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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The rocks of Yucca Mountain

crosssection2As I noted in last week’s posting, nuclear reactors produce high-level radioactive waste during their normal operation. This waste is not voluminous, but it can be dangerous and it needs to be sequestered in an out-of-the-way location for several millennia – in the 70 years since the first nuclear reactor was built there have been a number of suggested solutions for HLW disposal – some have advocated letting it melt its way into the Antarctic ice cap, others pushed for sinking containers into the deep and stable sediments of the ocean’s abyssal plains. Neither of these are now considered viable options (not to mention being forbidden by international treaty), leaving the nuclear nations with few options – the two that are being pursued by most nations are dry cask storage and burial in a deep geologic repository.

As radioactive elements decay to stability they give off radiation; when this radiation is absorbed by matter it deposits energy, which raises the temperature of whatever it is that’s absorbing the energy – the energy given off by radioactive decay is called decay heat and it can be significant. NASA makes use of this to help power their spacecraft – their plutonium-powered radio-isotopic thermal generators (RTGs) produce enough energy via radioactive decay to keep the spacecraft running for decades. Spent reactor fuel produces even more decay heat than do NASA’s RTGs, which is why fresh spent fuel is kept immersed in water at first.

The first fission products produced are predominantly short-lived radionuclides and they quickly decay to longer-lived atoms. This means that the rate at which energy is given off – and the degree of heating – drops over time. After six months the spent reactor fuel is substantially cooler – both thermally and radiologically – than it was when first removed from the reactor. During these first months – actually for the first few years – enough heat is given off that the best coolant is water; with time the heat production is low enough that water is no longer needed as a coolant. This is the point at which the two storage options – dry cask versus geological burial – come into play.

In dry cask storage the spent fuel is placed into huge concrete and steel casks. The decay heat permeates through the cask and thence into the atmosphere – the casks may be warm, but the spent fuel remains cool enough to remain undamaged. I visited a dry cask storage facility in Lithuania several years ago – the casks were at least 10 feet tall and 6 feet in diameter and weighed several tons. At the moment there are a number of nuclear power plants authorized to use dry cask storage and more are contemplating it as they run out of room in their spent fuel pools. The biggest fly in the ointment is that dry cask storage isn’t a permanent solution to the HLW problem – it takes millennia for the longer-lived nuclides to decay away and, at best, the casks will likely last only a century. In fact, some are showing signs of physical decay already and may last only a few decades. Simply put, dry cask storage is not a long-term solution for HLW disposal.

This is the reason that a number of nations are looking at deep geologic repositories for HLW disposal. Put the waste in a deep and stable rock formation, the thinking goes, and it will remain isolated from the environment for as long as we need. As of this writing, five nations (Belgium, France, Korea, Sweden, and Switzerland) are using geologic repositories, and facilities in another several nations are either under construction or have applied for operating licenses.

When it comes to geologic repositories, not all rock is created equal. Rock is not always a solid, impermeable mass – rocks crack and break, some rocks are fractured and faulted, and some are naturally porous. Since over time water will dissolve just about anything, we will want to try to keep water away from our radioactive waste – that way it can’t corrode the canisters, can’t dissolve the spent fuel, and can’t carry the radioactivity into the environment. Ideally, the rock used to contain high-level radioactive waste for hundreds of millennia should be able to keep that waste isolated from the environment – it should be tough, impermeable, and dry.

Some rocks disqualify themselves fairly quickly. Limestone, for example, is soluble, which is why there are so many beautiful caves in limestone country – Mammoth Caves is one of the best-known. Sandstone is porous and conducts water nicely, which rules it out, and both shale and slate fracture quite easily. Most high-level waste repositories that are in operation or that are planned are in granite – a tough, impermeable igneous rock. But there’s another approach that can be used – isolating the waste in a medium that’s impermeable but that will flow around the waste containers, sealing it in for the ages. That’s the approach being taken at the Waste Isolation Pilot Plant (WIPP), which is dug into a thick layer of salt. Over the years, the salt will deform and flow around the waste containers, locking in the waste for millions of years. Other HLW repositories are placed in clay formations, which will also flow around the waste, or into plastic sedimentary rocks. In all of these cases the idea is to put the waste into a layer of rock that will keep water away from the waste and that will keep the waste away from the environment for the requisite amount of time.

The rock at Yucca Mountain is neither granite nor a plastic material that will entomb the waste – Yucca Mountain is made primarily of rocks called tuff and ignimbrite – also called pyroclastics. Both of these rocks are the remnants of ancient volcanic eruptions – volcanic ash and debris that fell and that solidified into a solid mass. Both of these rocks can be very porous – some tuffs have a porosity of over 50% (meaning that half of their volume consists of pore space) – but they are not necessarily permeable. In other words, they might be built like a sponge, with lots of pores and holes, but the pores aren’t well-connected so there aren’t many pathways for water to easily flow. Not only that, but over time other minerals have formed in much of the pore space, blocking what flow paths do exist.

What all of this means is that the rocks of which Yucca Mountain is composed should do a fairly good job of keeping water away from the waste that would be stored there, but the rock might not be monolithic. In fact, virtually all rock everywhere is riven by cracks and faults and these can act as conduits to lead water from the surface to the spaces below. And, indeed, the rocks that comprise Yucca Mountain are fractured and faulted. The question is whether or not these will let enough water into the waste repository quickly enough to rot the waste storage containers.

The answer to this seems to be “it depends.” For example, water will flow more quickly along fractures and cracks, but the water will also deposit secondary minerals along this path and these minerals will eventually help to seal off the cracks (this same process is behind the beautiful crystalline linings of geodes). Both of these seem to have happened at Yucca Mountain in the past – geologists have found these secondary minerals lining fractures in the rock, indicating the past flow of water through the fissures, but the mineralization has also served to clog these conduits. So the question should be not so much “are there fractures” so much as “how much water can flow along these fractures?” In the case of Yucca Mountain we need to consider not only the permeability of these fissures, but also the amount of rain and groundwater available to percolate.

At present, Yucca Mountain is in the middle of a fairly dry piece of real estate where the annual rainfall is only a handful of inches annually. But climate changes over time – the Sahara was a fairly lush grassland within the last few tens of thousands of years – so we can’t guarantee that the current dry conditions will last until all of the radioactivity is gone. But that’s a topic for the next posting – which will be on the hydrogeology of Yucca Mountain. For now, let it suffice to say that the rock that makes up the mountain is not ideal due to its porosity and the occasional fracture and fault. However, secondary mineralization seems to have plugged the majority of the cracks and holes. Thus, while not as tough as granite and not as impermeable as salt or clay, the rocks of Yucca Mountain seem up to the task to which they might someday be put.

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