Featured Research

from universities, journals, and other organizations

Study could help improve nuclear waste repositories

Date:
September 19, 2013
Source:
Sandia National Laboratories
Summary:
Researchers are studying the movement of iodine-129 from spent nuclear fuel through a deep, clay-based geological repository. Understanding the process is crucial as countries worldwide consider underground clay formations for nuclear waste disposal.

Sandia researcher Yifeng Wang examines a clay sample from South Dakota as part of iodide experiments. A team of Sandia researchers is working to understand how fast iodine-129 released from spent nuclear fuel would move through a deep clay-based geological repository.
Credit: Randy Montoya

Here’s the question faced by a team of Sandia National Laboratories researchers: How fast will iodine-129 released from spent nuclear fuel move through a deep, clay-based geological repository?

Understanding that process is crucial as countries worldwide consider underground clay formations for nuclear waste disposal, because clay offers low permeability and high radionuclide retention. Even when a repository isn’t sited in clay, engineered barriers often include a compacted buffer of bentonite, a common type of clay, to improve waste isolation.

Iodine-129, a radioactive isotope with a half-life of 15.7 million years, is an important fission product in spent nuclear fuel and a major contributor to the predicted total radiation dose from a deep geological repository. So even a small improvement in the ability of clay to retain iodine-129 can make a difference in total dose predictions.

Some evidence indicates weak interaction between clay and iodide — a negatively charged predominant chemical species of iodine in geologic repositories, said researcher Yifeng Wang, who leads the study. Computer models haven’t been able to adequately explain clay’s chemical behavior with iodide, and the mechanism is difficult to study because the faint interaction is easily masked by measurement uncertainties.

“It seems there’s some kind of previously unrecognized mechanism that accounts for that kind of interaction,” said Wang, co-principal investigator for the Laboratory Directed Research and Development project to study radionuclide-clay interaction, now in its third and final year.

His team concluded the interaction, often disregarded as experimental noise, is real and that there might be engineering ways to improve clay’s ability to retain iodide.

Sandia team focuses on clay structure

The team — Wang and former co-principal investigator Andy Miller, who recently left Sandia; technician Hernesto Tellez; and year-round interns Jessica Kruichak and Melissa Mills — developed experiments with different clays, focusing on their structural characteristics. Past studies of iodide retention in clay concentrated on bentonite. Wang’s team instead studied several different clays, five with the same type of layered structure as bentonite.

Although industries are accustomed to using the plentiful and oft-studied bentonite, the team’s experiments show other clays have higher radionuclide retention capability and might isolate spent fuel waste better. Kaolinite had the best iodide retention of the five clays with layering properties. Wang said the team believes its work “can help us select a better clay material or combination of clay materials.”

Team members believe they discovered a mechanism for iodide-clay interactions that allows more accurate prediction of iodine-129 movement in a geologic repository. The finding was presented in May to the International High Level Radioactive Waste Management Conference in Albuquerque and was published in the conference proceeding.

The experimental data indicate iodide directly interacts with the tiny spaces between the layers of clay, called clay interlayer sites. That raises the question of how negatively charged iodide gets into those negatively charged interlayer sites, since like charges repel each other, similar to magnets of the same polarity. “So that contradicts the conventional concept,” Wang said.

The team got clues about what was going on by studying the problem at the nanoscale, 100,000 times smaller than the diameter of a human hair. At that scale, Wang said, the property of water changes in a way that enhances the pairing of ions.

Conclusion: ion pairing explains iodide reaction with clay

Ion pairing explains how iodide reacts with clay and moves into the pores despite the fact both iodide and clays are negatively charged.

The team postulates that iodide pairs with positively charged sodium to create a neutral ion pair. That occurs because of the enhanced ion association capability of water trapped in nanometer-scale clay interlayers, resulting in a pairing that helps iodide move into the interlayer by minimizing electric repulsion, Wang said.

Clay is densely compacted when it’s used as a barrier and can swell as it contacts with water. “That’s why people use clay materials and compact it,” Wang said. “It’s a good engineered barrier to isolate radionuclides.”

Retention properties increase with compaction, which makes the pores smaller, he said. “That’s another way to increase the effectiveness of clay materials,” he said.

But Sandia’s study also suggests measurements in labs could be more accurate. Usually researchers break up samples before they measure the solvency of a specific material. “We actually show the nano-pore confinement makes a big difference,” Wang said. “That means what you measure in the lab most of the time is not representative of an actual compacted material. The compacted material may in fact give you better retention.”


Story Source:

The above story is based on materials provided by Sandia National Laboratories. Note: Materials may be edited for content and length.


Cite This Page:

Sandia National Laboratories. "Study could help improve nuclear waste repositories." ScienceDaily. ScienceDaily, 19 September 2013. <www.sciencedaily.com/releases/2013/09/130919154255.htm>.
Sandia National Laboratories. (2013, September 19). Study could help improve nuclear waste repositories. ScienceDaily. Retrieved April 19, 2014 from www.sciencedaily.com/releases/2013/09/130919154255.htm
Sandia National Laboratories. "Study could help improve nuclear waste repositories." ScienceDaily. www.sciencedaily.com/releases/2013/09/130919154255.htm (accessed April 19, 2014).

Share This



More Earth & Climate News

Saturday, April 19, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Drought Concerns May Hurt Lake Tourism

Drought Concerns May Hurt Lake Tourism

AP (Apr. 18, 2014) Operators of recreational businesses on western reservoirs worry that ongoing drought concerns will keep boaters and other visitors from flocking to the popular summer attractions. (April 18) Video provided by AP
Powered by NewsLook.com
First Ever 'Female Penis' Discovered In Animal Kingdom

First Ever 'Female Penis' Discovered In Animal Kingdom

Newsy (Apr. 18, 2014) Not only are these newly discovered bugs' sex organs reversed, but they also mate for up to 70 hours. Video provided by Newsy
Powered by NewsLook.com
Ark. Man Finds 6-Carat Diamond At State Park

Ark. Man Finds 6-Carat Diamond At State Park

Newsy (Apr. 18, 2014) An Arkansas man has found a nearly 6.2-carat diamond, which he dubbed "The Limitless Diamond," at the Crater of Diamonds State Park. Video provided by Newsy
Powered by NewsLook.com
Deadly Avalanche Sweeps Slopes of Mount Everest

Deadly Avalanche Sweeps Slopes of Mount Everest

AP (Apr. 18, 2014) At least six Nepalese guides are dead after an avalanche swept the slopes of Mount Everest along a route used to climb the world's highest peak. (April 18) Video provided by AP
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins