| AVS 58th Annual International Symposium and Exhibition | |
| Actinides and Rare Earths Focus Topic | Thursday Sessions |
| Session AC+SS-ThM |
| Session: | The Surface Science of Actinides and Rare Earths |
| Presenter: | Wigbert Siekhaus, Lawrence Livermore National Laboratory |
| Authors: | M.A. Schildbach, Lawrence Livermore National Laboratory W.J. Siekhaus, Lawrence Livermore National Laboratory |
| Correspondent: | Click to Email |
Preferential uranium hydriding occurs frequently in narrow gaps. There are different hypotheses about its causes, one of which could be radiation-induced chemistry in gaps. Both 238U and 235U generate ionizing α, β, and γ radiation capable of vibrationally or translationally exciting, ionizing, or dissociating H2, all of which increase the reactivity of H2 with uranium. Dissociation of H2 is necessary to initiate hydriding, and it has been shown that the sticking coefficient of H atom is about 1200 times higher than H2’s[1]. Here we use the MCNPX radiation transport code to calculate the energy dependent electron flux generated from the 234mPa β- decay and from photoelectrons generated by brems-strahlung. We apply the code to gaps occurring in two 238U cylindrical pieces welded together and filled on the inside with LiD with a 100µm gap between 238U and LiD, and having a 100µm gap in 238U itself, typical for step-joint-welded uranium shells.
The MCNPX Monte Carlo Code - as configured now - tracks the life cycle of electrons throughout the material and calculates the electron flux as a function of energy, putting results into “energy bins” 1 keV wide. We find that at 2 keV (±.5 keV, the last energy bin) the calculated electron flux in the U-U gap is approximately 19 times larger than in the U-LiD gap, and fifty two times larger than in the U-air gap on the outside of the cylindrical shells. Cross-sections for electron-hydrogen collisions peak, however, below 1 keV energy. We establish the upper limit of the effect of electron-hydrogen collisions by extrapolating the MCNPX electron flux results from the last bin to energies as low as 1 eV by fitting a function to the flux between 2 and 20 keV. To calculate the fraction of H2 vibrationally or translationally excited, ionized, or dissociated per cm2/s , we integrate the product of the energy dependent cross sections (listed in reference [2]) and the energy dependent electron flux over the relevant energy range. The fraction of H2 molecules calculated to be dissociated is small, but significant during long-time exposure. Future work will extend the MCNPX code below 1keV (as is done for biological radiation damage), to avoid energy extrapolation.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
References
1. Balooch M, Hamza AV. “Hydrogen and water vapor adsorption on and reaction with uranium.” Journal of Nuclear Materials, 230, 3, 259-270, 1996.
2. Yoon JS et al., “Cross sections for electron collisions with hydrogen molecules.” Journal of Physical and Chemical Reference Data, 37, 2, 913-931, 2008.