Yucca Mountain – Packaging and Storing Radioactive Waste

t1larg.casks.nrcSo – thus far we’ve gone over a little of the history of the Yucca Mountain project and how both geology and hydrogeology can affect waste disposal. What I thought could be interesting today would be to talk a little about how the spent reactor fuel is packaged – both for transport and for disposal – because this is a third factor that has a profound impact on how well the waste can be isolated from the environment. Then, for the last installment in this series (next week) I’ll try to examine some of the claims both for and against the site to see how well they hold up.

To recap a little bit – fissioning a uranium atom splits it into 2 radioactive fission products. These accumulate as the reactor operates – adding more radioactivity as time goes on. As the reactor operates, though, the fuel “burns” up the uranium – after a few years the concentration of fissionable atoms drops to the point where it’s time to swap out the spent fuel rods for new ones. The spent rods are intensely radioactive so they’re normally stashed in spent fuel pools until they can cool off a bit – and in this case, “cooling off” means thermally as well as radiologically since the energy given off by the decaying fission products causes the spent fuel to heat up. But after a long enough time the fuel will cool off to the point at which it can be removed from the water and placed into huge casks that are placed in storage yards at the reactor sites – this is called dry cask storage.

At some point – if Yucca Mountain or some other high-level waste repository opens up – either the dry casks will be used for transport or the spent fuel will be transferred to transport casks that will be loaded onto rail cars or trucks and relocated to their final resting place. It’s these casks that will also be the penultimate barrier between the radioactivity within and the environment so they warrant a description.

First of all the things are huge. I saw some in Lithuania about a decade ago and they looked to be at least 10 feet tall and 5 feet in diameter. And since the physics of uranium fission are the same around the world (reactor design changes somewhat from place to place, but not enough to make a huge difference for commercial reactors) the characteristics of spent reactor fuel are reasonably similar as are the characteristics of the casks. In other words, the spent fuel casks in the US are huge as well.

In addition to providing protection to the spent fuel they are also designed to reduce radiation dose rates to an acceptable level – low enough to pose no risk to those sharing the road with the casks if they are transported by truck. But there’s a lot more to safely shipping waste than keeping rad levels down – the spent fuel casks must also be able to protect the waste while it’s in transit to the final disposal site, not to mention protecting it during its long millennia in storage. We’ll tackle these one at a time.

Spent fuel casks have to meet some stringent requirements to ensure that they don’t release highly radioactive fission products while they’re in transit to the final disposal site. Casks must be able to pass these tests without suffering a failure:

  • A 9-meter (30 foot) fall onto a hard surface
  • Puncture test where the container falls 1 meter onto a 6” steel rod
  • 30 minutes of being engulfed in an 800 degree C (1475 degree F) fire
  • 8 hours of immersion beneath 3 feet of water
  • 1 hour of immersion beneath 200 meters (655 feet) of water

These requirements are more than theoretical – in the 1970s Sandia National Laboratories tested some spent fuel containers with full-scale crashes to confirm that what looked good on paper and in the laboratory would work in real life. The most dramatic test was running a locomotive engine into a flatbed truck carrying a cask on it – the locomotive was pretty much destroyed while the cask, while damaged, survived and would not have leaked radioactivity into the environment. There’s a nice video on YouTube showing the locomotive test and others – these videos alone ought to allay any doubts about the ability of these casks to protect spent fuel while it’s en route to the disposal site.

Physical ruggedness is nice, but there’s more to keeping radioactive waste safe than protecting it from collisions – once delivered to the site the casks have to help keep the waste isolated from the environment for up to a million years and that takes a lot more than strength. Rust and corrosion will attack the strongest container – all they need are the right conditions and enough time to work. Not only that, but metals behave differently (and chemical reactions proceed more quickly) at higher temperatures – such as those produced by the decay of fission products. So the thermal effects also have to be factored in when designing the things.

So here’s the bad news about long-term disposal of spent reactor fuel – and the containers meant to hold it. Nobody knows how a container is going to hold up over even 100,000 years, let alone a million years (the time span required by EPA). We can do our best to design something with the lowest possible corrosion rate and we can do our best to design in a high level of structural strength – but no matter how we try to artificially age these materials in the lab we can only guess at their long-term performance. Let’s face it – all of human history is only about 5000 years and the Pyramids are younger than that. We can assert the longevity of our designed structures all we want, but we have no direct experience with anything so long-lived. Of course we can put other barriers in place as well – and likely will – but anything artificial suffers the same drawback, that all of human history is far shorter than the period of time for which we’re hoping to isolate the waste.

