The pandemic has certainly slowed America and the world to a crawl. But we will get back to some form of a working society, hopefully with a better perspective on what’s important. The temporary drop in carbon emissions from the lockdown of industrial and commercial activities around the world is one example, and has shown what is possible by decarbonizing society.
For that, nuclear power is essential. And nuclear waste must be dealt with, relatively quickly and easily, which it can be. There are not that many reasonable paths forward, and deep borehole disposal is one of them.
Elizabeth Muller, CEO of Deep Isolation, announced the completion of a Post-Closure Safety Analysis for their concept of deep borehole disposal of nuclear waste. Deep Isolation is a company out of Berkeley that is working to dispose of nuclear waste in deep boreholes in the Earth’s crust, safer and at a lower cost than existing strategies.
Schematic of a deep horizontal drillhole repository. A vertical access hole is drilled to depth, ... [+]
A Post-Closure Safety Analysis investigates and quantifies the safety and operation of the total system, and is required by law before any attempt at licensing.
Their Safety Analysis indicates that spent nuclear fuel from commercial reactors can be safely disposed using this method, and that it is viable in a wide-range of scenarios and conditions, as long as the repository is properly sited and carefully characterized, constructed, operated, and sealed.
The safety analysis also indicates that the long-term safety would more than comply with the regulatory maximum annual dose requirement of 10 mrem per year (0.1 mSv/yr) to an individual at the surface drinking potentially contaminated water from a well over the waste at some time in the distant future. This dose limit governs all nuclear repositories and clean-up sites.
The time scale covered by the analysis starts with repository closure, contains the thermal period (when things are still hot), and then extends to ten million years, an amazingly long time that covers any dose, even if anything significant got out.
The overall disposal system for spent nuclear fuel (see figure) includes a single or an array of deep horizontal drillholes bored into the host rocks using off-the-shelf directional drilling technology.
Individual nuclear fuel assemblies are encapsulated in customized, corrosion-resistant canisters, which are placed end-to-end into the relatively small-diameter, cased and potentially backfilled horizontal disposal sections of the drillholes.
The deep borehole disposal process begins with a vertical access hole drilled and cased from the surface through confining geologic units to a point a few hundred meters above the target repository depth.
The hole and surface casings are to guide the drilling and to protect freshwater aquifers. Below the kickoff point, a smaller-diameter hole is drilled that gradually curves until it is nominally horizontal. The radius of curvature is large enough to avoid any impedance during casing installation and waste canister emplacement. After the casing in the curved section is cemented in place, a final smaller-diameter drillhole continues near-horizontally for a few hundred meters to several kilometers.
The horizontal repository portion has a slight upward tilt that provides additional isolation, and isolating any mechanisms that could move radioactive constituents upward. They would have to move down first, then up, something that cannot occur by natural processes deep in the crust.
For larger-diameter canisters, the horizontal section may be drilled in two stages: a first small-diameter stage for characterization and testing of the disposal section followed by a reaming operation to create a diameter large enough to house the canisters.
This casing is also cemented in place, potentially with monitoring systems embedded or attached to it, which communicate real-time data about the repository condition to the surface during the pre-closure and evaluation periods.
About 10 drillholes are required to dispose of the waste from operating a 1,000 MW nuclear power plant over about 30 years. Globally, there are over 450,000 metric tons of nuclear waste that await disposal, or re-use in future fast reactors, followed by disposal of that waste. And there is currently no operating disposal solution for either.
Radiation release for the 3 most important nuclides, (I-129, Cl-36, Se-79) with a fault intersecting ... [+]
To say these analyses are complicated is an understatement of astronomical proportions. Refer to the report for details, but it models the key radionuclides over space and time, under different conditions, with and without through-going faults, with and without canister breaching, all at different depths, temperatures and pressures.
The figure above is an example of these analyses. It shows that, even under the worst conditions, radioactivity releases are a thousand times less than the normal background radiation of anywhere in America. Normal background in America is over 300 mrem/yr (3 mSv/yr), and is above even the top of this graph.
The idea of deep borehole disposal for nuclear waste is not new, but Deep Isolation is the first to consider horizontal portions of the wells as well as vertical, and is the first to actually demonstrate the concept in the field showing that the technology is not just theoretical. The field demonstration occurred last year when it placed and retrieved a waste canister from thousands of feet underground.
As geologists, we know how many millions of years it takes for anything to get up from that depth in the Earth’s crust, especially in tight rock formations like shale. And we have plenty of tight rocks in America.
So what better way to use this technology than to put something back into the Earth that you want to stay there for geologic time. “We’re using a technique that’s been made cheap over the last 20 years,” says Muller. “We could begin putting this waste underground right away.”
Like all leading climate scientists, Muller understands that the world must increase its use of nuclear energy to seriously address climate change or succeed in any Green New Deal, and knows that solving the nuclear waste problem is essential.
Indeed, when queried, the “waste issue” is all that most people really worry about.
