Oxidation of Silicon Carbide Composites for Nuclear Applications at Very High Temperatures in Steam

Round 1

Reviewer 1 Report

Nuclear fuel cladding plays a vital role in nuclear power plants. The commonly used materials for it are mainly zirconium and its alloy now. SiC composites have good strength and radiation resistance at high temperature, thus, attracting great attention in nuclear fuel research. In this paper, authors studied the oxidation behavior of SiC composites at high temperature in steam. The results are meaningful. It provides us with a reference maximal temperature that can be withstood by advanced nuclear SiC-based fuel cladding material to face accidental situations in LWRs. It can be accepted after some questions.

1. Some small mistakes need to be corrected, such as “Opila and Jacobson [8] discuss the strong effect of low-level impurities from alumina furnace tubes on the oxidation kinetics.” in page 4. It may be more appropriate to use "discussed" than "discuss”. “A vast number of papers has been published on silicon carbide and its behavior at high temperatures”. Pay attention to the singular and plural expressions of verbs. Similar issues should be noted elsewhere in the manuscript.

2. The author said “The thermal expansion coefficient of amorphous SiO2 is lower than of SiC and of β-cristobalite phase higher than of SiC”. It may be confusing, please change the expression. The same situation was also in the expression “More than 200 different crystal structures (polymorphs) are reported with the two major polytypes of silicon carbide being the metastableα-SiC having hexagonal crystal structure (similar to Wurtzite) and the thermodynamically stableβ-SiC with a cubic zinc blende crystal structure (similar to diamond).” It may be clearer if you can change the expression. Similar issues should be noted elsewhere in the manuscript. It may be confusing to read if the sentences are too long.

3. The author said “First bubbles at the sample surface were observed at 1820°C”, and “Such bubbles are formed due to the pressure buildup by the formation of gaseous reaction products like H2 and CO at the SiC/SiO2 interface.” Before 1820°C, there were also H2 and CO2 generating in the coating. Though their rates were small, why were there any bubbles  observed, even if one.

4. It seemed that the reaction between SiC and steam at high temperature to form methane and silica should also be added into the manuscript to make the it more complete, didn’t it?

5. According to Fig. 9, large number of bubbles were observed in Fig. 9a. Is there any relationship between it and the formation of Si-O-H and the gas? The author said it was the production of pressure buildup. The gaseous reaction products will be transferred by the steam flow from the surface immediately. Then can they form the pressure buildup? Was there possible that it was just the melting of silicon oxide. After all, the melting temperature of SiO2 is only around 1725°C. They may also form the similar morphology.

6. The author said the smoke at 1840°C was most probably caused by severe volatilization of Si-O-H…… It is also reported that these are less stable and oxidizes to silicon dioxide in air or water. So can it be stable as smoke in oxidizing water steam atmosphere in the experiments? Or can you prove the smoke was exactly the Si-O-H?

7. The SiC matrix was chemically vapor infiltrated by a CVI process after deposition of a thin PyC. So why did the author deposite PyC first on it. Why not just deposited the SiC coating on the SiC fiber? The author said “This is the consequence of low interfacial shear strength due to the presence of the pyrocarbon interphase”. And “the reaction of steam with the pyrocarbon interphase as well as the much higher effective surface of the fiber-matrix composite should be the main reasons for the accelerated oxidation kinetics resulting in high gas release rates.” The existence of pyrocarbon interphase seemed to have some negative effects. And it did not seem to be observed where the pyrocarbon was from SEM. So what was its effect? Why chose it if the author had to deposite a pre-coat?

8. Why the author specifically explained that some pieces of spalled-off …… a mass loss of the sample of 51 mg. Was not the author concerned about the gas release rate rather than the quality? In addition, where were the pieces from?

9. The caption of Fig. 6 seemed to be inconsistent with the description in the manuscript.

10. Generally, when we do several sets of experiments, we usually control one variable to be different for comparison. Test SiC-04 was run for three hours and fast cool-down in argon gas flow. Why?

11. What was between graphite and silicon carbide in Fig. 10.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The topic of this manuscript is interesting and the paper is well organized.

My objection is about the similarity of this manuscript with that cited as reference 4. The authors asserted that this paper contains two more experiment respect to the previous one, but only one is completely new since the second one is exactly in the same operating conditions already published. Moreover, the paper contains very similar substantial parts of the text and some figures are exactly the same.

I understand that this manuscript is mostly based on the work presented at the TOPFUEL 2021 Conference, but I think that the authors should at least revise the paper changing large parts of the text, selecting different figures when it is possible or modifying them. The authors should also clarify the novelty of this manuscript mainly in relation to the reference 4.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

In this manuscript, the oxidation experiments at very high temperatures in steam atmosphere were conducted for advanced nuclear grade SiC f /SiC CMC cladding tube segments. The transient experiment was carried out until serious local degradation of the sample at maximum temperature of approximately 1845°C. The isothermal tests for one hour-three hours at 1700°C in steam with final quenching by water and cool-down in inert atmosphere were run. A lot of research results of value were obtained. Despite the overall interest, the paper is needed to do some revisions before being considered for publication.

(1) In 3.2. test facility and conduct, “Three identical isothermal tests (SiC-02, SiC-03, and SiC-05) were conducted for one hour at 1700°C.” Why is the three identical isothermal tests (SiC-02, SiC-03, and SiC-05) designed in this study, please author give the idea.

(2) In table 1 (test characteristics, post-test examinations conducted, and selected results), the mass changes before and after tests for sample SiC-02 and SiC-05 are different under the same test condition, SiC-02 with weight loss -60mg, SiC-05 with weight gain+110mg, please give the explanation.

(3) Page 7 line 239, “They show the formation of bubbles (3 rd and 4 th image) and smoke (5 th image)”. Please make the arrow mark on the bubbles and smoke of the image.

(4) Page 8 line 253, “ ….. the external CVD SiC layer was consumed/removed, resulting in the high gas release rates as shown in Figure 3.” This inference is lack of enough evidence supporting, please provide more relevant results.    .

(5) In figure 8, there is a greatly distinction in the surface morphology of sample between SiC-02, 03 and 05 after 1 h isothermal oxidation at 1700°C, please explain the reason.

(6) Page 13 line 349, “This is the consequence of low interfacial shear strength due to the presence of the pyrocarbon interphase, which does not seem to be affected by the oxidative atmosphere.” It is suggested that the experiment of the fiber in matrix push-out will be made, so as to obtain the quantitative interfacial shear strength.

 

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have revised the manuscript according to the reviewers' comments and suggestions and it can be considered for publication