ML20205K516
| ML20205K516 | |
| Person / Time | |
|---|---|
| Issue date: | 04/09/1999 |
| From: | Advisory Committee on Reactor Safeguards |
| To: | |
| References | |
| ACRS-T-3071, NUDOCS 9904130256 | |
| Download: ML20205K516 (113) | |
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OFFICIAL TRANSCRIPT OF PROCEEDINGS O NUCLEAR REGULATORY' COMMISSION' ADVISORY COMMITTEE ON REACTOR SAFEGUARDS'
Title:
461ST ADVISORY COMMITTEE ON REACTOR SAFEGUARDS (ACRS)
TRO4 (ACRS; RETURN ORIGINAL TO Be"n"ri:TE l' M/S T-2E26 '
Dockei No.: ?d,;7yc 1
Work Order No.: ASB-300-736 D l
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LOCATION: Rockville,MD DATE: Friday, April 9,1999 PAGES:377 - 476 9904130256 990409 307 PDR ANN RILEY & ASSOCIATES, LTD. l
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DISCLAIMER UNITED STATES NUCLEAR REGULATORY COMMISSION'S ADVISORY COMMITTEE ON REACTOR SAFEGUARDS APRIL 9, 1999 The contents of this transcript of the proceeding of the United States Nuclear Regulatory Commission Advisory
( ) Committee on Reactor Safeguards, taken on April 9, 1999, as reported herein, is a record of the discussions recorded at the meeting held on the above date.
This transcript had not been reviewed, corrected and edited and it may contain inaccuracies.
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377 1 UNITED STATES OF AMERICA
( 2 NUCLEAR REGULATORY COMMISSION
)
3 ADVISORY COMMITTEE ON REACTOR SAFEGUARDS 4 ***
5 461ST ADVISORY COMMITTEE ON REACTOR SAFEGUARDS 6 (ACRS) 7 ***
8 USNRC 9 11545 Rockville Pike, Room T-2B3 10 Rockville, Maryland 11 Friday, April 9, 1999 12 The subcommittee met pursuant to notice, at 10:43 13 a.m.
14 MEMBERS PRESENT:
() 15 DANA POWERS, Chairman, ACRS 16 GEORGE APOSTOLAKIS, Member, ACRS 17 JOHN BARTON, Member, ACRS 18 MARIO FONTANA, Member, ACRS 19 THOMAS KRESS, Member, ACRS 20 DON MILLER, Member, ACRS 21 ROBERT SEALE, Member, ACRS 22 WILLIAM SHACK, Member, ACRS 23 GRAHAM WALLIS, Member, ACRS 24 MARIO V. BONACA, Member, ACRS 25 ROBERT E. UHRIG, Member, ACRS
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378 1 PROCEEDINGS
- 2 (10
- 43 a.m.]
LJ 3 DR. POWERS: Let's come into session. This is the 4 third day of the 461st meeting of the Advisory Committee on 5 Reactor Safeguards. During today's meeting the committee 6 will consider the following: proposed ACRS reports; impact 7 of the use of high burnup or oxide fuel on the revised 8 source term; relationship and balance between PRA results 9 and defense-in-depth; reconciliation of ACRS comments and 10 recommendations; report of the Planning and Procedures 11 Subcommittee; future ACRS activities; and we shall work on 12 reports.
13 A portion of today's meeting may be closed to 14 discuss organizational personnel matters that relate solely n
i ) 15 to the internal personnel rules and practices of this v
16 advisory committee in matters the release of which would 17 constitute clearly unwarranted invasion of personal privacy.
18 The meeting is being conducted in accordance with 19 the provisions of the Federal Advisory Committee Act. Mr.
20 Richard P. Savio is the Designated Federal Official for the 21 initial portion of the meeting.
22 We hav received no written statements or requests 23 for time to.make oral statements from members of the public i 24 regarding today's session.
25 A transcript of portions of the meeting is being G ANN RILEY & ASSOCIATES, LTD.
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379 1 kept and it is requested that speakers use one of the
()
< 2 3
microphones, identify themselves, and speak with sufficient clarity and volume so they can be readily heard.
4 I think the first item on today's agenda is to 5 dis' cuss an SRM that we received from the Commission 6 concerning high burnup fuel and MOX. I have asked Mr.
7 Tinkler from the RES staff -- and is Ralph going to be here?
8 MR. TINKLER: I understood he was.
9 DR. POWERS: I have asked them to attend in much 10 the role of invited experts to assist us in developing our 11 response to this SRM, so feel free to come up and join us.
12 I think everybody knows Mr. Tinkler -- Charlie. I guess 13 I'll have to be wired in or something.
14 Dr. Meyer has arrived.
() 15 You are here in the role of invited experts, so 16 you should feel free to pipe in, both of you, at any time 17 something moves you to speak.
18 This is a quotation out of the SRM -- " Consider 19 the impact on the revised source term if high burnup or 20 mixed oxide fuel were used in place of standard uranium 21 fuel."
22 The committee has indeed written on this subject 23 of high burnup fuel, both on its reactor accidents 24 implications and its source term implications. So much of 25 the focus of this discussion is going to be on the subject
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380 1 of MOX fuel I think most of the members are aware that.the
[~h '2 Department of Energy is contemplating the disposition of d
3 weapons grade plutonium by converting it to a mixed oxide 4 fuel. Most of the members I think are aware that the use of 5 mixed oxide fuel is not done in the United States now, but 6 it is done in Europe and certainly contemplated in Japan.
7 It would be a substantial change in the way we 8 have manipulated fuel in this country to go to mixed oxide 9 fuels. The face of it, it's not such a heroic change from a 10 technical point of view with respect to things like 11 neutronics and ruel behavior because in fact fuel over the 12 normal course of burnup becomes mixed oxide in the sense 13 that plutonium is generated.
14 However, it would be a mistake to think that the
() 15 experiences with higher burnup fuel that results in the 16 generation of plutonium yields a product that is identical 17 to mixed oxide fuel.
18 Many of the concerns associated with mixed oxide 19 fuel arise out of a concern over proliferation and an 20 independent group called The Nuclear Control Institute has 21 been in the process of looking in a good deal of depth at 22 some of the problems that might arise in connection with the 23 use of mixed oxide fuel and a fellow named Dr. Lyman has 24 done some analyses to attempt to assess risk.
25 I caution you that he has not undertaken what I t' ANN RILEY & ASSOCIATES, LTD.
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381 1 would call a formal risk assessment. He has gone through 2 and done what I would call a scoping assessment of risk in a (VT 3 fairly technically sophisticated fashion. He has certainly 4 used origin codes to look at the inventories that you would 5 have in fuel and he has used the max code'to look at the 6 consequences should you release radionuclides from this fuel 7 in an accident.
8 The bridge between inventories and consequences 9 has not been done in a mechanistic fashion. Rather he has 10 looked at publications produced by the NRC, at source terms 11 from plants similar to those that would use the mixed oxide 12 fuel, selected release categories out of those, and used 13 release fractions applied to the revised inventories.
14 The changes in inventories that occur in mixed i( ) 15 oxide fuel -- clearly there is more plutonium. Plutonium as 16 we know is not high on the list of radionuclides that pose 17 consequences, but because there is plutonium in the fuel and 18 some amount of it one does get other actinides, and the 19 biggest differences that one sees in the inventories, the 20 ones that are striking, really have to do with actinides, 21 the actinide americium and curium.
22 These radionuclides do cause pause. When one goes 23 through and looks at the consequences of various release 24 fractions of the actinides, one finds they can be extremely 25 potent if they are released in any abundance. Typically
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382 1 'they are not found-to be released in abundance from fuel, 2 but should they be'they have substantial potential 3 radiological consequences.
4 The truth of the matter is we don't know much 5 about the release of the actinides. They get mobilized from 6 the fuel in a way that can result in release to the outside 7 ,
poorly, only at the highest of temperatures or by dispersal 8 events. The NUREG 1150 or the NUREG 1465 type model of the 9 source term really admits to a substantial uncertainty 10 concerning the release fractions of these.
11 If people recall the history of that document, 12 release fractions were specified for the actinides. Based 13 on a lot of discussion they were subsequently reduced, so 14 there are low release fractions here but there is a
() 15 substantial uncertainty.
16 When one things about actinide releases the 17 experience of Chernobyl comes to mind. Many people will 18 remind you that something on the order of three and a half 19 percent of the actinides were released in the Chernobyl 20 accident. One has to remember, however, that the Chernobyl 21 accident was a unique experience, a different type of 22 accident, one that involved a substantial dispersal of fuel, 23 actual fuel particles mechanically released from the plant.
24 As there were explosions, things threw the fuel out. I 25 In fact, in a recent re iew that went back and
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383 1 looked at all the data that had been collected since the I'h 2 accident it was found that in every case the actinide U
3 release and the fuel release were identical. That is, the 4 actinides were being released not by heating them up and 5 vaporizing them but rather they were being released because 6 you were dispersing fuel, and when I say identical, of 7 course there is some uncertainty bound but this three and a 8 half percent release means that only some very small 9 fraction, perhaps a tenth of a percent, could possibly have 10 been released by a vaporization process that separated the 11 actinides from the fuel. The NUREG 1465 source term would 12 say the same thing. It's hard to mobilize these things.
13 They are refractory in their nature. They don't vaporize 14 very well -- and they did not in the Chernobyl accident.
'O is ,/ 15 Chernobyl released those radionuclides because of 16 the dispersal event. That does not eliminate the --
17 DR. WALLIS: Would you try and put that in 18 perspective with the releases of some of the other --
19 DR. POWERS: Oh, certainly. The releases of the 20 noble gasec of course were virtually quantitative. Releases 21 of iodine, a volatile radionuclide, was on the order of 67 22 percent. ,
23 DR. WALLIS: So it was a lot.
24 DR. POWERS: Releases of cesium were on the order 25 of 50 percent. Releases of thorium appeared to be very
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-384 1 nearly quantitative. Release of ruthenium, a metallic
() 2 3
fission product that ordinarily is thought to have a very low volatility, were on the order of 13 percent.
~
4 Again this is sometimes attributed'to a unique 5 feature of the accident in that it burst the core fault and 6 exposed this fuel-to an oxidizing environment. I, myself, 7 don't think that is peculiar to the Chernobyl accident. I, 8 myself, believe that in fact the progression of the severe 9 accident would expose fuel frequently to strongly oxidizing 10
~
environments, but you can see those release fractions, those 11 notable radionuclides are all much higher than this three 12 and a half percent.
13 Just because the fuel -- the actinides were-14 released by a dispersal event does not eliminate them from
() 15 concern. There are dispersal type events hypothesized for 16 severe accidents at conventional Western plants.
17 In truth, ice condensers may be particularly 18 susceptible to dispersal events. It is not that you just 19 have a dispersal. You must have a dispersal and a 20 containment failure in not necessarily simultaneously but 21 close enough in time that-the dispersed material does not.
22 have an opportunity to settle out within the plant before 23 you have the failure of the containment.
24 Ice condensers have weak containments. Some of 25 the kinds-of dispersal events that could-involve. releases of ANN-RILEY_& ASSOCIATES, LTD.
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385 1 the radionuclides as a dispersion rather than a vaporization 2 process -- certainly ex-vessel steam explosions, high
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3 pressure melt explosion events are dispersal events.
4 Yes, Mario?
5 DR. FONTANA: Is the concern -- there are several 6 things that happen in an ice condenser that kind of work 7 against each other. The ice bays themselves could act as 8 shock absorbers with respect-to the steam explosion and so 9 on, but the high pressure melt ejection or steam-explosion 10 could blow up the ice so you don't have much decay -- what 11 is that they are concerned about?
12 DR. POWERS: Typically by the time any of these 13 dispersal events will occur, the ice is largely melted and 14 you have a large volume of water down below the reactor 15 vessel.
16 DR. FONTANA: Okay.
17 DR. POWERS: Now that of course is good and bad.
18 Water is a coolant but it is also the medium for steam 19 explosion and it also acts in a slug capacity.
20 I think, and maybe one of our invited guests would 21 care to comment because I think he's actively involved in 22 looking at ice condensers and dispersal events, some of the 23 concerns of course are the direct loading you get from 24 events, but I think a lot of the concerns have come from 25 dispersal of fuel and material directly against the steel
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386 1 shell containment.
~
/D 2 MR. TINKLER: Yes. We are in the process right V
3 now of addressing direct containment heating and high 4 pressure melt ejection for ice condenser plants.
5 It turns out that the entrainment of debris into 6 the bulk containment atmosphere isn't so much a concern for 7 the ice condenser plants as it is the accompanying hydrogen 8 combustion, which is affiliated with many of these scenarios 9 because there's station blackout and there is no control for
-10 hydrogen during those events.
11 The dispersal in our analyses, which is largely a 12 heat transfer issue, isn't the principal mechanism that is 13 causing the challenges -- primarily the hydrogen.
14 Now there is another issue associated with the (j 15 debris transport to the containment shell or liner because 16 the refueling -- excuse me, the in-core instrument tunnel 17 exits into an instrument room which is adjacent to the 18 shell, but that has been considered and there is some data 19 from past DCH tests on that.
20 I would also comment somewhat on this general 21 issue because I do have a view on this and it is that the 22 fragmentation of molten fuel associated with dispersal into 23 fine particulation it seems to me is a stronger effect from 24 strong steam explosions than from the high pressure melt 25 ejection, which is largely an entrainment process. We do f3 ANN RILEY & ASSOCIATES, LTD.
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387 1 measure fragmentation of debris in the DCH tests and we
() 2 3
typically see particle sizes which on a weighted basis, mass weighted basis, obviously are large.
4 You know, we see a millimeter or larger particles.
5 We do see part of the distribution of smaller particles but 6 we have trouble measuring particles much smaller than about 7 50 microns.
8 The steam explosion tests which have been done in 9 the Faro and Krotos Program, they look at the post-steam 10 explosion particle sizes and they do see a distribution of 11 small particles but even in the stronger steam explosions it 12 is a relatively modest amount of the debris which is 13 fragmented to the sub-20 micron size.
14 DR. POWERS: I think it is fair to say that --
(O) 15 well, historical background,. WASH-1400 credited steam 16 explosions to be an enormously important source term event.
17 They envisioned the steam explosion producing a lot of very, 18 very fine material that remains suspended for long periods 19 of time.
20 Subsequently examinations of that said well, there 21 is a peculiarity of steam explosions. Yes, they can produce 22 fine material but they also produce fine water droplets and 23 fine water droplets have the capacity to sweep fine 24 particles out of the atmosphere, and there has been a 25 general trend to discount steam explosions as an important ANN RILEY & ASSOCIATES, LTD.
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388l 1 source of radionuclide release in event of a reactor n '
( 2 accident even though ex-vessel possibilities of steam V;
3 explosions probably have actually gone up in people's minds.
4 There's just not a source term consequential thing.
5 High pressure melt ejection -- there's been a 6 limited examination in the source term characteristics of 7 it, but it is evident that there are multiple processes 8 going on. There are dispersals that yield the millimeter 9 size particle events. There are other mechanical processes 10 going on like the effervescence of dissolved gas out of 11 melts that create another size particles, and then there is 12 a certain amount of vaporization processes and entrainment 13 processes.
14 My only point in this slide is that, yes, you have (n) 15 to worry about dispersal events when you think about 16 actinide releases.
17 The Commission's SRM asked us to look at the 18 impact of higher burnup fuel and MOX. With respect to the 19 revised accident source term, NUREG 1465 I think a lot of 20 this is just background, that was a revised source term 21 intended to replace TID 14844 for licensing and regulatory 22 processes. It is to describe a source term for a relatively 23 severe accident.
