MFN Scholar: Emily Siska

Malcolm F. Nicol Graduate Scholar Research Activities 2014-2015

Novel Development of Waste Forms for the Long Term Storage of Nuclear Waste. Insertion of Radionuclides into Silica Sodalite.

United States nuclear power policy has an open-cycle approach which produces ~27 tons of waste/year/reactor. Although glass and ceramic waste forms have proven to be durable and sufficient at immobilizing a wide range of radionuclides; there is still a need to explore alternative ones. Radioisotopes including 129I, 85Kr, 135Xe, and 99Tc have extremely long half-lives and need to be immobilized in wasteforms that will be stable over geological time periods. This project aims to explore novel waste forms to immobilize radionuclides such as these.

Zeolites are naturally occurring and synthetic aluminosilicate minerals. They are composed of naturally abundant, non-toxic materials. From observation of their natural analogs they have shown to be stable in environmental conditions over geological time periods and accommodate guest species, some of which are larger than their pore windows, naturally. Generally, zeolites have a crystalline structure made of a rigid framework of pores connected by channels. However, under certain conditions the framework can become flexible. This flexibility is the mechanism that allows for the diffusion of larger guest atoms/molecules. Molecular dynamics simulations show that lattice vibrations brought on by pressure and temperature can make the structure flexible enough to allow for the diffusion of large molecules.

The starting point of the project was the synthesis of a siliceous zeolite – sodalite. As a pure SiO2 mineral it is non-reactive, chemically stable material with the simplest composition. Synthesis involves a hydrothermal process of a mixture of amorphous silica, and an aqueous structure directing agent. The structure-directing agent can be pyrolyzed in air, leaving an empty framework ready to accommodate guest species.

Siska pic 1

Internal resistive heater testing

We aim to immobilize radionuclides by insertion into a zeolite powder. Encapsulation is accomplished by the application of high pressure and temperature to the host lattice and guest species. This not only induces flexibility in the lattice, but facilitates the diffusion of the guest species through the lattice. High pressure is generated with a diamond anvil cell (DAC) while high temperature is produced by internal and external resistive heaters. With the help of HPCAT scientists we have designed and fabricated both internal and external resistive heaters that are used simultaneously. Experimentally we can determine optimal pressure and temperature regimes at which certain radionuclides will diffuse into and through the sodalite cages.

Siska pic 2

Block heater and ruby system during experiment at Sector 16, APS

Diffraction data collected at APS, HPCAT is used to monitor changes in cell parameters; which indicate whether the intended guest molecules have been inserted into the zeolite.

Siska pic 3

Online heating experiment at Sector 16-IDB, APS. Left: Jesse Smith. Middle: Barbara Lavina. Right: Curtis Kenney-Benson

HiPSEC and HPCAT have been great facilitators for this research project and continue to be an invaluable source. Under the Malcolm Nicol fellowship, I have conducted experiments in house and at APS, HPCAT. The compression of sodalite with empty cages was performed using a pressure transmitting media (PTM) that was too large to go into the cages (even under pressure). This will help us determine the workable range of the material and how it behaves under pressure. We also pressurized the sample with a penetrating PTM and heated it for an extended period of time. Sodalite with filled cages should exhibit a different compression behavior than empty sodalite. The zeolite insertion will be monitored by studying its differences in compression rate.

Experiments:

  • June 2014: APS 16-IDB: SOD in fluorinert and insertion of Ar in SOD at high PT
  • December 2014: ALS: SOD in Fluorinert
  • March 2015: APS 16-IDB: SOD in Ne
  • July 2015: APS 16-IDB: SOD_SC in Fluorinert and SOD_SC in Ar

Workshops and Professional Schools attended while on the fellowship:

  • HPCAT workshop on high-pressure time-resolved synchrotron techniques, Argonne National Laboratory, 2014
  • School for Synchrotron Crystallography: Introduction and Experimental Methods, ChemMatCARS, The University of Chicago, Argonne National Laboratory, 2014
  • 2015 IUCr High-Pressure Workshop, Brazilian Synchrotron Light Laboratory, Campinas Brazil, September 12 – 15 2015