Gd2O3 Doped UO2(s) Corrosion in the Presence of Silicate and Calcium under Alkaline Conditions
, Javier Giménez 1, Ignasi Casas 1, Jordi Llorca 1 and Joan De Pablo 1,2Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis article is well structured and interesting.
It concerns the oxidative dissolution of material simulating the chemistry of an irradiated fuel and focuses on the role of trivalent elements.
Before publication, I suggest the following developments / complements:
1) It is also important to integrate and discuss works from the literature on the same subject but studying alteration through a chemical and not an electrochemical method. The results are consistent and complementary. Casella's work, for example, is interesting in this respect.
Suggested reference: Casella A, Hanson B and Miller W (“The effect of fuel chemistry on UO2 dissolution”) Journal of Nuclear Materials 476, PP 45-55).
2) The role of cluster involving oxygen vacancies is from my point of view not fully demonstrated. From this point of view, it would be necessary to have at least information on the initial stoichiometry of the samples (bulk). Measuring the crystal lattice parameter can provide information on the nature of the structural defects and clarify how the +III ionic charge is compensated in the solid. The presence of oxygen vacancies is a possibility but U(V) can also form while maintaining oxygen stoichiometry. XPS does not allow structural defects to be qualified. Can the authors complete their characterizations (XRD, Raman) and specify the nature of the structural defects? Otherwise, the interpretation must be qualified.
Suggested reference: The combined influence of gadolinium doping and non-stoichiometry on the structural and electrochemical properties of uranium dioxide. (Kim, J and coll. Electrochimica Acta 2017, 247, pp 942 – 948).
3) Page 4 and figure 2: The simultaneous presence of calcium and carbonates could also lead to the formation of ternary complexes that are even more stable than the carbonated complexes of uranium VI. Moreover, at basic pH the ligands (OH) can also play a role. Have the authors tested a system with Ca, Si and carbonates? What will then be the predominant effect between the possible formation of very soluble ternary complexes and the inhibitory role of Ca and Si? This point could be discussed and considered as a perspective.
Suggested reference: Evaluation of thermodynamic data for aqueous Ca-U(VI)-CO3 species under conditions characteristic of geological clay formation. Maia, FMS; Ribet, S; (...); Montavon, G. Applied Geochemistry, Janv 2021.
4) Page 5: “CVs are rapid experiments” but has the influence of the scan rate been evaluated by the authors and why was this rate of 10 mV/s chosen? Please clarify this.
5) Figure 7 and Ecorr measurements: What are the experimental conditions (aerated environment or under nitrogen) for acquiring these data and more generally what are the oxidants? Please clarify this.
To conclude the article presents interesting results under basic pH conditions on the role of fuel chemistry. I thank the users for taking into account the elements of discussion and the additions suggested above.
Author Response
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Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis paper studied the corrosion behavior of UO2 and Gd2O3 doped UO2 in alkaline solutions using electrochemical techniques. Results reveal that the increase in Gd2O3 doping level led to a reduction in the reactivity of UO2, and the results is a beneficial for the long-term management of spent nuclear fuel in a repository.
The results of this paper are quite interesting and worth to be published. The reviewer only have some small comments before it could be published. Here are some suggestion to revise.
1. We uauslly write the weight percentage as “wt.%” rather than “% wt.”.
2. Why use the “0.1 mol.dm-3 NaCl” as the electrolyte for this work?
3. Why use logI as the Y axis in Figure 3? It would be better to use Current vs. Time. I don’t think the current for UO2 case reaches a steady state after 1000s based on Figure 4.
4. THE XPS data in figure 5, why the peak move towards right as the Gd2O3 increases?
5. Please mark the peaks in Figure 6.
Author Response
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Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors present a very interesting work on the corrosion behaviour of UO2 and the effect of doping, Gd2O3 in this study. Doped UO2 is considered the most relevant advanced nuclear fuel and the understanding of the stability of this material in a nuclear repository is of significant importance. The manuscript is well structured and articulated. The approach that the authors are following is very interesting and complementary to standard dissolution experiments in which the dissolution rate of elements from the matrix in defined solutions is determined from concentration evolution of certain elements in the solution over time. The approach is well described in the introduction by referring to sufficient number of important journal articles. The experiments are very good explained, and the results are presented and discussed conclusively by enough tables and figures.
However, I want to address a few points that deserve some amendments or revisions from my point of view:
Page 1, line 35-38: Here, the authors describe that doping UO2 fuel is improving the reactor performance. I want to encourage the authors to give at least one example why Gd-doping was selected for this study. Is it because Gd is in the middle of the rare earth element series and the ionic radius fits best to the uranium species? Another and from my point of view more important reason is that Gd is a perfect neutron absorber and therefore considered in the development of advanced nuclear fuel.
Page 2, line 61-64: Here, the authors describe what the focus of this work is and to be honest I miss a very strong argument here why the authors preferred the electrochemical approach for this study. Here, a sentence is needed why the authors didn’t perform standard dissolution experiments in presence of silicate and calcium. I am pretty sure that the dissolution experiments won’t work with calcium and silicate. But if that is the case it needs to be mentioned, I guess. And what do we learn from this approach that is not accessible from the standard dissolution experiments in respect to reference 12, e.g. information about quantification of oxidation state ratios. That means, it is possible to extract quantitative information of the dissolution mechanism if I got it right.
Page 3, line 72/73: Here, the preparation of the material is described. But I miss any information how the material looks like. Is the Gd incorporated into the UO2 structure forming a solid solution and therefore, homogeneously distributed? Or do the pellets look like a two-phase mixture in which UO2-domains exist next to segregated Gd2O3. That can easily be measured by XRD and SEM and I am pretty sure that the authors did these measurements before performing the experiments. I recommend adding in chapter 3 a small chapter on the characterisation of the pellets.
Page 3, chapter 2.3 Electrochemical techniques: The authors give short summary about the electrochemical experiments performed in this work. I recommend informing here which information one can derive from each type of measurement. That would clarify why the series of experiments is essentially needed.
Page 3, chapter 2.4 solution: Is there any shift in the pH detected during the experiments? That would be important information, too.
Results and discussion: This chapter is very strong.
However, I am wondering if it is possible to reflect the results derived from these studies to the formerly performed dissolution studies, described in reference 12 and to a work that a German group published recently (Kegler et al.: Accelerated dissolution of doped UO2‑based model systems as analogues for modern spent nuclear fuel under repository conditions. MRS Advances, 8 (2023) 255–260). I guess in sum these investigations give a good view on the entire picture of the dissolution behaviour of doped advanced fuels.
Additionally, I have some additional very small remarks:
Page 1, line 32: to check the format of the numbers in the description of the U(IV)U(V)-oxide; numbers in lower case.
Page3, line 96: to add “recorded”: “All potential values are recorded versus…”
Page 8, line 235: “experiments” instead of “experiment”.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThank you for the corrections.
Reviewer 3 Report
Comments and Suggestions for AuthorsDear authors,
thank you very much for the work you put into this very relevant topic. And your efforts to the reviewers comments are highly appreciated.
Congratulaion to this phantastic work!
