Initial Multidisciplinary Study of Oxidized Chromium-Coated Zirconium Alloy for Fuel Cladding of SCW-SMR Concept: Weight-Gain and Thermal Conductivity Measurements and Coating Cost Evaluation

Round 1

Reviewer 1 Report

This work presents a study of the corrosion resistance of chromium coatings in conditions representing a supercritical water-cooled reactor. Also discussed is the impact of the oxide film on the heat transfer capabilities of the cladding material system. Major revisions are needed before this work can be published. The novelty of the work is difficult to discern, given that several sources are already cited in the text which discuss the oxidation behavior and thermal conductivity of chromium coatings. Additionally, relatively little data and discussion is presented on the deposition and microstructural characteristics of the coatings, two factors which will have significant impact on the performance metrics studied herein. It is recommended to discuss the characterization of these present coatings in greater depth, and reduce the emphasis on cost analysis, especially considering that the analysis presents a different deposition method than the one used in the study.

Several additional points by section:

1. (Lines 2-4) The title should be updated to better reflect the contents of the paper, specifically mentioning both the oxidation and thermal conductivity aspects.

2. (Line 77) Referring to Zr-2.5Nb as Zircaloy may be misleading for readers, as this is commonly used when discussing Zircaloy-2 and Zircaloy-4. Use zirconium alloy or Zr alloy instead to avoid confusion.

3. (Line 95) Can you clarify what level of grit was used for the SiC paper when preparing substrates?

4. (Line 101) Providing further clarification on the deposition parameters could be helpful for the field, i.e. deposition temperature, deposition pressure, target dimensions, bias pulse frequency. Was the process optimized to arrive at these parameters? If so, explaining this optimization can add further value to the field.

5. (Line 108) Were there any uncoated/shadowed areas following the deposition, due to the sample holder geometry? This will be important for measuring weight gain of the chromium coating without artificial inflation from uncoated Zr regions. Replacing or supplementing figure 2 with digital images of the sample holder mechanisms will clarify this point. Adding digital images of the as-deposited samples may also help.

6. (Line 116) Additional characterization of the chromium coating microstructure (i.e. surface SEM imaging, phase ID XRD, grain boundary etching, surface roughness measurement) should be included in the results and discussion to fully understand the coating performance.

7. (Lines 121-122) “The oxygen signal within the chromium layer (yellow line in Figure 3 (b)) is notable and uniform, and suggests incorporation during the coating process.” This comment needs further confirmation, it is likely that the overlap of the oxygen Kɑ (0.525 keV) and the chromium Lɑ (0.573 keV) peaks skewed these results. Quantitative EDS mapping may help resolve these peaks.

8. (Line 131-132) “Other specimen were tested at the same time and not reported here, including chromium-coated Zr-1Nb, Zr-1.2Cr-0.1Fe and titanium coupons.” Is this meant to say Zr-2.5Nb? Have steps been taken to confirm that corrosion products from the other alloy materials did not negatively impact the Cr oxidation?

9. (Line 156-158) “After each exposure, the specimens removed from the autoclave were visually inspected. The specimens were rinsed with ethanol and dried in a stream of hot air until no change from water vaporization was observed.” Have previous studies been conducted to confirm that no damage or spallation of the oxide is induced during this process?

10. (Lines 163-167) “Before the test, the surfaces of the bare (uncoated) coupons were shiny metallic while the surface of coated coupons were dull by comparison. After exposure to SCW, the surfaces of the coupons were dulled with the cloudy appearance characteristic of an oxide film. The colours of the oxide films were different and varied by base metal and coating. No sign of spalling was observed on exposed coupons.” Are digital images of the samples before and after autoclave testing available? This would help demonstrate the difference in appearance and bolster the argument that no spalling was observed. Post-autoclave characterization such as surface and cross-section SEM imaging is also recommended.

11. (Lines 174-176) “To put this number into context, the as-received pressure tube was oxidized in similar SCW conditions, and the weight gain at the end of 48 h was about 89 mg/dm².” It seems odd to compare the results of 1150 hours of testing with 48 hours of testing, even with such a sharp contrast in oxidation rates. If data is available for the uncoated baseline at a full 1150 hours, it would make for an interesting comparison. This data could even be plotted in Figure 5 alongside the Cr coating results.

