Peer review of: Understanding the interface interaction between U3Si2 fuel and SiC cladding (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

This is an interesting and original paper. It provides new data on the fuel clad interaction in 
support with the development of accident tolerant fuels considering a concept with a SiC/SiC 
composite clad and a fuel compound U3Si2. Data provided are thermodynamic data obtained by 
both DFT calculations and experimental measurements. 
The manuscript is clear and well presented. It provides new information and data presented are 
well supported and explained. Therefore I recommend publication. 
I have however a few questions and comments to clear before publication. 
1- Please check the English language throughout the manuscript. 
2- The authors present interaction data between SiC and U3Si2 and calculations of reactions and 
defect formation energies supported by experiment. However, no information is given on the 
kinetics of these reactions. Experiments performed at 1000 °C for 100h showed no interaction. 
However, can the authors ascertain the fact that there is a critical temperature for initializing the 
SiC/U3Si2 interaction? Is it only a question of energy at this level or also a question of time? 
3- At 1200 °C, the authors present results showing interaction between SiC and U3Si2 as also 
predicted by calculations. Only results obtained after 100 hours were presented. What would be 
obtained for longer duration? Is the kinetic modelling of the considered interaction mechanism is 
planned by the authors? Moreover, the authors say that due to the formation of phases at the 
SiC/U3Si2 interface, at a point, they are no longer in contact, therefore there are no reactions any 
longer except Si diffusion into U3Si2. 
Is it really possible to conclude that in a real fuel/clad configuration there would no be any 
possibility to continue having some interactions? Is it possible to say that the formation is really 
homogeneous throughout the interface? Moreover concerning the Si diffusion, what would be its 
impact on longer duration? Could it be detrimental to the clad integrity? 
4- Experiments were performed using CVD SiC, are more representative experiment using SiC/SiC 
composites with more representative geometries planned? Moreover are longer test duration 
considered? 



Reviewer #2 (Remarks to the Author): 

Reviewer report on the article “Understanding the interface interaction between U3Si2 fuel and SiC 
cladding” by Vancho Kocevski et al. 

The manuscript ““Understanding the interface interaction between U3Si2 fuel and SiC cladding” 
represents a combination of experiment and first principles calculation addressing chemical 
compatibility U3Si2 fuel and SiC cladding. Currently the U3Si2/SiC composite fuel/cladding system 
is considered a very promising option of international accident tolerance fuel initiative. To assess 
U3Si2/SiC as a possible candidate the chemical reaction between fuel and cladding are 
investigated using the density functional theory approach. Authors used this approach to establish 
phases formed at the interface from the calculated reaction energies. In addition the defect 
formation energies in U3Si2 and SiC in the presence of third phase have been calculated. The 
obtained information was used to understand U3Si5, UC and U20Si16C3 phases formation at the 
atomic scale. Further, the results and their interpretations are supported by presented 
experimental observations. The analysis looks like a very interesting and promising approach to 
initial estimate of interactions, phases formation and general stability of complex interfaces in 
harsh environment. 
In summary, this manuscript is of unquestionable interest for general auditorium of the Nature 
Communications readers. The story is well written and easy to read. I recommend publication in 
Nature Communication after some revision. 

Major questions: 
1. Equation 1, 2, 3 should be presented right at the place they are first mentioned, not at the end, 
in the Method section. To reviewer’s opinion it will make the text much easier to read. 
2. Authors discuss diffusion processes using information of the defects formation energies only. 
However, the defects propagation over the energy barriers separating two metastable or stable 
states is an important component of the diffusion process. It looks like some discussion here will 
be in place. 
3. The defects formation energy discussed in the text are calculated in the bulk phases. While 
presence of different phases is incorporated through chemical potential. However, often the 
defects formation energy at actual (sharp, physical) interface is very different from the one in the 
bulk. I believe that, it will make the analysis stronger if the authors present a reason why these 
types of defects are not taken into consideration. 



Reviewer #3 (Remarks to the Author): 

Understanding the interface interaction between U3Si2 fuel and SiC cladding by Kocevski et al., 
examines the chemical interaction between U3Si2 fuel and SiC cladding by DFT computation and 
diffusion couple experiments. Calculation of reaction energies demonstrate that U3Si5, UC, 
U20Si16C3, and USi phases can form at the interface. Defect formation energies calculation reveal 
that a thermodynamic driving force for generating defects in both fuel and cladding exists when 
U20Si16C3, U3Si5 and UC are in equilibrium together. Diffusion couple experiments were also 
carried out to document the formation of U3Si5, UC, and U20Si16C3/USi phases at the interface. 

While the subject matter of this paper could be important for the international accident tolerance 
fuel (ATF) program, the manuscript fails to provide any “understanding” beyond what one can gain 
from a simple assessment of U-Si-C ternary phase equilibria with some assumptions in atomic 
mobility – as far as potential phases that can form. In fact, the computational component of this 
work, while detailed in thermodynamics driving force and defect formation, omits the actual 
“diffusion kinetics” aspect. A better understanding would require, for example, knowledge of 
atomic mobility. 

Documentation of experimental work is limited and confusing. Authors do claim that “The 
materials of the couples also did not adhere, separating during the disassembly of the jigs.” Still it 
would be helpful to show, in one micrograph, SiC, U3Si2 and interaction layer in-between, so that 
the reader is spatially oriented. 

Will the actual applicaton of this fuel system be under high purity Argon? Any presence of oxygen 
(e.g., at the interface) would drastically change the thermodynamics and diffusion kinetics. 

This manuscript is more suitable for Journal of Nuclear Materials, as it represents more of an 
incremental work, and does not provide comprehensive and/or transformative “understanding.”