In the primary loop of most pressurized water reactors (PWRs), the coolant does not boil except near the top of the core where a small amount of nucleate boiling is sometimes allowed. Under these conditions, the thermal-hydraulic behavior of the core can be described by the equations of single-phase flow, which were introduced in Chapter 15. In a reactor core, single-phase flow is defined as fluid flow where no phase change occurs. Single-phase fluids include both liquids and gases, but the entire fluid must be in the same physical state, and separate phases (such as water and steam) are not allowed to coexist at the same time. When the density of a fluid is independent of the surrounding pressure, the fluid is said to be an incompressible fluid and the flow of the fluid is said to be an incompressible flow. Liquid metals, which are used in liquid metal fast breeder reactors (LMFBRs), have almost no real compressibility, and for all practical purposes, the density of these metals can be considered to be constant. However, ordinary water that is used to cool light water reactors, such as PWRs and boiling water reactors (BWRs), has a small amount of compressibility, and its density is temperature dependent. This temperature dependence at atmospheric pressure is shown in Figure 16.1. At the saturation point of water, the density of liquid water is also pressure dependent. This dependence is shown in Figure 16.2. Notice that while the density of water is 958 kg/m3 at atmospheric pressure (~100 kPa) and 100°C, its density is only 720 kg/m3 at 15.5 MPa and 310°C. Thus, its density in the primary loop of a PWR is roughly one-third less than it is at atmospheric pressure. Finally, for a PWR steam generator operating at 5.5 MPa and 270°C, its density is about 770 kg/m3. Its actual dependence is tabulated in various fluid property tables and is also available on the Internet. A URL where its properties are tabulated as a function of temperature and pressure can be found at The density of water at atmospheric pressure. In reality, reactors operate at much higher temperatures and pressures, and the density of water generally decreases under these conditions. For example, in a PWR core at 15.5 MPa and 300°C, its average density is about 720 kg/m<sup>3</sup>. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315226231/546aed26-a09d-44a2-9db7-325797f2403e/content/fig16_1.jpg"/> <target id="page_610" target-type="page">610</target>The density of liquid water at the saturation point as a function of the system pressure. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315226231/546aed26-a09d-44a2-9db7-325797f2403e/content/fig16_2.jpg"/>