Design of the Test Section for the Experimental Validation of Antipermeation and Corrosion Barriers for WCLL BB

Round 1

Reviewer 1 Report

The reviewer thinks that the paper is well-written and is useful for readers. I thus would like to recommend the publication of this paper in the present form.

Author Response

 

Referee #1: The reviewer thinks that the paper is well-written and is useful for readers. I thus would like to recommend the publication of this paper in the present form.

Thank you for your revision 

Reviewer 2 Report

After decades’ efforts in experimental and theoretical studies, the obscureness and lack of understanding of hydrogen isotopes transport behavior within plasma facing components and materials for future fusion reactors still represent a major technological challenge and risk for the realization of fusion fuel self-sustainability. Although design activities of fusion reactors such as EU DEMO and CFETR are being proceeded using assumed tritium transport, or more specifically, permeation data, discrepancy between the simulation results from real-world values at reactor scale is never completely clear. Thus, state-of-the-art technologies must be applied to reduce tritium permeation (and surface corrosion if a liquid blanket) so as to further increase design margins.

 

This manuscript reports the design of a test section comprising primarily a reduced-scaled mock-up of the WCLL breeding blanket for DEMO, to be installed in the TRIEX-II facility. The design of the mock-up preserves the geometric characteristics of the WCLL coolant pipes, while keeps a structure succinct enough to allow for PLD/ALD coating as well as handy experimental operations. This is a fine and delicate design even from the first glance, which the reviewer believes will be the key to the value of the results of future experiments. The experimental campaign staged into separate operation in gas phase and liquid phase is also very logical and well-planned. 1-D analysis results for the two operation phases are reported in Section 5 to support the test section design. In general, the manuscript is well written and presents itself ready for publishing. The reviewer enjoys reading the current paper and looks forward to more papers reporting the authors’ follow-up work on this test section.

 

Just a few minor suggestions proposed below.

• In the abstract part, in the sentence “deeply investigate permeation of Q2 through PHTS …” Q2 should refer to hydrogen isotopes H and D, yet this is not a conventional terminology and may be unfamiliar to the readers thus bring difficulty in understanding. A bit explanation in brackets or simply using “hydrogen isotopes” to replace Q2 will suffice.
• The abbreviation PRF is never explained in the context. It should be “permeation reduction factor” I guess? Need to mention its full name the first time PRF shows up in the manuscript. Also, from line 105-106, the statement “it is possible to detect … even at high PRF” seems not very straightforward to me. The tubes with maximum possible length are chosen and this will certainly lead to a maximized permeation area. Yet the permeation reduction factor PRF is the ratio between the permeation fluxes w/ and w/o barriers that is independent from the permeation area. The statement here could be fixed to be more precise.

Author Response

Referee #2: After decades’ efforts in experimental and theoretical studies, the obscureness and lack of understanding of hydrogen isotopes transport behavior within plasma facing components and materials for future fusion reactors still represent a major technological challenge and risk for the realization of fusion fuel self-sustainability. Although design activities of fusion reactors such as EU DEMO and CFETR are being proceeded using assumed tritium transport, or more specifically, permeation data, discrepancy between the simulation results from real-world values at reactor scale is never completely clear. Thus, state-of-the-art technologies must be applied to reduce tritium permeation (and surface corrosion if a liquid blanket) so as to further increase design margins.

This manuscript reports the design of a test section comprising primarily a reduced-scaled mock-up of the WCLL breeding blanket for DEMO, to be installed in the TRIEX-II facility. The design of the mock-up preserves the geometric characteristics of the WCLL coolant pipes, while keeps a structure succinct enough to allow for PLD/ALD coating as well as handy experimental operations. This is a fine and delicate design even from the first glance, which the reviewer believes will be the key to the value of the results of future experiments. The experimental campaign staged into separate operation in gas phase and liquid phase is also very logical and well-planned. 1-D analysis results for the two operation phases are reported in Section 5 to support the test section design. In general, the manuscript is well written and presents itself ready for publishing. The reviewer enjoys reading the current paper and looks forward to more papers reporting the authors’ follow-up work on this test section.

