A Review of Zirconolite Solid Solution Regimes for Plutonium and Candidate Neutron Absorbing Additives
Round 1
Reviewer 1 Report
The authors have presented the review of Zirconolite solid solution regime for plutonium and candidate neutron absorbing additives.
The study lays emphasis on Should the decision be made to immobilise the UK Pu inventory through a campaign of Hot Isostatic Pressing (HIP) in a zirconolite matrix, prior to placement in a geological disposal facility (GDF), a suite of disposability criteria must be satisfied. A GDF safety case should be able to demonstrate that post-closure criticality is not a significant concern, by demonstrating that such an event would have a low likelihood of occurring and low consequence if it were to occur. In the case of ceramic wasteforms, an effective means of criticality control may be the co-incorporation of a requisite quantity of a suitable neutron absorbing additive, either through co-immobilisation within the host structure, or encapsulation of discrete particles within the grain structure. Following an initial screening of a range of potential neutron absorbing additives, a literature-based assessment of the solid solution limits of a number of potential additives (Gd, Hf, Sm, In, Cd and B) in the candidate zirconolite (CaZrTi2O7) wasteform is presented. Key areas of research that are in need of development to further support the safety case for nuclearized HIP for Pu inventories are discussed. My specific comments are as follow.
1. Discuss the significance of the current study in some more details in the introduction part.
2. You can equip your study with comparison table of related studies and their results.
3. Double check all the Figures in terms of their clarity and quality.
English level has been fine overall some minor changes are required.
Author Response
Please see attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The present work devotes to the zirconolite solid solutions regimes for Pu immobilization and feasible neutron absorbing additives. Even though the aim of the work was to provide an overview of the status of the UK immobilization program for Pu inventories, it will be of great interest to all the community working in the field of high-level waste utilization. The crystal chemistry of the zirconolite polytype modifications is described. Main works on the Pu incorporation in to the zirconolite phases are outlined and summarized. The solid solution limits of potential neutron absorbing additives (Gd, Hf, Sm, Cd, In, B) in zirconolite are described. The influence of the dopants and regimes on the phase formation and solid solution limits is considered. The problem of congruent leaching of fissile materials and neutron absorbing phases is discussed. The text is clear.
Author Response
We thank Reviewer #2 for their kind appraisal of our work.
Reviewer 3 Report
The work is devoted to a review of neutron absorber candidates that can be applied as additives in zirconolite matrices containing radioactive Pu. The relevance of the work is determined by the significant stockpiles of PuO2 formed as a result of spent nuclear fuel reprocessing (140 teHM (tonnes-heavy-metal-equivalent)). Immobilization of radionuclides must meet certain standards, among which safety and environmental compatibility conditions are key. For PuO2-type waste there should be criticality conditions, which define the maximum possible filling of the immobilizing matrix with the target waste and regulate the presence of neutron absorbers. These conditions should make it impossible to start a chain reaction. The authors have considered such elements as Gd, Hf, Sm, Cd, In, B as absorbents, evaluated the results of selected additives in terms of cost, absorption efficiency, influence on the structure (monophase or multiphase materials formation), thermal stability, joint leaching rate of both the target radionuclide and neutron absorber, evaluated the stability of structures with different mass content of additive. The HIP method was chosen as the method of formation of the final matrix form. The work is of great interest from the point of view of generalizing the available results and solutions aimed at creating ceramic matrices based on zirconolite capable of immobilizing Pu together with a neutron absorber. I recommend the article for publication. I would like an answer to a number of clarifying questions: 1. Why was the HIP method chosen as the main one considered for the synthesis of ceramics? Much of the scientific literature is based not so much on the creation of materials, but for the methods of their creation. Among the most popular and researched at the moment methods pulsed electric current sintering, PECS or plasma-activated sintering, PAS, allowing to obtain materials that are devoid of some structural flaws.
2. The work often touches on the concept of a single hardware design for the entire waste treatment process, which is associated with significant structural and technological constraints. Why did you focus on this option?
3. I would like to see results and a description from the leaching mechanisms. Please emphasize the following works. 10.1016/j.jnucmat.2020.152314, 10.1016/j.jeurceramsoc.2022.02.007, 10.3390/ma12162638, 10.1016/j.jnucmat.2020.152314
4. What is missing is a large, extensive table with matrix parameters (leaching rate, compressive strength, density, hardness, method of producing ceramic matrices). A detailed justification of why cerconolite and perchlor are better than ceramics?
