The goal and scope of this study
Breakaway oxidation can occur during the high temperature oxidation test and is related to the embrittlement of the coating. The high-temperature oxidation test and subsequent ductility test are necessary to evaluate the breakaway oxidation phenomenon, and the guide to high-temperature oxidation and post-quenching ductility test was published in 2011. Each draft regulatory guide contains the content on the test method for breakaway oxidation, post-quench ductility and methodology of analytical limits.
The single test method and dissociative oxidation results that were suggested in draft regulatory guidance 1261 are written on paper. In the next chapter, the previous theories of dissociative oxidation and the literature survey of normal and high temperature oxidation performed with the dissociative oxidation test are presented in the next chapter. The results of the experiment are compared with well-established data to identify whether they are reliable.
The entire content of the paper is therefore as follows; literature review, test method, test equipment modification and verification test results.
- Basis of zirconium alloy oxidation
- Relationship between oxidation rate and temperature
- Phase diagram of zirconium-oxygen
- Thermodynamic properties of zirconium-oxide
- High temperature corrosion of zirconium alloy
- Structure of zirconium alloy after high temperature oxidation
- Hydrogenation phenomenon of zirconium alloy
- Breakaway oxidation
- Basis of breakaway oxidation
- Breakaway oxidation mechanism
The oxidation of metal is very sensitive to temperature and the graph in Figure II-2 shows the comparison between the weight gain and temperature of oxidation over Zircaloy-4 during 2500 days in the autoclave. The zirconium-oxygen binary phase diagram is shown in the left side of Figure II-3 and the pseudo-binary phase diagram is shown in the right side of Figure II-3. Ellingham/Richardson diagram is the graph of the Gibbs free energy of oxide as Figure II-4.
The red line in Figure II-4 is the Gibbs free energy which is the reaction that appears when Zr turns into ZrO2. The reaction occurs spontaneously when H2/H2O ratio increases above red line in the Figure II-4. The oxygen content of zirconium (left) and microstructure of zirconium alloy observed by microscope (right) at room temperature after the high temperature oxidation are shown in the Figure II-5.
As mentioned earlier, the right side of Figure II-5 can be viewed from the outside in order, oxide – α layer – β layer. Zirconium alloys experience quenching because the ECCS injects water in the LOCA situation, where quenching water is usually used for the high temperature corrosion test for similar conditions. The oxide layer and the α-Zr(O) layer become thicker as the oxidation time increases; this can be confirmed in Figure II-6.
Oxidation time is the shortest in the (a) and becomes longer after (d), the mark Ⅰ indicates the α-Zr(O) layer located between the brown color oxide layer and yellow color. The brown oxide layer and α-Zr(O) layer are the thickest in the (d) when oxidation time is the longest in the Figure II-6 13. Zirconium hydride forms outline shape as Figure II-7, zirconium alloy has the following effects in the zirconium cladding before when zirconium alloy absorbs hydrogen: hydrogen embrittlement due to excess hydrogen or its localization in a blister or edge loss of fracture toughness, delayed hydride cracking (DHC), acceleration of corrosion and acceleration of radiation growth 6.
The relationship between hydrogen content and oxidation time is shown for each zirconium alloy in the left side of Figure II-8, and the relationship between oxide layer thickness and oxidation time is shown in the right side of Figure II-8. HANA-5, 6, Alloy-A(ZIRLO) and Zircaloy-4 are used to test the data on Figure II-8, ZIRLO and Zircaloy-4 data may be focused on 12b. Burst oxidation can be observed in high temperature corrosion of zirconium alloy, and the high temperature oxidation test specimen has white oxide after burr oxidation as shown in Figure II-9.
Overall scheme of equipment
Specimen cutting
Specimen cleansing
The specimen holder
Steam supply equipment
Experimental method
- Purging test train
- Thermocouples
- Temperature control
- Thermal benchmark
- Temperature distribution of a specimen
- Weight gain benchmark
Install the new steam supply equipment
The connection method is prepared by using the PVDF connector between the steam supply device and the high temperature furnace system. The problem has arisen because on new steam supply devices that do not use a standard size. Stretching the hose is easier than compressing the hose, 1 inch (25.4 mm) and 3/4 inch (19.05 mm) IDs are used as standard which are larger than the normal size of flexible hose.
The 3/4 inch part is inserted into the stretched flexible hose with covering vacuum grease and secured with the stainless bands and cable ties. The ferrule set is inserted into the 1/2 inch part for fixing the pipe, and the metal side of the pipe is covered by the heater to prevent condensation of steam. There are additional problems that the new steam supply equipment does not have its own power control method.
