A Simplified Nuclear “Waste” Digester

Each of the Democratic candidates in the last presidential election, as part of the Nevada primary, had an opportunity to declare their everlasting hatred for the idea that “nuclear waste” would ever be stored at a remote site of desert called Yucca Mountain.

Eventually one of them was elected.

And he carried through on his promise, essentially bypassing the 1982 Nuclear Waste Policy Act and billions of dollars of investment by saying that Yucca Mountain wouldn’t ever be opened and that “nuclear waste” wouldn’t be stored there.

There’s a little problem with that, see–each of the utilities that operate nuclear reactors have been paying a dollar of tax for each megawatt-hour of electricity they’ve generated since the early 1980s. That $1/MW*hr has really added up since then, to the tune of about $25 billion.

So President Obama appointed a “Blue Ribbon Commission” made up of several former politicians, some political activists, some utility types, and one nuclear engineer to determine what we should do if we weren’t going to build Yucca Mountain. The commission was announced in February 2010 and has been meeting and taking testimony and holding public meetings on a regular basis since then. Now they’re expected to report their recommendations:

1. Store all the spent nuclear fuel in one place somewhere . . . temporarily.

2. Build something very much like . . . Yucca Mountain.

Which really makes you wonder a bit why we spent $10 billion designing and testing Yucca Mountain only to abandon it. The administration is pretty vague on this point, saying that the “science wasn’t there” whereas many people I’ve spoken to in the nuclear industry tell me it’s the most studied place on the planet.

I’m a bit on the fence myself. For years I opposed Yucca Mountain for the reason that I’m fairly convinced that “nuclear waste” isn’t really waste at all, but potentially untapped wealth. I didn’t think we should go toss all that barely-consumed nuclear fuel in the ground. But if the Blue Ribbon Commission is going to say we need a “geologic repository” like Yucca Mountain, well then, why not Yucca Mountain?

Seems to me that $10 billion is an awfully large amount of money to go to waste. I keep putting nuclear “waste” in quotes not in an attempt to be cheeky, but rather as a recognition that most people see this issue fairly differently than I do. They see nuclear “waste” as just that, a waste, whereas I see it as an under-appreciated opportunity.

So it was with great delight that I opened my emails this morning and saw one from a friend at the Oak Ridge National Lab talking about a recently released report titled “Fast Spectrum Molten Salt Reactor Options“. Now I’ll readily admit that a title like that probably isn’t going to cause many people to reshuffle their vacation reading list, but let me tell you a little bit about why I think you should care about what’s in that paper. In short, it describes some very compelling technology to not only solve the nuclear “waste” problem but how to turn it into a financial opportunity.

In the United States and over all the world, our nuclear reactors today are based on using uranium very inefficiently. We only consume a small fraction of the energy content in the uranium (less than 1%) before we remove it and throw it away. We don’t do this because we’re stupid or evil, we do it because it’s very difficult to get the vast majority of the energy out of the uranium. My nuclear engineering friends will probably cringe when they hear me say this, but it’s because most of the uranium doesn’t “burn good”. Only a little bit “burns good” and that’s the part we “burn”.

Building nuclear reactors that “burn” the rest of the uranium is hard, because we have to build a totally different kind of reactor from the kinds we have today. These reactors use “fast” neutrons instead of “slow” neutrons in the reactor. All of our reactors today use “slow” neutrons, and again, there’s some very good reasons for that. We’re not using “slow” neutrons because it’s a bad idea–in fact, using “slow” neutrons solves a lot of problems.

But when it comes to getting uranium to “burn good”, you have to get into using “fast” neutrons and dealing with the challenges that go along with doing that. And that’s why this paper is so interesting–it describes a way to build reactors that use fast neutrons but are much simpler and safer than other ideas we’ve had on how to build reactors that use fast neutrons.

The most basic difference is that the Oak Ridge paper suggests using liquid nuclear fuels instead of solid nuclear fuels. That solves a lot of problems right from the outset. Liquid fuels are mixed up in big batches, by remote control, with very simple procedures. Solid fuels also have to be mixed up in big batches by remote control but then have to be fabricated into shapes, typically pellets or spheres. Getting the fabrication just right is a major cost and technical challenge. That’s just not a problem for liquid fuels.

Another big difference is how you move the heat generated by nuclear fission out of the reactor. In the standard ideas for a “fast” neutron reactor, liquid sodium metal coolant has been used to transfer the heat generated in the solid rods from the inside of the reactor to the outside. Liquid sodium is very good at absorbing heat–as engineers would say, it’s very “thermally conductive“, but it’s not very good at holding a lot of heat–as an engineer would say, it doesn’t have a lot of “thermal capacity“. So you tend to need a lot of sodium to move the heat from the inside of the reactor to the outside.

Sodium metal has another big problem. It’s super-reactive and burns on contact with air or water, so you have to keep all this sodium away from the two most common things on our planet. If you watch the video I linked, you should know that this was a small piece of metallic sodium. These sodium-cooled fast reactors have hundreds of tonnes of the stuff, in a heat exchanger with guess what? High pressure water. No kidding.

The material used for the fuel in the Oak Ridge report is different. It’s a salt, which means it’s in a class of materials that are the most stable and non-reactive known to man. And since the fuel is a liquid, it is its own vehicle for moving that heat from the inside of the core to the outside. It doesn’t react with air and water. These salts aren’t as good as sodium at moving heat (thermal conductivity) but they hold a lot more heat than sodium (thermal capacity). This means that they have the potential to be more compact and less expensive.

