As 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.