24 It goes through and utilizes the results of a lot 25 of research sponsored by the NRC and release fractions and f ANN RILEY & ASSOCIATES, LTD.
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-389
- 1. release rates for release from the fuel cladding gap, 2 release during the core degradatiori process that is called
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3 in-vessel release, release from the ex-vessel-processes, 4 which is largely release during melt-concrete interactions 5 and.something-that is called late in-vessel release which-6 people have generally considered to be-the revaporization of 7 radionuclides piping systems back into the containment-8 atmosphere.
9 These release rates and release fractions were 10 derived from a variety of analyses done many of them in 11 connection with NUREG-1150 that are'not exclusively for this 12 purpose. Most of those analyses were done with something 13 called the source term code package, which is a technology 14 that now is 15, 20 years ago old and is really not used to
() 15 16 any significant extent.
These calculational results were modified by the 17 process of expert elicitation to develop refinements on the 18 code predictions, but all of the predictions and all of the 19 analyses were predicated on the existence of conventional 20 urania fuel and what I would call moderate burnups.
21 I don't think that this is ever specified in any 22 great extent but perhaps less than 40 gigawatt days per ton 23 were what were envisioned at the time the source term code 2
.4 package and the expert.elicitations were done.
25 NUREG-1465 of course does get used in the
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390 1 regulatory process. It is used in connection with design 2 basis accidents and the evaluation of engineering safety (V) 3 systems and AP-600 was kind of the test casa for one of the 4 major uses of it. I think it was a. leo used for System 5 80-Plus. What they focus on is not the er' i re release 6 scenario but rather the release, the gap rt ease and the 7 in-vessel releases.
8 These are used to assess what the worst two hours, 9 as far as containment concentrations, are of suspended 10 radionuclides.
11 What this tells you is it's these early phases 12 that are going to be important for the applications of 13 NUREG-1150 and that rates count. Frequently when one things 14 about severe accident source terms, one says, well, you
() 15 release the material and I don't really care how fast it 16 comes out -- it comes out to some extent and that is the 17 number I need to work with.
18 Well, rates do count because of this worst two 19 hours.
20 The question I think the Commission is posing to 21 us are the extensive release, that is the release fractions 22 specified in NUREG-1465 and the release rates still 23 appropriate for MOX and high burnup fuel. Again, we have 24 written on this subject of high burnup fuel and I think our 25 . focus is on the MOX fuel.
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r 391 1 I have provided you with a variety of documents
() 2 3
related both to high burnup and MOX fuel. I can't claim that this is an authoritative examination of the literature 4 or even the most salient papers out of the literature. What 5 I can tell you is this is a cross-section of documents that C I myself found useful in developing my thinking on the 7 subject of MOX fuel and you have been provided those for 8 your benefit to examine.
9 When you think about release from fuel you think 10 in terms of mecha stic processes now. Fission products get 11 borne in grains of fuel. They must diffuse through the 12 grain of fuel to a boundary of the grain. That diffusion 13 $ process is affected clearly by the size of the grain.
14 Interestingly, it is also affected by the oxygen potential.
() 15 That is, fuel with higher oxygen potentials has higher 16 diffusion coefficients, and it is very understandable.
17 It says oxygen goes into the lattice, causes the 18 lattice to expand, creates a little more room among the 19 atoms so that fission products can diffuse.
20 Once a fission product has reached the boundary of 21 a grain, it has to diffuse to a grain edge and the reason 22 that it has to go to a grain edge is because it is at the 23 edge that it begins to communicate with the open porosity in 24 the fuel. How much edge you have and how open that is to 25 the diffusion paths do depend upon burnup, do depend on ANN RILEY & ASSOCIATES, LTD.
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392 1 oxygen potential.
2 The.burnup effects have been considered for a long
(}
3 time in modern fission product transport. In fact, the old 4 ANS standard on fission product'-- fission gas release from 5 fuel includes a burnup correction factor, and it is a 6 relatively potent factor. Well, once a fission product 7 reaches the grain edge it now has to diffuse through the 8 pore structure. The magnitude of that pore structure'is a-9 function of burnup. The ability of the fission product to 10 diffuse through it depends on what chemical species the 11 fission product adopts. Is it a volatile species, for 12 instance a cesium atom, or is it a relatively nonvolatile 13 species like a cesium uranate?
14 Once you have gone through the pore structure you
() 15 reach the fuel cladding gap and you flow to a breach in the 16 cladding. The c1:jding at some point in the accident has 17 actually gone and actually getting through the pore 18 structure is sufficient to reach the breach and enter into 19 the bulk flow.
20 Fission product entering into the bulk flow then 21 goes through a thermal gradient. It can nucleate to form 22 particles or it can condense on surfaces.
23 NUREG-1465, when it specifies release from the 24 fuel -- or its release fractions, is not telling you how 25 much got released from the fuel. It is telling you how much
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393 1 got released from the fuel and successfully negotiated these 2 pathways through the reactor coolant system to get into the
(}
3 containment, okay, so you cannot compare release fractions 4 that you measure with fuel in experiments against the 1465 5 numbers. You have to compare release fractions and 6 transport fractions to get to those numbers.
7 These processes of nucleation and deposition 8 depend on lots of variables, tcmperatures, chemical 9 speciation, oxygen potentials and the like.
10 I will attempt to analyze these now in a fairly 11 mechanistic fashion. There are tools out there for doing 12 this. Those tools by and large were never developed for 13 heroic changes in the type of fuel we look at. In general, 14 there was a stock kind of fuel. In general, the codes have
() 15 16 been developed for PWR fuels. They even make scant recognition that PWR and BWR fuels are differ'-t.
17 They tend not to take into account things like 18 heterogeneity of fuel due to the existence of plutonium or 19 localized burnup effects or oxygen potential effects. They 20 tend to be much more correlations of experimental data.
21 Cometimes that correlation of experimental data is 22 well hidden. That is, there is a lot of mechanistic 23 dressing around the correlation of experimental data. That 24 means that the capabilities to deal with hig. burnup are 25 modest. Some of the codes do in fact employ a burnup
[D
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- l. 394 1 correction that looks muchLlike that that is built into the
'2 NS 5.4 t:,tandard Some have somewhat more mechanistic 3 descriptions. Most of those focus exclusively on theEfuel 4 porosity -- that is, how much open space is there amongLthe.
5 grains-for fission products'that' succeed in getting to the 6 grain surface to vaporize and flow through to' reach the 7 outside world.
- 8. I don't know of any codes that recognize that 9 burnup affects oxygen potential. There are some efforts to 10 recognize that interstitial oxygen affects diffusion
.11 coefficients and some modest effort to look at the effect on 12 chemical speciation as you go to higher burnups.
13 There have been attempts in the rest of the world 14 to improve upon these codes, particularly with respect to
() 15 16 fission gas and volatile fission product release. The Transuranium Institute and the Canadians have been working 17 on fuels that or models that are extremely mechanistic for 18 the release of these highly volatile materials.
19 I am aware of nothing going on to improve our 20 models for the more refractory fission products, certainly 21 not for the actinides, but not even for things like 22- strontium and barium that we consider to be moderately 23 volatile.
24 Codes have been developed for conventional urania 25 fuel at fairly moderate burnups. High burnup fuel is jrst ANN RILEY & ASSOCIATES, LTD.
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1395_
1 'different. :I think that is-the thing that caught us by 2 surprise. Maybe.Dr. Meyer would-like to comment also on
[
3 this area.
4 The high burnup fuel has differences because of' 5 microstructure and oxygen potential..=Once you reach 6 sufficient burnup you develop something that is called a rim 7 region. It 's a microstructurally different region.
8 Whereas normal fuel has grains typically on the order of 12 9 to 15 microns in size, the rim region has grains on the 10 order of 1 micron in size. It is highly porous, a large 11 amount of porosity into it, and you can see what all this is 12 leading to.
13 You have got very tiny grains, so fission products 14 borne in tiny grains have a small distance to move, large
() 15 16 amounts of surface area enter into a highly porous region.
They can release quickly from *. hat rim region into the fuel 17 cladding gap.
18 DR. MILLER.: Dana, what is the physical reason for 19 the dramatic change in grain size, or is that beyond the 20 scope of this lecture?
21 [ Laughter.)
22 JR. POWERS: More importantly, Don, it's beyond 23 the scope of my kr.owledge.
24 There are attempts to model the development of 25 this rim region. I have seen some modelled. It is a little ANN RILEY &' ASSOCIATES, LTD.
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396
- 1 difficult to do, because you can see it is running against
[~')
(/
2 surface energy. Surface energy would make these grains big.
3 DR. MILLER: Does this go through this transition 4 rather quickly with burnup or does it kind of move slowly?
5 DR. POWERS: Why do you ask all hard questions, 6 Don?
i 7 DR. MILLER: I guess you are doing your Master's 8 exam here. I was giving you a quiz.
9 DR. POWERS: I think that it is fair to say that 10 most of the thinking is that the development of this rim 11 region is occurring from the day that you start irradiation 12 of the fuel. The appearance of it, so that you can detect 13 it microstructurally, is a fairly abrupt event occurring --
14 and this has a lot to do with many, many variables -- but I
() 15 would say somewhere around maybe 45-50 gigawatt days per ton 16 you start seeing a clear rim region and it just starts 17 growing on you. These are kind of round numbers and 18 what-not, because it is a microstructural thing.
19 DR. MILLER: So if gathered something out of here, 20 at 50 megawatts per metric ton, things are changing 21 dramatically.
22 DR. POWERS: I think if you will recall some of 23 Dr. Meyer's presentations before the committee and the 24 subcommittee, he too would say that somewhere in that region ,
25 there seem to be some differences occurring in lots of I ANN RILEY & ASSOCIATES, LTD.
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397 1 things.
2 Roughly correct?
(}
q 3 MR. MEYER: Yes.
4 DR. POWERS: Feel. free to disagree and.what-not.
5 It's an area of some debate.
6 MR. CRONENBERG: But isn't that pretty small too, 7 Dana? I mean we are talking about a rim, we are talking 8 about 5 percent of fuel or so.
9 DR. POWERS: I mean yes. The rim grows and if I 10 get up to eight -- eighty gigawatt days fuel, the rim is a 11 huge volume. The 50 megawatt day fuel, it's a tiny volume.
It's on the outside so it -- as it grows in width it gets 13 pretty significant.
14 DR. MILLER: Well, how wide is this at say 50?
() 15 16 microscope.
DR. POWERS: At 50? You barely see it in a 17 DR. MILLER: It does affect the surface 18 dramatically.
19 DR. POWERS: That's right.
20 DR. MILLER: That's a key issue.
21 DR. POWERS: Well, it affects things so you get 22 somewhat higher fission gas release in that region.
23 DR. SEALE: Dana --
24 MR. MEYER: Could I jump in at this point and say 25 that in addition to this visible rim effect, I think that
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398 1 even in the bulk of the fuel that you have this growing accumulation of fission gases which are accumulating in
(~}
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2 3 bubbles on the grain boundaries so I don't think that the 4 rim is responsible for all of the high burnup effects'that 5 you see.
6 You are beginning to get a microstructure that is 7 almost linked together by pores on boundaries, even in the 8 bulk, inside of the rim.
9 DR. MILLER: So suffice to say the diffusion 10 effects, or diffusion coefficients on fission products 11 changes substantially overall.--
12 MR. MEYER: Yes. If you have a model that 13 characterizes release by a diffusion coefficient then the 14 effect of diffusion coefficient is changed substantially.
() 15 DR. SEALE: Dana, I seem to remember back in the 16 deep dark misty past of fast reactor fuel concerns where 17 high burnups were the rigor rather than the potential for 18 the future that there was a microstructure re: orientation 19 of the fuel with radially aligned grains that were 20 zone-refined into the fuel over operation that essentially 21 the long axis was in the direction of the temperature I 22 gradient and all those other good things.
23 Is this the prenatal or is the beginning of that?
24 DR. POWERS: No, I think this an entirely.
25 different phenomena. We have a noted expert on exactly that
(~}
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399 1 with us. Ralph Meyer has cut his technical teeth on some of
)
/~) 2 those issues. He might want to comment, but I think that O
3 really is a temperature issue and this is really an-4 irradiation issue here.
5 DR. SEALE: Okay.
6 DR. POWERS: Dr. Meyer, would you care to comment?
7 MR. MEYER: No that is enough on that. The LMFBR 8 fuels operate at higher center line temperatures. So that 9 is --
10 DR. SEALE: Yes. Yes. Than,. you. Because you 11 gave me a rational basis to abandon my predispositions.
12 DR. POWERS: Yes, we specifically -- light water 13 reactors, they specifically demand that be no center line 14 melting.
(r')j 15 DR. SEALE: Below temperature.
16 DR. POWERS: Mixed oxide fuel is a real mystery.
17 Plutonium dioxide is complete admissible in uranium dioxide.
18 They make up powders that prese them together. They center 19 them, they don't get homogeneity. They get heterogeneous 20 regions of locally high plutonium. Plutonium burns 21 preferentially in the irradiation. You get locally high 22 porosity. There are some interesting neutronic effects.
23 Again, the point is MOX is just different.
24 DR. MILLER: So I will find the reasons for all 25 that embedded in these papers.
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400 1 DR. POWERS: Dozens of them.
() 3 2 DR. MILLER:
DR. POWERS:
So I do not have any questions then.
Dozens of them. Well, chemistry.
4 unlike thermal-hydraulics, is not a predictive science, and 5 we rely heavily on experimental data. And our models are 6 built, as I said, in general, all release models somewhere, 7 somehow have at their core a correlation of experimental 8 data.
9 This other question is -- What have we got for 10 high burnup fuel? What have we got for MOX fuel? There are 11 some tests going on there, some tests in France called 12 VERCOURS. Also, in France, there is the PHEBUS program.
13 VERCOURS tests tend to be small tests where they are heating 1
14 pellets in furnaces under somewhat controlled conditions, j r \
15 and they have done this with MOX and high burnup fuel.
16 We contemplate in this country having yet a 17 different type of fuel with yet different types of programs, 18 and that is fuel with burnable poisons. There are neutronic 19 effects when you put plutonium in fuel and you need to have 20 burnable poisons. And sotne proposals are to include 21 coatings of a zirconium boride in the pellets. Well, tt e 22 French don't do that and they haven't really experimented 23 with that.
24 The PHEBUS progran is conducting a test 25 specifically to look at the release of refractory
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401 1 radionuclides, _ including actinides. It'is an attempt to get
() 2 13 fuel to a very'high temperature in an' advanced state of
-degradation. And this particular testLis scheduled for'the
- 4 current calendar year.
1 5 I think when we were at the Quadripartite meeting 6 we heard some words about some proposed Japanese tests.
7 Myself, I don't know anything about this, but IL think: they 8 harken back to the kinds of experimenta on fission product 9 release that one of our esteemed colleagues got to oversee 10 at Oak Ridge. I think the will be very similar. And If 11 ~ think the intention is to look at some high burnup fuel 12 there.
13 DR. WALLIS: Dana, you keep talking about low 14 volatility. Well, that is all right if their chemistry is
() 15 all right. They could be combined with something else which-16 makes them gaseous, if they are in the right chemical form.
17 DR. POWERS: .We are working in an environment of 18 extremely high temperatures here and chemistry does 19 simplify. We have not identified for most fission products, 20 certainly, in most of the actinides, anything that'is 21 extremely volatile. We have identified volatile forms. The 22 actinides do tend to go - for instance, _ americium goes from 23 an AM-203 to product a volatile species which is AMO, but 24 these are fairly high temperature phenomena.