The electric power controller is prepared to control the steam supply equipment called as slidecs (voltage transformer).
Theoretical steam generation rate
The recommended steam flow rate is 0.5 ~ 30 mg/cm2∙s 3, so the steam generation rate should be converted to the generation rate per unit area. The maximum steam production is 24.7 mg/cm2∙s (35 g/min) at a power of 1300 W, which is the maximum power of the steam supply equipment. Obviously, this maximum steam generation is not achieved due to heat loss, transformer efficiency, etc.
But steam production is expected to take the right range of the recommendation, the criterion will be met.
Measurement of steam flow rate
- Axial temperature distribution
- Circumferential temperature distribution
- Comparison of the weight gain
- Surface condition after high temperature oxidation test
- Microstructure
The possibility of fracture oxidation of zirconium alloy cladding at high temperatures around 1000ºC under small and large LOCA has been a concern in recent years. The standard, 10CFR50.46, proposed by the NRC, requires setting the breakaway oxidation criterion based on actual performance of the fuel cladding and confirming the criterion by the periodic testing of cladding tubes used in the facility. High temperature oxidation of zirconium alloy has different trends with low temperature oxidation because there are several differences between low and high temperature oxidation.
Phase transformations of zirconium and its oxide also occur at high temperature; the characteristic phenomena of zirconium alloys can be obtained by inducing a phase transformation. The transition of the oxidation kinetics results in surface discoloration and this phenomenon is called "breakaway oxidation". A radiant heating system with a steam and water supply system is used for the test to simulate the LOCA condition represented by high-temperature steam oxidation of a rapidly heated liner and subsequent water quenching.
The steam supply equipment is one of the important systems for the high temperature oxidation test because the high temperature oxidation test must be performed in a steam environment. The results of ZIRLO weight gain measurement are consistent with the well-established data and these results are successfully verified by the high temperature oxidation test. Wavy interface between metal and oxide is estimated to be the precursor to breakaway oxidation phenomenon and the microstructure after the high temperature oxidation test is shown similar to most of previous research.
When zirconium alloy experiences the high-temperature environment with the vapor state in the accident situations, the high-temperature oxidation properties of zirconium alloy show different oxidation kinetics, which are different from normal oxidation kinetics due to the phase transition, oxidation rate, etc.
A composition of a representative zirconium alloy is introduced in Table II-1, and the information is also introduced in Table II-2, which is the minimum breakaway oxidation time in each alloy. The occurrence of a wavy structure forms the lateral crack in the oxide and then the vertical crack. After cutting the specimen, wash the specimen in running tap water and remove burr C.
An alumina support is also placed between the sample and the holder in the experiment, a picture of the sample, alumina and Inconel holder is shown in Figure III-4. The zirconium alloy sample can react with air, especially with nitrogen molecules, under high temperature conditions. Steam is usually used to flush the test set in an experiment. Figure III-5 is an image of a sample when the test set is flushed for 10 minutes and tested using the normal test procedure.
The temperature range in the experiment is a maximum of 1200 °C (or some experiment tested above this range), and the choice of thermocouples is recommended for K or S type thermocouples in the draft regulatory guide 1261. The criterion in the draft regulatory guide 1261 recommended calibration point is followed 800ºC and 1000ºC, so the recommendation is met. The recommended method of temperature control is presented in draft regulatory guide 1261, and the method is briefly presented as follows;.
The furnace control thermocouple is used to estimate the temperature of the sample in the escape oxidation test. If the experimental weight gain is not within ±10% of the CP correlation, the thermal benchmark should be retested in the guideline. The initial steam supply equipment has a twisted long pipe in the system, and the heated pipe provides heat to the flowing water for the supply of steam.
Additional water is used by a thin water pipe in the initial steam supply equipment, which is expected to cause periodic transitions. As already mentioned, temperature distribution in the sample is one of the recommendations in the test guidelines. The results of observing the appearance of the ZIRLO sample after high-temperature oxidation are shown in Figure V-3.
The oxidation time of the microstructure in Figure V-5 is longer than the oxidation time of the microstructure in Figure V-4. The Metal / Oxide interface is flat interface in the sample, the precursor of oxidation fracture is not observed in Figure V-4. The Metal / Oxide interface is wavy interface in Figure V-5, the precursor of oxidation fracture is observed in Figure V-5.
Flat interface between the metal and the oxide is observed in the early period of the high temperature oxidation test, but wavy interface between the metal and the oxide is observed after the oxidation breaking out; flat to wavy transition of the interface was reported by other authors to be the precursor of the fracture oxidation.