12. (Figure 5) Are there any suspected reasons for the drop in weight gain at 1000 hours? This seems to imply that either the oxide or the coating may have spalled.

13. (Figure 7) It would be helpful to highlight the location of the thermocouples in this diagram (specifically showing where the thermocouples would contact the control specimens during the test itself)

14. (Lines 267-268) “To quantify the thermal conductivity of a coated sample, a benchmark test is per formed with a polished sample (benchmark sample) that is not coated.” Is this benchmark sample composed of the same Zr-2.5Nb alloy as the coating substrates, or a different material entirely?

15. (Table 3) Are measurements of the as-deposited Cr coating thermal conductivity available? This would help argue the point that heat transfer is being reduced by the increased oxide thickness, especially when compared to the previous literature that was cited in the introduction to this section, where it is argued that Cr coatings increase heat transfer over bare Zr alloys. Include the thermal conductivity of the uncoated Zr in this table as well for clear comparison. If available, thermal conductivity measurements of post-oxidation Zr would provide further insight.

16. (Section 6, cost analysis) This section seems out of scope given the early stages of coating development and testing in this work.  Additionally, the thickness values are based on the measured oxidation rate of coatings deposited by PVD, while the cost analysis discusses the implementation of powder-based deposition. The differences in microstructure and surface uniformity between these methods could results in different oxidation behaviors, and at the very least this needs to be mentioned as a caveat. If possible, comparing the cost of these powder-based deposition methods against PVD methods such as magnetron sputtering could more clearly illustrate the compromises involved in choosing a method for commercial implementation.

17. (Lines 296-298) “Furthermore, the VPS processes, which is a direct current (DC) plasma spray process (Figure 9), is expected to be more suitable to coat a zirconium-based alloy (fuel cladding) with a Cr (99.99% purity) coating.” Coating purity should not be the main driving force for the decision to use plasma spray, as high purity targets can also be used for magnetron sputtering to increase coating purity. The use of a vacuum chamber backfilled with inert gas also reduces the likelihood of Cr reduction or oxidation, while avoiding the challenges of metal powder handling. Please justify the choice of vapor plasma spray deposition beyond coating purity, i.e. deposition time or scalability for instance.

Comments for author File: Comments.docx

The writing is clear and concise throughout the manuscript, but it is recommended to read through the paper sentence by sentence to catch any minor errors that were missed during editing.

For example, Line 481: "When the life of SCW-SMR of fuel cladding is expected to last 5 years..."

With that said, there are no substantial spelling or grammatical errors. 

Author Response

Overall disposition: One of the objectives of this manuscript is to introduce the multidisciplinary aspect to the research. We should be thinking ahead of how we are going to adapt the knowledge to the real world (rather than only focusing on pure science). Cost analysis is very relevant in the practical world. In this manuscript, our coating cost estimate is extremely novel. The authors would like to present the methodology to the larger group of audience. This methodology to estimate the coating cost is useful for other types of coating especially when the parameters become more available in the future.

1. (Lines 2-4) The title should be updated to better reflect the contents of the paper, specifically mentioning both the oxidation and thermal conductivity aspects.

Disposition: The title has been modified to “Initial multidisciplinary study of oxidized chromium-coated zirconium alloy for fuel cladding of SCW-SMR concept: Weight-gain and thermal conductivity measurements and coating cost evaluation”.

2. (Line 77) Referring to Zr-2.5Nb as Zircaloy may be misleading for readers, as this is commonly used when discussing Zircaloy-2 and Zircaloy-4. Use zirconium alloy or Zr alloy instead to avoid confusion.

Disposition: The term zirconium alloy is used to replace zircaloy.

3. (Line 95) Can you clarify what level of grit was used for the SiC paper when preparing substrates?

Disposition: Text is added on page 3 to read “The surfaces of the coupons were first polished using 360 grit SiC paper followed by 600 grit SiC paper”.

4. (Line 101) Providing further clarification on the deposition parameters could be helpful for the field, i.e. deposition temperature, deposition pressure, target dimensions, bias pulse frequency. Was the process optimized to arrive at these parameters? If so, explaining this optimization can add further value to the field.

Disposition: See section 2.2 and the coupon dimension was specified in section 2.1.

