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October 2010

Radiation Tolerance? It's All in the Chemistry!

Materials studies could lead to tailored materials for immobilizing nuclear waste

Rad Radiation destroys the ordered atomic arrangement in Gd2Ti2O7 (left), while atomic order is mostly preserved in Gd2Zr2O7 (right).  These ceramics are candidate materials for immobilizing nuclear waste. Gd, Ti/Zr, and O are represented by green, grey, and red spheres, respectively. Enlarge Image

Results: In the quest for safe methods of storing nuclear waste, scientists at Pacific Northwest National Laboratory, Oak Ridge National Laboratory, and Curtin University, Australia are investigating radiation effects on candidate materials for immobilizing plutonium and other actinides. They used molecular dynamics simulations to compare radiation damage accumulation in two ceramics and determined three requirements for the ideal radiation-tolerant ceramic:

  1. It must have mechanisms to dissipate the energy of radiation without changing the ordered arrangement of ions so that there is minimal increase in volume.
  2. There must be empty spaces in this arrangement, so that some of the ions knocked out by radiation can find a place to get back in line.
  3. Some of the ions must be able to move fast to clean up any residual radiation damage.

Their results explained why gadolinium zirconate is much more radiation tolerant than gadolinium titanate.

Why It Matters: To meet the world's increasing energy needs and provide reliable "around the clock" electricity, nuclear energy is a strong contender, but reactor safety, nuclear non-proliferation, and radioactive waste management are important concerns. Ceramics that can survive in the extreme environment of radiation, elevated temperature, and stress without losing their crystal structure hold the promise of acting as a reliable first line of defense by locking up such toxic elements.

To evaluate ceramic waste form performance over the time scales needed to store nuclear waste, around 100,000 years, scientists need a thorough understanding of the fundamental physics at the atomic level. In this study, the scientists showed that subtle differences between the two ceramics they investigated led to drastically different structural and mechanical response as radiation damage accumulates.

"One of the neat things about this ceramic family is that you can manipulate its chemistry," says Dr. Ram Devanathan, a PNNL materials scientist and lead author of the paper. "You can create a material with a wide range of properties by changing the positively charged ions, creating maybe 100 different possibilities with different properties. The question is what ceramic is optimal for safely storing radioactive waste, and how does chemistry influence your decision?"

Methods: Two ceramics were studied with identical arrangements of ions. These were gadolinium titanate (Gd2Ti2O7) and gadolinium zirconate (Gd2Zr2O7), both pyrochlore ceramics.  When the ions were repeatedly knocked out of position to simulate radiation damage, the zirconate pyrochlore "healed" itself. The zirconate pyrochlore has a mechanism to shuffle the positive ions in a way that does not increase energy. Devanathan likens it to bowling.

"Imagine that the bowling ball shuffled the pins around on impact instead of knocking them down. That's what we see at the atomic scale: the ions are rearranged in almost the same ordered rows as before, so there must be some mechanisms that accommodate disorder that way without raising the energy." 

The oxygen ions in the zirconate can move around fast enough in their clean-up job to keep up with the rate of damage.

When the chemistry is changed by substituting titanium for zirconium, the rearrangement of ions raises the energy and makes the material swell.  The ordered arrangement collapses into a glass-like disordered state that is less effective in locking up radioactive elements.

The scientists used atomic-level computer simulations to study the change in structure and properties of pyrochlore in response to different mechanisms of damage buildup.

"In lab experiments, all mechanisms occur together, and we can't tell which is the most critical for changing the properties of the ceramic," Devanathan noted. "With computer simulations we can separate mechanisms, study them in isolation, and zero in on the most effective."

One thing the team learned was that there was a highly effective damage mechanism that involved knocking the positively charged ions out of position. This process was so energetic it could also spontaneously knock a few negatively charged ions out of position and shuffle other positively charged ions nearby. In the zirconate, this spontaneous generation of secondary defects in response to the primary defect is part of the self-healing process by which the radiation energy is dissipated and volume expansion is avoided.

What's Next: The scientists are looking at other ceramics that share the same characteristics as gadolinium zirconate to further test their hypothesis about radiation tolerance.

Acknowledgments: This research was supported by the U.S. Department of Energy Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Part of the research used the Molecular Science Computing Facility in EMSL (Environmental Molecular Sciences Laboratory), a national scientific user facility.

Reference: R Devanathan, WJ Weber, and JD Gale. 2010. "Radiation Tolerance of Ceramics - Insights from Atomistic Simulation of Damage Accumulation in Pyrochlores." Energy and Environment Science 3(10):1551-1559.

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