On the other hand, the engineered packages aren’t the only barrier between the radioactive waste and the environment – and we actually do have one data point about the ability of rock to hold radioactive waste for prolonged periods of time. In fact, what we have is the remnants of a natural nuclear reactor that achieved criticality in what is now the nation of Gabon (in Western Africa) about two billion years ago. The details of how the reactor (called the Oklo reactor) formed and operated are fascinating, but there’s not enough room in this posting to go into the details. For the purposes of this, let it suffice to say that in two billion years, virtually all of the fission products have remained in place. This is in spite of the reactor zone being located in fractured and porous sandstone that was below the water table more often than not – in fact, if the reactor zone were not completely saturated with water the reactor could never have operated. So – remembering the last two posts – porous and water-saturated rock are not well-suited for waste disposal. But in spite of this, the fission products have remained in place for two billion years. This bodes well for the ability of Yucca Mountain (or whatever location ends up with the spent fuel repository) to safely isolate the waste until it decays to stability.

So here’s the bottom line with regards to the waste containers. First, they certainly seem capable of safely storing spent reactor fuel for the length of time that they’re stored at the reactor plants and multiple tests have shown they can protect the waste while it’s en route to wherever it will be disposed of. But no matter how well we design the containers – no matter how convincing our computer models and calculations might be, there’s no guarantee that they’ll last the million years that is the current standard for the waste site. But that doesn’t mean that Yucca Mountain is incapable of storing radioactive waste safely for that length of time – the natural nuclear reactor in Oklo shows that even radioactive waste that’s stored in porous and water-saturated sandstone can remain in place for the eons. This bodes well for the Yucca Mountain site’s ability to retain our radioactive waste for a paltry million years or so.

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Yucca Mountain – hydrogeology

bigstock_Water_Drop_Blue_8453Although I’m a radiation safety professional I studied geology as an undergrad and for my MS (my doctorate is in Environmental Science). This included several classes in hydrogeology – these were some interesting classes, and I added to the experience by doing some work as a field hydrogeologist for a project at a Department of Energy facility in the early 1990s. Although I’m not a professional hydrogeologist, between my classes and field experience I’ve at least got some familiarity with the basic concepts of field geology, with groundwater modeling, and with some of the practicalities of work in the field. But before applying this to Yucca Mountain, I’d like to go over some of the fundamentals of hydrogeology (you can find much more in a wonderful publication by the United States Geological Survey).

The biggest of these is sort of obvious – water flows downhill. Or, to be precise, water flows down-gradient; from an area of high pressure to an area of lower pressure. This means that groundwater can flow uphill, just as water can flow through pipes from the ground floor to higher floors in a building, if the water pressure is high enough.

Although water normally doesn’t exist in the form of underground rivers or streams it does flow through sediments. But water in a river or stream will flow hundreds or thousands of kilometers in a year; water flowing through an aquifer might flow only a few meters in a year. And unlike surface water where you can pump seemingly unlimited quantities of water (up to the capacity of the river or stream of course), most aquifers are more limited in capacity.

There are a number of factors that affect the speed at which water flows through an aquifer and the rate at which it can be pumped from the ground. One factor is the porosity of the aquifer – the percentage of the rock or sediments that is pore space. An aquifer with higher porosity (all else being equal) will hold more water. But there’s more to an aquifer than how much water it will hold – if the pores are interconnected the water can flow easily from one pore to the next, if they are isolated then water remains locked in the pore space. So permeability is another important factor – clay can actually have a high degree of porosity but, since the pores are blocked off from each other, it is highly impermeable; so much so that clay is frequently used to seal landfills to isolate whatever they hold. Groundwater flows slowly through low-permeability rock and sediments – flow velocities can be as low as centimeters per year. One other thing – permeability depends on more than just the interconnection between pores – cracks and fractures can pipe water through the rock at high velocities, adding to the permeability.