5. (Line 108) Were there any uncoated/shadowed areas following the deposition, due to the sample holder geometry? This will be important for measuring weight gain of the chromium coating without artificial inflation from uncoated Zr regions. Replacing or supplementing figure 2 with digital images of the sample holder mechanisms will clarify this point. Adding digital images of the as-deposited samples may also help.

Disposition: New Figure 5 is added to give ideas of how the as-deposited sample looks like versus the sample after exposed to SCW. The uncoated area should provide a large oxygen diffusion path producing ZrO2 (white). In another case, the uncoated Zr alloy was disintegrated completely in SCW environment.

6. (Line 116) Additional characterization of the chromium coating microstructure (i.e. surface SEM imaging, phase ID XRD, grain boundary etching, surface roughness measurement) should be included in the results and discussion to fully understand the coating performance.

Disposition: SEM micrograph of as-deposited Cr-coated Zr-2.5Nb specimen is added, see Figure 3 (a).

7. (Lines 121-122) “The oxygen signal within the chromium layer (yellow line in Figure 3 (b)) is notable and uniform, and suggests incorporation during the coating process.” This comment needs further confirmation, it is likely that the overlap of the oxygen Kɑ (0.525 keV) and the chromium Lɑ (0.573 keV) peaks skewed these results. Quantitative EDS mapping may help resolve these peaks.

Disposition: EDS mapping is added to Figure 3.

8. (Line 131-132) “Other specimen were tested at the same time and not reported here, including chromium-coated Zr-1Nb, Zr-1.2Cr-0.1Fe and titanium coupons.” Is this meant to say Zr-2.5Nb? Have steps been taken to confirm that corrosion products from the other alloy materials did not negatively impact the Cr oxidation?

Disposition: There were all coated with chromium of similar thickness obtaining from the same PVD coating batch. No cross-contamination is expected.

9. (Line 156-158) “After each exposure, the specimens removed from the autoclave were visually inspected. The specimens were rinsed with ethanol and dried in a stream of hot air until no change from water vaporization was observed.” Have previous studies been conducted to confirm that no damage or spallation of the oxide is induced during this process?

Disposition: Deionized was used to rinse the oxidized coupons. Text is modified. Thank you.

10. (Lines 163-167) “Before the test, the surfaces of the bare (uncoated) coupons were shiny metallic while the surface of coated coupons were dull by comparison. After exposure to SCW, the surfaces of the coupons were dulled with the cloudy appearance characteristic of an oxide film. The colours of the oxide films were different and varied by base metal and coating. No sign of spalling was observed on exposed coupons.” Are digital images of the samples before and after autoclave testing available? This would help demonstrate the difference in appearance and bolster the argument that no spalling was observed. Post-autoclave characterization such as surface and cross-section SEM imaging is also recommended

Disposition: New Figure 5 is added to disposition this comment.

11. (Lines 174-176) “To put this number into context, the as-received pressure tube was oxidized in similar SCW conditions, and the weight gain at the end of 48 h was about 89 mg/dm².” It seems odd to compare the results of 1150 hours of testing with 48 hours of testing, even with such a sharp contrast in oxidation rates. If data is available for the uncoated baseline at a full 1150 hours, it would make for an interesting comparison. This data could even be plotted in Figure 5 alongside the Cr coating results.

Disposition: Further down the same paragraph, another method of comparison using a penetration depth with a normalized exposure time was proposed. It reads “In the present SCW study, Cr-coated Zr-2.5Nb showed improved corrosion resistance over the original Zr-2.5Nb pressure tube coupon, with a penetration depth of only 0.03 µm/d versus 2 µm/d, assuming linear oxidation kinetics”.

12. (Figure 5) Are there any suspected reasons for the drop in weight gain at 1000 hours? This seems to imply that either the oxide or the coating may have spalled.

Disposition: This is Figure 12 from previous paper by Khatamian in 2013 (not from the present study).

13. (Figure 7) It would be helpful to highlight the location of the thermocouples in this diagram (specifically showing where the thermocouples would contact the control specimens during the test itself)

Disposition: Figure updated to show thermocouple arrangement on control specimen and text updated to quantify the spacing between the thermocouples.

14. (Lines 267-268) “To quantify the thermal conductivity of a coated sample, a benchmark test is per formed with a polished sample (benchmark sample) that is not coated.” Is this benchmark sample composed of the same Zr-2.5Nb alloy as the coating substrates, or a different material entirely?

Disposition: The benchmark sample is made from Zr2.5Nb material. This has been added to the text.

