Underground tests dig into how heat affects salt-bed repository behavior
Study to refine computer models, inform policymakers for future spent
nuclear fuel disposal

Date:
November 4, 2021
Source:
DOE/Sandia National Laboratories
Summary:
Scientists have just begun the third phase of a years-long
experiment to understand how salt and very salty water behave near
hot nuclear waste containers in a salt-bed repository.



FULL STORY ========================================================================== Scientists from Sandia, Los Alamos and Lawrence Berkeley national
laboratories have just begun the third phase of a years-long experiment
to understand how salt and very salty water behave near hot nuclear
waste containers in a salt- bed repository.


========================================================================== Salt's unique physical properties can be used to provide safe disposal
of radioactive waste, said Kristopher Kuhlman, a Sandia geoscientist and technical lead for the project. Salt beds remain stable for hundreds
of millions of years. Salt heals its own cracks and any openings will
slowly creep shut.

For example, the salt at the Waste Isolation Pilot Plant outside Carlsbad,
New Mexico -- where some of the nation's Cold War-era nuclear waste is
interred - - closes on the storage rooms at a rate of a few inches a
year, protecting the environment from the waste. However, unlike spent
nuclear fuel, the waste interred at WIPP does not produce heat.

The Department of Energy Office of Nuclear Energy's Spent Fuel and Waste Disposition initiative seeks to provide a sound technical basis for
multiple viable disposal options in the U.S., and specifically how heat
changes the way liquids and gases move through and interact with salt,
Kuhlman said. The understanding gained from this fundamental research will
be used to refine conceptual and computer models, eventually informing policymakers about the benefits of disposing of spent nuclear fuel in
salt beds. Sandia is the lead laboratory on the project.

"Salt is a viable option for nuclear waste storage because far away
from the excavation any openings are healed up," Kuhlman said. "However, there's this halo of damaged rock near the excavation. In the past people
have avoided predicting the complex interactions within the damaged salt because 30 feet away the salt is a perfect, impermeable barrier. Now,
we want to deepen our understanding of the early complexities next to the waste. The more we understand, the more long-term confidence we have in
salt repositories." Trial-and-error in the first experiment To understand
the behavior of damaged salt when heated, Kuhlman and colleagues have been conducting experiments 2,150 feet underground at WIPP in an experimental
area more than 3,200 feet away from ongoing disposal activity.

They also monitor the distribution and behavior of brine, which is
salt water found within the salt bed left over from an evaporated 250-million-year old sea. The little brine that is found in WIPP is 10
times saltier than seawater.



========================================================================== "Salt behaves much differently when it's hot. If you heat up a piece of granite, it isn't that different," Kuhlman said. "Hot salt creeps much
faster, and if it gets hot enough, the water in brine could boil off
leaving a crust of salt on the waste container. Then that steam could
move away until it gets cool enough to return to liquid and dissolve
salt, possibly forming a complex feedback loop." In other words, the scientists are looking at whether the heat from spent nuclear fuel could
help enclose waste containers, and even protect them from the corrosion
that salty water can cause.

Planning for the experiment's first phase began in 2017, using existing horizontal holes at WIPP. During this "shakedown" phase, researchers
learned what equipment to use in subsequent experiments. For example,
the first heater, which worked like a toaster, did not get the nearby
salt hot enough to boil brine, said Phil Stauffer, a geoscientist with
an expertise in combining computer models and real-world experiments
who is leading Los Alamos National Laboratory's contributions. However,
the second heater the team tried, an infrared model, was effective;
it worked more like the sun.

"When we put the first radiative heater into the first borehole, as part
of the shakedown phase, it turns out the air didn't allow the heat to efficiently move into the rock," Stauffer said. "Then we switched to an infrared heater, and the heat moved through the air with little energy
loss. In the early numerical simulations, naively we just put in heat;
we didn't worry about how the heat got from the heater into the rock."
How brine and gases move through salt During the experiment's second
phase, the team drilled two sets of 14 horizontal holes into the side
of a hall and inserted more than 100 different sensors into the holes
around the central horizontal hole containing the heater. These sensors monitored the sounds, strains, humidity and temperatures as the salt
was heated and cooled.



