ABSTRACT
The development of SFRs has been possible thanks to the attractive nuclear, physical, and even some chemical properties of sodium. Sodium is the most common element of alkali metals. Sodium has only one stable isotope: 23Na. Neutron flux leads to the formation of radioactive isotopes: 24Na (half-life is 14.98 h), inducing the necessity to wait for decay before some interventions on primary circuits, and 22Na (half-life is 2.6 years), to be taken into account during the decommissioning stage. This low activation is also a very attractive property of sodium for nuclear use in sodium fast reactors (SFRs). Sodium is in the liquid state at 98°C and the boiling point is 883°C. This wide range in the liquid state at atmospheric pressure is responsible for the high thermal inertia of the sodium system, a favorable safety feature. The density of sodium is always less than that of water. It has a value of around 850 kg/m3 at 400°C. The density of the liquid phase is higher than the solid phase (volume expansion is about 2.7% during solidification). Due to this characteristic, it is necessary to follow specific procedures for melting sodium in storage vessels or containers. The viscosity of sodium at 400°C (310 Pa·s) is of the same order as the viscosity of water at 100°C (280 Pa·s). Due to such similarity between density and viscosity of sodium and water, it is possible to carry out experimental studies with water to simulate hydraulics of sodium components. The thermal conductivity of sodium is very high: about 76.6 W/m/K at 573 K. Comparatively, the conductivity of water varies from 0.6 W/m/K at 20°C to 0.465 W/m/K at 350°C (at a pressure of 150 bars), whereas the conductivity of sodium is between 100 and 150 times higher at atmospheric pressure. In view of its high boiling point, sodium is considered as not very volatile. The fact that sodium is not very volatile has several consequences: in normal operation, evaporation rapidly attains an equilibrium level (condensation = vaporization). Consequently, in various gas plenums, and particularly in the main vessel, the mass transfer toward the colder roof of the slab is rather limited, particularly in the presence of argon (due to its low thermal conductivity). Nevertheless, the SFR operational feedback shows that sodium aerosols are deposited in the upper structures or narrow gaps. Hence, it is necessary to take care of the vapor and aerosol traps in the cover gas to prevent any related issues. Due to this low volatility, the sodium flames are very short and the heat produced by the fire is rather low: thus, it is possible to extinguish the fire by spreading a powder mixture of Na carbonate, Li carbonate, and graphite. Sodium, like all metals, has a very low electrical resistivity. These attractive conduction properties are widely used in sodium technology: instrumentation, level probes, flow measurements, electromagnetic pumps, and leak detection. The speed of sound in sodium varies little with temperature. Sound waves therefore propagate very well through sodium. This property is widely used in all metrology and visualization techniques in sodium and compensates the opacity in sodium for in-service inspection operations. The influence of temperature on sonic velocity is high. Using this, the temperature could be also deduced from the sound velocity in sodium. Since sodium has a tendency to lose its external electron, it will have very significant reducing characteristics, as the case with all alkali metals. It reacts exothermically with water, potentially with violence, as a function of local conditions. This reaction with water producing sodium hydroxide and hydrogen gas is strongly exothermic (162 kJ/mol of Na) and extremely fast. For these reasons, sodium-water reaction (SWR) that can occur in steam generator (SG) units is considered as an important safety issue and several measures are developed and implemented to mitigate this event. Nevertheless, this reaction with water is favorably used for the development of cleaning processes for structural material wetted with sodium during handling operations and, moreover, for the conversion of large
amounts of sodium into sodium hydroxide at the end of the reactor operation, during the decommissioning phase. Solid sodium quickly oxidizes in air and liquid sodium burns in air over melting point, that is, 98°C, if it is spread out in air and over 140°C in other cases; it forms sodium peroxide Na2O2 or, with limited oxygen, the oxide Na2O.