5. a lot of papers with links older than 5 years, please increase modern links in the work
6. Not enough feedback on the modern ISO currently used for leaching and fmc-mechanical methods for investigating the properties of ceramics
7. Why do the authors regard plutonium waste as radioactive waste? I ask to add a block to the review - the obtaining of products (A radioisotope thermoelectric generator (RTG, RITEG)). zirconolite is a good natural-like matrix as a candidate for such a product 10.1016/j.jhazmat.2019.02.016
The article was written by a native speaker, no remarks
Author Response
Please see attachment.
Author Response File: Author Response.pdf
Reviewer 4 Report
The msnuscript shows the review of zirconolite chemistry and evaluation of potential solid solution regimes for Pu immobilisation, which basically contains the recent development about zirconolite in these application fields. But the structure of paper needs to be further improved. Some problems need to be solved.
1.From Line 145 on, in the review, some data or figures need to be concluded as the fugures in the manuscript. This should contain some (at least four figures) to review the development of zirconolite materials in many fields.
2. The review paper structure needs to be improved.
3. The english grammmar should be further improved.
The msnuscript shows the review of zirconolite chemistry and evaluation of potential solid solution regimes for Pu immobilisation, which basically contains the recent development about zirconolite in these application fields. But the structure of paper needs to be further improved. Some problems need to be solved.
1.From Line 145 on, in the review, some data or figures need to be concluded as the fugures in the manuscript. This should contain some (at least four figures) to review the development of zirconolite materials in many fields.
2. The review paper structure needs to be improved.
3. The english grammmar should be further improved.
Author Response
Please see attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Accept in its present form.
English language has been fine.
Author Response
We thank the Reviewer for their comments and for accepting in current form.
Reviewer 3 Report
The authors have completely ignored all the comments I made to improve the publication. It's too bad to realize that colleagues from UK and USA have not responded to them in any way.
1. As I have written before, there is a lack of data in the area of physico-chemical and physical-mechanical characteristics. without it, the general conceptual character of the overview is lost.
2. A similar review has already been described by Orlova Albina from Lobachevsky NNSU in Materials. Please take this fact into account.
3. Plutonium is a very dangerous and harmful man-made metal on earth. And I am surprised by the fact that the authors did not disentangle the methods of leaching and releasing plutonium from the presented matrices. no mechanisms have been disentangled.
4. Regarding plutonium-based RITGs, plutonium oxide RITGs are currently in use. Why haven't the authors investigated and dealt with the fundamental issue of using zirconia matrices for such devices? https://mars.nasa.gov/msl/home/ ,
DOIhttps://doi.org/10.1039/D2NA00306F
5.for example, for deep space exploration. As you can see, there is a very high risk of accidents when such vehicles take off from the surface of our planet. It is necessary to get the strongest and best possible structures on the RITEGs' basis.
6. I have not seen an elaborate comparison table with different matrix methods (HIP, CIP, SPS and others)
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Author Response
Dear Reviewer,
We are grateful for your comments, however, I have significant reservations with the comment provided insofar as the requested changes would incur a significant modification to both the scope and objectives of the manuscript. The article was intended to provide an overview of solid solution mechanisms of Pu and, in tandem, Gd, Hf, B, In, Sm and Cd within zirconolite-structured phases. The article was written in a logical order to reflect these aims and we are confident that these points were discussed meaningfully. The article did not intend to, as many of your comments requested, provide a discussion and summary: of mechanical properties, synthesis methods and aqueous durability. To provide a thorough review of these properties in any meaningful detail would require at least two separate articles; therefore, I feel it prudent to disregard these as they would require months of additional work to say the least. My leading reservation with the comments are based on the fact that the article did not claim to discuss these properties and at no point were they alluded to in any great detail. This can be reflected by the extremely positive nature of Reviewer #2's comments. I am confident that a second review is simply not necessary. We do not wish to make significant modifications to our paper based on unreasonable comments regarding subject matter that was never claimed to be discussed - particularly: dissolution mechanisms, fabrication methods and mechanical properties. It was not the aim of the present work. Happy to discuss further, Kind regards Dr Lewis BlackburnRound 3
Reviewer 3 Report
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