Molecular Dynamics Simulations of Xe Behaviors at the Grain Boundary in UO2
, Qingqing Wang 2, Lu Wu 2 and Huiqiu Deng 1,*Round 1
Reviewer 1 Report
The manuscript is devoted to the study of the migration of interstitial Xe atoms and the growth of Xe bubbles in UO2 using computer simulations in the framework of the molecular dynamics method. It is shown that it is energetically more favorable for gas bubbles to segregate in the region of a symmetric tilt grain boundary Σ5 than in bulk UO2. The migration of Xe atoms in this boundary is characterized by a lower potential barrier compared to bulk UO2. The results of the calculations are not fully clear, and some points need to be clarified, as indicated below. Additionally, English language editing is needed for this.
1) English language is poor. Lots of incorrect word usage, misspellings, wrong endings, word order, tautology, etc.
Some examples are:
“The pressure of internal pressure of”
UO2 should be UO2
lattice constant a0 should be a0
“bubble evolution mechanism of Xe bubble”
“to study the bubble evolution mechanism of Xe bubble, … is used for the simulations of Xe bubble”
What does “structure of migration of Xe atom” mean?
“energy calculated for the formation energy”
“driven force”
There is no description of E: “E decreases with a maximum value”
ΔE is not described
“is used to calculated,”
“to contrast” is better change to “to compare”
There is no need to mention the simulation method again in the Results section: “The nucleation and growth of Xe bubbles at an early stage was simulated by molecular dynamics method.”
etc.
Authors should thoroughly revise the text of the manuscript and improve English language.
2) Table 1:
The caption is incorrect: it says “Xe is incorporated at different sites in UO2 grain boundary”, but apparently these are calculations for bulk UO2. Almost all Table 1 caption in this manuscript is simply copied from Table 1 caption in ref. [3].
There is no description of the symbols in the first column.
There is no description of the symbols in the first line.
What is the meaning of this sentence: “Contrast amount is …” ?
How do your calculations compare with the data in Table 1 from ref. [3]?
3) The superfluous description of the GB structure theory (lines 110-128) must be removed, it is not necessary in the context of the article and is contained in almost any textbook on the structure of solids. Instead, it is better to give an illustration of the GB itself at different temperatures, especially if it is discussed that the structure of the GB changed with increasing temperature: “At 2000 K, the Σ5(310) grain boundary is consisting of a more distorted triangular pattern, and the middle gap becomes larger.”
4) The X-axis in Figure 1 is labeled incorrectly. Should “Number of Xe atoms” be right?
5) The description of formula (2) contains errors.
First it is stated that “formation energy of the Xe atoms is defined as … ”, but later it is described as “the total energy of simulation supercell”
Two designations, N and n, are used, which apparently mean the same thing.
If is the total energy of the supercell without Xe atoms, what is N means?
Please provide correct description of the formation energy of Xe atom as it done in J. Phys.: Condens. Matter 26 (2014) 105501 or in ref. [8].
6) Figure 3 and Table 2 duplicate each other, one of them should be removed.
7) “Figure 4. Comparison of nucleation configurations …” of what?
8) “the volume of Xe bubble increases approximately linearly with the increasing number of Xe atoms, with a trend of rapid growth. However, at higher Xe concentration, the rate of growth will slow down with the increasing of Xe atoms.” Where is interval of linear growth and interval of “higher Xe concentration” in Figure 5(a)? These dependencies are approximately linear everywhere. Please correct the further explanation on lines 298-302.
9) The authors should explain in the text of the manuscript why the pressure in the bubble in the GB decreases while increasing the number of Xe atoms.
10) From data in ref. [3] pressure for 1500 K and 75 Xe atoms in bulk lies in the range from 2 to 4 GPa. In these calculations, if we take the average value for temperatures of 1000 and 2000 K to compare with 1500 K and extrapolate to 75 atoms, then we get a value of ~12.5 GPa. Bubble volume of 75 Xe atoms in bulk for 1500K in ref. [3] equals 4 nm3, and in these calculations for 1000-2000K only 2-2.7 nm3 if extrapolated to 75 atoms.