========================================================================== Melissa Mills, a Sandia geochemist, made a special salt-concrete seal
for testing the interactions between cement and brine.

Among the sensors used were almost 100 temperature sensors, like those
found in home thermostats, so researchers could measure temperature
through time at locations around the heater. Yuxin Wu, a geoscientist
from Lawrence Berkeley National Laboratory, also installed fiber-optic temperature sensors, strain gauges and electrical resistivity imaging.

Charles Choens, a Sandia geoscientist, used special microphones, called acoustic-emissions sensors, to listen to the "pop" of salt crystals as
they expand while heated and contract while cooling, Kuhlman said. The
team used these microphones to triangulate the location of the popping
salt crystals.

"Those pops are evidence of the transient permeability of the salt
bed -- the cracks between the salt crystals, which brine can percolate through." Kuhlman said. "When you heat it up, it closes those little
cracks. When the salt is hot, the permeability goes down, but when
it cools down, the cracks temporarily open up and the permeability
increases." To test the flow of gases through the damaged salt, the researchers injected small amounts of rare gases, such as krypton and
sulfur hexafluoride, into one borehole and monitored their emergence
in another, Kuhlman said. "When the salt was hot, the gases didn't go
anywhere. When we turned the heat off, the gases permeated the salt and
came out in another borehole." Similarly, the team injected lab-made
brine into one borehole with a small amount of the element rhenium
and blue fluorescent dye as "tracers." The team is monitoring for the
emergence of the liquid in other boreholes, which will be sampled at
the end of the test.

"The goal with the fluorescent dye -- once we drill out post-test
samples -- is to map where the tracer went," Mills said. "Obviously,
we'll be able to say that it went from one borehole to the other, if we
detect a rhenium signal, but we won't know the path it took. Also, brine
will interact with minerals in the salt, like clay. The fluorescent dye
is a visible way to identify where the liquid tracer actually went in
the field." In the third phase, which began in mid-October, the team
will be drilling a new array of nine heated boreholes, building on what
they learned in the prior phases of the experiments.

Working in challenging conditions underground The team has learned a lot
from the first two phases of the experiment, including the best heater
type, when to drill the boreholes and just how corrosive the brine is,
Stauffer and Mills said.

"The first two phases involved a lot of equipment testing; some has
failed, and some was sent back to the manufacturer," Mills said. "We've
also learned to keep back-up equipment on hand because salt dust and
brine destroys equipment.

We need to double-seal things because the brine can seep down insulated
wire and then equipment dies. It's been a process to learn how to work
in the salt environment." Kuhlman agreed. "Many things can go wrong when
you take sensitive lab equipment and put it in a salt mine. We went back
and read the reports from the WIPP experiments in the '80s. We want to
learn from the past, but sometimes we have had to make our own mistakes."
The researchers are collaborating with international partners to use
the data from this project to improve computer models of the complex
chemical, temperature, water-based and physical interactions that take
place underground.

This will improve future modeling of nuclear waste repositories globally.

Ultimately, the team would like to scale up to larger and longer
experiments to obtain data relevant to future salt repositories, said
Kuhlman and Stauffer.

These data, supplementing already collected data, would inform repository designers and policymakers about the safety of permanently disposing heat- generating nuclear waste in salt repositories.

"It's been really intriguing and interesting, for me, to work on a
project that is so hands-on," Mills said. "Getting to design and build the systems and going underground into WIPP has been really rewarding. Doing research in an active mine environment can be a challenge, but I've
been proud to work down there and implement our ideas." Sandia National Laboratories is a multimission laboratory operated by National Technology
and Engineering Solutions of Sandia LLC, a wholly owned subsidiary
of Honeywell International Inc., for the U.S. Department of Energy's
National Nuclear Security Administration. Sandia Labs has major research
and development responsibilities in nuclear deterrence, global security, defense, energy technologies and economic competitiveness, with main
facilities in Albuquerque, New Mexico, and Livermore, California.

========================================================================== Story Source: Materials provided by
DOE/Sandia_National_Laboratories. Note: Content may be edited for style
and length.


==========================================================================


Link to news story: https://www.sciencedaily.com/releases/2021/11/211104081502.htm

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