Mini-Reactor Proliferation-Resistant Fuel with Burnable Gadolinia in Once-Through Operation Cycle Performance Verification

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1) very timely, sound investigation method/s, technically well-written

Author Response

Please see attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Please see the attached document. 

I completely understand the authors put a lot of effort into this manuscript, but I believe that the results are not properly generated. My major concern is that the results are not generated correctly and any conclusion the authors might have could simply be invalid.

Comments for author File: Comments.pdf

Author Response

Please see attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper presents an interesting study on a "single-load" core design for an SMR using LEU and Gd as burnable poison. The paper is well-structured, results are clearly presented and are supported by the data.

Remarks on content

- Concerning the MINI21 design: what is, as of today, the best "reference" for the design (giving an idea of the concept, not only neutronically), the business case, ... The design is not taken up into the NEA SMR dashboard. If the authors would score the MINI21 on the different categories of the NEA dashboard, how would the MINI21 score?

- Concerning the MINI21 design: who is driving this design? Why? Up to now, this is a "paper reactor" (and those always work and are built on-schedule, within-budget etc). What are the "industrial" challenges. I miss some critical reflection on this.

- p1, l41-42: the fresh fuel is UOX so it is a bit weird to read here on requirements on the Pu vector. Maybe already clarify that this is the Pu vector of the spent fuel. And to corroborate the phrase in the conclusions on this requirement (at the start Pu239/Pu is 100% and decreases but the mass is irrelevant for proliferation issues), it would be nice to see a table/graph of total Pu mass and fraction of Pu239/Pu as a function of burnup.

- p2, l 45: it would have been nice to see a comparison of the spent fuel properties (radiation emission, decay heat, radiotoxicity) of the MINI21 vs a classical PWR (normalized to fuel mass) for example.

- p2, l58: LEU+ with 7.5wt% U235. This is (a little bit) above the "classic" 5%. Is it worth the trouble? What would be the impact on the core design if you would restrict yourself to 5%? Because with 7.5wt% you enter the "twilight zone" on transportation, storage etc of the fresh fuel (transport containers, criticality risk, ....).

-p2, l60: the core seems far from the "optimal cylinder" dimensions. Why?

-p2, Table1: this "differential" loading with different Gd contents makes fabrication of pellets and assembly of pins/assemblies complex. Has this been discussed with a fuel vendor on feasibility? If so, are there QA methods/inspections possible to check a sealed pin whether it corresponds to desired loading?

-p4, l121-132: Two questions: why the reduction in isotopes taken into account (can't be a computational bottleneck compared to MCNP)? On what did you base the time-stepping of the depletion algorithm? I can "observe" from the graphs that it is a fixed time step in BU (I think), but no finer grid at start-up (to capture Sm/Xe transient)?

-p5, Table 3: surprised to see B in the list. From the description of the MINI21, I (implicitly) assumed that BU would be counteracted by Gd alone. If there is boric acid used, what is the let-down curve?

-p5, l162: If I understand correctly, the difference in attainable BU between no-Gd and yes-Gd is 4 GWd/t. That still is >6% of the final BU. Acceptable from economics point of view?

-p8, section 3.4: why do you not include Pu236? When you load Np237, you will create some Pu236 by (n,2n) reactions on Np237 (via Np236m). It will not be much, but the decay chain for Pu236 is rather troublesome (radiation emission) and this could have an impact on handling/storage.

-p9, section 3.4: a crucial ingredient I'm missing in this paper is the control rod worth over the cycle. Especially with depleting burnable poison, the CRW will vary a lot over time. A neutronics study is not complete without this. So please provide differential/integral curves for typical CR positions for some moments in time (or in BU) during the cycle.

Suggestions for textual improvement/typos

- p1, line 28-29: phrasing is a bit weird (FAs are implemented .. and are shorter than ...)

- p2, line 49: what is the definition of "controlled nuclear material" in your context? IAEA? US NRC/DOE/NNSA? And what are the implications of that?

- p2, l67: You refer to Table 1 which gives the Gd content but you haven't detailed the Gd in your text (comes afterwards).

- p2, Figure 1: please provide some guidance on what the reader is supposed to see. a): what are the 'w' positions? Why are there two colors for 'w'? What's in the white, corner position? b): too small, too dark

- p3, l75: please put the '2' and '3' in real subscript. This day and age, that should be possible with any word processing software.

-p3, l78-79: "keff must be below 1.10". Why? Please explain what the physics reason is behind the constraint.

-p3, l84: I assume the "reflecting boundary between L3 and L4" is in your model. There is no "high-albedo" material as sheet in between L3 and L4 pellets, I assume?

-p3, l86: what is "novel" in this design? The different wt% Gd?

-p3, l87: I would put the isotopic breakdown in a table together with the thermal capture cross section and maybe already the spent fuel Gd composition as well. (in section 3.3 you can refer back to this table)

-p3, l96-97: please explain why the presence of Gd in the spent fuel is a "reactivity penalty"

-p4, l103: what is the "v-by-c" method? Variation by ?

-p4, l108: explain OTOC

-p5, Section 3: please consistently use pcm as a unit for reactivity. Not mk.

-p5, l152: define the "Doppler coefficient". And I assume (I hope), it should be negative (unless you define it as dk/dT = -K_D/T -> that's why definitions are important)

-p6, l165: "outout"

-p6, l176: this L/A definition is "hard to read". Do I understand it correctly that for each of the six horizontal layers, you calculate the average power and relate that to the total average power (total power/6)?

-p6, Table 4: based on my comment above, I would transpose table 4. Since then the "layers" are rows (as they are in your core)

Comments on the Quality of English Language

Some typos and clarifications (see comments section).

Author Response

Please see attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for addressing my comments and suggestions.

I have one last comment:

-     The authors mentioned that the isotopes include 96% and above contribution to k-eff. I am not sure how this number was obtained without conducting depletion simulations with all and partial list of isotopes. But even with incorporating these FPs together with the actinides it seems that at t=0 there is a 4000 pcm uncertainty on k-eff. I believe it to be too high and it may even be amplified further with burnup. This part requires more detail response and justification.

I understand that the authors do not want or intend to perform more simulations and at this point I am not asking them to do so.

However, any justification for not including other isotopes should be included in the main body of the paper. 

Author Response

Please see attachment.  Thank you again for your support in refining our manuscript.

Author Response File: Author Response.pdf