So again, why should you care about these little engineering facts? Because you’ve got a personal $83 share in the $25 billion waste fund that has been collected over the last 30 years to deal with nuclear “waste”, and this liquid-salt-based nuclear reactor could be the means to dealing with that waste in a way that will make money rather than consume it.

I thought of an analogy that might help explain how we got into the situation we’re in with nuclear “waste”. Imagine there’s a vending machine that carries snacks. You see a snack that you want and put $1.00 into the machine. The snack falls into the bin and you retrieve it and then you hear some change fall into the coin return. You happily recover the change and notice that it has a value of about 60 cents, but there’s a catch. The coins are in another currency.

In this analogy the vending machine is today’s common type of nuclear reactor–a light-water-cooled reactor running on enriched solid-uranium fuel. The money you load into the machine is the fissile material (uranium-235) that is consumed by the reactor. The snack that you retrieve is the electricity that was generated by that uranium-235 as it was fissioned over a period of time. And the change that you recovered, in a foreign currency, is the plutonium that was made in the reactor while it made electricity.

So to give a simplified view of what we’ve been doing in our nuclear reactors for the past fifty years or so, we’ve been buying snacks (electricity), spending money (uranium-235), and generating change (plutonium) in a foreign currency. We’ve managed to make quite a bit of plutonium is all those years and it’s one of the main drivers around the issue of nuclear waste.

A fast reactor promises to be another kind of vending machine, one into which you can load the foreign money, buy a snack, and get as much or more foreign money back in return. How can this be? Sounds too good to be true, right? Well, it’s possible because only in a fast reactor can cheap, abundant, nearly worthless uranium-238 be fissioned and release its energy. And this is because plutonium is less likely to absorb a neutron without fissioning in a fast reactor than in a thermal reactor.

So we’ve accumulated a fairly substantial amount of the “foreign currency” of plutonium in fifty years of nuclear operations and in the United States at least, we haven’t “spent” any of it. Not because we’re stupid or anything, but mostly because it hasn’t been worth it to build the machines that would burn plutonium. We tried. A lot. But it didn’t work out. Part of the problem might have been the approach we took. In any case, we keep on burning uranium-235 and accumulating plutonium.

That’s why this paper is so interesting. The salt-based fast reactor can fix a lot of problems that the sodium-cooled, solid-fueled fast reactor faces.

The biggest one is safety. I’ve already mentioned how the salts are stable and unreactive and the sodium is dangerous and explosive. But another reason is that things get, shall we say, “twitchy” in a fast reactor versus a reactor that uses slowed-down neutrons. Twitchy is NOT a good quality to have in a nuclear reactor, and it’s one of the bigger reasons we use slowed-down neutrons in all our reactors. Fast reactors based on salt can remove most of the “twitchiness” from fast reactors compared to the ones built around sodium.

Meltdown is another big issue when you’re dealing with a solid-fueled reactor, because the structures of the reactor aren’t designed to hold melted solid fuel. It’s too hot. So it can potentially melt through the big steel vessel that holds the fuel and the sodium if there is a meltdown.

This is going to sound strange, but the best way to address the issue of meltdown is to use a fuel that’s already liquid. Meltdown then becomes a meaningless concept because you have a fuel that’s intended to be used in a liquid state. Surprisingly, the temperature of the liquid salt fuel is a lot lower than the temperature at which solid fuel would melt. So the metal structures of the reactor have no problem holding liquid salt fuel, whereas they wouldn’t be able to survive the temperatures of solid fuel that had melted. In addition, there’s no sodium coolant with which to potentially react.

Fast reactors aren’t going to be particularly cheap though. They have some real drawbacks in general versus reactors that use slow neutrons. It is unlikely we would build them if we weren’t interested in doing something about the “waste” issue.

If tossing nuclear “waste” in the ground was cheap and not a big deal, maybe we wouldn’t worry about all this. Just because we have all this “foreign currency” doesn’t mean we have to “spend” it. But anti-nuclear factions have consistently made the issue of long-lived nuclear “waste” one of the central planks of their arguments against nuclear power, which leads engineers like me to try to figure out clever solutions rather than perhaps challenging whether or not it’s a really significant problem in the first place.

If we are going to deal with all the plutonium we’ve accumulated in the last fifty years, then consuming it in a fast reactor will get rid of it much faster than waiting 24,000 years for 50% of it to decay away. Plus we’ll get to make electricity (snacks) and hopefully everyone sees the value of that.

There is $25 billion in the Nuclear Waste Fund of the United States. Right now it is legally obligated to go towards the opening of Yucca Mountain. Not “some waste repository”, but Yucca Mountain. This will not change until the 1982 Nuclear Waste Policy Act is amended or repealed, regardless of the actions of the President. To change the NWPA, you have to convince 39 states that you have a better plan for nuclear “waste” than Yucca Mountain, and that will likely be an uphill battle for any Congress or administration.

But, if the 1982 NWPA is changed, then the monies in the Nuclear Waste Fund could be applied to building fast reactors that will consume and destroy the long-lived components of the waste, particularly plutonium. And if that direction is taken, the salt-based fast reactor is a much better choice than the sodium-cooled, solid-fueled fast reactor. It will be safer, cheaper, more-effective, and more flexible.

(this post was originally published on Forbes)



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