15. (Table 3) Are measurements of the as-deposited Cr coating thermal conductivity available? This would help argue the point that heat transfer is being reduced by the increased oxide thickness, especially when compared to the previous literature that was cited in the introduction to this section, where it is argued that Cr coatings increase heat transfer over bare Zr alloys. Include the thermal conductivity of the uncoated Zr in this table as well for clear comparison. If available, thermal conductivity measurements of post-oxidation Zr would provide further insight.

Disposition: No, unfortunately the measurements of the as-deposited Cr coating are not available.

16. (Section 6, cost analysis) This section seems out of scope given the early stages of coating development and testing in this work. Additionally, the thickness values are based on the measured oxidation rate of coatings deposited by PVD, while the cost analysis discusses the implementation of powder-based deposition. The differences in microstructure and surface uniformity between these methods could results in different oxidation behaviors, and at the very least this needs to be mentioned as a caveat. If possible, comparing the cost of these powder-based deposition methods against PVD methods such as magnetron sputtering could more clearly illustrate the compromises involved in choosing a method for commercial implementation.
    Deposition: Please see the first paragraph in section 6.
17. (Lines 296-298) “Furthermore, the VPS processes, which is a direct current (DC) plasma spray process (Figure 9), is expected to be more suitable to coat a zirconium-based alloy (fuel cladding) with a Cr (99.99% purity) coating.”

Coating purity should not be the main driving force for the decision to use plasma spray, as high purity targets can also be used for magnetron sputtering to increase coating purity. The use of a vacuum chamber backfilled with inert gas also reduces the likelihood of Cr reduction or oxidation, while avoiding the challenges of metal powder handling. Please justify the choice of vapor plasma spray (VPS) deposition beyond coating purity, i.e. deposition time or scalability for instance.

Deposition: VPS is not chosen because of its purity, see the first two paragraph in section 6 which reads… “The coating selection for the oxidation experiments was based on familiarity of chromium coating with the PVD method to conduct the initial R&D. Ongoing R&D will consider alternative coating processes such as a future in-house VPS method. The differences in microstructure and surface uniformity between these coating methods could results in different oxidation behaviors. In addition, neutron economy will have to be assessed. These performance indicators would need to be evaluated in relation to coating method aspects such as commercial scalability and deposition time. Due to re-source availability and future in-house development a VPS method is chosen for initiating R&D coating economics for the SCW-SMR concept.

The VPS processes is expected to be applied to coating the outer surface of a zirconium-based fuel cladding of a 64 element fuel bundle in an SCW-SMR, similar to that shown in Figure 9. The VPS Process, which is a direct current (DC) plasma spray process (Figure 10), is also assumed to use a 99.99% purity Cr powder for evaluating the cost of coating”.

Reviewer 2 Report

The metallic chromium coatings deposited by PVD methods are well known for improving the corrosion and oxidation resistance of zirconium alloy claddings. To expand the scope of their application in SCW SM reactor it is still necessary to perform study on the evolution of the structure of the coatings under supercritical water conditions.

The authors have done original work on the systematic study of the effect of physical vapor deposition of chromium on Zr-2.5Nb alloy. Oxidation tests were performed on coated samples at 500 C and 18 about 25 MPa in a refreshed autoclave. The effect of the oxide on heat transfer and hydraulic resistance is also discussed in this study. This study evaluates the coating cost of fuel cladding with chromium in a vacuum plasma spray process.

Formal aspects:

The English is good.

Graphical material is presented in good quality.

Statistical errors are reported.

The abstract and highlights: fully explain the content of the paper.

References are adequate.

Article is acceptable for publication.

Comments for author File: Comments.docx

Author Response

Thank you for your review. Your comments/concerns are highly appreciated.

Reviewer 3 Report

The presented manuscript “The study of oxidized chromium-coated zirconium-based alloy for fuel cladding of SCW-SMR concept: A preliminary integrated analysis” contains good findings. However, this manuscript requires some improvements in following areas:

·         Abstract needs to be revised. Need to address a) need of this work, b) novelty, c) methodology suggested, d) results obtained (numerical).

·         Introduction section is very poorly written. Revise it and add the relevant information.

·         I didn’t find relevant literature work to justify the research gap. Relevant literature must be added to justify the research gap and need of the present study.