25' We don't have anything with the actinides that is
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402 1 the equivalent of ruthenium going to the tetroxide, which is
() 2 3
the difference between a material that is virtually not volatile to one that will vaporize in a warm hand.
4 Well, the data that come from these tests is scant 5 and all I have been able to do is intercept some e-mail 6 traffic to show you some results that are coming apparently 7 out of the VERCOURS program. They are looking at the 8 effects of burnup and MOX. I have listed there a couple of 9 tests that are getting talked about the Internet now, 10 literally over the Internet. So I would hesitate to say 11 these are validated and peer reviewed data.
12 Here are a couple of attempts to look at 13 relatively high burnup fuel. This they call their high 14 burnup, this they call up low burnup test. And what they A
i
) 15 have are the cesium release fractions here.
16 This is a test with MOX fuel. Now, the first 17 thing that you notice from this, these experimental results, 18 you can't tell a damn thing from it. There is no neasure of 19 experimental error. There is no test that is replicated.
20 So when we look at this high burnup, low burnup, and that is 21 a very modest difference in burnup there, and we see 26 to 22 18 percent, I cannot attest to you that there is any 23 difference between those numbers at all. If they redid this 24 test six times, they may have gotten numbers up in the 26 25 percent.
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403 1 DR. FONTANA: There is just one test each?
() 2 3
DR. POWERS:
right now in the database.
That's right. That's where we stand And, in fact, if you look at the 4 release rates here for these experiments, yes, you see some 5 differences, but I can't attest to you that it is -- there 6 are huge numbers of variables that affect release. They can 7 control a fraction of those. There are limits in your 8 ability to control things and tests at high temperatures 9 never go the way you anticipate. That is a fundamental high 10 temperature chemistry rule. And so there are minor 11 variations here. But on the fact of it, it certainly 12 suggests that there is a burnup effect.
13 This burnup effect disappears if I take this fuel 14 up to complete melting. You get -- for cesium, you get to
() 15 100 percent release, you aren't go to release any more. The 16 rate effect persists.
17 The difference we see for MOX is impressive. It 18 is completely different. It starts releasing cesium 19 earlier. It releases it at a faster rate, and it releases 20 at a greater extent throughout the experiment. It would 21 surprile me if we could attribute that strictly to a 22 stochastic variability in the test. But, again, I have not 1
25 a demonstration of that.
24 DR. FONTANA: The statement that you made a couple 25 of sentences ago, that the effect disappears when you take
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404 1 fuel to melting, is that for the two-rows or,all~ three rows?
() 2' 3
DR.-POWERS:
everything.
'In' fact, .Mario, is it is true for:
4- DR. FONTANA: Okay. You would think so.
5 DR. POWERS: For the volatiles, if I melt it, I:
6- tend'to release all of the volatiles, okay, 90 percent of 7 the volatiles and it is all gone. That is'not.true of the 8 actinides and you will notice I have no data on actinides 9 here.
10 There are a few data points floating around on 11 ruthenium, but, basically, the French have simply not 12 reduced all the data. And they do these things with gamma, 13 scans, it takes forever to reduce some of the data. I'think 14 Dr. Kress can attest to you at lengtn.about both the cost.
) 15 and the time required to reduce data from some of~these 16 tests. Plus, after you do that, the data reduction, there 1 is an elaborate effort required to make sure that you have 18 not lost something and that you are properly interpreting.
19 it. A lot of things called fudge factors come in here l 20 magically.
21 DR. WALLIS: When you compare these, I mean are 22 they in the same form, or do you say fuel, whether they are 23 exactly the same --
24 DR. POWERS: Pellets.
25 DR. WALLIS: -- geometry and history and so on?
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7
- _L-.--. _
405 1 DR. POWERS: Yes.
/~ \ 2 DR. WALLIS: So there is no reason to assume that Q
3 they were madu differently or anything? There are all kinds 4 of variables.
5 DR. POWERS: There are a huge number of variables 6 and the same for the fact that they look at pellets. To say 7 that those variables have been controlled, no, not a chance.
8 These guys, when they do irradiated release experiments, you 9 get what is made available to you. And there is a problem.
10 I mean if somebody gives you a fuel rod and you pick a 11 pellet out of that, and you want to do a replicate test, the 12 next pellet down is different, it has a different 13 irradiation history.
14 And people take steps to try to correct for that.
( 15 They used paired rods, half pellets. There are a lot of 16 steps. You.never control everything. And, in fact, if you 17 look at some of the databases that were prepared that lie at 18 the heart of one of our release models called COURSOR, and I 19 do invite members, after drinking a little bit, to 20 interrogate Dr. Kress on the history of the development of 21 the COURSOR model, because it is an amazing description of 22 the realities of science.
23 DR. WALLIS: There is one pellet in each of these?
24 I don't understand. ,
25 DR. POWERS: I think these -- well --
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406 1 DR. WALLIS: How is it possible to run a program 2 of any integrity whatsoever with one pellet? That doesn't 3 make any sense.
4 DR. POGERS: The experiments are a little more 5 elaborate than what I have described, but I think you have a 6 write-up on them. You can see the details.
7 DR. WALLIS: It just seems incredible.
8 DR. POWERS: What you have to understand is that 9 these are hot pellets, and there are --
10 OR. WALLIS: It takes an act of Congress to get 11 one pellet, is that what that is?
12 DR. POWERS: Well, I understand it takes a hot 13 cell and a high temperature furnace. Now, hot cells you 14 have got, and high temperature furnaces you have got, but
() 15 now you have got to have a hot cell with a hot temperature 16 furnace. Those are rare things.
17 DR. WALLIS: There is not a shortage of pellets?
18 DR. POWERS: It is a shortage of experimental 19 techniques and these are expensive to do. This is an 20 expensive, expensive process.
21 DR. WALLIS: Once anyone has done one experiment, 22 he doesn't want to do it again.
23 DR. POWERS: You can't afford to, usually.
24 DR. KRESS: We would typically get three 25 experiments a year, at most.
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~
c 407 1 DR. POWERS: On a' good year. On a good year.
2 I just wanted to put un nd show you some of the (VT 3 complexities and the chemistry involved when you start 4 talking about MOX fuel and especially MOX fuel with burnable 5 poisons. Chemical phenomena are driven by this solid state 6 reaction. Hexavalent plutonium just loves to go into 7 solution in urania. It has -- it would be by itself happier 8 at high temperatures to be trivalent. That is not so happy 9 in urania. But when . occurs driven largely by entropic 10 cunrid_rotions, it yields oxygen. That gives you a higher 11 oxygen potential. It also gives you vacancies. Vacancies 12 accumulate and create porosity.
13 DR. FONTANA: I hate to show my ignorance, but 14 what is O sub zero?
() 15 DR. POWERS: Oxygen on a oxfgen site.
16 DR. FONTANA: Oh, okay.
17 DR. POWERS: This is all done in what is called 18 quarter notation. If you are not familiar with it, I will 19 be glad to explain it to you offline. Here I put it up to 20 emphasize the complexity here, that these are solid state 21 and gas state and heterogeneous reactions.
22 If we have a coating, oxygen can be consumed by a 23 burnable poison coating, to put zirconium into the lattice 24 and create boric oxide. Boric oxide can react with fission 2
.5 products. This reaction actually is driven by reducing the
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408 1 cesium pressure and reducing its volatility substantially at
() 2 3
the cost of increasing the iodide volatility.
My point here is there is a lot of complexities 4 and most of these complexities were not considered in 5 formulating the NUREG-1465 model of the source term. What 6 was considered in formulating 1465 was to attempt to get a 7 relative bounding, but not outrageously' bounding estimate on 8 the fission product releases that could occur in a reactor 9 accident.
10 So, in formulating our advice to the Commission on 11 this matter, I think we have to bear in mind, that, yes, 12 this kind of detail was not recognized. The real question, 13 is 1465 bounding or not? So I have prepared a draft letter 14 that I think you have in front of you in which I have taken
() 15 -- made two suggestions.
The first is that when we go in to look at 16 17 applications to use MOX, when we go in to look at even high 18 burnup fuel, that really we ought to do this on a 19 risk-informed basis, that we ought not try to adhere to the 20 old deterministic regulations. I think they serve us poorly 21 in that we cannot access the improved understanding and 22 technologies we have available if we were to live with the 23 TID 14844 source term. That, in fact, we should require the 24 use of something like the NUREG 1465 revised source term, 25 because that gives us access to the new technology.
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409 1 And, second, we need to have the analytic. tools
()
V 2 and the experimental data to answer the question -- Does'the 3 revised source term still pose an adequate bound.for the 4 purpos s that we do safety analyses in connection with 5 design basis accidents or in connection with the assessment 6 of the adequacy of engineered safety devices? Or do we need 7 to put corrections in to accommodate some of this new 8 experimental data -- experimental analytic data? I don't 9 know the answer to that. I only know that things are 10 different than what they were when the thing -- when the 11 model was set up.
12 DR. WALLIS: Was the model confirmed by TMI or 13 something, where there was just some --
14 DR. POWERS: It has been looked at with TMI and it r N.
15 is another one of those things that you can say the model is
(]
16 bounded, okay, that the releases probably -- were definitely 17 as bad at TMI as the model would predict. The model has 18 been -- the model is inherently based on the experimental 19 data. In fact, I think it is a truism there were no 20 experiments that did not at one time or another get 21 incorporated into the correlation.
22 DR. WALLIS: But what experiments? I mean full 23 scale releases in a real plant is not --
24 DR. POWERS: Is not in the offing, it just did not 25 occur. But these varied between fairly large PBF tests,
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410 1 fairly small, but somewhat integral tests conducted at Oak 2 Ridge Laboratory, scale experiments conducted at Batelle, in
)
3 pile tests done at Sandia.
4 DR. WAL'LIS: And they all fit with some model?
5 DR. POWERS: As a correlation.
6 DR. KRESS: And data out of a test in Germany 7 where it used very small, not even real fuel, just simulated 8 fuel.
9 DR. POWERS: That's right. Yes. Sasha 10 experiments.
11 DR. FONTANA: Backing up a little bit. Does using 12 MOX fuel go along with high burnup necessarily?
13 DR. POWERS: No. No. And, in fact, in France 14 they put burnup restrictions, tighter burnup restrictions on I
) 15 MOX than they do on conventional fuels.
16 Now, I do not understand all the rationale for 17 that, but it may be simply a lack of experience with the 18 fuels. There are some differences in the fuel and fuel clad 19 behavior, but most of them seem to accrue to the benefit. I 20 mean MOX is what is called a softer fuel than conventional 21 fuels.
22 DR. WALLIS: It seems to me all MOXs aren't equal, 23 you can different proportions.
24 DR. POWERS: Yes. Absolutely.
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[411 1l -together.
2 DR. POWERS: ' Absolutely.
)
- 3. DR. WALLIS: Sandwiches.of MOX, all' kinds of 4 stuff, a big variety.
5 DR. POWERS: Absolutely. .--And it is interesting,.
6 you will read in.some of.the papers.that 1 the kind of mix 7 that we are contemplating for the DOE dispersal is one'off 8 the worst from a fission product release-standpoint.
9 I turn to our invited experts and ask if: they have
-10 any comments that:would help the Committee.
11 MR. MEYER: I would like to comment on the 12 temperature ranges here and your concern with the higher' 13 releases of volatile radionuclides at low temperatures.
14 There are low temperatures and there are low temperatures.
() 15 If we confine our attention for a moment to fuel 16 temperatures during normal operation then there is not much 17 of an effect of mixed oxide on observed fission gas release 18 other than the indirect effect of the higher temperatures'at 19 a given power rating because of the reduced thermal 20 conductivity of the mixed oxide fuel. And in.a Halden 21 report as recent as 1995, Tony Turnbull has concluded that 22 he can't find any difference in the release of fission gases 23 in.this temperature range between MOX fuel and UO2 fuel.
24 In 1995 when we at the NRC fii'st introduced a high 25 burnup. correction-factor for fission gas release, and then f
(
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412 1 high burnup was gigawatt days per ton in our minds, this
~N
[b 2 3
correction factor was in fact based on data from mixed oxide fuel, because we didn't have LWR type fuel data with MOX, so 4 we relied on LMFBR data. Also, the ANS 5.4 standard for 5 fission gas release is -- uses data from, as I recall, from 6 19 gigawatt days per ton up to the mid-50s, all of which are 7 from 25 percent plutonium mixed oxide fuels.
8 So I completely agree with all of the reason to 9 expect some differences, but, ao a practical matter, there 10 doesn't seem to be much difference up to burnups around 45 11 or maybe 50 gigawatt days per ton in the normal operating 12 range.
13 Now, the relevance of this to NUREG-1465 is that 14 the gap release in 1465 is used for the fuel handling
() 15 accident and the fuel handling accident is an accident that 16 doesn't involve any kind of transient or high temperature 17 operation of the fuel.
18 And so if you include in the gap release, data 19 from -- in this intermediate temperature range, above the 20 normal operating range and up into this range of the 21 VERCOURS testing, or the HI or the VI testing, then you are 22 putting in there data from a temperature range that is not 23 applichble to the fuel handling accident.
24 The other applications for source term in 25 licensing are -- another one is the reactivity initiated
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w~ _
413 1 accident, .the rod' drop accident specifically in a PWR. :Juxi
() 2 3
NUREG-1465 says it-doesn't apply to that accident because there is something unique in the transient itself that seems 4 to~ shake loose a lot of~ fission gas. And so there you are, 5 I think, dependent on a totally different database, that.is 6 one.from the pulse reactor test programs rather than the 7 high temperature laboratory experiments.
8 And the third application of NUREG-1465 or of a.
9 source term, then, is to the' loss of coolant accident, And 10 of the loss of coolant accident, according - .the way we 11 analyze it according to Part 100 of the regulation, requires 12 the assumption of a substantial core melt involved in 13 producing the source term that you use for this event, even 14 though the event itself is analyzed according to 50.46 and
() 15 Appendix K doesn't -- doesn't result in core melt. And so 16 in that case, you are now into the early containment release 17 for the source term.
18 So I just want to express a little concern about 19 this intermediate temperature range because I don't disagree 20 for a minute with the technical discussion that has been 21 presented, but this-intermediate temperature range.doesn't 22 have very much of an application in regulatory analysis.
23 DR. POWERS: It was certainly'a focus of an awful-24 lot of discussion before this Committee in connection with 25 AP600, Ralph.
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414, 1 MR. MEYER: Okay.
2 DR.. POWERS: So now why is that a smallLconcern?.
[
3- Was it just because it'was'new?
4 MR. MEYER: I guess'I am not aware of those 5 discussions. Can you help, Charlie?
6 MR. TINKLER: No, actually, and I was going to 7 comment with respect to the significance of the data. While 8 it is interesting. data, the VERCOURS data, 1780 degrees 9 Kelvin, from a risk perspective, when you are doing these 10 severe accident calculations, once the core gets to 1780 11 degrees Kelvin, it is on the way up. It doesn't hang around 12 at 1780 degrees Kelvin for very long because you are deep.
13 into the oxidation escalation part of the sequence. So, I 14 guess --
() 15 16 DR. KRESS:
MR. TINKLER:
You have already set'it off.
You have set it off and you are on 17 your way up, and you are not. stopping at 1800 degrees 18 Kelvin, and you are not going to hang around there very long 19 either. So --
20 DR. POWERS: I would think that in looking_at what 21 little I know about these tests, that actually I would 22 interpret them as having been through the oxidation
~
23 transient and now on the recovery.
24 MR. TINKLER: Oh.
~
25 DR. POWERS: I'mean they are more suited as a
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415' if . description of the~ recovery phase. Now, thcy artificially.
