On building and interpreting physical models: the NRC now has to face what went wrong in modeling the AP1000 nuclear reactor design (UPDATED on September 24th & 30th, 2011)

16 September 2011

This post is to tell you what has happened so far since August 16th, 2011.  That was the day that I made a presentation to the NRC’s Advisory Committee on Reactor Safeguards (ACRS)’s Subcommittee on the AP1000 Design Certification  summarizing the main points in the two letters I had sent to them, boiling it down into two main issues:  Issue #1 “Forgetting About Heat of Solar Radiation on the Exterior Surface of the Concrete Shield Building” and Issue #2 “Forgetting about solar radiation on exterior surface of physical models of evaporative cooling used to validate WGOTHIC computer code.”   The WGOTHIC computer code is used to analyze how heat is removed via the steel containment — on the AP1000 passive plant, unlike on all the other PWR reactors in the entire world, removal of all the reactor decay heat using the reactor safety systems ultimately relies upon rejecting heat to the surrounding ambient air via cooling of the steel containment shell.  So everything rides on the WGOTHIC computer analysis getting heat transfer calculations right during the design phase of the AP1000.

Links to the letters, slides,  and the text of the presentation were provided in my earlier post on this blog entitled “Forgetting About the Sun.”  That’s also the title under which the presentation is listed in the table of contents of the transcript of the meeting now available on the NRC’s webisite.  I don’t suggest you bother to read the portion of the  transcript on my  presentation, though — perhaps it is due to a lisp or my soft voice, but the transcription of my portion is ridden with errors to the point of often being useless (a representative error:  “passive plant” transcribed as “plastic plant” )  But I do suggest that you look at the following URL that links to a single pdf document of my slides integrated with the text of the < 5 minute talk I prepared:   SterrettACRSPresentation16August2011TalkTextWithSlideImages

To give a little background:  The ACRS is an independent committee at the NRC composed of people from academia and industry that advises the five-member NRC Commission.  ( The ACRS’s purpose is described here:  http://www.nrc.gov/about-nrc/regulatory/advisory/acrs.html and the five current Commissioners are pictured here: http://www.nrc.gov/about-nrc/organization/commfuncdesc.html )   Now, the significance of bringing up an issue about the AP1000 design at this point in the process is that the ACRS is supposed to be independent of the staff.  Thus, they have the power, though it is not exercised too often, of advising the NRC Commissioners to act contrary to the recommendation of the NRC staff, such as recommending against granting final design approval when the staff has recommended granting it.  The Commissioners make the final decision, though.

The AP1000 design is now in its 19th revision, and the urgent desire that some in industry have to get the design certified so that construction can begin is evidenced by the fact that “industry stakeholders”, as they are called in the regulatory realm, devised a way to do an end-run around NRC regulations that otherwise would have required that nobody start building parts of a new nuclear power plant that are important to safety until the plant design is final.  The NRC cooperated in providing a legal means to do this, with an argument I really don’t agree with.  So, around 2006/7 something called a “Limited Work Authorization”  was created that permits the license applicant to carry out  “activities including pile-driving and foundation work for structures, systems or components with high importance to safe operation and security at a nuclear power plant.”   ( http://insidehealthpolicy.com/20070418592557/Health-Documents/Text-Document/nrc-issues-final-rules-on-limited-work-authorizations-worker-fatiguedrug-testing-provisions/menu-id-219.html ).

The “regulatory analysis” of the rule, including its history, is here:  http://www.pogoarchives.org/m/gc/regulatory-analysis-new-rule.pdf     It’s like a lot of energy regulation that occurred during the Bush-Cheney administration (it weakens regulations for corporations in the energy industry), and I’m not happy about this one at all.  It seems to me the danger of granting Limited Work Authorizations cited in the regulatory analysis:  that it would cause certain public perceptions, including “that the NRC has decided to approve an application before formal evaluation of the application is complete”,   has come to pass.

What the regulatory analysis did not mention explicitly, but should have, are the well-known dangers to public safety that this kind of regulation leads to:  it creates situations where enforcement of regulations governing design  that would be costly in schedule or capital suffers because of social, political, and/or economic pressures.  Since lapses in regulating design changes that really should be made affects public safety, I just don’t see why such a consideration shouldn’t be important in considering what falls under the NRC’s authority.  The “streamlined” process, known as the “Part 52” or “one-step” process — which is the process under which the AP1000 and the new reactors at the Vogtle site applied for their certifications and licenses — was supposed to avoid this by requiring that a new reactor be fully designed and approved prior to carrying out such construction activities.  The goal seems to have receded far beyond the horizon.