Just a few minor suggestions proposed below.

• In the abstract part, in the sentence “deeply investigate permeation of Q2 through PHTS …” Q2 should refer to hydrogen isotopes H and D, yet this is not a conventional terminology and may be unfamiliar to the readers thus bring difficulty in understanding. A bit explanation in brackets or simply using “hydrogen isotopes” to replace Q2 will suffice.

Thank you for your comment. The terminology has been updated according to your suggestion.

• The abbreviation PRF is never explained in the context. It should be “permeation reduction factor” I guess? Need to mention its full name the first time PRF shows up in the manuscript.

Thank you for your comment. The terminology has been updated according to your suggestion.

 

• Also, from line 105-106, the statement “it is possible to detect … even at high PRF” seems not very straightforward to me. The tubes with maximum possible length are chosen and this will certainly lead to a maximized permeation area. Yet the permeation reduction factor PRF is the ratio between the permeation fluxes w/ and w/o barriers that is independent from the permeation area. The statement here could be fixed to be more precise.

Thank you for your comment. As the Reviewer stated, the PRF is the ratio between two fluxes, hence it is independent on the permeation area. However, the need of having also a large permeation area is due to the fact that the measurement is carried out by a quadrupole mass spectrometer, which can provide the amount of permeated moles of hydrogen isotopes in the unit time (e.g., mol/s); in order to have a signal which can be measured by the mass spectrometer, a sufficient permeation area has to be guaranteed. In order to catch your comment, the sentence was modified as follows: “…so that it is possible to detect a hydrogen or deuterium flux (Q₂, Q=H, D). In fact, even if the PRF is independent on the permeation area, the measurement of the flux is carried out by a quadrupole mass spectrometer, which can provide the amount of permeated moles of hydrogen isotopes in the unit time (e.g., mol s^-1); in order to have a signal which can be measured by the mass spectrometer, a sufficient permeation area has to be guaranteed.”.

 

Author Response File: Author Response.docx

Reviewer 3 Report

I really would like to congratulate with the authors for the work done. The paper is worthy of publication in Applied Sciences. Just some minor revisions, listed in the attached file, are necessary.

Comments for author File: Comments.pdf

Author Response

Referee #3: I really would like to congratulate with you for the work done. The paper is worthy of publication in Applied Sciences. Just some minor revisions, listed below, are necessary.

1. Introduction

• The development of antipermeation and corrosion barriers may have an impact on the WCLL BB design, in particular in the capture of the system requirements as well as in the assessment and definition of its interface with the primary heat transfer system and/or LiPb loop. In fact, thermal-hydraulic, safety and manufacturing aspects have to be taken into account, among the others, when introducing coatings. Hence, these implications should be mentioned at least in the introduction, in order to enforce the importance of this activity within the field and stressing its wide range of influence. To this end, a couple of reference may help to link the reported activity to the holistic BB design approach, governed by the systems engineering:
• A. Spagnuolo et al., Development of load specifications for the design of the breeding blanket system, Fusion Engineering and Design, 157, 111657, 2020, DOI: 10.1016/j.fusengdes.2020.111657.
• A. Spagnuolo et al., Systems Engineering approach in support to the breeding blanket design. Fusion Engineering and Design, 146, Part A, pp. 31-35, 2019, DOI: 10.1016/j.fusengdes.2018.11.016.

Thank you for your comment. Part of the introduction was rewritten according to your comment and the two references have been added.

2. Coating technologies and manufacturing strategies

• Have been the two fabrication technologies investigated (PLD and ALD) compared also from the cost/economic point of view? Is there any study and/or comparison (even in literature) on this aspect? Moreover, have the two fabrication processes a comparable efficiency? Please briefly comment theses points in this section, making also use of references if appropriate.

Thank you for your comment. There is no particular set cost for running a cycle of these systems; the cost is due to the initial capital cost and to the operation of the machine. A preliminary studies was carried out and integrated in the text, scaling up of the devices is foreseen for instance in EUROfusion activities.