() 2 3
held it, and it won't held. And you arel absolutely; correct, the temperature escalation will wipe out some apparent.
4 chemical effects, because, I:mean you get on a steep slope, J- 5 you can't tell-the difference.between slow and fast'in some 6 ' respects.
7 But I think the preponderance of the literature,:
8 even for the low temperature range where the Halden 9 apparently can't see anything, other people see things. And 10 they, of course, have to be given some credence becauce 11- their papers are getting into the public domain.
12 MR. MEYER: Well, you have to get the temperature 13 right and that's -- I mean, clearly, the accumulation of 14 fission gas in a mixed oxide fuel rod is going to be higher.
() 15 because it is going to run a little higher temperature and 16 that is going to pump the gas out. But we are accustomed to 17 talking about release fractions as a-function of 18 temperature, and when you get it in those terms is when it 19 is difficult to see the difference in the low temperature 20 range.
21 DR. KRESS: Yes, the problem with those, Ralph, is 22 that release fractions as a function'of temperature were.all 23 developed by tests.like Dana mentioned where they.were held 24 at temperature.
25 MR. MEYER: 'Right.
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416 1 DR. KRESS: If one looks at most of the release
/~D 2 models, they are diffusive like in nature, and it depends on V
3 how long you are at temperature, how much you get released.
4 MR ., MEYFR: Yes.
5 DR. KRESS: If it translates the results into a 6 diffusive like model, which may, like the ones in Victoria, 7- and then superimpose on that a temperature transient, you 8 get an entirely different basis with respect from I guess 9 the COURSOR like model.
10 DR. POWERS: Let me interrupt and, first of all, 11 ask if Dr. Kress can continue the chairing of the meeting 12 because George and I have to excuse ourselves.
13 DR. KRESS: Yes, sir.
14 DR. PC.!ERS : And, also, to pose the question, I
() 15 16 don't think that we want to plunge into a great deal of detail in our letter to the Commission, technical detail. I 17 think we do need to get the advice from our invited experts 18 on what -- how they would care to respond to the question 19 that has been posed. The question that has been posed is 20 fairly bare, in fact, I quoted the entirety of the question.
21 And I believe the invited experts have reviewed the 22 transcript, they will knoa that that is all we have. So, if 23 you can pursue that as well as some of the technical details 24 that would be useful. I think we have to go.
25 DR. KRESS: Well, with that, I will just at this
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417 1 point turn it over to the invited experta and-say, what
T 2 advice do you have for us?
(V 3 DR. WALLIS: Well, could I ask something before we 4 do that?
5 DR. KRESS: Yes.
6 DR. WALLIS: Someone behind me mentioned LOCA, 7 Appendix K, something which I have asked in my naive way 8 about. I mean this strange regulatory world where you are 9 required to calculate transients and nothing ever melts, and 10 then you are suddenly supposed to turn around and assume a 11 substantial melt always seemed to me preposterous and 12 ridiculous. And it doesn't matter whether it is'MOX or not.
13 You can still make the same preposterous and ridiculous 14 assumption. Who cares? If that is the regulatory world,
[Ji 15 who cares what the fuel is?
MR. MEYER: Well, it is a design basis accident.
16 17 DR. WALLIS: So if that is the law, who cares 18 about what the fuel reality?
19 MR. MEYER: Well, the name of the game is to 20 postulate one of the worst credible accidents that you can 21 think about, the loss of coolant accident, and at the same 22 time, sort of the worst source term that you can think 23 about, and patch those together in order to achieve this 24 level of protection. This is historic, almost prehistoric.
25 We have always done it this way. It doesn't seem ridiculous
()
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418 1 to us, but it is not mechanistic.
j)/
9 2 DR. WALLIS: 'But there is a huge gap. I mean, 3' obviously, there is a huge logical. gap between assuming --
4 you have to calculate it is a nothing melt and then make 5 this large leap to assuming a huge melt. That is such a 6 ridiculous --
7 MR. MEYER: Well, --
8 DR. WALLIS: It needs a different adjective. It 9 is such a remarkable leap that it doesn't matter what the 10 fuel is. If you are going to make that kind of gross 11 assumption, all the details are irrelevant.
12 MR. MEYER: The design basis accident, in the 13 first place, is there to establish adequacy of emergency 14 core cooling systems. And so you have these mechanistic
() 15 16 requirements on limited damage to the core in order to give as much guarantee as you can that the emergency core cooling 17 systems are going to function properly. And then in our 18 defense-in-depth kind of attitude, then we leap ahead and l
19 say, okay, -- l 20 DR. WALLIS: Are you invoking comething which we 21 don't understand, defense-in-depth?
22 DR. KRESS: You are saying why should the 23 containment --
24 DR. WALLIS: So you are appealing to some higher 25 power like defense-in-depth. Okay.
l I
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419 1 MR. TINKLER: It is the design basis conditions
] 2 for successive layers of defense-in-depth. There is a DBA 3 calculation for the containment. There is one for the core 4 in the ECCS and there is yet another one for off-site dose.
5 DR. WALLIS: You are justifying what is. But if 6 you are going to worry about what the fuel really is, then 7 you are going to have to go to a different level of logical 8 consistency.
9 DR. FONTANA: In other words, it is not exactly 10 risk-informed. The detail thet you are asking for with 11 respect to the fuel behavior may be much finer than what is 12 required on the basis of the accident best defined.
13 MR. TINKLER: Well, you could distinguish between 14 what is an appropriate source term for design basis and what
() 15 16 should be done as part of an integrated risk assessment.
You can make that distincti'on, and the Committee can make 17 that distinction. There is some subjectivit; to boundary 18 conditions imposed for the design basis.
19 And I would like to just clarify something as far 20 as the staff's interpretation of 1465, because Dana did use 21 the word " bounding" a few times. And if you look at the 22 language of 1465, it is pretty clear it is representative.
23 It is not intended to be bounded -- bounding and says that 24 explicitly. There are aspects of it that you might say are 25 conservative because it focuses on certain kinds of
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i 420 1 sequences, but it is not intended to be bounding. And in 2 that respect, it is meant to be a substantial challengu to
( {
3 the mitigation of off-site does. And, really, that is about 4 what you should draw from its application.
5 DR. WALLIS: If you knew it was MOX, you wouldn't 6 make -- why would you make any other guess?
7- DR. KRESS: I will tell why. The reason you do 8 these things that they talk about is because that was a 9 reasonable way to develop a set of regulations and a set of 10 criteria on which to judge the designs in the absence of-11 real information which would be an integrated risk analysis 12 with full knowledge and characterization of the 13 uncertainties.
14 If you go use this type of approach, or an LWR,
() 15 with the standard fuel that we have and the standard design 16 elements, then later come back and do a risk assessment, the 17 risk assessment is the proof of the pudding that this 18 process works, because you send up with a set of reactors 19 that meet your safety goal criteria, your risk criteria. So 20 it is an indirect validation that this process works.
21 And now we are asking, will this process also work 22 for a different kind of fuel, a different kind of burnup?
23 And the question is moot, I mean is not answerable by 24 saying, well, if you just throw in the same criteria in the 25 design basis, are you still going to bound the risk?
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421 1 DR. WALLIS: So you are saying this very arbitrary
() 2 3
assumption that was made about releases turned out --
DR. KRESS: Was based on knowledge --
4 DR. WALLIS: -- when you did some more complicated 5 analyses to be okay?
6 DR. KRESS: Yeah. But it was based on, these were 7 based on judgments and good thinking and conservatism that 8 relied on experience with the light water reactor without 9 MOX fuel, without high burnup.
10 DR. WALLIS: You are validating those judgments 11 and leaps of --
l-12 DR. KRESS: Faith. By the actual PRA, because it 13 is the only way you can do it.
14 DR. WALLIS: By the PRA, which you also believe, I 15 mean it is a much better tool.
16 DR. KRESS: Well, that is the other question, how l
17 much are you going to believe the PRA, and what is its {
18 uncertainty?
19 DR. WALLIS: So you really saying for PRA type 20 world you need to be much more sophisticated in how you 21 model the releases?
22 DR. KRESS: That's right.
23 DR. WALLid: That's why you have to worry about 24 it.
25 DR. KRESS: That's right. And that is where the
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422 1 real risks are determined. .
2 DR. WALLIS: That if you stick with the Appendix K
)
3 world, there probably would never be a~ reason to do that.
4 DR. SHACK: We don't plan to design a reactor for 5 MOX, the reactor is there. You are going to put the MOX in 6 it.
7 DR. KRESS: That's right. But the question is --
8 DR. SHACK: Now you have to figure out --
9 DR. KR/ ,: Well, the queucion is, should we allow 10 that?
11 DR. SHACK: Right. I mean what is the delta risk 12 in doing that?
13 DR. KRESS: Yes. And the only --
14 DR. SHACK: We are past design basis.
() 15 16 DR. KRESS: And'there is no way to answer that by going to the design basis.
17 DR. SHACK: That's right.
18 DR. KRESS: You have to rely on something else.
19 DR. WALLIS. Well, you could. You could legally 20 say design basis is still valid, forget it.
21 DR. KRESS: Yeah, you could, if you had the 22 database. And that is the other part of this discussion, 23 does the design basis still bound -- that was the question, 24 does it still bound the utilization of MOX in high burnup i
25 fuel? That is the question that we have been asked. j ANN RILEY.& ASSOCIATES, LTD.
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423 1 DR. WALLIS: I guess in a legal sense ~it-might 2 today.
3 DR. KRESS: Might. I don't -- I mean I am not 4 sure.
5 DR. SEACK: I think the answer probably is it 6 does. Right? At least that is certainly the answer you had 7 better get.
8 MR. MEYER: If you go all the way back.to the 9 general design criteria, it probably does. Some of the 10 implementations of those criteria into specific numerical 11 regulations may have to be t 4 Vered with.
12 DR. UHRIG: Are t. sere not some experiments being 13 carried out, individual pins and assemblies in Watts Bar?
14 MR. BARTON: No, that's tritium. That's tritium,
( 25 yeah.
16 DR. SHACK: Besides, nobody plans to melt the core 17 in a reactor to find out what happens.
18 DR. SEALE: That's right.
19 DR. KRESS: You are not going to get any data out 20 of that.
21 DR. SEALE: But as long as it is still water 22 moderated, it looks like a light water reactor design, the 23 general design criteria probably apply.
24 DR. KRESS: But I think the real way I have view 25 this is to say, is to run a risk assessment with the fuel ANN RILEY & ASSOCIATES, LTD.
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424 1 .that we now have, in the particular plants we are going to
() 2 3
stick things in, it is plant-specific, and say, all right, does it make any difference or does it make an acceptable 4 difference if I stick in MOX and high burnup fuel? Ari the 5 only way I can do is by calculation, and the only way I can 6 do it by calculation is to have enough data to validate the 7 fission product release in the transport models that are in 8 the code.
9 DR. SHACK: Just coming back to acceptance 10 criteria, suppose you could do that and you found that you 11 got a delta risk at this plant, but it was still within the 12 range of risks at all the other plants?
13 DR. KRESS: Then you make your judgment. Is this 14 okay?
( 15 DR. FONTANA: Yeah, but those plants weren't 16 licensed on those kind of calculations. You have to start 17 over.
18 DR. KRESS: It is like a change to the licensing 19 basis.
20 DR. FONTANA: Yeah.
21 DR. KRESS: It is almost like that.
22 DR. UHRIG: Tom, you threw high burnup fuel and 23 MOX together. They are not necessarily together. You are 24 not going to run MOX up to a high burnup fuel, are you?
25 DR. KRESS: It is two separate issues.
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425 1 DR. UHRIG: Okay. I was confused.
2 DR. FONTANA: I think it depends on a strategy 3 that you want to do. If you are just trying to take as much 4 weapons fuel and make it as unusable as possible, what I 5 would do is not burn it up very much, and just push a lot 6 more through t:1e system. If you are trying to change the 7 isotopic ratio, I guess --
8 DR. SEALE: You want to spread those neutrons 9 around.
10 DR. FONTANA: Yeah, that is what you want to do.
11 I don't know how they are going to do it. They will 12 probably -- most economically, I guess.
13 DR. UHRIG: What you are advocating is a low 14 burnup?
() 15 DR. FONTANA: Well, if you try to push as much 16 weapons fuel through the system and make it unusable, then 17 you would want to do that.
18 DR. KRESS: And the other part --
19 DR. UHRIG: It is recovered from the spent fuel.
20 DR. KRESS: The other part of this, the other 21 consideration is how much of the total core is going to be 22 MOX, and how much of the total core is going to go to these 23 high burnups? I mean that is -- you are talking about a 24 fraction of the core is what you are dealing with.
25 MR. TINKLER: That is the point that we feel
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426 1 should be considered. That we have data to 47 gigawatt days 2 or a 62 watt day per ton limit, three cycle core. At the v
7 beginning of the cycle, none of the fuel is about 47, at the 4 end of cycle, only one-third of the core is over 47, 5 two-thirds of the core is under that burnup limit, so --
6 DR. KRESS: It is not always kosher to assume you 7 are even at the end of the cycle when you are going to have 8 the accident.
9 MR. TINKLER: But, you know, it is -- I guess we 10 would say for those kinds of loads, we don't think for 11 either the DBA or the severe accident you would ever be able 12 to see the difference above the kinds of releases we have 13 for the data that we have.
14 Now, we think there is some -- we are not sure --
'n
) 15 notwithstanding the VERCOURS data, we are not sure that the s
16 effect is clear from the existing data on burnup. We are 17 willing to concede that it is possible at high burnups it 18 could exist. But even from the risk perspective, with the 19 severe accident, we have -- a total 1465, a total 75 percent 20 release of the iodine, 75 percent release of the cesium. It 21 is invessel, ex-vessel.
22 DR. KRESS: Yeah, but you only count the invessel.
23 MR. TINTLER: Well, that is for the DBA.
24 DR. WALLIS: Who cares if you are going to release 25 so much?
~
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427 l 1- MP TINKLER: But for the severe accident risk 2 assessment, you would consider all of it. So you would be
(}
3 up to 75 percent of the core inventory.
4 DR. KRESS: That is releases.
5 DR. WALLIS: Within uncartainties, the difference 6 between 50-75 percent is nothing, within uncertainty about 7 uhat is going on.
8 MR. TINKLER: Well, you are arguing that the 9 design basis accident is pretty severe and I would agree.
10 DR. WALLIS: Well, it is big accident and that is 11 it. That's what this Committee keeps telling me, don't 12 worry about the details. A LERF is a LERF and it doesn't 13 matter whether it is a little LERF or a big LERF.
14 MR. TINKLER: Well, without adequate dose
/~%
() 15 mitigation, the DBA looks an awful lot like a severe 16 accident. It is pretty indistinguishable.
17 DR. KRESS: That's exactly right. So, you know, 18 those are the issues that we have to debate, and they are 19 legitimate.
20 MR. TINKLER: Without beat!.ng the obvious, though, 21 for plants now allowed to go up co 62, current approach is 22 to allow TID 14844 f or adjustmen'., it is hard to see that 23 tnat is a better approach as o3 posed, you know, an 24 impediment to the use of 1465. We do see substantial 25 benefits for the use of 1465 in other areas, . safety C\ ANN RILEY & ASSOCIATES, LTD.
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428 1 enhancement, as well. The most recent case with Perry, the 2 bufferir; af the sump, we~ consider it a good improvement.
