GEMS specific HERACLES v.0.2. database for U, TRU and FP speciation

The HERACLES-TDB has been compiled to support modeling of U and fission products (FP) solid and gaseous speciation during pyroreprocessing of spent nuclear fuel. The database covers molar thermodynamic properties of compounds of actinides, fission products, and minor actinides covering the elements as shown above. At present, the data for over 610 condensed compounds (including melts and liquid condensates and over 360 gaseous (also charged) species are provided. The gas phase is treated as an ideal mixture of ideal gases; for melts the non-ideality of mixing is taken into account using models like Redlich-Kister, NRTL, UNIQAC, Bregg-Williams, Compond Energy Formalism, etc. In total GEMS has around 30 Mixing Models implemented.

For each compound or gas, the stoichiometry formula is provided together with the molar enthalpy (of formation from elements at their standard states), absolute entropy and heat capacity at standard state (T_r = 298.15 K and P_r = 1 bar). In most cases, the molar Gibbs energy of formation at T_r was calculated from ΔH⁰(T_r) and S⁰(T_r) and entropies of elements at standard state. Necessary auxiliary thermodynamic data were taken from (Cox at al., 1989).

Values of standard molar Gibbs energy function ΔG⁰(T_r) of a compound, needed for calculation of equilibria at elevated temepratures, can be calculated from the following equation [Karpov et al., 1989]:

$g_T^0 &=& G_{T_r}^0 - S_{T_r}^0 * (T-T_r)-T \sum_{i} Mn_i a_i$

where G⁰(T_r) and S⁰(T_r) are the standard molar Gibbs energy and absolute entropy at T_r.

The terms Mn_i are given by equation:

$Mn_i &=& \dfrac{T^{n_i}}{n_i (n_i + 1)} + \dfrac{T_r^{n_i + 1}}{T (n_i + 1)} - \dfrac{T_r^{n_i}}{n_i}$

where ni are power coefficients: n₀ = 0; n₁ = 1; n₂ = -2; n₃ = -0.5; n₄ = 2; n₅ = 3; n₆ = 4; n₇ = -3; n₈ = -1; n₉ = 0.5.

Empirical coefficients a₀ - a_i refer to the polinomial heat capacity function on temperature. In most of the cases i=5 corresponding to the Haas-Fischer equation (see eq. below). The data on Cp⁰= f (T) are provided for each compound in the database in the temperature range of 298 K – 3000 K. For gaseous species, methods of statistical thermodynamics were applied, hence the temperature interval considered is much broader and in some cases reaches 6000 K.

$Cp(T) &=& a_0 + \sum_{i} a_i T^{n_i}$