How does humanity plan to permanently store high-level nuclear waste that can remain radioactive for several thousand years? The answer isn't as simple as digging a hole in the ground. No, geologically unique locations must first be scouted out.
Short-term storage versus long-term storage
The suitability of a given disposal option depends on the waste form, volume and radioactivity of the waste. Generally speaking, options for disposal of nuclear waste include near-surface disposal and deep geological disposal. High-level waste (HLW, which includes spent nuclear fuel rods) is subject to much stricter disposal criteria than low-level waste. The exceptionally long-lived nature of high-level radioactive waste requires that it be sequestered far from human activity for very long periods of time.
There are currently no HLW storage facilities in the world. Instead, HLW is stored in interim storage, typically on-site at the location it was produced. This process usually begins with the underwater storage of high-level waste in spent fuel pools for 10-20 years, after which the fuel is transferred to dry cask storage, which entails storing the waste in radiation-shielded vessels known as castor containers.
Where on Earth can we store HLW?
The most attractive method for disposing of high-level nuclear waste seems to be deep geological disposal, which relies primarily on the thickness of natural geological barriers to isolate the radioactive properties of the waste. The only operational deep geological disposal facility - the Waste Isolation Pilot Plant in New Mexico - does not store high-level waste.
Many countries, including the US, have invested significant time and energy into identifying suitable sites for final repositories commissioned specifically for HLW, but few have made any definitive decisions. For example, the United States' proposed repository at Yucca Mountain, Nevada was shelved amidst political opposition and concerns over the site's geological stability (bye bye, Yucca Mountain Johnny). A list of repository sites and their current statuses is available here.
Sweden and Finland are among the more advanced in terms of planning for the disposal of spent fuel. In 2009, SKB (Sweden's nuclear fuel and waste management company), announced the decision to build its final repository for spent nuclear fuel at Forsmark. It is projected to be completed by 2023, and is slated to be one of the first permanent disposal sites for high-level waste ever to be built.
What constitutes a nuclear vault?
The conception that nuclear waste, especially high-level nuclear waste, is stored in a man-made vault per se is something of a misconception. In reality, decades of research have led to the conclusion that the best "vaults" or "storage facilities" for high-level nuclear waste are naturally-occurring geological formations. An informational leaflet released by SKB in 2008 describes this concept as follows:
The final repository [will be made]of materials occurring naturally in the earth's crust. As far as possible the idea is for storage to resemble natural conditions, thereby avoiding any undesirable effects resulting from the use of foreign materials. The final repository will be built in accordance with the multi-barrier principle. Three barriers (the canister, the buffer and the rock) will together prevent radioactive substances from the spent nuclear fuel from harming people of the environment.
The entire process is known as the "KBS-3 Method". A concise description of deep geological disposal (as well as other nixed nuclear disposal options, such as shooting waste into space) can be found at the World Nuclear Association. A video schematic of a deep geological storage facility - in this case, the under-construction Onkalo waste repository in Finland - can be viewed above.
Once the HLW is secure, how will we prevent future generations from tampering with it?
Once the nuclear waste has been stored hundreds of meters below the surface and the tunnels have been backfilled, the task of keeping the waste safely isolated from man and the environment would be left to an immense natural geological barrier. Indeed, this is the appeal of deep geological repositories. Political dynamics, wars, human expansion would play little part in the long-term storage of the waste.
The very concept of deep geological storage acknowledges the reality of geological time and the ephemeral nature of man's existence relative to that time scale. Centuries from now, the location and purpose of deep geological repositories could very well be forgotten, so passive institutional controls (PICs), or markers and methods designed to warn and inform future civilizations about the buried HLW. You can check out the Waste Isolation Pilot Plan's PIC set-up here.
Also, PICs will have to contain a linguistic and symbolic component - we can't assume that future generations will speak any modern languages. The WIPP's PIC planners have a novel solution for this far-off language barrier:
Since 1991, the United States Department of Energy has been working with a team of linguists, scientists, science fiction writers, anthropologists and futurists to come up with a warning system. [...]The team intends to etch warnings and informational messages into the granite slabs and pillars. This information will be recorded in the six official languages of the United Nations (English, Spanish, Russian, French, Chinese, Arabic) as well as the Native American Navajo language native to the region, with additional space for translation into future languages. Pictograms are also being considered, such as stick figure images and the iconic "The Scream" from Edvard Munch's painting. Complete details about the plant will not be stored on site, instead, they would be distributed to archives and libraries around the world. The team plans to submit their final plan to the U.S. Government by around 2028. [WIPP's Wikipedia page via Science Illustrated]
So yes, "nuclear storage facility futurist copywriter" will be a hot job in the next hundred years or so. For more reading on and schematics of deep geological facilities, check out the following brochures from SKB, the United States Nuclear Regulatory Commission's website, and the WNA's website.
Top photo of excavation at Onkalo via Posiva.