[}
3 DR. MILLER: You said the fuel was one-third MOX, 4 is that on a -- basis?
5 MR. TINKLER: I'm sorry, I was referring to 6 one-third high burnup. One-third high burnup.
7 DR. MILLER: Okay. I thought you said MOX.
8 MR. MEYER: But the third also applies to the MOX.
9 In fact, what we expect --
10 DR. MILLER: That is on a fuel basis? Is that 11 regional or --
12 MR. MEYER: No. When the weapons material is 13 designed to be put into a PWR, we expect the design to be no 14 more than a third of the assemblies.,
r
( 15 DR. MILLER: Initially?
.w 16 MR. MEYER: At any time. At any time.
17 DR. MILLER: A third of the assemblies will be 18 MOX?
19 MR. MEYER: Yes.
20 DR. STALE: Each load would be one-third MOX.
21 MR. MEYER: Yes.
22 DR. SEALE: At most.
23 MR. MEYER: Yes, j 24 DR. FONTANA: Would these be restricted to the 25 lower burnup areas?
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429
-1 MR. MEYER: Not necessarily. Not- necessarily.
2 DR. MILLER: Just'in the normal rotation?
.D 3 DR. SEALE: If it is the shuffle.
4 MR. MEYER: There would be sor:e consideration of 5 the vessel effluents because of the harder neutron spectrum 6 from the MOX assemblies, so you might not load them out of 7 the periphery.
8 DR. UHRIG: I probably asked the wrong question 9 here. Would they be restricted to the total burnup of 10 something less than say 47 or whatever?
11 MR. MEYER: I can't say at this time. Just 12 offhand, I don' t see why the y would have to be, provided 13 that the database were available to justify it. And I think 14 this is exactly where the French are going. They have this
() 15 notion of MOX parity, they call it. They want the same 16 burnup limit for their MOX fuel as they have for UO2 fuel, 17 and according to their concerns, the missing data are for 18 'che reactivity initiated accident. The Cabri data, Cabri 19 just hasn't tested high enough burnup MOX fuel rods yet.
20 DR. FONTANA: Is that because of smaller beta?
21 MR. MEYER: There are a lot of things. If we get 22 into this, it will get confusing.
23 Could we go back to your draft letter?
24 DR. FONTANA: Okay.
25 MR. MEYER: I have one concern, and I think maybe ANN RILEY & ASSOCIATES, LTD.
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430' 1 Charlie has a comment'about the letter, and'it is'a concern 2: that is' reflective of the comments that I have already made.
3 Where it says, "We.are aware of' experimental studies that 4 show there to be enhanced release of fission gases to the 5 fuel cladding gap."
6 DR. KRESS: What page-are you on? I 7 MR. MEYER: I don't know.
8 DR. KRESS: Oh, it is on -- in our. letter itLis 9' line 45.
10 DR. SEALE: Yes.
11- MR. MEYER: That would be it, yes. My only 12 concern is that someone who might not be familiar with the 13 details would cor clude that tha release rate for MOX fuel 14 was higher than for UO2 fuel because we-use release rates, I
() 15 16
- mean these fractional values to quantify this.
Well, let me back up and try and say that over 17 again. I will start over again.
18 DR. KRESS: Okay.
19 MR. MEYER: During normal operation, which is 20 being discussed in this sentence, the increased release to 21 the fuel cladding gap appears to be determined by the 22 temperature effects rather than by changes in the-release 23 rates.
24 DR. KRESS: Becausefit tends to reach equilibrium.
25 MR. MEYER: That is because the thermal-ANN RILEY & ASSOCIATES, LTD.
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431 1 conductivity of hsX is lower, that at a given power level'it
' /T - 2 runs at a higher temperature, and because of the higher V
3 temperature --
4 DR. KRESS: It reaches equilibrium.
5 DR. WALLIS: If you run it at the same power you 6 could get more releases because it would be a higher 7 temperature.
8 MR. MEYER: That is correct.
9 DR. WALLIS: So it is true that normal operation, 10 if it is at the same power level, there would be --
11 MR. MEYER: Yas. Yes. You know, maybe it 12 wouldn't mislead anybody. It just --
that is my only 13 comment.
14 DR. WALLIS: Why don't they operate at lower power 15 level?
16 DR. KRESS: But your concern is this --
17 MR. MEYER: You might -- what if, you know, if you 18 stuck in a phrase like because of its somewhat higher 19 temperatures? Just stuck that in.
20 DR. KRESS: We will take that under consideration.
21 MR. MEYER: Okay. Thank you. Charlie may have 22 had a comment.
23 MR. TINKLER: Well, I already made my comment.
24 With respect to the issue of fuel dispersal versus 25 vaporization, while it is true they are fuel dispersal ANN RILEY & ASSOCIATES, LTD.
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432 1 events in a general sense, I think there is -- I make'a 2 distinction between steam explosions which involve a strong 3 shockwave -- the potential for fragmentation versus fluid 4 entrainment process, a little different from the ability to 5 finely fragment molten UO2 with a fluid entrainment process, 6 as opposed to a strong shockwave passing through molten 7 material, are two different processes.
8 DR. KRESS: I agree with you completely.
9 MR. TINKLER: And it is true that when we measure 10 debris, we get distribution, but some of that stuff we 11 measure in a DCH test is concrete dust that we create when 12 we blow stuff out of the vessel. It is not UO2, it is not 13 actinides, it is dust.
14 DR. KRESS: Now, at one time there was a 15
( consideration that the fuel actually had to dissolve to a 16 gas state.
17 MR. TINKLER: Right. <
18 DR. KRESS: And that as you dispersed it at high j l
19 pressure and low pressure, that that gas goes out of 20 solution and that is part of fragmentation, as opposed to an 21 entrainment process. Has that been -- I i
22 MR. TINKLER: Its ability to be -- it is hard to 23 tell because the thermite we use in these tests is pretty 24 effervescent, okay.
25 DR. KRESS? Is effervescent.
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433 l 1 MR. TINKLER: Yeah. So if you are drawing your i
'"') 2 conclusions about the effervescence from thermite, you might x_) '
3 be --
4 DR. KRESS: That is what I was counting, in other 5 words, you are calling it the thermite really.
6 MR. TINKLER: And if you look at the Argonne test 7 where they used UO2 results, just a few noted reactor melts, 8 it is a pretty small percentage, down in the 20 microns or 9 less. But 20 microns is still pretty big particles for a 10 dispersal off-site.
11 DR. KRESS: Especially, you know, those settle out 12 pretty quick, 20 microns. So, you are right, those are big 13 particles.
14 MR. TINKLER: Those are big particles.
,m
',s 15 DR. KRESS: I don't recall what we are asking in (v
16 this letter, because I forgot -- I haven't read it in a 17 couple of days.
18 Well, we appreciate the input. We will -- when we 19 are scheduled to --
20 MR. SINGH: I think sometime this afternoon.
21 DR. KRESS: This afternoon.
22 MR. SINGH: Possibly, or maybe tomorrow morning, I 23 am not sure.
24 DR. KRESS: So I guess at this time we are scheduled to break for lunch. Okay.
25 ANN RILEY & ASSOCIATES, L TD .
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434 1 Well, I aopreciate the valued experts and we will
() 2 3
take your advice into consideration. Thank you very much.
At'this time I am going to declare a recess for 4 lunch.
5 [Whereupon, at 12:10 p.m., the meeting was 6 recessed, to reconvene at 1:25 p.m., this same day.]
7 8
9 10 11 12 13 14 16 17 18 19 20 21 22 23 24 25
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435 1 AFTERNOON SESSION 2 [1:25 p.m.]
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3 DR. KRESS: Can I call the meeting to order, 4 please? Dana and George are indisposed at the moment.
5 MR. BARTON: Are you sure that that is all that is 6 wrong with them?
7 DR. KRESS: No.
8 DR. SEALE: They may even be impaled by now.
9 DR. KRESS: The next item on the agenda was to 10 talk about the little working group's product on the role of 11 defense-in-depth in a risk-informed regulatory system. It 12 started out being a hard paper that is being developed by 13 our imminent fellow, Jack Sorenson, and then we worked on it 14 and produced a paper jointly authored by Jack and George and r
15 Dana and I to be presented at some conference or other, I
.(
16 have forgotten which conference.
17 And then I was tasked with pulling together the 18 ideas in the paper and carrying it to the extent of a 19 possible letter on the role of defense-in-depth in a 20 risk-informed regulatory system. George and I and Dana were 21 supposed to develop letter jointly and put it forth as a 22 possible Committee position. We]l, things didn't quite work 23 that smoothly.
24 I developed the first draft, nobody liked it. I 25 developed a second draft, nobody liked it. We have gone f]
4
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436 1 back and forth. Dana and George aren't here, but you can --
(N 2 your handout has attached to it some of the e-mails we have b
3 traded back and forth between George,' Jack and me and Dana.
4 And that can give you a flavor of the problems and the areas 5 of dispute.
6 What I suggest we do now is read the letter and 7 read the attachments so you know what the problem is. I 8 don't have much hopes, really, of getting a letter out this 9 time. Maybe we can get a start and get a feel from the rest 10 of the Committee on what ought to go in it.
-11 Dana tells me he has some compromised positions 12 that he thinks may reconcile most of.the problems. I 13 haven't seen that yet, but when he gets it, I would like to 14 See it.
f
( )s
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15 The things I would like to point out to you mostly 16 that are areas of contention are, number 1, I have taken the 17 foolhardy or bold step, depending on your viewpoint, of 18 defining yet again another -- producing another definition 19 of defense-in-depth. That is wrought, fraught with land 20 mines and all sort of things, so that is one area of 21 contention.
22 The other area of contention is -- it is a novel 23 definition that allows you put necessary and sufficiency 24 limits on defense-in-depth and I have outlined to do that, 25 and there is a lot of contention as to whether that is
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437 1 practical, workable, doable, those sort of things. And I
('"N 2 have even put into the letter an example of how to apply c) 3 this, and the example has to do with the balance between CDF 4 and containment failure as a defense-in-depth concept. And 5 that also is one of the areas of contention.
6 So having flagged some of the points, there are 7 others, but those are the major ones, I think, maybe you 8 could spend the time now while we are waiting for Dana and 9 George, because I don't think we can advance very far 10 without them. Maybe you could read this whole package and 11 then we would be prepared to discuss it further.
12 Does that sound like a good way to --
13 DR. WALLIS: It is not very different from draft 14 number 4 dated March 29th.
15 DR. KRESS: No, it is not.
[V) 16 DR. WALLIS: I had some comments on that which I 17 haven't shared with you yet.
18 DR. KRESS: Okay. Well, we would be pleased to 19 have those, too.
20 DR. WALLIS: I notice some of the points have 21 disappeared.
22 DR. KRESS: Well, that may have happened, yeah.
23 Maybe other people had the same problems.
24 DR. SEALE: One other comment here. If anybody 25 has any other examples --
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438 1 DR. KRESS: That is the other point, yes.
2 DR. SEALE: On page 2, I think it be worthwhile to
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3 try to identify those.
4 DR. KRESS: It would be nice to have those, yes.
5 DR. SHACK: Again, if anybody could explain the 6 examples that are already there.
7 DR. SEALE: That would help, too.
8 DR. SHACK: That would help, too.
9 DR. KRESS: Yeah, that is why we need --
l 10 DR. SEALE: I am not sure I read that first one 11 the way -- as being correct.
12 DR. SHACK: I am pretty sure I read it as b'aing 13 incorrect.
14 DR. SEALE: Yeah, I think that is --
() 15 16 DR. KRESS:
DR. SEALE:
The original drafts had no examples.
Yeah, but that is not the steam 17 generator.
18 MR. BONACA: What example, you are talking about 19 the one in the back?
20 DR. WALLIS: Well, the 50.59 --
21 MR. BARTON: Yeah, these two here. These two, 22 these examples. l 23 DR. SEALE: The first example.
24 DR. KRESS: Well, I can make a stab at it. I 25 didn't put these examples in, they were supplied to me. The
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. - . . .. . - . . . . .. .. . . I
439 1 problem is that is an arbitrary choice.
[~ \ 2 DR. SHACK: What is an arbitrary choice' V
3 DR. KRESS: Not to choose risk-informed limits on 4 the leakage.
5 DR. SHACK: No, the viewgraph we saw said you 6 should define the accident leakage limits based on risk 7 information. It was directly contrary to the statement.
8 DR. KRESS: Oh. Well, we will wait till Dana 9 comes back. I didn't like either one of the examples, to 10 tell you the truth.
11 DR. WALLIS: You keep saying that defense-in-depth 12 compensates for uncertainty, controls risk -- all that.
13 That is just an assumption, no one has ever proved it to me.
14 DR. KRESS: Oh, I fully admit --
() 15 16 DR. NALLIS:
DR. KRESS:
Intuitively, it is reasonable.
I fully admit that what I have done is 17 concocted a concept of defense-in-depth and concocted a 18 definition that may or may not have any basis in anybody 19 else's definition there.
20 MR. BONACA: Well, when I received your letter, 21 okay, first of all, I thought that it was important, because 22 what you did -- I mean I really believe that 23 defense-in-depth had to be a concept that we should espouse 24 to some degree, some level, okay. Now, what we have in 25 place today is so fundamentally deterministic that I have
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440 1 got a problem with it, too.
[~'i 2 DR. KRESS: Yeah, that is --
V 3 MR. BONACA: And I can understand that the 4 commitments to the current defense-in-depth are an 5 impediment to the application of PRA, and I totally agree <
6 with that.
7 On the other hand, I think what we are trying to 8 do to establish new criteria for defense-in-depth, which 9 would allow, in fact, a better technology to be applied, and 10 then to articulate a better --
11 DR. KRESS: That is more or less the motivation.
12 MR. BONACA: Yes, and I agree with that entirely.
13 I mean because --
14 DR. KRESS: You have put your finger on exactly
() 15 16 the problem and the motivation.
DR. SEALE: The camel that we call 17 defense-in-depth trE v is a product of the time in which the 18 only guidance we had for designing reactors were the general 19 design criteria and we did not have -- we did not rely on 20 much more than bounding -- I won't even say calculations --
21 evaluations, bounding evaluations of plant accident 22 behavior. And that is a very different breed of cat, or it 23 ought to be, from what defense-in-depth would be today.
24 MR. BONACA: Yes. See, the issues, for example, 25 are like, you know, common mode failure. We all agree that
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441 1 it is a fundamental problem. Therefore, in the past, not
(')
V 2 having a better technology, we devised diversity, 3 redundancy, et cetera, to deal with those issues in a very 4 structured way, that was the best we knew. Now, PRA is much 5 more capable of identifying those linkages, so we would 6 propose different means of addressing the same concern. But 7 you would still want to design through those kinds of new 8 ,
technology against common mode failure. So, the objective 9 - so, you know, I thought that your paper was on target.
10 Okay.
11 I don't kra4 how it has evolved now, but --
12 DR. KRESS: It still has that in it. And so, you 13 know, I have had to fight to keep it in.
14 MR. BONACA: Those are the examples, however, I
() 15 16 would suggest we put in.
DR. KRESS:
Like, for example --
That would be a good one.
17 MR. BONACA: Issue, you know, common mode failure 18 is a concern. In the past, deterministic has been using 19 these, you know, precepts because there was nothing better, 20 but today.you have a technology that we know much more 21 capability, that can be applied, and, you know, that is an 22 example that we could make of how you would build this new 23 -- again, so, anyway.
24 DR. KRESS: There is also --
25 DR. SHACK: . What would we have to know to apply
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442 1 your concept .to'the decision on a containment spray for an 2 AP600?
('J'{
3 DR. KRESS: You would have to know -- you would 4 have to have two things, and these don't exist. This is 5 part of the problem. You would have to know what is an 6 acceptable value of, say, LERF. You would have to have a 7 value that the Commission says this is the acceptable LERF.
