Underground tests dig into how heat affects salt-bed repository behavior

Newswise – ALBUQUERQUE, NM – Scientists from the National Laboratories of Sandia, Los Alamos and Lawrence Berkeley have just begun the third phase of a year-long experiment to understand how salt and very salty water behave near hot nuclear waste containers in a salt storage facility.

The unique physical properties of salt can be used to safely dispose of radioactive waste, said Kristopher Kuhlman, a Sandia geoscientist and engineering lead on the project. Salt beds remain stable for hundreds of millions of years. Salt heals its own cracks and all openings slowly creep shut.

For example, the salt at the waste isolation pilot facility outside of Carlsbad, New Mexico – where some of the country’s Cold War-era nuclear waste is being buried – closes in storage rooms at a rate of a few inches per year to keep the environment from harm protect the garbage. In contrast to spent nuclear fuel, the waste buried at WIPP does not generate any heat.

The Department of Energy Office of Nuclear Energy’s spent fuel and waste management initiative aims to lay a solid engineering foundation for several viable disposal options in the U.S., and in particular how heat changes the way liquids and gases move through and with salt Salt interact, said Kuhlman. The knowledge gained from this basic research will be used to refine conceptual and computational models and ultimately educate policy makers 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 from the excavation all openings have healed,” said Kuhlman. “However, there is this halo of damaged rock near the excavation. In the past, people have avoided predicting the complex interactions within the damaged salt, as the salt forms a perfect, impermeable barrier 9 meters away. Now let’s deepen our understanding of the early complexities besides waste. The more we understand, the more trust we will have in salt deposits in the long term. “

Trial and error in the first experiment

To understand the behavior of damaged salt when heated, Kuhlman and colleagues at WIPP conducted experiments at a depth of 2,150 feet in a test area more than 3,200 feet from ongoing disposal activities. They also monitor the distribution and behavior of brine, which is salt water found in the salt bed left over from an evaporated 250-million-year-old sea. The small brine that occurs in WIPP is ten times saltier than sea water.

“Salt behaves very differently when it’s hot. When you heat a piece of granite it’s not much different, ”says Kuhlman. “Hot salt creeps up a lot faster, and when it gets hot enough the water in the brine can evaporate and leave a crust of salt on the waste bin. Then that vapor could move away until it becomes cool enough to re-liquefy and dissolve salt, potentially creating a complex feedback loop. “

In other words, the scientists are investigating whether the heat from spent nuclear fuel could help enclose waste containers and even protect them from corrosion from salt water.

Planning for the first phase of the experiment began in 2017 using existing horizontal holes at WIPP. During this “shakedown” phase, the researchers learned what equipment to use in subsequent experiments. For example, the first heater that worked like a toaster didn’t make the salt nearby hot enough to boil brine, said Phil Stauffer, a geoscientist experienced in combining computer models and real-world experiments who made the contributions. However, the second heater the team tried, an infrared model, was effective; it worked more like the sun.

“When we installed the first radiant heater in the first borehole as part of the shakedown phase, it turned out that the air did not let the heat get into the rock efficiently,” said Stauffer. “Then we switched to infrared heating and the heat moved through the air with little loss of energy. In the early numerical simulations, we naively just worked up a sweat; We didn’t care how the heat from the heater got into the rock. “

How brine and gases move through salt

During the second phase of the experiment, the team drilled two sets of 14 horizontal holes in the side of a hall and inserted more than 100 different sensors into the holes around the central horizontal hole with the heater. These sensors monitored noise, strain, humidity, and temperatures as the salt heated and cooled.

Melissa Mills, Geochemist at Sandia, created a special salt-concrete seal to test the interactions between cement and brine.

The sensors used included almost 100 temperature sensors, such as those found in home thermostats, so the researchers could measure the temperature in places around the heater over time. Yuxin Wu, a geoscientist from Lawrence Berkeley National Laboratory, also installed fiber optic temperature sensors, strain gauges, and electrical resistance imaging.

Charles Choens, a geoscientist with Sandia, used special microphones called acoustic emission sensors to hear the “pop” of salt crystals that expand when heated and contract when cooled, Kuhlman said. The team used these microphones to triangulate the location of the popping salt crystals.

“These cracking noises are proof of the temporary permeability of the salt bed – the cracks between the salt crystals through which the brine can seep,” said Kuhlmann. “When you heat it up, it closes those little cracks. When the salt is hot the permeability drops, but when it cools the cracks open temporarily and the permeability increases. “

To test the flow of gas through the damaged salt, the researchers injected small amounts of noble gases like krypton and sulfur hexafluoride into one borehole and monitored their leakage in another, Kuhlman said. “When the salt was hot, the gases went nowhere. When we turned off the heating, the gases penetrated the salt and emerged in another borehole. “

Similarly, the team injected laboratory-made brine into a borehole with a small amount of the element rhenium and a blue fluorescent dye as a “tracer”. The team is monitoring fluid leakage in other wells that will be sampled at the end of the test.

“The goal with the fluorescent dye – once we’ve drilled out samples after testing – is to map where the tracer went,” said Mills. “Of course we can say that it went from one borehole to another when we discover a rhenium signal, but we don’t know which way it took. In addition, brine interacts with minerals in the salt, such as clay. The fluorescent dye is a visible way to see where the liquid tracer actually was in the field. “

In the third phase, which began in mid-October, the team will drill a new series of nine heated wells, building on lessons learned from the previous phases of the experiments.

Working underground in difficult conditions

The team learned a lot from the first two phases of the experiment, including the best type of heater, when to drill the wells, and how corrosive the brine is, said Stauffer and Mills.

“The first two phases involved a lot of equipment tests; some failed and some were sent back to the manufacturer, ”said Mills. “We have also learned to keep backup equipment in stock because salt dust and brine destroy equipment. We have to double seal things because the brine can seep through insulated wires and then the equipment dies. It was a process to learn how to work in the salt environment. “

Kuhlmann agreed. “A lot of things can go wrong when you take delicate laboratory equipment and bring it to a salt mine. We went back and read the reports from the WIPP experiments in the 1980s. We want to learn from the past, but sometimes we had to make our own mistakes. “

The researchers are working with international partners to use the data from this project to improve computer models of the complex chemical, temperature, water and physical interactions that take place underground. This will improve future modeling of nuclear waste repositories worldwide.

Ultimately, the team wants to scale up to larger and longer experiments to get relevant data for future salt deposits, said Kuhlman and Stauffer. This data, which complements data already collected, would inform repository planners and political decision-makers about the safety of the permanent disposal of heat-generating nuclear waste in salt storage facilities.

“It was really fascinating and interesting for me to work on such a hands-on project,” said Mills. “It was really worth it to design and build the systems and go underground to WIPP. Research in an active mining environment can be challenging, but I take pride in working down there and putting our ideas into practice. “


Sandia National Laboratories is a multi-mission laboratory operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the National Nuclear Security Administration of the US Department of Energy. Sandia Labs has important research and development roles in the fields of nuclear deterrence, global security, defense, energy technologies, and economic competitiveness with principal locations in Albuquerque, New Mexico and Livermore, California.


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