Harnessing Geothermal Energy Potential from High-Level Nuclear Waste Repositories

Round 1

Reviewer 1 Report

The authors present an interesting research that introduces a proof-of-concept study using a numerical model of a binary-cycle geothermal system powered by the decay heat produced by high-level nuclear waste. Although the topic presented is relevant both from technical and scientific points of view, there are several aspects that should be worked in more detail for completeness. Below are several points to consider for this purpose:

Introduction Section.  In order to support the sentence “To the authors’ knowledge, this is the first study to systematically explore the possible use of decay heat in the repository setting.” A comprehensive literature review should be included.

The proposal is feasibly in theory considering only the energy point of view, but more information about safety issues, health problems, final disposal of energy conversion technologies, etc. should be provided.

Van Genuchten can be encountered twice in Table 1.

Section 3.3. Emplacement tunnel at a depth of 300 – 1000 m. Was this depth range varied in your calculations? Does this length influence the length of the vertical heat exchanger? Please explain in more detail.

Line 159. “There is no flow at the canister surface”, Do you mean heat flow or fluid flow? Please complete accordingly.

General comment for Section 3.3. Considering the information provided this section is not clear enough about the type of geothermal resource, hot dry rock or hydrothermal.

Section 3.4. It is not clear the heat transfer model for VGHE. In the current the heat transfer model belongs to a steady-state one-directional heat conduction for a plane wall, however since you have a pipe (approximately a U-tube with a coil around the canister) the model should be radial system using inner and outer diameters. In this same regard, initial temperature Ti is for the complete length of the VGHE or only for a portion? The model presented should be considered as transient or steady state?

Define clearly in this section the fluid flowing inside the VGHE. In the section describing the ORC you mention this for the primary loop, but for better understanding include the information here.

Section 3.4.3 Providing an illustration (Figure 4) of heat-VGHE-Heat Pump from your own study would be more suitable for understanding your methodology. Include numbering of each relevant energy stream.

Define the acronym SNF/HLW

Strictly speaking, there is not an isentropic type compressor or expansion devices, the thermodynamic process is what can be treated as isentropic. Please consider this comment in your manuscript and for Table 3 and Table 4, Compressor Type: Isentropic and Pump Type: isentropic. Compressor Type refers commonly to: rotary screw, rotary vane, reciprocating, scroll, etc.

Use the same terms and variables in equations, for example Eq (4) W stands for work or power but in eq. (5) the term N appears for power.

Section 3.4.4. Equation (6) is not typed correctly; the mass flow terms are not included accompanying the specific energy terms. A similar issue can be found for equation (7), where the mass flow term is not included accompanying the outlet specific energy.

Equation for isentropic process in the pump is incorrectly presented, the isentropic term should appear in the numerator part.

In energy equations please use the same nomenclature, since the energy efficiency for the ORC uses and upper dot and for COP is not included.

Section 4. Results. From the data shown in Figure 6. The initial Bentonie-Canister temperature is around 170°C, I understand that the initial temperature underground has this value, therefore the correct utilization will depend on the geothermal temperature and will be suitable only for specific geothermal regions. Please add more information in this matter since this temperature value was not discussed previously.

Table 6. Adding heat input values to the organic Rankine cycle would complete the information provided.

Section 5. Discussion. The comment provided in the third paragraph of this section related to the economic feasibility should be reconsidered since not economic feasibility was carried out.

Author Response

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Reviewer 2 Report

This paper attempts to design a binary cycle geothermal system powered by the heat produced by HLW within the geological repository. This study was carried out by using a numerical model. The system description, methodology, results of COP, power and efficiencies are described adequately. However, the following questions need to be addressed. I would recommend minor revisions before this paper can be accepted.

11. In the introduction, a binary-cycle geothermal system powered by the heat produced by HLW is proposed. However, the advantages of this proposed technology should be compared with relevant technologies by reviewing more literature.

22. Assumptions and assumed parameters should be referenced. For example, in section 3.1, it states that “We assumed the spent fuel from a typical PWR core, containing fuel assemblies with a total thermal power of 3300 MWth. The power history used for the calculations is approximated as 4 years of operation at full power, with a cooling time of 4 years. We consider four assemblies per canister.”

33. References of Table 1 and Table 3 should be clarified. The assumed efficiencies of pump, heat exchanger and turbine in Table 4 also need to be referenced.

44. In section 3.3, it is better to study grid independence test.

55.  In Figure 6, it would be beneficial to include legends to help readers in clearly distinguishing between different lines.

66.   In section 4.1, besides providing a straightforward description of the temperature and heat output, it is important to include a more in-depth analysis.

This paper is well-written, but it can still be further improved to be more clear and concise.

Author Response

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Reviewer 3 Report

In the research work described in this paper, the changes in long-term heat extraction for power generation using nuclear waste heat as a heat source for artificial thermal storage have been investigated using numerical simulations. The topic of the current study is consistent with Energies. However, the novelty of the current work, the quality of the images, the implication and explanation of findings in the analysis results need to be further improved. I recommend a revamp and resubmission to be able to suit a journal of such repute. Specific comments are as follows:

1. Clear statements of the novelty of the work should appear briefly in the Abstract and Conclusions sections, implication and explanation of findings also should present in the Abstract.

2. Introduction for the use of nuclear waste and the development trend of related research is not explained, which is very important to reflect the innovation of research, in the introduction should clearly state the shortcomings of related research and the innovation of this study, and give the basis.

3. Please provide the basis and detailed information for the meshing in the article (Figure 3).

4. In order to allow readers to more clearly understand the conceptual design process, a detailed flow chart of working principles should be provided in Section 2.

5. The modeling process in Section 3 should give reasonable assumptions so that readers can verify the feasibility of the solution and the reliability of the numerical results.

6. Sections 4.2 and 4.3 describe the details of the working process of the vapour compression heat pump cycle and the ORC cycle, it is recommended that the details of the cycle process are described in section 2 and that the results section only shows the results of the study.

7. The presentation of results on system performance in different recession periods in Section 4 is lacking, and it is recommended to add.

8. Whether the current study can support the reliability of the conclusions in the paper by experimental data.

Minor editing of English language required.

Author Response

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Round 2

Reviewer 1 Report

Authors have addressed several modifications to the original manuscript submission providing a concise explanation for each point. The manuscript was improved for being accepted.

Reviewer 3 Report

Thanks to the author's answers and the revision of the manuscript。 The manuscript is improved and most of my comments adequately addressed.

However, the quality of the language still needs to be further improved by avoiding "We ..." as much as possible, which should be expressed in the passive voice, thus improving the scientific quality of the language.

In addition, the clarity and formatting of some of the figures need to be adjusted to improve the readability of the pictures. The significance and results of this study should be highlighted in the conclusion section.

Suggested acceptance with minor modifications.

Minor editing of English language required.