8 You are above, it is not acceptable; you are below it, you 9 are okay. That it item number 1.
10 DR. SHACK: Okay, I give you that number.
11 DR. KRESS: Yes, okay. There is something else --
12 DR. SHACK: Numbers I can supply to you easily; 13 DR. KRESS: There is something else you have to 14 have to apply my concept and that is, we all know that when 15 j
) you calculate that LERF, because it is a calculated value, 16 what you have to go through to get it, that there is a 17 relatively large uncertainty in it. And when you -- the 18 thing you have to have in addition to the number is an 19 acceptable value on that uncertainty, how big it can be. So 20 that is another policy issu=.
21 I don't want to know the number just in the LERF, 22 I want to know how much uncertainty you are willing to 23 accept. That's two things you have to have. You give me 24 those two things, and then you have to do enough 25 defense-in-depth in order to meet both of them. And that is
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443 1 enough information to fix all the defense-in-depth you need.
2 Now, the point i s ., that I am making here, is the
(
3 value that is acceptable for uncertainty may very well 4 depend on the absolute value thac you have for LERF, or may 5 even depend on the absolute value you have for CDF, which is 6 part of LERF. So there is not necessarily one value, it may 7 be variable. The bigger those are, the less uncertainty you 8 are willing to tolerate. The lower they are, maybe it is 9 all right.
10 So this is an unknown function that somebody has 11 to develop as a policy statement. It is not easy to come 12 by. Policy things are never. EJt if you had those two 13 things, then my concept deals with using defense-in-depth to 14 both realize a risk objective, which is the LERF, and to
() 15 vievelop your level of confidence with the uncertainty, to 16 deal with both of those things, which is what 17 defense-in-depth is supposed to do, it compensates for --
18 DR. WALLIS: Well, I agree with what you say, but 19 your write-up mixes up these concepts. I mean acceptable 20 risk and acceptable level of uncertainty are two concepts.
21 DR. KRESS: Two separate --
22 DR. WALLIS: At some level, those are essential 23 design criteria.
24 DR. KRESS: They would have to be design criteria.
25 DR. WALLIS: Defense-in-depth with multiple layers
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444
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.1 'of protection is one way to meet those criteria. As is a 2 different thing, it is1a'different animal completely from-
{}
3 acceptable risk.
4 DR. KRESS: I agree. I may have --
5 DR. WALLIS: You put them all in the:same 6 sentence.
7 DR. KRESS: Well, I may have phrased it wrong'and 8 it may need some clarification. But you are right, they are 9 separate things. The defense-in-depth provisions you do are 10 a way to meet these things.
111' DR. WALLIS: And as far as we have an intuitive 12 sense that that is the best way to meet it. But I haven't 13 seen the existence proof that it is the best.
14 DR. KRESS: Well, I do make a point 1in there.
() 15 DR. WALLIS: There are otherLways perhaps.
16 DR. KRESS: I do make a point in there that the 17 concept is that if you have these two numbers that I talked 18 about, uncertainty acceptance and LERF acceptance, --
19 DR. SHACK: And you are confident that you model 20 uncertainty is small enough that'the uncertainty that you 21 can compute and measure is meaningful.
22 DR. KRESS: You have to do something.about that.
23 You know, there's lots of issues here. But my' point wasf 24 that the re > son.you put more and more defense-in-depth on ac 25 design is because you'are uncomfortable with the uncertainty D- ANN RILEY & ASSOCIATES, LTD.
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445 1- .in your risk' numbers. .Therefore, your. comfort level goes up
. when you;put'more and more defense-in-depth.
~
2
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3 I reinterpreted that to'say there.must then be an 4 inverse correlation between the amount'of uncertainty and 5 the amount of defense-in-depth. The'more defense-in-depth 6 you have, the less uncertainty in your risk metric. That'is 7 an assumption in here.
8 DR. WALLIS: We are also comfortable --
~
9 DR. KRESS: And that hasn't been proven. That is
.10 an inference on my part. And if that is not true, why you 11- might as well just throw thic in the garbage, too. So there 12 is a huge leap of faith there that I mean to meant to point-13 out to you.
14 DR. SEALE: Tom, I take issue one.of-your comments
-( 15 here. You know, earlier, George made the_ point as he went
'16 through his cornerstones argument that the further you went 17 from the initiating event in a sequence, through to the 18 consequences, the greater the uncertainty was because you 19 were propagating more and more uncertainties through each 20 other.
21 DR. KRESS: That's true. That's true.
22 DR. SEALE: Okay. You said just now that,the more 23 defense-in-depth you had, the less uncertainty you had.
24 DR. WALLIS: No.
25 DR. KRESS: Yes, I did. I said that.
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446 1 DR. SEALE: And I would say the more comfort you
()
G' 2 have, you don't have less uncertainty, it is just that you 3 are more and more comfortable that the real world is 4 somewhere on the good side of where you are because of your 5 defense-in-depth.
6 DR. WALLIS: I think I know why are more 7 comfortable, that was what I was going to say, is that you 8 don't think in terms of probabilistic things and 9 uncertainties. Even if a mathematician could prove to you 10 that with three layers, there is less uncertainty than with 11 six, with proper design.
12 DR. SEALE: Yes.
13 DR. WALLIS: You tend to think in terms of either 14 it fails or it doesn't. You tend to think in terms of
() 15 yes-no. And so you much more comfortable to have six 16 yes-noes than three, because that is the way you are sort of 17 constrained to think. You don't think in terms of 18 probabilistic stuff.
19 DR. KRESS: I will put forth a hypothesis. That 20 hypothesis is the more defense-in-depth you apply, in the 21 traditional sense of defense-in-depth, the less uncertain is 22 the final number you have on the risk metric. That is a 23 hypothesis that I thoroughly believe and am putting forward.
24 I know how to. prove it, you can prove it with a PRA.
25 DR. WALLIS: That is not true at all.
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447 1 DR. KRESS: Well, that is a hypothesis.
2 DR. WALLIS: Not true at all. If you had two
(}
3 layers and you tested them ad infinitum, till you were 4 absolutely how they would behave, that would give you less 5 uncertainty than if you had six layers that you were 6 uncertain about every stage, obviously.
7 DR. KRESS: No, no. No, no. No.
8 DR. WALLIS: Having more layers doesn't make you 9 any more certain.
10 DR. KRESS: Yes, it does.
11 DR. WALLIS: No.
12 DR. KRESS: If each layer has its own level of 13 uncertainty associated.
14 DR. WALLIS: But they may have greater levels of
() 15 uncertainty than two layers which you really design 16 properly.
17 DR. SHACK: I think he is adding a third layer to 18 the two perfect layers.
19 DR. KRESS: That's exactly right 20 DR. WALLIS: No, no, no, no, no.
21 DR. KRESS: That is what I am doing. That is 22 exactly what I am doing.
23 DR. SHACK: He is not substituting three imperfect 24 layers for --
25 DR. KRESS: No , I am not.
Q
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448 1 DR. WALLIS: You aren't. allowed to that. You.
() 2-3 aren't allowed:to do that.
DR.:KRESS: Oh, yes, I am.
4 DR. SHACK: That's what defense-in-depth means.
5 DR. WALLIS: Oh, well, then --
6 DR. SHACK: That is alternate. design procedure, to 7 use-three imperfect layers in place of two perfect.
8 DR. KRESS: Yeah, but:that is something else 9 altogether.
10 DR. SHACK: That is a different beast.
11 Defense-in-depth says I do the best I can with two.
12 DR. KRESS: That's right. And then --
.13 DR. SHACK: If that is not enough, I had a third.
14 DR. WALLIS: No, but I don't think that ir the
() 15 case. I don't think that is the case.
16 DR. SEALE: You may have more comfort, but you 17 don't have more -- less uncertainty.
18 DR. SHACK: I sure as hell'do.
19 DR. KRESS: I do. Yes, I am glad I have got.one 20 guy agreeing with me on that one.
21 DR. SEALE: No , no.
22 DR. KRESS: I can prove it. This is a hypothesis 23 that can be proved.
24 DR. WALLIS: No.
25 DR. KRESS: And I can prove it using a PRA.
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449 1 DR. WALLIS: Gee whiz.
2 DR. MILLER: I think Bob is right, but the
(
3 probability of having a problem is less. You have more 4 uncertainty on a less -- lower probability.
5 DR. WALLIS: Yes, that's right. I agree with 6 that.
7 DR. MILLER: Uncertainties are added in quadriture 8 and the probability is a product.
9 DR. KRESS: You are right, but you are talking 10 about a different --
11 DR. WALLIS: You are mixing up uncertainty and 12 probability.
13 DR. KRESS: You are mixing -- well, you guys are 14 mixing it up.
0)
( 15 DR. WALLIS: No, no, no, no. You get a C for 16 that.
17 DR. KRESS: You wait till I write my --
18 DR. WALLIS: You can say anything you like about 19 me.
20 DR. KRESS: I am going to. You are guys are wrong 21 about this.
22 Now, the mathematician here is going write 23 something down for us. Now, the quadriture doesn't work 24 here, John. You have got to apply the same quadriture to 25 two separate situations, though. You can't use it for a ANN RILEY & ASSOCIATES, LTD.
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450 1 static thing. I am adding more defense-in-depth.
() 2 3
DR. WALLIS:
DR. MILLER:
You have reduced the risk.
You have reduced the risk by adding 4 three layers.
5 DR. WALLIS: You haven't. reduced the uncertainty.
6 DR. MILLER: And you reduced -- the uncertainty, 7 in your mind, is reduced, I agree with that. But the 8 uncertainty in the end result is actually --
9 DR. WALLIS: Well, I guess you have to define 10 uncertainty --
11 DR. KRESS: How can you guys say that?
12 DR. SHACK: If you are saying sigma isn't smaller, l i
13 that is one thing. If you are saying the uncertainty on 14 whether it fails or it doesn't fail, is a different beast.
() 15 We are not talking about whether si is larger or smaller.
16 We are asking whether the uncertainty in the failure 17 probability is larger or smaller, and that is different.
18 DR. SEALE: Well, that is what I was calling the 19 comfort.
20 DR. SHACK: Well, that is the uncertainty we are 1 I
21 talking about here. You have got to define what your j l
22 objective is, and that is to prevent failure. j I
23 DR. SEALE: That's right.
24 DR. SHACK: And then the question, what is the 25 uncertainty on the failure?
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451 1 DR. WALLIS: Well,.I guess we have to define what 2 we mean by uncertainty more carefully.
(}
3 DR. SEALE: That's right, 1 DR. WALLIS: Maybe that's why --
5 DR. SEALE: That is the case.
6 DR. WALLIS: If Tom defines it his way, he may be 7 able to justify it.
8 DR. KRESS: Well, I have to admit it, I defined it 9 as sigma. Sigma on the LERF, in terms of sigma, you know, 10 it may be not be actually --
-11 DR. WALLIS: Sigma.
12 DR. KRESS: Sigma. But it is the same thing as 13 some multiple of sigma on the LERF.
14 DR. WALLIS: Now, you could put in another layer l
) 15 which is extraordinarily uncertain and not very effective, I 16 don't think that helps you very much in terms of certainty.
17 DR. MILLER: No.
18 DR. SHACK: No , but it doesn't hurt you.
19 DR. WALLIS: Well, in terms of risk, it doesn't.
20 But in terms of uncertainty, --
21 DR. MILLER: It doesn't hurt you either. Relative 22 uncertainty still goes down.
23 DR. WALLIS: It is relative.
24 DR. KRESS: Yeah. That is what I was trying to 25 say.
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452 l
l 1 DR. MILLER: Well, then we agree. j
() 2 3
DR. KRESS: The relative uncertainty gets smaller, you have more defense-in-depth.
4 DR. WALLIS: Okay. So your revised paper is 5 probably more acceptable than the first one. Maybe I --
6 maybe the C was in my mind.
7 DR. KRESS: I don't know. But, anyway, this is --
8 DR. MILLER: Now, let's read tne paper, 9 DR. SHACK: I think, you know, your approach is a 10 useful way to think about defense-in-depth.
11 DR. KRESS: Yes, I know, whether you can actually 12 quantify it and put it --
13 Dk. SHACK: Right. Now, whether it provides a 14 useful way to make policy and decisions, I think is a 15 very --
16 DR. KRESS: And that is one of the points of 17 contention between one, too.
18 DR. SEALE: That is a real different question.
19 DR. KRESS: Dana particularly thinks it is not 20 useful in that sense. And that may -- that is a legitimate.
21 DR. SHACK: You would have -- you know, I mean 22 unless you like create debating societies.
23 DR. KRESS: That is one of the points, yeah.
24 DR. SHACK: Sooner or later, somebody is going to 25 have to make a judgment.
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453 1 DR. KRESS: But, you know, the way to start with 2 something that is as difficult as this is to get an
-v 3 overriding principle that you believe in.
4 DR. SHACK: Uh-oh.
5 DR. KRESS: And then work backwards and say, what 6 can we do with this to make it practically implementable?
7 That, I am asking -- I am not going to do that. All I want 8 to do is say this is the principle, let the staff work on 9 how to implement it.
10 DR. WALLIS: Well, I don't think you can ever 11 justif. it as a principle. You can say that acceptable risk 12 and acceptable levels of uncertainty are tha basic design 13 goals and they, you could say, were principles.
14 Defense-in-depth is one approach to meeting them. The other
,m
) 15 one is to do a much more thorough design and testing of 16 fewer elements and have less defense-in-depth but more 17 substance to the actual defense at each layer. That meets 18 the design objectives.
19 DR. KRESS: Yes, well, that is almost -- that is 20 actually covered in there, because what you are doing is you 21 are either reducing the uncertainty or reducing the risk 22 metric, one or the other.
23 DR. WALLIS: So that is what you have to show when 24 you involve defense-in-depth. You have to say that with 25 this defense-in-depth I get a better -- a better something.
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454 1 DR. KRESS: Well, that is the hypothesis.
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2 DR. WALLIS: But you can't appeal to 3 defense-in-depth by itself without'a measure of reduced 4 uncertainty.
5 DR. SHACK: No , but this definition covers that.
6 DR. KRESS: I thought I covered that.
7 DR. SHACK: If you make the first two barriers so 8 reliable and so uncertain, then when you compute to see if 9 you have got livable uncertainty, you say --
10 DR. WALLIS: That is not the way it is invoked, 11 though. It is invoked saying we have got these various 12 things, we have got to maintain defense-in-depth. It 13 doesn't say anything the certainty of the barriers. It says 14 that more barriers is good. That 4.s the --
() 15 16 there.
DR. KRESS: I did amend -- I hope that wasn't in 17 DR. SEALE: Part of the reason you do 18 defense-in-depth,
- hough, is that you don't know for sure 19 whether you are playing with a full deck. I mean --
20 DR. KRESS: That is one of Dana's.
21 DR. SEALE: You have got a completeness problem.
22 DR. APOSTOLAKIS: You are chairing. You dc.1't 23 like it?
24 DR. KRESS: Not in this. This is -- I have got 25 too much of a conflict of interest to chair this. But I
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455 1 guess Dana has, too.
()
\,s 2 DR. SHACK: Yeah, like I say, who.are we going to 3 hand it to that doesn't?
4 DR. APOSTOLAKIS: Well, you want to give it to a 5 person who is. neutral.
6 DR. KRESS: George would be more neutral.
7 DR. APOSTOLAKIS: I am neutral.
8 DR. KRESS: Yeah, let's turn this over to George.
9 You haven't been in on this debate we have already had, so 10 we would be interested in your comments that are coming in 11 cold.
