Dependence of the extraction capacity of neutral bidentate organophosphorus compounds on their structure: a quantum chemical study

Abstract

Correlations between the extraction capacities and molecular structures of organophos phorus compounds (reagents for extraction of transplutonium elements from spent nuclear fuel) were studied using a quantum chemical approach. The results of calculations are in qualitative agreement with experimental data. The approach proposed can be used for analysis of the extraction properties of all classes of organophosphorus compounds and also for predic tion of the most efficient organophosphorus extragents with preset properties.Processing of spent nuclear fuel (SNF) in order to extract uranium and plutonium followed by recovery into fresh nuclear fuel is an important step in a closed fuel cycle. At present, the SNF processing technology used in the Russian Federation is based on the PUREX process, 1 which was initially elaborated to provide plutonium ex traction for military purposes from irradiated uranium lumps. Large scale PUREX processing of SNF for nuclear power engineering, especially at high nuclear fuel burnup, leads to accumulation of large amounts of liquid high level waste (HLW). 2 Therefore, in connection with an increase in the amount of processed SNF and taking into account toughening of requirements for handling of HLW, in the Russian Federation it is planned to fractionate liquid HLW and extract the Cs, Sr, and long lived ac tinide fractions in order to provide separate solidification and subsequent long term storage of the corresponding products. Among methods for HLW fractionation, the leading position is occupied by the extraction techniques involving organophosphorus compounds (OPCs) as the most promising extragents. 3-6The search for highly efficient OPCs is an urgent sci entific and practical problem. In particular, it includes establishment of the dependence of the extraction capac ity (EC) of OPCs on their structure as one of the main goals. To this end, the extraction constants of various elements are often correlated with certain characteristics of the reactivity of extragents, 7,8 e.g., the atomic group electronegativities, 9 the Taft constants σ* (see Ref. 10) and the Kabachnik constants σ P (see Refs 11 and 12, in both cases assuming additive substituent effects), the elec tron density (charge) on the functional atom, the ioniza tion potential, 13 the core s level energies, 14 as well as with the basicity established from the energies of test reactions with the simplest electron acceptors 10,15-18 or from quan tum chemical calculations of the proton affinity energies or the energies of complexation with Li + and LiH (see Refs 14 and 19).Neutral OPCs (NOPCs) is the first class of compounds for which the "EC-structure" correlations were studied systematically. Phosphates, phosphonates, phosphinates, and phosphine oxides have the same reaction center (phorsphoryl group) and form complexes with identical composition (actinide : NOPC ratio) while their ECs sig nificantly increase in this order. A correlation equation was proposed, which relates the parameters lnK ex (K ex is the effective extr...