A Critical Look at the Need for Performing Multi-Hazard Probabilistic Risk Assessment for Nuclear Power Plants

Round 1

Reviewer 1 Report

This is a very interesting review article on multi-hazard risk assessment. The review, in my opinion, is less systematic compared with a standard survey, but the non-standard structure does make the article more readable. Most of it serves a very good tutorial type article that can be very useful to young researchers in the field of safety engineering. 

Regarding technical content, the main issues are:

1. Section 2 is rather brief. Although the primary focus of the article is on multi-hazards, I believe a thorough discussion/tutorial for single hazard PRA is necessary and useful as it serves the basis for understanding multi-hazard PRA. Things the authors should consider including, for example, include basics of how PRA is conducted, especially on the quantification since they feed into Section 4, and ideally with a simple and intuitive example.
2. Section 3 and 4 are generally well written, but how the two methods discussed in 4.1 and 4.2 links to the idea of a generic framework is not clearly articulated.  
3. Section 5 is rather difficult to follow. I think some upfront knowledge may help the reader to understand the method.

Other concerns:

• Figure 3 is taken from the cited source. Is permission granted to avoid copy right breach?
• Ref[30] and [31] are not accessible. In fact, I don’t think these citations are necessary as long as the search method is being described.
• Typos need to be thoroughly identified and corrected, e.g. line 215, extra period after addressed.

Author Response

Thank you for your review. We found your comments extremely helpful to improve the paper in terms of readability and clarity. We also tried to address the comments, as it can be seen below.

Regarding technical content,

1. Section 2 is rather brief. Although the primary focus of the article is on multi-hazards, I believe a thorough discussion/tutorial for single hazard PRA is necessary and useful as it serves the basis for understanding multi-hazard PRA. Things the authors should consider including, for example, include basics of how PRA is conducted, especially on the quantification since they feed into Section 4, and ideally with a simple and intuitive way.

We add a short paragraph to give an idea of how we perform PRA to the reader. This paragraph includes the definition of event tree and fault tree and introduces how we obtain those. Moreover, two frequency results and NRC comments are at the end of Section 2 to get insights into the order of magnitude.

2. Section 3 and 4 are generally well written, but how the two methods discussed in 4.1 and 4.2 links to the idea of a generic framework is not clearly articulated.

We added a short paragraph in Section 4 to introduce how we can perform multi-hazard PRA and why we explained why a generic framework is needed. Besides this brief justification, we added a new sub-section (i.e., 4.3) to explicitly underline expectations from a generic framework.

3. Section 5 is rather difficult to follow. I think some upfront knowledge may help the reader to understand the method.

NRC provides guidelines to reactor designers, either for LWRs or non-LWRs, for preparing a licensing application. These guidelines differ greatly between LWRs and non-LWRs. We added a paragraph to make this section more understandable. This new paragraph introduces what we need for reactors to be approved by the NRC in the US.

Other,

1. Figure 3 is taken from the cited source. Is permission granted to avoid copy right breach?

To avoid delays, we reproduced the figure and cited the source.

2. Ref [30] and [31] are not accessible. In fact, I don’t think these citations are necessary as long as the search method is being described.

We added a description regarding how we performed to search and removed the references.

3. Typos need to be thoroughly identified and corrected, e.g. Line 215, extra period after addressed.

We walked through the whole paper to correct typos.

Reviewer 2 Report

The authors present an interesting paper on a timely matter.  Some suggestions are offered below to strengthen the paper and bring greater clarity.

Abstract line 20

I think of supply chain issues as not an immediately realizable problem as suggested that these could impact off-site emergency equipment availability in some emergency.  Seems a bit contrived.

Abstract, line 29

You don’t always see references in abstracts but since these are included a reference might be warranted for the non-LWR licensing (ref. 22).

The authors state climate change as the main driver for this shift in how we do PRAs. Yet their examples are nearly all non-climate driven events, and global warming is only mentioned once more in the paper in an offhanded manner. The conclusions emphasize human action as a more significant threat to micro reactors but is not substantiated earlier in the paper.  In effect the intro says its climate change driven, the body of it uses earthquake/tsunami, and the conclusion direct human actions. All are valid to varying degrees but seem not so well substantiated by presenting this way.