12 DR. SEALE: Why don't you stay there for five 27 minutes so George knows where we are, rather than having to 14 start over.
r-(3) 15 DR. KRESS: I would rather have him start over.
16 MR. BARTON: That is probably why we never got 17 this thing out.
18 DR. APOSTOLAKIS: Did you actually read the 19 letter?
20 DR. SHACK: Don't worry, if we want to start over 21 again, we will.
22 DR. APOSTOLAKIS: What is the major objection?
23 DR. KRESS: To the letter? It appears to be --
24 DR. APOSTOLAKIS: Let me start from here. Have 25 you guys read the paper? Are there any objections to the
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456 1 paper?
() 2 3
MR. BARTON: Jack's paper?
DR. APOSTOLAKIS: Yes. There are.
4 , DR. SHACK: Well, I don't have any objections, I 5 have read the paper.
6 MR. BARTON: It is a good paper.
7 DR. APOSTOLAKIS: Anybody has a problem with.the 8 paper?
9 DR. WALLIS: We are talking about detail.
10 DR. APOSTOLAKIS: Of the paper or the letter?
11 DR. KRESS: They are confused. They don't realize 12 that there is a paper and a letter.
13 DR. APOSTOLAKIS: No , everything knows there is a 14 paper.
O 15 DR. KRESS: Well, some of them --
()
16 DR. SHACK: Yes, well, --
17 DR. APOSTOLAKIS: Okay. Now, I will take -- I 18 will put on the table an extreme view, okay, which Tom may 19 not like. I would be very happy, I think it would be a 20 useful contribution to the ongoing debate among the 21 Commissioners and senior staff to send a paper with a very 22 short letter that says here is our -- I like the opening 23 thing. Oh, you took that away?
24 AP part of our continuing --
25 DR. KRESS: Dana hated that, a
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457 l' -DR. APOSTOLAKIS: What1did you like, what did'you-() 2 3
~put there?.
DR. KRESS: I don't remember. Dana hated it.
-4 DR. APOSTOLAKIS: As part of our continuing.
5 interest in contributing to issues, --
i 6 DR .. KRESS: Yes, s,-^ thing like that.
7 DR. APOSTOLAKIS: -- please see the attached!
8 . paper. Thank you very much. Dana Powers, Chairman of the 9 Committee. One sentence.
10 DR. FONTANA: I move we change chairman again.
11 DR. APOSTOLAKIS: Or maybe two.
12 DR. WALLIS: The attached paper --
13 DR. APOSTOLAKIS: In other words, this is 14 something that will go upstairs very quickly, will make a
() 15 contribution, and we can address Tom's other ideas, w', 'h I 16 think, you know, are a significant advance, but they reca.e 17 debate.
18 DR. ;1UiCK: The paper leaves you with the 19 structionalist approach and the rationalist approach. Tom 20 has come down with both feet.
21 DR. KRESS: Yeah. Gotten down out of the clouds 22 to a lower level of clouds.
23 DR. SEALE: Yes.
24 DR. APOSTOLAKIS: So I would like to see whether 25 people are willing to go along with that. And we are not
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4
458 1 rejecting this letter.
\ 2 DR. KRESS: It is a rationalist case.
3 DR. APOSTOLAKIS': . We are not' rejecting it.
4 DR. SHACK: We know which camp you are in.
5 DR. KRESS: Okay.
6 MR. BARTON: What are we doing with it, if we are 7 not rejecting? What is your plan for this letter?
8 DR. APOSTOLAKIS : Well, thib letter can go up in 9 June or July after the members express concerne via e-mail, 10 you know, so that Tom will have input to think about.
11 DR. SEALE: Not only that, but you are also doing 12 a little bit of evangelistic work on the Commissioners from 13 a less partisan perspective if they see the paper, have a 14 chance to digest it and so forth, and then get exposed to
() 15 coming down with both feet in one camp'or the other.
16 DR. APOSTOLAKIS: Right.
17 MR. SAVIO: We have an SRM to respond to May 17th.
18 DR. APOSTOLAKIS: Well, the paper is probably good 19 enough for that.
20 DR. KRESS: April could be the response.
21 MR. BARTON: A response for the SRM.
22 DR. APOSTOLAKIS: Yes, the SRM doesn't say much.
23 MR. MARKLEY: Does it answer the question in the 24 SRM, though.
25 DR. APOSTOLAKIS: Which is?
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459~
1 MR. MARKLEY: 'The/ balance between-PRA andL 2 . defense-in-depth.
L3- 'DR. APOSTOLAKIS: Well, option t.J,.:that is J what
~
4 it is, It says.use structuralist at the,very top and 5 rationalist below. I am willing to defend it.
6 DR. SEALE: It is not a simple questi'on.
7 DR. APOSTOLAKIS: It is not a-simple question.
8 DR. SEALE: If.you send something like the paper 9- to the Commissioners and say we are thinking.about it, this 10' is the way the problem is being structured. We are going'to 11 examine the issues in greater detail, and we_will get back 12 to you when we have more to say.
13 DR. APOSTOLAKIS: That's right. I think option 14 two addresses the concern of the Commission. And we will
() 15 come back with a letter later.
16 DR. SEALE: Yeah.
17 DR. WALLIS: I just want to make sure that we do 18 come back, that it not one of these things we complain-19 about.
20 DR. APOSTOLAKIS: This letter will not die, 21 Graham. This letter has a lot of sweat -- what-is it, 22 blood, sweat and tears. And I don't think it should die 23 either. I mean --
24 DR. SEALE: There is'even some consternation in 25 it.
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460 1 DR. APOSTOLAKIS: Maybe we can impose a deadline
(}
V 2 on ourselves, that maybe by the July meeting at the latest, 3 this goes up.
4 Dana, I just proposed, but evidently there is a 5 lot of objection to this letter. That we send the paper 6 which our colleagues do not object to, with a very short 7 cover letter to respond to the SRM we ha're, and then inform 8 them that we are continuing to work on the subject and, you 9 know, something is coming up soon.
10 DR. POWERS. Can you give me a thumbnail 11 description of the nature of the objections to the letter?
12 DR. KRESS: They are a lot like some of yours.
13 DR. POWERS: Obviously sage objections then.
14 DR. KRESS: So if you remember what yours are,
( 15 there is a flavor of a lot of that in the objections I am 16 getting here, plus some others like I may put forth a 17 hypothesis that some of the people don't even think is true, 18 the basis for the concept, in other words. That hypothesis 19 is that the more defense-in-depth you have, the less 20 uncertainty you have in the final risk -- with the 21 hypothesis. There are a number of people here that are 22 telling me that that is not true, it just the opposite.
23 DR. POWERS: I thought that the nature of your 24 hypothesis was in reality that is what should be true.
25 DR. KRESS: It should be true.
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461 1 1 DR. POWERS: And that -- the situation is, in 2 fact, that defense-in-depth as originally formulated is an
(~'))
3 archaic concept.
4 DR. SEALE: That's right.
5 DR. POWERS: And that it has to evolve in the face 6 of this new technology. And I thought that the --
7 DR. KRESS: That a better representation of what I 8 was trying to say.
9 DR. POWERS: That the hypothesis that is 10 articulated in here is that in a perfect world, there would 11 be an inverse correlation.
12 DR. KRESS: Absolutely.
13 DR. POWERS: But that when we found that there was 14 not such an inverse correlation, that was a point at which
() 15 we were misapplying defense-in-depth.
16 DR. KRESS: That is a wonderful -- a better 17 explanat on of my concept.
18 DR. MILLER: Could you explain what the imperfect 19 parts of the world are that --
20 DR. POWERS: Well, I think we see them when we 21 speak to applying defense-in-depth to the safety analysis of 22 digital electronic systems.
23 DR. MILLER: That is because you have potential 24 for common mode failures, is that it?
25 DR. POWERS: I think we see arbitrary appeals to
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462 1 defense-in-depth there. I think your potential for common
()
G 2 mode failures and their impact is something that lies 3 legitimately in the domain of PRA to assess and evaluate for 4 you and make decisions on whether you need redundant and 5 diverse systems. To come back and say I need redundancy, 6 diversity or something else in the name of defense-in-depth 7 is a misapplication of the modern concepts of 8 defense-in-depth.
9 DR. SEALE: It was appropriate in the --
10 DR. POWERS: It was appropriate when we lacked the 11 tools.
12 DR. SEALE: That's right.
13 DR. POWERS: It is inappropriate now. That is the 14 concept. Now, Tom and I have had a running dialogue on this
() 15 16 now for three months with some of the more vitriolic e-mails passing back and forth.
17 DR. KRESS: Those are -- some of them are in here.
18 DR. POWERS: He does go on and offer -- Tom is 19 interested in seeking resolution, a definitive path through 20 the morass in this letter, and I have been a reluctant 21 dragon on this. I see the virtue of trying to cut through 22 this. But I think it requires more thought, and where we 23 last left it was that the letter necessarily has to be 24 longer. But I think I would not be in favor of producing a 25 letter that simply said here is our paper and we are working r~ ANN RILEY & ASSOCIATES, LTD.
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463 1 on this more. I think this concept of inverse correlation
/N 2 has to appear.
b 3 DR. KRESS: I agree.
4 DR. POWERS: Now, much of the rest of it on the 5 . definition of acceptance criteria ar the need for those, I 6 think that has to appear.
7 DR. KRESS: I agree.
8 DR. POWERS: Now, the definition of new risk 9 measures and the call for a definition of acceptance 10 criteria there --
11 DR. KRESS: It doesn't have to --
12 DR. POWERS: I think is something that deserves 13 longer-term thought.
14 DR. KRESS: Yes, I agree with that, too, Dana.
() 15 16 DR. POWERS: So my feeling is much more optimistic maybe, that we can produce a letter that I think expresses 17 truisms and facts, and alert the Commission to this pitfall 18 that they have on the road to risk-informed regulation.
19 And, in fact, I have suggested there are three others that 20 need to be attended to. One of those three others is in 21 fact subsumed in the existing letter, and that it is these 22 ak measures and their acceptance criteria, I think is one 23 that needs to.
24 We have just gotten out discussions where this 25 precise topic came up,'and it is evident that we need not
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464 1 only to square our risk measure , but to square our thinking
(~') 2 about acceptance criteria with international acceptance
.\J 3 criteria.
4 DR. KRESS: Yes, that's true.
5 DR. POWERS: And at least say why --
6 DR. KRESS: Why they are different.
7 DR. POWERS: Why we differ and why we should not 8 go to the most restrictive of acceptance criteria that you 9 find in the field.
10 So I guess my advice to you, Tom, is no, let's not 11 gc for a minimalist letter. Let's take the salient points 12 and those that are more speculative about cutting thr 'ngh 13 the morass. Sure, we can work on those and set a deadline.
14 I suspect it is the fall, but I don't think it should be
() 15 16 beyond the fall.
DR. APOSTOLAKIS: Is the letter going out at this 17 time, though?
18 DR. POWERS: The letter is scheduled to go up next 19 month.
20 DR. FONTANA: Next month?
21 DR. POWERS: In May.
22 DR. APOSTOLAKIS: If we already know that we agree 23 on certain things and we want to put them in the letter, why 24 don't we send it at this time. They asked us, we know that 25 they are concerned about it. We don't have to go by the
()
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465.
1 deadline of the SRM.
% 2 DR. SHACK:
(d 3 maximum late date.
No, I think he was just giving you the 4 DR. APOSTOLAKIS: No, but I think we should -- I 5 mean, Tom, do you feel.you can draft a letter based on this 6 discussion?
7 DR. KRESS: I think with Dana's help.
8 DR. APOSTOLAKIS: Cutting and pasting 9 DR. KRESS: I think with Dana's help. Not cutting 10 and pasting. With Dana's help.
11 DR. APOSTOLAKIS: A new letter.
12 DR. KRESS: Well, it would be -- I think it would 13 have just a bit of a different flavor and it would be -- it 14 wouldn't be so dogmatic in the sense as this is what
() 15 16 defense-in-depth is, this is the way you have to handle it.
It would be more like these are the attributes you should 17 have in a proper defense-in-depth concept. And cast it in 18 that way. And then you would say, it would be no longer a 19 hypothesis that says if you apply defense-in-depth properly, 20 the more you put on, the more the uncertainty should be 21 reduced. It would have just a little different flavor _o 22 it. It would preserve much of the stuff that is in there.
23 It wouldn't come down as this is the way it is, the way it 24 has to be done type thing. It would be when you develop a 25 defense-in-depth policy, it needs to have these kinds of
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466 1 characteristics to it.
) 2 DR. WALLIS: I am concerned though because I think
{"'J
~-
3 this has potential to be a very significant letter, and I am 4 concerned about rushing into a letter the way we do, rushing 5 into a consensus, which we do wonderfully on some things, so 6 important, without time to mull it over and say, well, do we 7 really do mean that, or is there a better way to do it? And 8 I am sorry it has taken some time already. But I would feel 9 uncomfortably, personally, about endorsing ~something which 10 is sort of a compromise of things, various things people 11 think at the moment, rather than something which is really 12 solid.
13 DR. KRESS: You would like to spend more time.
14 DR. WALLIS: Yes, I think so.
() 15 DR. APOSTOLAKIS: So you are arguing for May.
16 DR. WALLIS: I don't want to put it off much at 17 all, but I do think --
18 DR. APOSTOLAKIS: No, May, we have to send 19 something.
20 DR. WALLIS: I am nervous about rushing into a 21 decision in a day.
22 DR. APOSTOLAKIS: In May we have to do it.
23 DR. WALLIS: That's fine, we will definitely do 24 that.
25 DR. APOSTOLAKIS: Well, all right.
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467 1 DR. POWERS: Well, let's turn charter Tom and Dana
'T to struggle with some words and see what we can come up with
[d 2 3 Saturday afternoon.
4 DR. SEALE: I sure would like to hear Dana's 5 evolution of the relationship between layers and 6 uncertainty, because Tom's was a little jarring.
7 DR. POWERS: Well,'I think you look at the paper, 8 you see that this distills fairly logically in the paper 9 that there has to be a change and that there are two 10 categorizations of change, two categorizations of 11 defense-in-depth that appear. One is the dogmatic overall 12 and the other is a consciously applied based on quantitative 13 analysis, which tends to be on the microscopic side.
14 I think we can -- I mean I think that is the
() 15 inverse correlation area. Tom has a more mathematical 16 definition than appears in the paper, and it his attempt to 17 define necessity and sufficiency. And necessity and 18 sufficiency I think is the thing that we don't see in the 19 arbitrary appeals to defense-in-depth.
20 DR. APOSTOLAKIS: I think in my mind it is very 21 simple. The only reason defense-in-depth deserves a name in 22 the new era is that many times in the past we have been 23 wrong, and I would like to have that feeling that I have 24 something to protect me.
25 DR. WALLIS: That is the yes-no thing.
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468 1 DR. APOSTOLAKIS: Now, the rest --
() 2 3
DR. SEALE: That is a completeness question.
DR. APOSTOLAKIS: Yes, exactly, it is the 4 completeness question. Now, we can debate it for a long 5 time, you know, where we can come up with an exact 6 definition and so or But if that was not a factor, I would 7 argue for the rationalist approach all the way.
8 DR. WALLIS: Even then the question of how many 9 layers you need depends on how comfortable you feel about 10 your likelihood of being wrong.