Something else to keep in mind is that water can be corrosive and it’s a solvent, so if we’re trying to isolate radioactive waste – to keep it safe for millennia – then it behooves us to try to keep water away from it. This is why designers do their best to keep water out of all hazardous materials landfills and disposal sites – not just radioactive waste facilities. And since high-level waste can remain fairly highly radioactive for centuries or longer (and since it can also contain things like plutonium and americium), we need to do our best to try to keep water away from the waste containers for the same length of time.

One last thing to consider is that virtually every place on Earth has a water table. Water falls on the surface as rain or snow, the water flows downhill to collect in lakes, rivers, and streams, and water can leak out of the bottoms and sides of the rivers, lakes, and streams into the soil. This water will percolate downwards until it can’t descend any further – usually because it fetches up against an impermeable layer of rock or soil (clay, for example). The water will collect there and will begin flowing downgradient. Sometimes the water table is only a few feet underground, in other places it’s thousands of feet down.

So let’s put this together to try to figure out what sort of hydrological conditions will be good for a high-level waste repository. And remember that the whole idea is to keep water away from the radioactive waste as long as possible and, when contact with water is inevitable, to minimize its duration. So what conditions are we looking for?

The first condition should be to have the waste as far from water as possible. Satisfying this means in part putting the waste as far from the water table as possible. In the case of Yucca Mountain the proposed waste location is both several hundred feet above the local water table and several hundred feet below the surface upon which rain and snow will fall. The depth to the water table means that there are no worries about water rising to flood the site from below – and that any leaking waste will take a long time to descend to the water table; the depth below ground means that precipitation has quite a distance to flow before reaching the waste.

So it’s nice that the waste will be stored several hundred feet belowground, but a more fundamental question is whether or not surface water can reach it – after all, if the rock is heavily fractured and permeable then water can be piped directly down from the surface, possibly reaching the waste in hours or days. On the other hand, if the rock is impermeable then the water will take so long to reach the waste that it’ll be absorbed by the intervening rock before it ever penetrates to the waste repository. Porous, fractured rock speeds the flow of water from the surface to the waste; low-permeability rock impedes it.

Something else can slow the flow of water as well – geometry and chemical reactions. The parched desert rock that overlies the Yucca Mountain repository soaks up water in two ways – first, water percolating through the rock will coat and fill the pore spaces it’s flowing through and until the pores are filled with water it won’t flow further. So even permeable rock – if it’s bone dry – will absorb quite a bit of water. As the water coats the pores it also reacts chemically with the minerals that comprise the rock. Feldspar, for example, is a dry mineral that hydrates to form clay minerals. So groundwater in contact with feldspar will be absorbed by the feldspar, forming clay. There are other hydration reactions as well – most dry minerals will weather to form hydrated versions. Water flowing slowly through rock will undergo these hydration reactions until all of the available minerals are converted – only then will water start flowing through the rock, if the new minerals haven’t expanded to block the passages, which sometimes happens.

So think of the rock overlying the waste repository as a sponge – if you trickle water slowly onto a sponge, the sponge will slowly expand as it absorbs the water. The sponge swells and swells until it’s saturated – only then will water start to drip from the bottom. And – in the absence of large fractures – surface precipitation will not reach the waste repository until the rock overlying the repository site is saturated and converted to hydrated minerals. Cracks and fractures are a different story – but even here, if the flow path is sufficiently circuitous or if the fractures are sufficiently small, the water will react with dry minerals before it can reach the waste.

So we know that the Yucca Mountain repository site is situated several hundred feet from both the water table and the surface precipitation, but what about the rock? Well, it’s reasonably porous and it has some fractures – not the ideal rock to form an impermeable barrier. Geologists have found fractures and faults in the overlying rock, as well as evidence that hydrous minerals have formed in the past. This means that not only will the rock hold a lot of water, but there are some flowpaths to get that water to the waste location. On the other hand, the rock pretty dry, so the water has to first fill up the pore space and then hydrate the minerals – something that can take hundreds of thousands of years in a climate as dry as Nevada’s. So from this standpoint, the hydrogeology of Yucca Mountain might not be perfect, but it’s pretty good.

The kicker is that, while we know what today’s climate is like, we don’t know what it will be like in a thousand years. If it stays dry then the waste will be in good shape. But if the climate changes and puts Nevada in the monsoon belt then all bets are off. This is not disqualifying, by the way – but it would call for relying on engineered barriers as well as natural ones to help keep the waste dry. The engineering is something we’ll look at in a future posting.

