*2.1. Review of NUPEC Test Facility*

Figure 2 depicts the NUPEC test facility, where the bundle void measurement tests were performed. The test loop was a high-pressure and high-temperature recirculation loop, of which the design pressure and temperature were 19.2 MPa and 635.15 K, respectively, which covered PWR (Pressurized Water Reactor) operating conditions. Once the coolant flowed into the heated section of the bundle, which had an active heated length of 3.658 m, the coolant was electrically heated from the heater rods, and the phase change occurred. At three different locations in the heated section, which consisted of a lower (2.216 m), middle (2.669 m), and upper point (3.177 m), respectively, the void fraction was measured within four central subchannels of the bundle. The steady-state bundle test consisted of three different test series, which were distinguished by test assembly ID, namely B5, B6, and B7, respectively. As listed in Table 1, the test series B5 and B6 utilized an identical 5 × 5 assembly. However, in the case of B7, the assembly contained a thimble rod, which described the guide tube at the center of the test section. The uniform axial power shape was applied to B5, but B6 and B7 used a cosine axial power profile. Each assembly was equipped with three different types of spacer grids, which consisted of a simple spacer grid, a spacer grid with no mixing vane, and with mixing vane, respectively. A more detailed description of the specification of the test facility is available in Reference [8].

**Figure 2.** NUPEC test facility [8]: (**a**) whole scheme of the test recirculation loop; (**b**) scheme of the test section where bundle void measurement tests were performed.


**Table 1.** Specification of the bundle test section [8].