Work under a “Limited Work Authorization” has already begun on building an AP1000 reactor in Georgia, at the Vogtle site.   A few photos are here: http://nuclearstreet.com/nuclear_power_industry_news/b/nuclear_power_news/archive/2011/09/14/new-vogtle-construction-pictures-of-units-3-and-4-091403.aspx      (You can identify where the new work is being done easily from the red color of the recently disturbed earth.)

To get an idea of the confidence that Southern Nuclear has in what they are doing at Vogtle 3 and 4 (using funds already assessed and taken from ratepayers for it), even as the ACRS’s Subcommitee on the AP1000 is still reviewing the technical basis of the AP1000 design, here is a promotional/informational video about how things are progressing at the Vogtle site:


Around August 10th the NRC staff completed its report to the NRC Commission, called an FSER (Final Safety Evaluation Report), and headlines around the world were that the NRC staff had approved the AP1000.  As I mentioned in an earlier post, the NRC moved quickly to clarify the situation:    http://public-blog.nrc-gateway.gov/2011/08/11/one-step-closer-to-the-ap1000-finish-line/  but confusion remains.  The company that wants to build the reactors (Southern) was quoted as saying that “This action [the NRC staff completing a FSER (Final Safety Analysis Report) for the AP1000] provides clarity and certainty toward the historic issuance of a license for the first new units approved for construction in the United States in more than 25 years.”  ( http://www.freerepublic.com/focus/f-news/2762697/posts  )

Where does this leave the Advisory Committee on Reactor Safeguards’ Subcommittee on the AP10000?  The pressure is on, both ways:  on the one hand, there is pressure to get “closer to the ap1000 finish line” (in the words of the NRC blog post); on the other hand, there is pressure from the standpoint of professional integrity, to make sure that if there is an issue that needs to be looked at to provide reasonable assurance as to the safety of the reactor, that they discuss it thoroughly and make an appropriate recommendation.

Since the August 16th, 2011 meeting, I received no feedback, but that is not surprising; members of the public are permitted to ask questions and register concerns, but no answer is owed them personally.  The next news would be at the next relevant ACRS meeting, which was the meeting of the full ACRS committee, held from September 8th – 10th.  The AP1000 subcommittee report was scheduled for 1:15 to 2:45 pm on Thursday the 8th, which is just when I teach (1:30 pm – 2:50 pm, Tuesdays and Thursdays).  I asked that things be moved around a bit so that I could listen in to the segment of the meeting on the AP1000 in case they might mention the issue I had raised, but was told no changes could be made at that point.  I was offered the opportunity to speak at 3 pm for about 10 minutes.  Here are the notes I referred to in speaking for a little over 5 minutes on the topic to the full ACRS Committee on September 8th:


This is Dr Susan G Sterrett, of Carnegie Mellon University.  Prior to my academic career, I worked in the nuclear power industry, including work in structural mechanics and work in fluid systems design.  Although I did some work on Westinghouse passive plant designs, I never worked specifically on the AP1000.  I obtained the information referred to here from the materials made available to the public on the NRC’s website.

To summarize the presentation last time, there were two issues:

Issue #1    Radiation on Concrete Shield Building not accounted for in analyses.  When sun shines, surface temp is higher than ambient air, whereas at night it will radiate heat back to night sky and depending on conditions, surface temp can be lower than ambient air. 

Issue #2   The Large Scale Test and the Small Scale Tests  [physical model tests] used to validate WGOTHIC [the computer code used for the calculation of peak containment pressure] were outdoors, in the sun.   The main effect these tests were used for was a coefficient representing evaporative losses, and the sun aids evaporation.

I’d like to begin by saying a few brief things about each of these. 

First, regarding issue #1:

– Responses by the AP1000 designers that “normal” thermal loads don’t need to include the extra 30 or 40 degrees or whatever it may be, due to heat of solar radiation, don’t make any sense to me.   Surface temperatures in the sun exceed ambient air temperatures in normal plant operating conditions.   

