High temperature thermo-chemical properties evolution of NaF-NiF2 system

Abstract

In Molten Salt Reactor (MSR), Ni based alloys are more suitable for handling of the molten fluoride salts because of formation of corrosive resistive NiF2 layer over the structural material components. A ternary mixture of LiF-NaF-KF (FLiNaK) can be used as coolant salt for transfer of heat from the reactor core. NiF2 layer of the Ni-based alloys shows reactivity towards NaF at reactor operating temperature. NaNiF3(s) is the single complex ternary compound in the NaF-NiF2 system. The compound has been synthesized and characterized. The thermodynamic stability of NaNiF3(s) has been experimentally measured in terms of Gibbs energy of formation using Galvanic cell technique and its measured ΔfG°m(T) was obtained as.ΔfG°m(T) [NaNiF3(s)]/kJ mol−1 = (−1242.9 ± 2.3) + (0.2312 ± 0.0039) × (T/K).The heat capcity and high temperature enthalpy increment was also experimentally measured using DSC and high temperature calorimetric technique, respectively and the values were obtained as.Cp,m(J·mol−1·K−1) = 117.74 + 0.0498 × T −1.07351 × 106/T2(303 ≤ T/K ≤ 828).(HT-H298)(J·mol−1) = −40270.6 + 120.4 × T + 0.0234 × T2+717669.7/T.Effect of oxygen impurity over the stability of the compound has been investigated by calculating the chemical potential diagram of Na-Ni-F-O system. The binary phase diagram of NaF-NiF2 system and the ternary phase diagram of Na-Ni-F2 system has been calculated based on Gibbs energy minimization technique using the experimental thermodynamic data.