Abstract
This chapter is mainly focused on illustrating some introductory progress on thermal hydraulic issues of supercritical water, including heat transfer characteristics, pressure loss characteristics, flow stability issues, and numerical method. These works are mainly to give a basic idea of elementary but important topics in this area. An analytical method was proposed up to predict the heat transfer coefficient and friction coefficient based on the two-layer wall function. Flow instability experiments have been carried out in a two-parallel-channel system with supercritical water, aiming to provide an up-to-date knowledge of supercritical flow instability phenomena and initial validation data for numerical analysis. The natural circulation instability of supercritical water was also investigated in the experiments.
TopIntroduction
The Supercritical Water-Cooled Reactor (SCWR) is a high temperature, high pressure water cooled reactor that operates above the thermodynamic critical point (374°C, 22.1 MPa) of water. The reactor outlet temperature can be as high as 500°C which can increase the thermal efficiency up to 40% or even more. Supercritical water is a single phase fluid without two-phase interfaces, leading to simplification of the primary system, no need for steam generator, pressurizer and main circulation pump of the PWRs, and no need for the inner circulation pumps, steam separator and dryer of BWRs. As for the safety advantages, due to the characteristics of the supercritical water, no phase change would occur in the SCWR core under nominal conditions, therefore there is no DNB risks like in the PWRs (See 2015 GIF Annual Report).
Gaps exist in applying supercritical fluid into nuclear reactors. Among them, the heat transfer, pressure loss and flow stability are some typical and key thermal hydraulic issues to be solved. The significant changes in the thermal physical properties may have big influence on the flow resistance characteristics. When the wall temperature is greater than the pseudocritical point and bulk fluid temperature is below, large variation of fluid property will happen across the boundary layer. In such situation, the wall heat transfer coefficient will be dependent on the heat flux. As the increase of heat flux, the wall heat transfer coefficient is decreased. The friction coefficient will not depend solely on the bulk Reynolds number, but also bonds tightly with the physical property of the boundary layer. Experiments of supercritical water needed to be performed and data needed to be collected and compiled to develop the suitable empirical or semi-empirical correlations. Meanwhile, the theoretical analysis is relatively few, attributed to the fact of the difficulty in dealing with the complex variation of the physical properties.
The large density difference between the core inlet and outlet raises doubts about the possible occurrences of flow instabilities similar to those observed in BWR. As the licensing of SCWR will probably require, at a minimum, demonstration of the ability to predict the onset of instabilities, it is necessary to understand the instability phenomena in SCWR, to identify the important variables affecting these phenomena, and ultimately to generate the maps identifying the stable operating conditions of different SCWR designs. The large density difference before and after the pseudocritical point of supercritical water also leads to good natural circulation capability which could be used in the passive core residual heat removal system. Still, the natural circulation could see the possibility of system flow instability under some circumstances which need attention in designing relative safety system.
In this chapter, four sections would be organized to illustrate some introductory progress on thermal hydraulic issues of supercritical water. These sections are:
Section I: The heat transfer characteristics of supercritical water
Section II: The pressure loss characteristics of supercritical water
Section III: The flow stability issues of supercritical water in parallel channels
Section IV: The natural circulation flow stability issues of supercritical water
TopBackground
The Supercritical Water-Cooled Reactor (SCWR) nuclear energy system is a combination of the advanced nuclear reactor technology and the updated supercritical fossil boiler technology. Compared with the current water-cooled reactor NPPs whose average plant efficiency is only around 30% due to use of obsolete subcritical turbine-generator system, the SCWR has significant economical and technical advantages. The Generation IV International Forum selected SCWR as one of the six most promising Generation IV nuclear energy systems in 2002 after technical investigation and comparisons among more than 100 initiative reactor types for several years.
Key Terms in this Chapter
Near-Critical Point: The narrow region around the pseudocritical point where all the thermo physical properties of a pure fluid exhibit rapid variations.
Critical Point: The point where the distinction between the liquid and gas phases disappears, i.e., both phases has the same temperature, pressure, and volume. The critical point could also be called as critical state.
Supercritical Fluid: Normally, the fluid with its pressures and temperatures higher than the critical pressure and critical temperature. However, in this chapter, the supercritical fluid includes both the temperature below and above the critical temperature as long as the pressure is above the critical pressure.
Deteriorated Heat Transfer: The heat transfer performance characterized with lower values of the wall heat transfer coefficient compared to those at the normal heat transfer.
Pseudocritical Point: The point at a pressure above the critical pressure and at a temperature where the specific heat capacity reaches its maximum value at this particular pressure.
Wall Function: A kind of theory based on the wall boundary method to describe the near wall velocity and temperature profile.
Flow Instability: A kind of dynamic phenomena with its flow fluctuates regularly or irregularly, leading to other parameters fluctuate accordingly and could bring about damage to the system or equipment.