– Some people felt that the effect of sunshine would be negligible.  The fact is that there are a vast number of cases — not just a handful , but many– where concrete roadways, bridges, ramps, and other structures such as airstrips have buckled.  In those cases, the risk to public safety is not large; they just close down the highway, bridge or airport until they can demolish the old buckled portion and replace it.  The shield building is completely different.  It supports the PCS [Passive Containment Cooling System] water tank, and it forms the passageways for the airflow needed for reactor decay heat removal after an accident.  So the attitude that in ignoring the effects of solar radiation on the AP1000, what is being done on the AP1000 shield building SC modules conforms to the normal practices for concrete used in these other industries should not be an acceptable response, if it is even true.  The technical papers I cited about analysis of concrete indicate that this complacent attitude is most certainly not the norm at other federal agencies and institutions such as Oak Ridge or the NIST.  (I chose papers that I thought would not be difficult to obtain; please let me know if you have any problem obtaining them, though.  I can send you a copy.)

Secondly, regarding issue #2:  There were several remarks by subcommittee members dismissive of this concern, based upon their intuition that the sun shining on the physical model would tend to be unhelpful, rather than helpful, in providing cooling.   I understand the puzzlement here, but first let me point out that Westinghouse [the plant designer] is always saying that the worst case is cold, and in fact when I first raised this issue, they said that if they counted in the extra heat from the sun in their analysis it “would help.”  So,  it is hard to see how you could accept those statements and yet have an intuition that sunlight would make evaporation worse, i.e., to question a claim that sunlight might possibly in some cases aid evaporation. 

I personally think no generalizations about whether a certain increase or decrease in any single factor will help reduce containment pressure or not can be made, since the effect on containment cooling involves humidity and other factors that affect conduction through the wetted film.  Radiation is only one of the mechanisms involved.   If there is intense sunlight and very low humidity, it seems to me that is very different from the case of the interior of the shield building where there is little sunlight and high humidity.  Using  ambient air temperature to correlate cases isn’t going to be valid — I think anyone who understands the basics of heat transfer has to agree with that.  The test just didn’t measure the things needed to make inferences to other cases. 

All I did say, or what I meant to say, at any rate, was that if they are going to use the large scale [physical model] test to determine coefficients of evaporative effectiveness, then whether or not it is in sunlight needs to be taken into account, since sunlight aids evaporation.  I don’t think anyone should find that an objectionable statement.  

Second, I point out that the purpose of the large scale test is limited to certain aspects, mass and heat transfer effects, not overall containment temperature and pressure response.  It’s not really a scale model;  it doesn’t use physical similarity.  Here I am going on the nonproprietary reports and the FSER [AP1000 Final Safety Analysis Report].  It was emphasized in the FSER Chapter 21 discussion in 21.3.8 of the Rev 15 FSER, that 

This experiment [the Large Scale Test] is designed to induce similar containment dome heat transfer processes and circulation/stratification patterns inside containment as in the AP600; however it is not meant to simulate specific AP600 accident scenarios. The large scale test data is used to validate the WGOTHIC computer code, which will be used to analyze the AP600 containment.” 

I note also the main conclusion of the test, which was that

“evaporation was the primary mode of heat removal from the outside of the vessel (approximately 75 percent of the total).”

I think the situation is this:  because the tests did not separate out heat of solar radiation, which was present in the LST but will not be present in the AP1000 wetted steel containment sheel, from the effects of the ambient air temperature and the humidity of the air, we just don’t know.  How are you going to figure out how to account for the fact that the LST was outdoors?  You can’t use WGOTHIC to answer this, and the test data isn’t helpful, either, since they did nothing to measure solar radiation.  How does one measure solar radiation?   By pan evaporation — at least, that is how it is done in meteorology.  They needed to do that in the LST, but they didn’t.  At least based on the information I have access to, all they measured were temperatures.  You have more information than I do, but if that is true, then:  the informantion needed is just not there.  The test concluded that the most dominant factor is cooling via evaporation, but assumed it was the same whether the equipment was in the sun or in the dark.  It looks to me as though WGOTHIC uses coefficients for evaporative loss based on a test performed in the sun for which no data on how much of the evaporation was due to the sun was taken.  I hope that this helps in conveying the problem. 

