Currently, there are six Generation IV reactor systems under development worldwide: 1) Very-High-Temperature Reactor (VHTR); 2) Sodium-cooled Fast Reactor (SFR); 3) SuperCritical Water-cooled Reactor (SCWR), 4) Gas-cooled Fast Reactor (GFR), 5) Lead-cooled Fast Reactor (LFR); and 6) Molten Salt Reactor (MSR). Of these six systems, Canada has decided to pursue the SCWR as its choice for a Generation IV reactor, with some research being conducted on the VHTR. One main objective of SCWRs is to increase the thermal efficiency of current nuclear power plants from the 30–35% range to approximately 45–50%. In order to accomplish this, SCWRs are being designed to operate well above the critical point of water at pressures of 25 MPa and reactor outlet temperatures up to 625°C. These operating conditions also make the SCWR, along with the VHTR and other Generation IV systems, suitable candidates to support thermochemical hydrogen cogeneration.

The design and operation of a facility capable of accurately and safely conducting experiments in supercritical water is a very expensive task. In order to facilitate our understanding of supercritical heat-transfer phenomena, modeling fluids such as carbon dioxide, refrigerants, ammonia and helium can be used to complement our knowledge of supercritical fluids. Some of these fluids, namely helium and carbon dioxide, have also been considered as potential working fluids in some special designs of reactors and power cycles.

The objective of this paper is to investigate the feasibility of using alternative working fluids such as helium and Refrigerant-134a (R-134a) by comparing the fluid and transport properties with those of water. Operating conditions of SCWRs are scaled into those of the modeling fluid, R-134a, in order to provide proper SCWR-equivalent conditions. The equivalent properties for helium, which is one possible coolant for the VHTR, are also discussed. The thermophysical properties for selected working fluids are obtained from NIST REFPROP software. The results indicate that the thermophysical properties of the fluids undergo significant changes within the critical and pseudocritical regions similar to that of supercritical water. A sensitivity analysis for the effect of temperature on selected thermophysical properties at various supercritical pressures was performed.

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