Chris Stanek

  • ACCELERATED CHEMICAL AGING OF NUCLEAR WASTE FORMS

    C.R. Stanek

    Los Alamos National Laboratory, Los Alamos, NM USA

    Selection criteria for nuclear waste form compositions have predominantly focused on that composition’s radiation tolerance and leaching resistance. However, the effect of transmutation of radionuclides, especially “short-lived” 90Sr and 137Cs, to chemically distinct daughter products (Zr and Ba respectively) will also have a significant impact on nuclear waste form stability. Due to the technical challenges associated with this studying problem, the topic of transmutation has received limited attention during the past 30 years of waste form development. In order to develop a predictive capability to design radiation tolerant and chemically robust nuclear waste forms, we must first address a fundament materials science question: What is the impact of daughter product formation on the stability of solids comprised of radioactive isotopes? To answer this question, a multidisciplinary approach integrating density functional theory calculations with the synthesis and characterization of small, highly radioactive surrogate samples has been developed to accelerate the chemical aging process [1].
    The chemical evolution that occurs during the lifetime of a waste form can be simulated by performing DFT calculations to assess phase stability as a function of composition – and therefore time. For example, we predicted that rocksalt BaCl may form via the decay of 137Cs in CsCl where all of the cesium atoms are the radioactive isotope 137Cs, an important, short-lived fission product (half life of 30 years and 137Cs undergoes β-decay to 137Ba) [2]. We termed the phenomenon of the formation of metastable crystalline daughter phases via the transmutation of a radionuclide in the parent phase radioparagenesis. That our first principles calculations predicted the formation of 137BaCl from the decay of 137Cs suggests that in situ daughter product formation may lead to non-intuitive defect structures or phases. That is, based on ionic bonding theory, one would expect the formation of rocksalt BaCl2 upon Cs decay, rather than BaCl, since Ba is a rigidly 2+ cation. This unusual BaCl phase has never been observed, perhaps because it has never been synthesized in this manner.
    In this presentation, details of the accelerated chemical aging approach are discussed as well as recent results for a range of materials systems, including: 109Cd1-xAgxS, 55Fe2-xMnxO3 and 177Lu2-xHfxO3. In addition, implications of in situ transmutation are also discussed, including unconventional defect chemistry, backward design of nuclear waste forms, exploration of novel materials and even the role of transmutation on DNA stability [3].

    [1] C.R. Stanek, B.P. Uberuaga, B.L. Scott, R.K. Feller and N.A. Marks, “Accelerated Chemical Aging of Crystalline Nuclear Waste Forms,” Current Opinion of Solid State and Materials Science, 16, 126 (2012).
    [2] C. Jiang, C.R. Stanek, N.A. Marks, K.E. Sickafus and B.P. Uberuaga, “Predicting from First Principles the Chemical Evolution of Crystalline Compounds Due to Radioactive Decay: The Case of the Transformation from CsCl to BaCl,” Physical Review B, 79, 132110 (2009).
    [3] M. Sassi, D.J. Carter, B.P. Uberuaga, C.R. Stanek and N.A. Marks, “Carbon-14 decay as a source of non-canonical bases in DNA,” Biochim. Biophys. Acta, 1840, 526 (2014).