One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of Sr-90 insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its beta-decay on the cement paste structure. We show that Sr-90 is stable when it substitutes the Ca2+ ions in C-S-H, and so is its daughter nucleus Y-90 after beta-decay. Interestingly, Zr-90, daughter of Y-90 and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for Sr-90 storage.

VL - 49 UR - http://pubs.acs.org/doi/10.1021/acs.est.5b02609 IS - 22 JO - Environ. Sci. Technol. ER - TY - JOUR T1 - Cement As a Waste Form for Nuclear Fission Products: The Case of 90 Sr and Its Daughters JF - Environ Sci Technol Y1 - 2015 A1 - Dezerald, Lucile A1 - Kohanoff, Jorge J A1 - Alfredo A. Correa A1 - Caro, Alfredo A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Andres Saùl AB -One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of (90)Sr insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its β-decay on the cement paste structure. We show that (90)Sr is stable when it substitutes the Ca(2+) ions in C-S-H, and so is its daughter nucleus (90)Y after β-decay. Interestingly, (90)Zr, daughter of (90)Y and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for (90)Sr storage.

VL - 49 IS - 22 ER - TY - JOUR T1 - Multiphase equation of state for carbon addressing high pressures and temperatures JF - Phys Rev. B Y1 - 2014 A1 - L. X. Benedict A1 - K. P. Driver A1 - S. Hamel A1 - B. Militzer A1 - T. Qi A1 - Alfredo A. Correa A1 - Andres Saùl A1 - E. Schwegler AB -

We present a 5-phase equation of state for elemental carbon which addresses a wide range of density and temperature conditions: 3g/cc<ρ<20g/cc,0K<T<∞. The phases considered are diamond, BC8, simple cubic, simple hexagonal, and the liquid/plasma state. The solid phase free energies are constrained by density functional theory (DFT) calculations. Vibrational contributions to the free energy of each solid phase are treated within the quasiharmonic framework. The liquid free energy model is constrained by fitting to a combination of DFT molecular dynamics performed over the range 10000K<T<100000K, and path integral quantum Monte Carlo calculations for T>100000K (both for ρ between 3 and 12 g/cc, with select higher- ρ DFT calculations as well). The liquid free energy model includes an atom-in-jellium approach to account for the effects of ionization due to temperature and pressure in the plasma state, and an ion-thermal model which includes the approach to the ideal gas limit. The precise manner in which the ideal gas limit is reached is greatly constrained by both the highest-temperature DFT data and the path integral data, forcing us to discard an ion-thermal model we had used previously in favor of a new one. Predictions are made for the principal Hugoniot and the room-temperature isotherm, and comparisons are made to recent experimental results.

VL - 89 ER -