How can such a significant discrepancy be explained if authors of ref. [3] state that all potential functions approximately similarly describe bubbles for a small number of Xe atoms? Moreover, we compare the bubble in the GB in these calculations and the bubble in the bulk in ref. [3]. It turns out that the pressure in the GB is higher, and the atomic volume is less than in bulk. What is the pressure and atomic volume in the GB region according to your calculations? It would also be useful for understanding to add dependencies for Xe bubbles in bulk UO2 in Figure 5.
11) In addition, in this work there is no description of the sample and calculation scheme for the bubble in bulk UO2. Corresponding text should be added to the manuscript.
12) “In the complete block, some studies have shown that the pressure is very high when Xe atoms are added, but the pressure is relatively low with the addition of Xe atoms [8].” Absolutely confusing sentence. Is pressure high or low when adding Xe atoms? What is “complete block”?
Comments for author File: 
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Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
To be honest, I am not the best reviewer to feel qualified to judge the paper's merits. I admit that the Xe (nobel gas) atoms can segregate into grain boundaries of UO_2 (metal-like) matrix, and that they experience smaller order at the boundaries than in the bulk, the latter tuned somehow by the temperature value.
First of all, I don't feel qualified to judge several physical potentials applied by the Authors within the MD technique employed, and how to discriminate the subtleties among them. Second, what is missing to the present reviewer is a type of (mini)review paper that is offered here, and that is able to unveil bubbles formation vs. grain (boundaries involving) growth in the overall principal (non)diffusional regime(s). Thus, a small suggestion would be to take into consideration the contents of: (a) M. Niemiec et al. ”Phase transformation kinetics in d-dimensional grains containing systems: diffusion-type model”, Physica A 248, 365 (1998); A. Gadomski, ”Kinetic approach to the nucleation-and-growth phase transition in complex systems”, Nonlinear Phenomena in Complex Systems 3/4, 321 (2000), if the Authors don't mind (to be put somehow in Intro or as appropriate, if useful). Technically, the Authors have to avoid incorrect descriptions within the axes' legends of figs 1-2 that are fairly inaccurate such as lnD(m^2/s) or that of horizontal axis of Fig. 1.
Overall, the paper, in spite of the reviewer's incompetence in judging potential's subtleties addressing details, looks publishable after a meaningful minor revision. It would also be advantageous to know whether the potentials employed belong to a class of entropic potentials, see J.M. Rubi, A. Gadomski, "Nonequilibrium thermodynamics versus model grain growth: derivation and some physical implications" https://doi.org/10.1016/S0378-4371(03)00282-6.
Last, is it reasonable to use the notion of molecular statics in this ms., see the 2nd line of Abstract, or in the 2nd line of p. 3: "molecular static/dynamic simulation" - it seems to me to be either obsolete or out of convention? (Is the statics a manifestation of the 3rd law of dynamics and/or the implication of the 1st law of thermodynamics, or both?)
Author Response
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Reviewer 3 Report
The paper is devoted to the computational study of a Xe/UO2 grain boundary. The article is scientifically sound and well-written; a good literature review is given. Overall, the presented research is an important step toward our fundamental understanding of processes occurring in uranium dioxide at the atomistic level. Thus, in my opinion, this manuscript can be published in Metals after minor revision.
- Page 1, line 16. “The bubble structure in grain boundary...”
How the bubble structure is determined? It should be explained.
- Table 1. The following abbreviations should be explained: “08-GB, 10-CM, 14-TB, 14-TM, 14-TC, 16-CC”.
- Page 4, line 173. “the number of Xe atoms in the bubble is too small to give us a good statistical result”.
This number should be specified.
- Page 4. All quantities in Eq 1 should be defined.
- Page 4. “The first step is to construct and relax the initial system”
How do the authors relax the system? It should be explained in the text.
- Page 4. “when the system relaxed at a given temperature”
How did the authors know that the system is fully relaxed? It should be explained in the text.
- Page 6. Figures 1a and 1b can be combined.
- Page 7, line 267. “it was needed several picoseconds for the system to relax”
How did the authors find this relaxation time? It should be explained in the text.
- Page 8, line 283. “This is due to the increase of temperature”
The simulations were performed in an NVT ensemble, i.e. at a constant temperature. Therefore, it is not clear why the temperature is changed? It should be explained in the text.
- Page 10. The pressures in the UO2 phase should also be calculated and drawn.
Author Response
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Author Response File: Author Response.pdf