EFDA_D_2NDPQ3 Internal Deliverable BB-9.3.6-T001-D005 Liquid Breeder cost-relevant inputs and PbLi Loops - 2017

 

• At lines 71-73 you state that no corrosion has been detected from experimental tests lasting, at maximum, 8000 h. Since the expected DEMO first BB lifetime is 2 fpy, corresponding to ~18000 h, and even more (5 fpy) for the second BB, is it possible to extrapolate from these results so to conclude that corrosion is never an issue? Please address this point.

 

Thank you for your comment. The corrosion was measured after 2000 h exposure, 4000 h and 8000 h and no EUROFER attack was observed or coating degradation, therefore it is possible to extrapolate that the coating will operate until 16000 h without degradation. However, in the frame of FP9 project it is planned to characterise the corrosion rate up to 16000 h. The answer has been detailed in Chapter 2.

 

• In sight of its application to the whole DEMO WCLL BB, has been the proposed coating procedure validated and/or discussed with the manufacturing group involved in the DEMO project? Please add some information (or reference, if any) on that.

 

Thank you for your comment. Preliminary discussion on how can be coated WCLL TBM/BB was started. Moreover, a dedicated procedure to validate the coating manufactured was developed, the Electrochemical Quality Test – EQT: EQT exploits the conductivity of the EUROFER and the high resistivity of the oxide coatings. The part is submerged in an electrolyte and by Electrical Impedance Spectroscopy (EIS) is possible to measure the resistance and capacitance of the system. By modelling the data, it is possible to extract an average defect density and coating resistivity on the whole coated component, even of complex shape, like a BBM. The paper was integrated with the validation procedure description.

Reference: EFDA_D_2NYJD4 - Internal Deliverable BB-5.3.1-T004-D002: Upgrade ALD and PLD device machine (Scale up of the system devoted to manufacture PLD and ALD coating)  

 

• Since you plan to cover all the welds with ALD coating, has been this aspect validated and/or discussed with the safety group involved in the DEMO project, in order to assess its applicability to the DEMO WCLL BB? Please add some information (or reference, if any) on that. Moreover, how this manufacturing process can affect the structural design/qualification criteria prescribed for welding? Please discuss this aspect, if possible.

 

Thank you for your comment. A preliminary discussion related coating manufacturing on all the welds with ALD was discussed, taking into account that the process will be performed at 100-200°C no impact on structural design can be expected.

4. Integration in TRIEX-II facility and experimental investigation

• Since in DEMO WCLL BB the LiPb temperature, in some regions, could be higher than 500 °C, could it be possible in principle to run experiments at highest temperature? If yes, there are limitations? Please comment on that point.

Thank you for your comment. In principle it is not possible to increase too much the temperature due to constraints in TRIEX-II. In particular, the maximum operative temperature of the pump is 530 °C, hence there is a low margin for experimental runs at temperature higher than 500 °C.

• Figure 3 is hard to see clearly. Could you please highlight with circles at least the main components?

Thank you for your comment. Figure 3 was updated including circles and new caption.

5. Transport analysis in support of the design

• Since big margins of uncertainty exist for T transport parameters (even several order of magnitudes), it could be interesting to perform parametric analysis in support of the WCLL mock-up adopting proper ranges of variation for some quantities. Do you have planned such a kind of analysis? Please comment on that.

Thank you for your comment. Additional parametric analyses, especially for the surface parameters of the steel, will be carried out as a next step in order to compare the numerical results with the experimental results. It is foreseen to perform multiphysics transport modelling considering also the LiPb velocity field with sensitivity analysis on the recombination constant of Eurofer (to include possible oxidation of the surface). Sensitivity analyses on the Sieverts’ constant will be taken in consideration on the basis of the experimental campaign outcomes for Sieverts’ constant measurement, which is now on-going at ENEA Brasimone research centre. To catch your comment, the following sentence has been added in the conclusions: “Additional sensitivity analysis on the surface parameters of EUROFER can be carried out in order to take into account possible oxidation of the steel.”

Author Response File: Author Response.docx