I have a lot more I could add, but assume that time is limited.  Just let me add one more thing:  if this doesn’t get done right before Design Cert is granted, that’s it!  _There is no check and balance on it until the real challenge comes in nature_ (Pause, repeat.)  Why?  Because, so far as I know, the ITAACS [Inspections, Tests, Analyses and Acceptance Criteria] are not going to provide a check on this error.  The ITAACS don’t test for heat removal capability from the containment by the PCS per se:  the ITAAC criterion is whether or not the piping and tank as installed will deliver a certain flowrate over the steel containment dome, that is all.  So they won’t find this kind of error.

The claim of adequacy of heat removal from the containment — which, in this passive plant, unlike in any operating plant, also performs the function of dumping heat to the ULTIMATE HEAT SINK,  —  is based on calculations of the flowrate needed that are performed using WGOTHIC that take things such as PCS flowrate as inputs to the calculation.  The fact that the PCS also functions as the system conveying heat from all the reactor safety systems to the ULTIMATE HEAT SINK (the ambient air) means that every other system performing post-accident heat removal relies on passive containment cooling to remove the heat it has extracted and is passing on —  and the validity of conclusions about the effectiveness of those systems in removing decay heat is based on calculations performed using WGOTHIC.   And, WGOTHIC analyses calculations about evaporative heat losses are exactly what is put in question by the errors that follow from forgetting about the sun. 

I’ll stop here due to time. 


Partway through it (I seem to recall it was at the point where I mentioned that the scale models were not constructed using principles of physical similarity, but I could have been wrong), the sound went dead, so I could not hear any responses, nor take any questions about what I had presented.  Later, I heard that my voice came through, though, so I did get to make the entire presentation to the full ACRS committee.  Someone who listened in on the part of the meeting I missed, which was right before I spoke, told me it seemed to him as though much of the discussion on the AP1000 segment of  that Full ACRS committee meeting concerned the issue of heat of solar radiation.  So it seems they are discussing it.  That’s good.   What will happen next?  I’ll be looking for hints, and will let you know.

UPDATE September 24th, 2011:  The transcript of the September 8th ACRS meeting is now available.  (http://pbadupws.nrc.gov/docs/ML1125/ML11256A117.pdf ) The committee seemed to take things pretty seriously, but, reading it, it’s hard not to worry that there doesn’t seem to be adequate time to address the issues they consider in satisfying detail.  Given that the plants asking for a license for a new nuclear plant based on the design certification of the AP1000 would be getting a license to operate the plant —  and that no city, county, or state governing body or legislature would be able to stop the owner of the plant from operating it even if all the citizens served by it didn’t want it to  — this is deeply unsatisfying.   We’re talking about the adequacy of using containment cooling as an ultimate heat sink — something that has not and is not being done on any operating PWR in the entire world — and we’re talking about using a means of determining the adequacy of such an ultimate heat sink based on analyses that contain a lot of uncertainty.

UPDATE on September 30th, 2011.

The ACRS letter on the AP1000 was considered significant enough that a PR Newswire letter went out on it last week.   These are actually press releases written by a company’s PR department that get sent to newspapers and magazines, who often publish them in their entirety, unedited, with a note in small print noting that it is an unedited press release.  Here is the URL of the press release: http://www.prnewswire.com/news-releases/acrs-concludes-ap1000-maintains-robustness-of-previously-certified-design-and-is-safe-130579313.html  I thought it interesting that it spoke only of “maintaining the robustness of the previously certified design”, rather than saying the revised design was in fact robust.

I looked at the ACRS letter a few days ago.  Here is my take on how they treated the issues I raised in my presentation “Forgetting About the Sun”:

In a nutshell:  The ACRS response to the issues I raised in their letter to the NRC Commissioners looks very thorough on the surface, but is actually quite evasive.   

The letter gives the appearance of having “resolved” an issue, when in fact they (Westinghouse and ACRS) have merely distracted attention away from it, and thereby evaded it.   Sometimes it is by ignoring an issue, sometimes it is by focusing on the wrong thing, sometimes it is by providing an answer that looks hopelessly optimistic without giving the basis for it, and sometimes it is by using vague language to make it look as though a topic has been addressed even though it has not.

The most unsettling thing (to me, and, I would hope, to others as well) is the formal issuance of a conclusion that appears to deal with an issue, but brazenly evades it.  Public safety depends on this committee  (Advisory Committee on Reactor Safeguards) and the NRC (Nuclear Regulatory Commission) being apolitical.

Details later, as I am otherwise engaged in the next few days, but I wanted to bring this post up to date.

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