Thermodynamic Properties for Rare Earths and Americium in Pyropartitioning Process Solvents

Abstract

A pyrochemical process is being developed to recover the longlived alpha-decaying radioactive nuclides from PUREX (plutonium uranium extraction), high-level wastes generated by the reprocessing of spent nuclear fuel. 1 These aqueous wastes contain fission products (noble metals, rare earths, transition metals, rubidium, strontium, cesium, barium, cadmium, tin, selenium) and actinides, the former being predominately short-lived, the latter containing many long-lived radionuclides. Removal of these alpha-decaying actinides from the residues reduces the long-term radiotoxicity and simplifies waste management. 2,3 Molten LiCl-KCl eutectic salt, cadmium and bismuth are used as solvents in the TRUMP-S (transuranic management by pyropartitioning separation) process. Process goals are to remove 99% of the actinides (U, Np, Pu, Am, and Cm) from chlorinated PUREX residues and to recover a product that is greater than 90% actinides by weight. The process separates the waste 4 into four fractions: actinides, rare earths, metals more noble than actinides, and metals more active than the rare earths. Separation of actinides and rare earths as a group from noble and active metals is accomplished by reductive extraction from the chlorinated high-level waste (HLW) residues into liquid Cd. The actinides are separated from the rare earths by electrorefining and countercurrent reductive extraction into liquid bismuth. A full set of consistent thermodynamic property measurements for the actinides and rare earths in the process solvents is required to predict actinide/rare earth separation. Data for the major trivalent actinides were reported previously. 5 Thermodynamic data obtained from measurements for the trivalent rare earths and americium are reported in this paper.Electrochemical potentials were measured and used to calculate the standard Gibbs free energy of formation of the metal chlorides in LiCl-KCl eutectic. Potential measurements at the LiCl-KCl/Cd and LiCl-KCl/Bi interfaces were made to calculate the activity coefficients for Am in the liquid Cd and Bi phases. These thermodynamic properties provide a basis for predicting composition in the salt and metal phases and of the separation products from electrorefining and extraction processes.Equilibrium potential measurements in LiCl-KCl eutectic have been reported previously for the actinides 5 and rare earths. 6 The previously reported rare earth data did not show the same degree of consistency with theory (i.e., calculated ion valence, standard potential temperature dependence) as for the reported actinide results.New data for the rare earths and Am are reported here. Improvements to the electrochemical cell design and experimental procedures were implemented for the experiments reported. For the rare earth chlorides, rare earth metal was deposited onto a Ta cathode to ensure a reliable electrical contact to the rare earth metal in equilibrium with the salt. A thoroughly characterized AgCl-LiCl-KCl salt was used in the reference electrodes. Relatively large amounts of Am a...

Determination of the activity coefficient of Am in liquid Al by electrochemical methods

“…Peak 2c (E A p $ À2.12 V vs. AgCl/Ag) associated with the anodic peak 2a (E A p $À1.97 V vs. AgCl/Ag) is typical of an exchange involving an insoluble product that is attributed to the Am(II)/Am(0) system. These results are consistent with those reported by other authors on Am behaviour in chloride media [17,18].…”

“…In fact, the lack of data on the Am activity coefficient in most of the solvents makes the comparison difficult. To our knowledge, only activity coefficients of Am in Cd and Bi at 450°C and 500°C have been published in the literature [17,21]. This study reports an estimation of the activity coefficient of Am in Al(l) at 700°C.…”

Electrochemical behaviour of curium(III) ions in fused 3LiCl–2KCl eutectic

“…Pyrochemical reprocessing methods are based on a good knowledge of the basic chemical and electrochemical properties of actinides [5][6][7][8][9][10][11][12][13][14] and fission products [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. This information is necessary for creating the effective technological process.…”

Study of thermodynamic properties of Np–Al alloys in molten LiCl–KCl eutectic

“…(17) and (18)), the variation of E 0 (Np 3+ /NpAl n ) was determined vs. the temperature and the values were recalculated vs. the used AgCl/Ag reference electrode according to the procedure described in [30].…”