## 2 Experimental Setup  
Isothermal tests at 1000 \({}^{\circ}\)C with varying oxygen flow rates from 20 to 200 mL/min were performed with Zircaloy-4 tube segments using a commercial thermogravimetric analysis (TGA). Abrupt temperature escalation, which leads to abrupt increasing mass gain in the beginning of the oxidation reaction, was expected, due to an intense exothermic reaction between oxygen and Zr metal. Mass gain and mass-gain-rate data obtained from TGA were used to describe the oxidation behaviors. In addition, an investigation of macro- and micro-graphs supporting TG analyses was performed. The preparation of samples and experimental procedures was as follows: The test samples were prepared by dissecting a long original Zircaloy-4 tube, and two holes with 1 mm outer diameters were drilled into the upper part of the tube, from which thermobalance was suspended. The dimension of the segment was 10 mm long, with a 9.5 mm outer diameter and a 7.9 mm inner diameter. The fabricated test specimen is represented in Figure 1.  
After the samples were prepared, the surfaces of the samples were polished and cleaned in an ultrasonicator using acetone. The commercial SETARAM TGA was used as a thermobalance for the oxidation tests. The inside of the test section temperature of the TGA was controlled by an s-type thermocouple, which was placed close to the bottom of the sample (3 mm distance).  
The tube was hung in a TGA using a series of platinum (Pt) wires. All the tests were started after the mass-gain data were stabilized. Argon was injected to remove impurities inside the TGA for 90 min while room temperature was maintained, and the temperature was raised to 1000 \({}^{\circ}\)C under an argon atmosphere. After the desired temperature was obtained, the temperature was stabilized for 10 min. Then, oxygen was injected as an oxidant and varied from 20 to 200 mL/min, according to the planned tests. The argon flow  
Figure 1: Preparation diagram of specimen inside the TGA.  
rate was adjusted to the corresponding oxygen flow rate to maintain an oxygen-to-argon ratio of 2:1 to protect the TGA. The most frequent oxidation time was 2 h, and several cases were conducted for an additional 4 h. The direction of gas flow was top to bottom, based on the samples (gravity direction). After oxidation, the supply of oxygen was stopped, and the temperature was lowered as far as possible under an argon atmosphere. The post-test samples were photographed and embedded in a mixture of resin and hardener. In addition, the samples were ground and polished for metallography. Macro- and micrographs were taken of samples after 2 and 4 h of oxidation. Some of the micrographs were made by overlapping a series of photos to avoid unclear parts in each picture.  
Moreover, a dummy test was performed with an alumina tube, which was manufactured with the same geometry according to a Zircaloy-4 specimen, to distinguish between mass gain from the reaction between oxygen and metal and other effects. Figure 2 shows the results of mass gain from the dummy test. It was observed that there was a small mass gain during the oxygen inflow period. If we assumed that there was no reaction between oxygen and the alumina tube, the mass gain could be considered to only be due to other effects. One of the possible effects which could lead to a low initial mass gain at the initial stage could be from the momentum force induced by the gas flow direction being from top to bottom. After the initial mass gain, a steady state was achieved and d(\(\Delta\)m)/dt converged to zero. The final mass gain was 0.08% after 2 h of oxidation time. This mass-gain value is negligible compared to the average mass gain of Zircaloy-4 oxidation tests (18.98%), so it can be assumed that the other effects on mass gain can be ignored. Consequently, the mass-gain data from Zircaloy-4 tests do not consider the results of the dummy test.