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The classification of steam as wet, dry, and superheated has its limitation. Consider the behavior of the system which is heated at a pressure that is higher than the critical pressure. In this case, there would be no change in phase from liquid to steam. In all states, there would be only one phase. Vaporization and condensation can occur only when the pressure is less than the critical pressure. The terms liquid and vapor tend to lose their significance.
At pressure that is higher than the critical pressure, water is in a special state, which is known as the supercritical fluid state. A supercritical fluid is a fluid that is at pressures higher than its thermodynamic critical values. At the critical and supercritical pressures, a fluid is considered a single-phase substance, although all thermophysical properties undergo significant changes within the critical and pseudocritical regions.
See also: Critical Point of Water
Pseudocritical line – pseudocritical points. Pseudocritical line consist of pseudocritical points, which are points at a pressure above the critical pressure and at a temperature (Tpc > Tcr) corresponding to the maximum value of the specific heat at this particular pressure.
See also: Supercritical Water Reactor.
The supercritical water reactor (SCWR) is a concept of Generation IV reactor that is operated at supercritical pressure (i.e., greater than 22.1 MPa). The term supercritical in this context refers to the thermodynamic critical point of water (TCR = 374 °C; pCR = 22.1 MPa) and must not be confused with the criticality of the reactor core, which describes changes in the neutron population in the reactor core.
Depending on the core design, the supercritical water reactor may be operated as a thermal reactor or as a fast-neutron reactor. The concept of the supercritical water reactor may be based on classical pressure vessels as in commercial PWRs or pressure tubes as in CANDU reactors. The pressure-vessel design of supercritical water reactors is developed largely in the EU, US, Japan, Korea, and China. In contrast, the pressure-channel design is developed largely in Canada and Russia. The pressure-vessel design allows using a traditional high-pressure circuit layout. The pressure-channel design allows the key features of passive accident and decay heat removal by radiation and convection from the distributed channels even with no active cooling and fuel melting. The use of multi-pass reactor flows makes reheating and superheating possible.
A once-through steam cycle has been envisaged for both pressure vessel and pressure-tube designs, omitting any coolant recirculation inside the reactor. It is similar to boiling water reactors, steam will be supplied directly to the steam turbine, and the feed water from the steam cycle will be supplied back to the core.
As well as the supercritical water reactor may use light water or heavy water as a neutron moderator. As can be seen, there are many SCWR designs, but all SCWRs have a key feature, which is the use of water beyond the thermodynamic critical point as primary coolant. Since this feature increases the peak temperature, supercritical water reactors are a promising advancement for nuclear power plants because of their high thermal efficiency (~45 % vs. ~33 % for current LWRs).
A supercritical fluid is a fluid that is at pressures higher than its thermodynamic critical values. At the critical and supercritical pressures, a fluid is considered a single-phase substance, although all thermophysical properties undergo significant changes within the critical and pseudocritical regions.
At pressures above the critical pressure, properties of water in the reactor change gradually and continuously from those we ordinarily associate with a liquid (high density, small compressibility) to those of a gas (low density, large compressibility) without a phase change. There is no change in the phase of water in the core. On the other hand, physical properties such as density, specific heat, specific enthalpy undergo significant changes, especially in the temperature range of the pseudocritical region (for 25 MPa between 372°C and 392°C). For example, in a typical supercritical water reactor:
- the density of supercritical water at the inlet and the outlet is about 777 kg/m3 (for 25MPa and 280°C) and 90 kg/m3 (for 25MPa and 500°C),
- the specific enthalpy of supercritical water at the inlet and the outlet is about 1230 kJ/kg (for 25MPa and 280°C) and 3165 kJ/kg (for 25MPa and 500°C)
The following figures show the behavior of water’s thermophysical properties near the critical (22.1MPa) and pseudocritical (25MPa) points. Near the critical point, these property changes are dramatic. In the vicinity of the pseudocritical point at 25 MPa, these property changes become less significant. At 25 MPa, the most significant property changes occur within ±25◦C around the pseudocritical point (389.4◦C). This region is known as the pseudocritical region. For convenience, below the pseudocritical point, fluid properties are considered to show liquid-like behavior, and above the pseudocritical point, they are considered to show gas-like behavior.