Review Reports

Round 1

Reviewer 1 Report

The authors should provide also the version of the COMSOL Multiphysics which they used in their analysis.

The authors should quantify what is the most of the mesh which have element quality higher than 0.5. Also, did the authors built the mesh using the automatic methods COMSOL offers, or they defined it by themselves?

The authors mention that the most efficient direct solver was the "PARDISO". Which solvers did they compare? It would be good to provide a graph (probably in the supplementary material) showing the differences between the solvers. Also, the technical specifications of the cluster should provided. The authors used both iterative and direct solvers, or only direct solvers? What was the convergence criterion used for all variables? More information should be provided in this section. 

The authors should provide quantitative results and not qualitative. For example, in the 3.1. section, phrases as "should not be too large", or "most regions of the fluid.." should be avoided. 

In the literature is available the boiling temperature of the molten salt?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The caption (so explanation what we see) has to be improved strongly for most figures.

My main criticism is on the meaning of this work. It reads in the manuscript ‘Since this SMDFR design is quite novel and no experimental data can be used as reference’ so the simulation results refer to something not yet constructed? So one cannot validate the model results. Will the SMDFR be built? This study seems a design study, as such valuable, but what is the follow up?

Line 209-210: The computer model is validated by examining the value of wall 210 resolution in viscous units and performing mesh sensitivity study. It does not seem the model is validated, validation is something accomplished by comparing against experimental data or proven more exact/better model.

Introduction: please add to the text what such units are used for.  E.g. something based on ‘If the reactor works as intended, the U-238 of a spent nuclear fuel element of a LWR (about 1 ton) could be completely dissolved in Cl-salt, including the problematic long-living transuranic parts. Complete breeding and fission could power a 1 GW thermal DFR for about 2.5 years. After that time the element would be completely converted into fission products and the need for storage in a final deposition for nuclear waste would be reduced from 1 million to about 300 years.’

At the end of the Introduction section it needs to be described what is intended to be achieved in this study, what simulation, why, the goal, are other simulations already published? For instance move the first part of the Conclusions section 4 to the end of the Introduction (noval-> novel design of the core (including the distribution zone, fission zone and collection zone)

207 of small modular dual fluid reactor is proposed and the thermal hydraulics characteristics of the 208 distribution zone is investigated by the numerical simulation using COMSOL Multiphysics with CFD 209 Module and Heat Transfer Module. The computer model is validated by examining the value of wall 210 resolution in viscous units and performing mesh sensitivity study.).

Line 37: a distribution zone and a collection zone, please indicate these in the figure

Line 42: for the chemical processing plant. What is this, please describe what, how

Line 48: in order to be more flexible. What is the increased flexibility, carn it be easier used in certain applications? Please comment in the text.

Line 50: control rod. Please comment why this is otherwise needed.

Line 70: much more fuel tubes:  MANY more fuel tubes

Line 70-75: ‘it is quite challenging

71 to contain all of them in the computer model due to the limited computational resources. Thanks to the 72 hexagonal configuration of the core, a fan of 30_ (Figure 4) around the central cylindrical axis, which 73 is only one twelfth of the complete geometry, is selected to present the full scale characteristics by 74 applying symmetry boundary conditions [10].The number of fuel tubes to be simulated is decreased 75 from 1027 to 100, among which there are 72 complete tubes.’

Please address the following items

• How long did a single simulation take, and on what computer resources
• There are 72 complete tubes, so 28 incomplete tubes, what does this mean, what are the implications?
• What symmetry was applied, e.g. D6h ???? (6 fold rotation and mirror plane ..)
• a fan of 30_ (Figure 4) around the central cylindrical axis: please show in Figure 3 which are these 30,. In other words integrate Fig 3 and Fig 4. From Fig 4 I find it difficult to see what symmetry element is applied

Line 90: the weakly compressible 91 Reynolds-Averaged Navier-Stokes (RANS) equations are adopted: is this what is an option in the COMSOL software suite?

Line 109: From the engineering point of view this mesh scenarios is acceptable: what do you mean by this, from an engineering point, please explain in the text.

Caption Fig 8 and Fig 9: what is the unit for the X-axis? Same question for Fig 14 and Fig 15.

Line 145-147: A majority of the molten salt tends 147 to go upwards along the fuel tubes and only a small part can pass the peripheral regions and reach the 148 central regions (Figure 10).

Figure 10 is not very clear for me, what I am seeing here, how does the geometry compare to previous figures, and can it be indicated where the peripheral and the central regions are located.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

No further suggestions and/or comments are required for the manuscript.

Reviewer 2 Report

To my opinion the revised version of this manuscript can be published.

Still, I would like to make a remark. As the reactor was not yet built, it is not possible to validate. For a very preliminary study I think this is OK, but the authors stating the computational resources were limited, in their reply to the first referee round they stated ' A single simulation takes about 31611 seconds when using “PARDISO” solver '  which is less than 10 hrs'  and that on a PC. Further studies should definitely be performed using other computer resources (much larger, taking a finer mesh, more tubes, to verify results), and for the TU Munich this cannot be a problem, for sure.