The Next Generation Nuclear Plant (NGNP) will most likely produce electricity and process heat, with both being used for hydrogen production. To capture nuclear process heat, and transport it to a distant industrial facility requires a high temperature system of heat exchangers, pumps and/or compressors. The heat transfer system is particularly challenging not only due to the elevated temperatures (up to ~ 1300K) and industrial scale power transport (~ 50 MW), but also due to a potentially large separation distance between the nuclear and industrial plants (100+m) dictated by safety and licensing mandates.

In contrast to single-phased forced convective heat transfer via ‘pumping a fluid', a thermosyphon (also called a wickless heat pipe) transfers latent heat through the vaporization / condensing process. The condensate is further returned to the hot end by gravity, i.e. without any requirement of pumps or compressors. Since the latent heat of evaporation is large, a considerable amount of heat can be transported with a minimal temperature difference from end to end.

It is easy to demonstrate that a sodium-filled thermosyphon can transport comparable amounts of thermal energy as a single-phase convective sodium loop, within the same diameter pipe. Sodium thermosyphon and sodium forced convective loops can both deliver comparable rates of heat transfer through a given size pipe. This can be easily demonstrated by considering the ideal rate of convective heat transport through a pipe without losses; modeled in terms of enthalpy by:

= ?H ? V = ?H ( /A) (1)

where is the rate of thermal energy transport per unit flow area (kJ/m2 s), ?H is the change of the specific enthalpy of the transport fluid (kJ/kg), V is the average flow velocity (m/s), is the mass flow rate (kg/s), ? is density (kg/m3), and A is the cross-sectional flow area (m2).

Two-phase heat transfer by a thermosyphon has the advantage of high enthalpy transport that includes the sensible heat of the liquid, the latent heat of vaporization, and possible vapor superheat. In contrast, single-phase forced convection transports only the sensible heat of the fluid. Additionally, vapor-phase velocities within a thermosyphon can be much greater than single-phase liquid velocities within a forced convective loop.

Figure 1 Enthalpy for saturated sodium: liquid and vapor [2]

Figure 1 exemplifies the enthalpy enhancement in heat transfer afforded by a two-phase thermosyphon verses a single-phase convective loop with sodium as the working fluid. The specific enthalpy (?H) of saturated liquid and vapor, relative to the solid at 298.15K, are illustrated as a function of temperature [2]. Assuming heat transfer from a high-temperature gas cooled reactor at 1300K (1027oC) to an industrial facility; the maximum single-phase heat transfer is given by the enthalpy gain from points A to B in Figure 1, or approximately 1,200 kJ for each kilogram of sodium. Compare with two-phase heat transfer from points A to B to C where the enthalpy gain is approximately 4,933 kJ per kilogram with no vapor superheat; over 3˝ times more heat per kilogram of sodium than the single-phase. The saturation pressure of sodium at 1300K is only 0.326 MPa thus pressure forces are minimized. Vapor flow through a pipe is limited by compressible ‘choke flow' when the vapor reaches its sonic velocity. For sodium vapor the sonic velocity is approximately 750m/sec at 1300K [3].

The limiting heat transfer rate for an ideal sodium thermosyphon operating around 1300K can therefore be estimated by equation (1):

= ?H ? V = (4,933) kJ/kg (0.8)kg/m3(750)m/s = 2960 MW/m2

Similarly, single-phase liquid sodium could transport the same rate of thermal energy with an average flow velocity of about 3.5 m/s, well within the capabilities of advanced liquid metal pumps. This simple analysis for sodium as the working fluid theoretically illustrates that a thermosyphon and a forced convective loop can both deliver comparable rates of heat transfer through a comparable diameter pipe. An inclined thermosyphon, however, has the luxury of controllable heat transfer without the need for high temperature pumping and can deliver the heat at the same approximate temperature as the source.

Ideally, a 15 cm diameter pipe could transport ~ 50 MW of power at 1300 K to a hydrogen production facility some distance away by serving either as a sodium thermosyphon or as part of forced-convective loop. A larger diameter thermosyphon could reduce the vapor velocity below limiting critical choke velocity. Therefore, the two-phase heat delivered by a sodium thermosyphon is comparable to that delivered by a single-phase forced convection loop.

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