Supercritical Fluid – Supercritical Water

Phase diagram of water

Phase diagram of water.
Source: wikipedia.org CC BY-SA

The classification of steam on wet, dry and superheated has its limitation. Consider the behavior of the system which is heated at the pressure, that is higher than the critical pressure. In this case, there would be no change in phase from liquid to steam. At 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 special state, that is known as 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 as a single-phase substance in spite of the fact that all thermophysical properties undergo significant changes within the critical and pseudocritical regions.

See also: Critical Point of Water

Supercritical “steam” is actually supercritical water, because at supercritical pressures fluid is considered as a single-phase substance. However, this term is widely (and incorrectly) used in the literature in relation to supercritical “steam” generators and turbines.
Pseudocritical line - Pseudocritical pointsPseudocritical 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.
Characteristics of SCWRs

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, that describes changes in the neutron population in the reactor core.

The supercritical water reactor may be operated as a thermal reactor or as a fast-neutron reactor, depending on the core design. The concept of the supercritical water reactor may be based on classical pressure vessel as in commercial PWRs or on 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, while the pressure-channel design is developed largely in Canada and in 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 and use of multi-pass reactor flows making reheating and superheating possible.

For both pressure vessel and pressure-tube designs, a once through steam cycle has been envisaged, omitting any coolant recirculation inside the reactor. It is similar as in 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 neutron moderator. As can be seen, there are many SCWR designs, but all SCWRs have a key feature, that is the use of water beyond the thermodynamic critical point as primary coolant. Since this feature allows to increase the peak temperature, the supercritical water reactors are considered a promising advancement for nuclear power plants because of its high thermal efficiency (~45 % vs. ~33 % for current LWRs).

Properties of Supercritical WaterA 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 as a single-phase substance in spite of the fact that 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 at 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 at the outlet is about 1230 kJ/kg (for 25MPa and 280°C) and 3165 kJ/kg (for 25MPa and 500°C)

supercritical-fluid-specific-heat-conductivityFollowing figures shows the behaviour of thermophysical properties of water 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 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 behaviour and above the pseudocritical point they are considered to show gas-like behaviour.

IFRAME to nist.gov – Steam Tables – Compressed Water and Superheated Steam

Reactor Physics and Thermal Hydraulics:

  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
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  9. U.S. Department of Energy, THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW. DOE Fundamentals Handbook, Volume 1, 2 and 3. June 1992.

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