranium?

Repo_co8 karma

Using thorium breeder reactors has a lot of promise, with some significant engineering challenges as well. Thorium is quite a bit more abundant naturally than uranium, making the fuel potentially more readily available. Using it as a breeder could potentially produce a tremendous amount of fuel too, which is always a plus. Also, thorium-232 has a lower atomic mass number than uranium-238, meaning neutron irradiation in a reactor would not produce higher actinides in significant quantities, making waste disposal and reprocessing potentially quite a bit more favorable. There would be a need for continuous chemical reprocessing to separate U-233 for reactor fuel from the thorium breeder however, which could be very difficult and expensive. It will be interesting to see the future of definitely has a lot of positives but can the functional challenges of plant operations be made economically viable?

[deleted]7 karma

I would also add that the production of U233 invariably produces small amounts of U232, which is an incredibly strong gamma emitter, making fuel fabrication significantly more expensive and dangerous. That, coupled with the fact that there is currently enough uranium to fuel the world's reactor fleets for the next couple hundred years, as well as promising advancements (although still a ways away) in extracting natural uranium from seawater which would lead to a nearly endless supply of uranium, makes a thorium fuel cycle unpalatable to most countries.

What will be extra interesting to watch is the potential emergence of some of the more highly-touted advanced reactor designs over the next several decades - particularly some of the small modular reactors - which run on high-assay low enriched uranium (HALEU). The fuel fab market is currently not equipped to handle a move towards significant amounts of HALEU, although as of last week, Centrus is on the hook to demonstrate production with existing US technologies by 2020. In any case, thanks for doing this AMA! Very interesting stuff.

Repo_co4 karma

Ohh no! An actual reactors expert! Hope none of my answers have seemed too fraudulent to this point haha. I'm really just a waste guy...

WagonFunf3 karma

What's the boundary pushing topic of your field?

Repo_co9 karma

In terms of used fuel reprocessing, there are several topics which are being actively investigated. Pyroprocessing, the dissolution of fuel into molten salt solutions and performing electrochemical separations has the potential for performing highly targeted chemical element recovery, particularly with high burnup fuel. In the solvent extraction realm, advances have been made to separate americium and curium from bulk, fission-produced lanthanides to produce more favorable final waste products. This includes americium oxidation, and mixed-ligand solvent extraction systems. These each promise to reduce overall process complexity and generated waste. Thanks for your question!

Lazerboy933 karma

How did become a nuclear engineer? How did you know this career path was right for you?

Repo_co7 karma

I was an undergraduate engineering student who didn't know what he wanted to do. My university had a small nuclear program, and waste was the issue that piqued my interest the most. Upon graduation, I applied for graduate school at a university with a larger nuclear engineering program. I don't know even now if the career path was right for me, but I still find it tremendously interesting, so I suppose that's a good sign!

Here's a list of universities that have nuclear engineering programs in the United States. I think all of them have undergraduate degrees as well. Good luck if you're interested!

skeeter042 karma

Doesn't the high level waste problem basically limit the use of Fission reactors ? Does the amount of waste produced in a year really amount to tons of HL waste ? Do you know if Yucca mountain is being used for storage ?

Repo_co6 karma

While I think that the lack of an actual disposal pathway for used nuclear fuel and high-level radioactive waste is a serious issue which will need to be resolved, I don't think that it presents that great of an inhibitor to nuclear power generation. The oft quoted figure is that all of the reactors in the United States produce about 2000 metric tons of waste per year. There is about 80,000 metric tons total so far from every kilowatt of power generated from nuclear power plants, which has been about 20% of the total electricity generated in the country for the last several decades. This is a remarkably small amount of waste for the amount of power produced.

Yucca Mountain is not currently being used as a disposal site for used nuclear fuel and high-level radioactive waste. A license application is currently being considered by the US Nuclear Regulatory Commission which must issue an operating license before it can start accepting waste.

Intellectualbedlamp0 karma

I heard a rumor on Reddit that Trump was attempting to reclassify the regulatory status of some nuclear waste in a certain location (sorry can't remember where), in order to be able to get rid of it by dumping it in the ground water.

I took it with a grain of salt because I haven't seen anything about it (even though I despise him), but was wondering if you knew anything/had heard of this?

Repo_co5 karma

Hmmm...I haven't heard anything about this issue and would need some more specifics before I offered a meaningful response. Sorry about that!