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Yucca Mountain

Yucca_Mountain_2In the mid-1990s I was on a technical advisory committee to the organization that was working to find a location for a low-level radioactive waste disposal facility that was to service several states in the Midwest. Even with stringent siting criteria there were a number of locations that would have been acceptable; what stopped the process was that the Midwest Compact decided to pull the plug – changes on the national level made it more attractive to continue using existing sites than to start up a new one. At present, there are a number of sites for the disposal of low-level radioactive waste, but the nation is still struggling to answer a question that many nations have already satisfactorily addressed – where to put the nation’s high-level radioactive waste. This process has recently come up in the news again. This is an important topic – one that warrants more than a single posting – so I’d like to discuss various aspects of the science behind high-level radioactive waste disposal in general, and of Yucca Mountain in particular, over the space of the next few weeks. But first I’d like to start with an overview of where high-level radioactive waste (HLW) comes from and a little bit of history on this topic. So history this week, then we’ll get into the science.

 

By definition HLW is “highly radioactive material produced as a byproduct of the reactions that occur inside nuclear reactors.” It includes both spent reactor fuel (fuel removed from the reactor because it no longer has enough fissionable U-235 to easily sustain a chain reaction) and waste produced during the reprocessing of spent fuel. HLW can be hot – both thermally and radioactively – and it has to be handled with care. Not only that, but some of the nuclides remain hot for decades to millennia so it has to be kept secure for at least as long as the Pyramids have been standing. There are a lot of factors that enter into storing HLW safely – too many to go into here, but they’ll be dealt with in upcoming posts.

All nuclear reactors produce HLW during their operation – a typical reactor will be refueled every year or two; during a refueling outage much of the fuel is shuffled around in the core and the remainder is removed. In other nations the spent fuel is reprocessed – plutonium that was produced during reactor operations is removed and residual fissionable U-235 can be removed as well; whatever’s left over is disposed of as HLW. Until recently plans were to dispose of HLW beneath Yucca Mountain, located in the Nevada desert. Here’s what happened.

In 1982, realizing that the US needed a long-term plan for high-level waste, the Nuclear Waste Policy Act was enacted into law. This law tasked the Department of Energy (DOE) with developing a repository, the Environmental Protection Agency was instructed to develop environmental safety standards and to evaluate a repository’s safety, and the Nuclear Regulatory Agency was told to develop appropriate regulations. DOE started a long process that led them to Yucca Mountain. Without getting into the details, by 1987 the DOE was beginning to investigate its suitability as a deep geologic repository and Congress approved it in 2002. Instead of solving the issue, though, this only seemed to serve as incentive to those opposed to the site – both inside and outside of Nevada. Both scientific studies and legal/political opposition continued until 2011, when Congress pulled the plug on Yucca Mountain. And there’s where the issue seemed to rest until just recently, when the issue came up yet again. At present it seems the issue isn’t settled after all (the US Court of Appeals for the District of Columbia recently voted 2-1 that the Yucca Mountain licensing process must continue) – but who knows what will happen next year.

At the same time, all of our nuclear power plants are continuing to operate and to produce HLW. At first it was stored on-site in wet storage – immersed in water. As the spent fuel pools began to fill up the nuclear power plants got permission to re-rack their fuel (the spacing of the spent fuel rods is carefully controlled to prevent the possibility of a criticality). With pools continuing to fill up – and still no place to permanently take the waste – the NRC authorized dry cask storage – taking the spent fuel out of the swimming pool and putting it into land-based casks. And that’s where matters stand today – over 50 individual reactor sites are storing their HLW on site in a combination of dry casks and re-racked swimming pools.

There have been a ton of claims and counter-claims about Yucca Mountain – the scientific aspects of these claims can be evaluated on their scientific merits; the social, economic, and political aspects are a little more complex. In the next few weeks we’ll look at things such as hydrogeology, seismic risks, waste container integrity, geology, and how they affect the ability of the site to safely contain HLW. The goal will be to present some of the claims made by both the pro- and anti-Yucca Mountain advocates, to review some of the basic science, and to try to use the science to evaluate the claims. I’ll try to leave the social, economic, and political issues out of the discussion since they lie outside my area of primary expertise – but at the very least we can try to dispense with the scientific and technical issues.

The post Yucca Mountain appears on ScienceWonk, FAS’s blog for opinions from guest experts and leaders.