• In the last paragraph of introduction, write the novelty of your work and selected conditions for experimentations.

·         Specifications of the used equipment should be presented.

·         Add the figure of the experimental setup.

·         Improve the quality of figure 2 and figure 3a.

·         What is the basis of the selection of mentioned input conditions? Specify them in detail.

·         Add table containing the experimental parameters along with recorded responses.

·         Results and discussion section is written well. However, compare the obtained results with similar past studies and justify the findings with proper technical reasons in results and discussion section.

·         Conclusion section needs to be revised. Write in bullet points.

• Mention the limitations and further scope of improvement in last section.

Author Response

General disposition to your comments:

Thank you very much for taking a leading role to review the manuscript coatings-2562414.

I am currently working to address the comments from all three experts in the field. However, I would like your further assistance in order to appropriately address comments from Reviewer #3.

I am not certain if this reviewer read the same version of the manuscript submitted on July 31^st. The set of comments though some are useful, most of them are convoluted. In fact, the comments listed from this reviewer could be applied to any manuscripts.

Most of them do not specify the exact point of how to improve our manuscript. I am trying my best to understand the questions and would like to ensure the reviewer read the same document when we communicate.

Please find an attached document for my response to Reviewer #3. It would be very helpful if he/she can review and provide clear comments.

Thank you in advance. From Kittima Khumsa-Ang.

• In the last paragraph of introduction, write the novelty of your work and selected conditions for experimentations.
• Response from author: The manuscript has already contained the importance of this study with regards to the specific type of Advanced Nuclear Reactor Concepts, which is Supercritical water-cooled reactor. This is the only concept that utilises light water that currently being used in the current fleet. Hence, the experimental conditions are extremely relevant to the expected operating conditions of such reactor when it is at full-power operation. Please read carefully and please provide specific points on how to improve the manuscript.
• Specifications of the used equipment should be presented.
• Response from author: See Section 3 a detailed description of the recirculating loop was provided. Please confirm that you are requesting for more information on the instrumentations.

Currently, the two paragraphs read

“The oxidation experiments were conducted in refreshed autoclaves in supercritical water at 500 °C to evaluate the corrosion behaviour of the pressure tube specimen coated with metallic chromium (Table 1). Other specimen were tested at the same time and not reported here, including chromium-coated Zr-1Nb, Zr-1.2Cr-0.1Fe and titanium coupons. The SCW refreshed autoclave consists of a 200 L Type 316 stainless steel feed tank, equipped with a gas bubbler for deoxygenating the feed water, a main-loop pump (Vindum, syringe pump) delivering flow from 1 to 20 mL/min at pressures up to 25 MPa, an Inconel 625 electrical preheater (Carbolite-Gero Model: EVT Tube furnace), and a 1 L Inconel 625 autoclave (Parker Autoclave Engineers) rated for 25 MPa and 575 °C. Downstream of the autoclave is a cooler, back pressure regulator (TESCOM, 4000 psi), and an ion exchange column (see Figure 4).

Coupons were inserted on a sample tree made of pre-oxidized 316L stainless steel and exposed to the test solutions at listed conditions in Table 2. The pH (Endress Hauser Ceragel Model CPS71D), conductivity (Endress Hauser Condumax Model CLS21D), and dissolved oxygen concentration (Orbisphere Electrochemical Sensor model 31110.02) of the feed water were monitored, as were the pH and conductivity of test solution exiting the autoclave.”

• Add the figure of the experimental setup.
• Comment from author: See Figures 4, 6 and 7.
• What is the basis of the selection of mentioned input conditions? Specify them in detail.
• Comment from author: See Section 3 for rationale of the selected test conditions (T, P and chemistry).
• Add table containing the experimental parameters along with recorded responses.
• Comment from author: See Table 2.
• Results and discussion section is written well. However, compare the obtained results with similar past studies and justify the findings with proper technical reasons in results and discussion section.
• Comment from author: What is the point to be improved? Please provide in more details.
• Conclusion section needs to be revised. Write in bullet points.
• Comment from author: What exactly is the point of improvement? Please specify in detail and please leave the choice of format to the authors. I would appreciate technical details to be added rather than formatting.
• Mention the limitations and further scope of improvement in last section.
• Comment from author: This is excellent point. I am working to incorporate this comment to the manuscript. Thank you.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Manuscript can now be accepted in current form