Line 184-188

You are taking a side in a two-century old debate amongst historians and sociologists when you claim that increased population will result in shortage and hazards. Since popularized by Malthus this approach has been widely debated but at scale the end result has always been that human adaptability has overcome shortages. We now live in a age with far more material wealth and the least abject poverty in history. By asserting this as fact you will have a portion of your audience be turned off to your arguments for little gain. The example of Fukushima is strong enough on its own, if you want a example of direct human caused hazards there are other examples particularly in armed conflict.

Lines 198-209

The examples begin to hint at the nuances the authors are trying to get at but then the last bullet is minus an example where all the others have one.  If I was cynical I might thing the authors with this construct stumped themselves to provide such.

Line 334 This diagram seems pointless as its all blank, it appears to be just a random wire diagram to describe what already was described in a short sentence.  Authors should consider the value or perhaps amplify the example.

Line 354-355

I am not sure what this is saying.  “applicable to real cases systematically and practically”  If not applicable to real cases then what?

Line 360

Different how?  Comparison is not clear. This section needs greater clarity.

Line 488-491

This increased need was only mentioned in the conclusions, it should be throughout the paper. Human made hazards are not mentioned at all. Direct human action is a far more serious and new threat than the increase in climate driven events that are stated as the main concern.

Author Response

Thank you for your review. We found your comments extremely helpful to improve the paper in terms of readability and clarity. We also tried to address the comments, as it can be seen below.

Regarding technical content,

1. Abstract line 20, I think of supply chain issues as not an immediately realizable problem as suggested that these could impact off-site emergency equipment availability in some emergency. Seems a bit contrived

We agree that the immediate effect on safety equipment may not be significant. As a result of that, we removed that statement from the abstract.

2. Abstract, line 29, You don’t always see references in abstracts but since these are included a reference might be warranted for the non-LWR licensing (ref22).

We’ve removed the references from the abstract.

3. Authors state climate change as the main driver for this shift in how we do PRAs. Yet their examples are nearly all non-climate driven events, and global warming is only mentioned once more in the paper in an offhanded manner. The conclusions emphasize human action as a more significant threat to micro reactors but is not substantiated earlier in the paper. In effect the intro says its climate change driven, the body of it uses earthquake/tsunami, and the conclusion direct human actions. All are valid to varying degress but seem not so well substantiated by presenting this way.

In general, the hazards are either from natural or human-made. In this paper, we focused on natural hazards like earthquakes, tsunami, or so on in this paper. We did touch on global warming it may lead to a change in the frequency of natural disasters. To clarify what we are trying to address, we did the walkthrough of the paper and clarified our goals by inserting and removing some parts as a minor update.

4. Line 184-188, You are taking a side in a two-century old debate amongst historians and sociologists when you claim it increased population will result in shortage and hazards. Since popularized by Malthus this approach has been widely debated but at scale the end result has always been that human adaptability has overcome shortages. We now live in a age with far more material wealth and the least abject povery in history. By asserting this as fact you will have a portion of your audidence be turned off to you arguments for little gain. The example of Fukushima is strong enough on its own, if you want a example of direct human caused hazards there are other examples particularly in armed conflict.

We updated the statement that causes unnecessary debate. Now, our goal should be more straightforward for the readers.

5. Line 198-209, The examples begin to hint at the nuances the aothors are trying to get at but then the last bullet is minus an example where all the others have one. If I was cynical I might thing the authors with this construct stumped themselves to provide such.

We revised the definitions and examples to give more insights to the readers.

6. Line 334, this diagram seems pointless as its all blank, it appears to be just a random wire diagram to describe what already was described in a short sentence. Authors should consider the value or perhaps amlify the example.

We removed the diagram and included a brief explanation instead.

7. Line 354-355, I am not sure what this is saying. “applicable to real cases systematically and pracitcally” If not applicable to real cases then what?

We’ve updated this statement since it confused the readers.

8. Line 360, Different how? Comparison is not clear. This section needs greater clarity.

NRC provides different guidelines to reactor designers and applicants depending on if they are licensing a LWR or a non-LWR. We added a background paragraph to improve the clarity.

9. Line 488-491, This increased need was only mentioned in the conclusions, it should be throughout the paper. Human made hazards are not mentioned at all. Direct human action is a far more serious and new threat than the incease in climate driven events that are stated as the main concern.

This is correct. We’ve removed the paragraph discussing microreactors since it was not within the scope of the paper.