Characteristics of SCWRs
SCWRs operate 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,
- 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)
From a neutronics point of view, the density of water is the most important factor. There is a significant change of neutron spectrum, which changes with the axial coordinate of the core. It is caused by the greater moderation in places with greater density. For this reason, a coupled neutronics – thermal hydraulics calculation is inevitable to obtain neutron flux distribution within the core. In each nuclear reactor, there is a direct proportionality between the neutron flux and the reactor thermal power.
Since the coolant (moderator) undergo significant change in density, the SCWR can also be designed as a fast neutron reactor. This property depends on certain reactor design.