11 DR. APOSTOLAKIS: Sure.
12 DR. WALLIS: So you face the same problem.
13 DR. KRESS: It's the same.
14 DR. WALLIS: And you can always say adding another
() 15 layer is better, but that is never any good, because you can 16 add infinite number --
17 DR. APOSTOLAKIS: That's why we have a specific 18 proposal in the paper.
19 DR. SHACK: No, but if you have no model 20 uncertainty, this does become a mathematical problem. I can 21 do the analysis if I don't have to worry about model 22 uncertainty.
23 DR. KRESS: You can do it precisely.
24 DR. SHACK: I can, yes.
25 DR. POWERS: And what we are saying is that we
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469 1 have become sophisticated in small areas. We.are sufficient 2 that the residual uncertainty of completeness is small and
[}
3 we are not even going to allow it arise.
4 DR. FONTANA: Let me come up with an old example 5 here and see how this fits. Design the reactor, put a 6 containment around it. We have got defense-in-depth. Then 7 somebody does a WASH 740 and finds a V sequence.
8 MR. BARTON: Find what?
9 DR. FONTANA: A V sequence, a bypass sequence.
10 The defense-in-depth that you thought you had wasn't there, 11 to the extent that you --
12 DR. APOSTOLAKIS: Well, that's right.
13 DR. POWERS: That's true. Keep going.
14 DR. FONTANA: All right. So if you were trying to
() 15 do a mathematic thing, you would be wrong because you forgot 16 something, or you didn't know.
17 DR. SHACK: But if I had no incompleteness --
18 DR. WALLIS: it is like steam generator tube 19 ruptures. You can put another box around the whole thing to 20 catch whatever comes out the steam generator tube, or you 21 can make better steam generators.
22 DR. KRESS: Absolutely.
23 DR. WALLIS: Now, you have got a choice. It is 24 not clear defense-in-depth is the obvious way to go.
25 DR. FONTANA: According to Tom's definition, I ANN RILEY & ASSOCIATES, LTD,
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470 1 think you are not allowed to make a crappy steam generator
(~3 2 because you put a box around it.
s_/
3 DR. SEALE: We made the comment before you guys 4 got back that the present, in-place, defense-in-depth 5 process is appropriate to the general design criteria'as 6 being the way we do the business. And that is not a bad 7 comment to make actually. And that as we have developed now-8 the models, the understanding _of the pieces and so on of the 9 process, we can attack this problem more mathematically and 10 begin to get to the kind of trade that you are talking 11 about.
12 To the extent that we have residual concerns about 13 completeness, you know, maybe there is a version of the Y2K 14 bug that bites fuel elements or something like that --
() 15 DR. APOSTOLAKIS: But completeness --
16 DR. SEALE: Completeness in the sense that there 17 is an accident out there that we haven't experienced yet.
18 To that extent we still feel that we need defense-in-depth.
19 DR. POWERS: No , I think he has got -- I think he 20 has upon an important point that we want to make, Tom,.is 21 that at no time were we speaking to the GDCs. That as a 22 generalized good engineering practice -- in fact, we touch 23- upon in the paper, defense-in-depth is a good idea. It 24 builds into your. plant the kind of redundancy that leads to 25 low core melt frequencies.
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471 1 DR. SEALE: Right.
2 DR. POWERS: We are speaking primarily to the
(}
3 issues of the safety regulation of plants where you have to 4 make decisions. And you want decisions to be necessary.
5 And having made those decisions, you want to make them 6 sufficient. And that is where we are having troubles 7 because we think we have a tool that for many, many kinds of 8 decisions is sufficient. And we find these arbitrary 9 appeals undermining its effectiveness.
10 MR. BONACA: Can we talk about the issue of -- for 11 example, I mean I was mentioning how I liked this paper, 12 personally, because it leaves defense-in-depth as a concept, 13 but at such a high level, and then it allows for new 14 technology to work itself cut. An example that should be
() 15 put in, for example, is protection for common mode. We 16 didn't know any better than looking at diversity, 17 redundancies, but today, if you look at the PRAs, they are a 38 much more powerful tool to use.
19 I think an example like that could help point out 20 exactly what we talk about in the structure of this.
21 Anyway.
22 DR. POWERS: Good.
23 DR. FONTANA: Can we get another copy of that 24 paper? I didn't bring mine with me. Thank you.
25 DR. APOSTOLAKIS: So what do we do now? I am 4N ANN RILEY & ASSOCIATES, LTD.
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472 q 1 really anxious to go through the first reading of the letter
() 2 3
on the Safety Goal Policy Statement because I want to get your input.
4 DR. POWERS: I guess we can accommodate that. I 5 can take five minutes to just ask Tom the outcome of the 6 discussion on MOX.
7 DR. KRESS: Yes, let me see if I can recall that.
8 When we called upon staff, or the invited experts to give us 9 whatever advice they would like on the ultimate question, we 10 got two kinds of advice. One of them had to do with going 11 into the MOX letter and carefully looking at the paragraph 12 starting on line 45 and be sure that that clearly says what 13 we want it to.
14 And the other type of advice we had was when we
() 15 think about this issue, that we need to keep in mind that 16 the actual implementation of higher burnup fuel and MOX, two 17 separate things, they are two separate things, is going to 18 be in such a way that there will be only a limited fraction 19 of the core in those states. And that if one is asking the 20 question of whether it is permissible or not, you need to 21 keep in mind that concept that there is actually a limited 22 amount.. And it turns out that that limited amount is pretty 23 big, it is like one-third of the core will be MOX fuel.
24 DR. POWERS: Well, if DOE has its way, it is even 25 bigger than that.
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473 1 DR. KRESS: Okay. And so,_you know, that may not '
2 be as much of a consideration as one might think. But it
[d'T 3 reduces things -- it reduces the-increase by two-thirds if 4 you are thinking about the whole core. I don't know how 5 much it reduces it, but it reduces it.
6 DR. POWERS: What it means is that the increase is 7 only a third a big as what you thought.
8 DR. KRESS: Might have thought.
9 DR. POWERS: If it is uniform. Now, 10 unfortunately, it is also the hot portions of the core.
11 DR. KRESS: And the same way with burnup, all the 12 fuel won't go to 60.
13 DR. POWERS: There it is really a third.
14 DR. KRESS: Yes. So that was the other thing that (O j 15 they -- and they still believe 1465 source term has 16 sufficient boundingness in it that they feel comfortable 17 with using MOX and stuff, but that left-me a little cold I 18 because I don't know where they derive their comfort from.
19 DR. POWERS: Yeah, I guess the question is, how 20 insistent are that they be in a position to demonstrate 21 that? Because what we have seen out of that particular 22 group of experts is a lot of pencil whipping and not much 23 data.
24 DR. KRESS: And my problem, we got into this long 25 discussion between the dichotomy between design basis and
[D N.,
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474 1 reality, real risk, and my concept on how to show this is,
()
LJ 2 in a perfect world, you would just run your risk analysis 3 with these phenomena in it is and see how much delta risk 4 you are going to get.
5 We don't have : hat because you don't have the 6 models to account for e effects and there is no data, 7 not enough database to elop empirical models. The only 8 recourse one has in the absence of such a database and 9 models is to try to see if you can bound the problem. And 10 one way to bound it would be to go into a risk assessment 11 using a melt core or something and take the third of the 12 core that is MOX and the third of the core that is high 13 burnup and bound the releases from those by some high 14 number, just from that part of it. Run your risk sequences
( ) 15 and see what that did to you in delta risk.
16 That would be one thing that they haven't done. I 17 don't know what that would do to you in delta risk, but they !
18 imply, the two guys imply that that probably would be an 4
19 acceptable delta risk. But to me that would be l 20 plant-specific and it depends on what type you are going to 21 put it in.
22 DR. POWERS: I guess, I mean what it does is it 23 presumes that phenomenology remains the same.
24 DR. KRESS: Yes.
25 DR. POWERS: And the experience with the high !
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l 475 k 1 burn, of' course, isEthat at least in one area itlisn't the
() 2 3 .
same.
IM1. KRESS: 'It presumes that you could do-4' something to the releases from the high burnup or the MOX-5 fuels, like arbitrarily increase them to some high level,.
6 and that the rest of the fuel would not be affected, the 7 rest.of the core would not be affected, you treat it like it 8 normally is. That is an assumption that, you know, is not 9 necessarily true.
10 But, anyway, short of that demonstration,-I can't
.11 buy off on it unless we have got data or models to show me 12 that the risk increase is acceptable.
13 DR. POWERS: Yes, I guess the question, I think-14 the question is, if we need to include a recommendation in
() 15 the letter that says we think that this is an issue that is 16 going to demand experimental validated conclusions and not 17 just an analyses.
18 DR. KREE:S : I think that is true, i
19 DR. POWERS: Okay. George, let's turn to your 20 pink pussycat here.
21 DR. APOSTOLAKIS: Okay. What I tried to do with 22 -- well, Mike and I worked on this, so what we tried to do 23 was to put the letter together in cuch a way'that all the 24 views that were expressed here yesterday are here. So you 25 see'I.am saying at some point that we are unable to reach a y
e ANN RILEY.& ASSOCIATES, LTD.
L ('S
. Court Reporters-1025 Connecticut Avenue, NW, Suite 1014 Washington,_ D.C. 20036 (202) = 842-0034
476 1 conclusion.
2 Now, yesterday, though, it is true that all the
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3 members except three thought that the idea of an l 4 over-arching policy was very good, was excellent. So if 5 today the Committee feels that this should be reflected in 6 the letter, 1 can easily take the same letter and give it a 7 different flavor and make it a conclusion, in which case, of 8 course, there should be a minority view. Now, I shouldn't 9 really read it, should I? Or just go over it paragraph by 10 paragraph? Or n:'/he just go ahead and read it?
11 DR. POWERS: Why don't you go ahead read it.
12 [Whereupon, at 2:22 p.m., the recorded portion of 13 the meeting was concluded.]
14 15 16 17 18 19 20 21 22 23 24 25
(( n ANN RILEY & ASSOCIATES, LTD.
\s / Court Reporters 1025 Connecticut Avenue, NW, Suite 1014 Washington, D.C. 20036 (202) 842-0034 I
REPORTER'S CERTIFICATE This is to certify that the attached proceedings before the United States Nuclear Regulatory Commission in the matter of:
NAME OF PROCEEDING: 461ST ADVISORY COMMITTEE ON REACTOR SAFEGUARDS (ACRS)
CASE NUMBER:
PLACE OF PROCEEDING: Rockville, MD were held as herein appears, and that this is the original transcript thereof for the file of the United States Nuclear Regulatory Commission taken by me and thereafter reduced to typewriting by me or under the direction of the court reporting company, and that the transcript is a true and accurate record of the foregoing proceedings.
Wu .
Mark Mahoney Official Reporter Ann Riley & Associat:s, Ltd.
- . . I O -
High Burnup Fuel and MOX SRM:
" Consider the impa tc on the revised source term if high burnup or mixed oxide fuel were used in place of standard uranium fuel."
O e
4 0
A e <eg g , . , ,,
i s
O Nuclear Control Institute o Assess risk in the use of MOX at commercial reactors
- ORIGEN for inventories
- Releases based on NUREG-1150 for Sequoyah
- Consequences with MACCS O
o Questions DOE analyses especially with regard to Actinides (Am, Cm)
- Actinide releases uncertain
- Chernobyl releases of actinides by dispers$l
-Ice Condenser plants may be susceptible to dispersal events such as steam explosions and high pressure melt expulsion.
O t
[
Q., . - - . _ ...-.- . - . - . -. - . . . .
REVISED ACCIDENT SOURCE TERM O
- (NUREG-1465) t o Release from a severe accident to replace tid 14844 o Defines release fractions and release rates for
I
- gap release
-invessel release ,
- ex-vessel release
-late invessel release O .
o Based on results from NUREG-1150
- Source Term Code Package
- Results of expert opinion elicitation
- low enrichment urania fuel at moderate burnups (<40 GWd/t)
- O v . . . . - - . . ~ ~ . . . - . . . . . , . . . . . . - - - . . . . . , -
. . \
0 O NUREG 1465 o Used for DBA and Engineered Safety System evaluation
- Gap release
-Invessel release
- Worst Two Hours Are the extents of release and the release rates O from NUREG-1465 still appropriate for MOX and High Burnup Fuel?
h O .
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i i ,
PAPERS -
O Hj. Matzke, " Oxygen potential in the rim region of high bumup UO 2 fuel", J. Nuclear Mater.,208(1994)18.
R.T. Primm, Ill, J.C. Ryman, S.B. Ludwig, " Storage of Assemblies Containing Mixed Oxide Fuel", ORNLITM-13424, April,1997.
B.D. Murphy, " Characteristics of Spent Fuel from Plutonium Disposition Reactors Vol. 3: A Westinghouse
. Pressurized Water Reactor Design", ORNL/TM-13170N3, July 1997.
K. Lass mann,'C. O' carroll, J. van de Laar, C.T. Walker, "The radial distribution of plutonium in high burnup UO 2 fuels", J. Nuclear Mater., 208(1994)223.
O C.T. Walker, M. Coquerelle, W. Goll, R. Manzel,
" Irradiation behavior of MOX fuel: Results of an EPMA investigation", Nuclear Eng. Design, 131(1991)1.
T. Fujimo, N. Sato, T. Yamashita , K. Ouchi, J. Nuclear Mater.,201(1993)70.
M. Ishida, Y. Korei, "Modeling parametric studies of the effect of Pu-mixing heterogeneity on fission gas release from mixed oxide fuels of LWRs and FBRs", J. Nuclear Mater.,210(1994)203.
l O
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__________________J
} RELEASE FROM FUEL o j o DIFFUSION THROUGH THE GRAIN TO BOUNDARY
- affected by oxygen potential and grain size o DIFFUSION TO GRAIN EDGE
- affected by burnup and oxygen potential
~
o DIFFUSION THROUGH PORE STRUCTURE O - affected by burnup and speciation o FLOW TO THE CLADDING BREACH .
o ENT OF FISSION PRODUCT INTO BULK FLOW )
~o NUCLEATION AND DEPOSITION
- affected by speciatior and oxygen potential
.O 4
_7
.s - --* '-~
' ~
ANALYSIS TOOLS O
o NRC TOOLS ARE NOT CAPABLE OF DEALING WITH MIXED OXIDE
- heterogeneity
-localized burnup
- oxygen potential o LIMITED CAPABILITIES TO DEAL WITH HIGH BURNUP
- fuel porosity O - oxygen potential
- interstitial oxygen effect on diffusion coefficients
- species vapor pressures o BETT'ER RELEASE MODELS AVAILABLE FOR GASES & VOLATILES o NOTHING PROMISING NOW FOR OTHER RADIONUCLIDES O
l
4 0 .o HIGH BURNUP FUELIS DIFFERENT
- microstructure a rim region of high porosity; small grain ~
size .
- oxygen potential a higher diffusion coefficients j changed speciation i MIXE XI E FUEL O
- heterogeneity and local burnup
- Pu burns preferentially
-locally high porosity
- neutronic effects e
- O j .
. . - - . - . - .. . . - . . . - ~. . . A l . . : .T*: . ' '.T... . .
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O EXPERIMENTS o VERCOURS l
- release from irradiated pellets
- no work with ZrB2 coated pellets o PHEBUS FPT-4 1
- release of low volatility radionuclides O o JAPAN Tests (?)
o
, . . ." - - * ' * .. . * .- _ ; .; J *: * * ?; =_'
- 1 * *;*, 'f.1; * ';' - * * ' - * " * *
- e s.
1
.O FUEL BURNUP Low Temp.
Gwd/t Ca Release
- UO2 53 26%
UO2 47 18%
MOX 41 58%
1780 K Plateau for over 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> O .
4 4
- O. .
g
.y . .: . . . . =
5 O BURNABLE POISONS d
2 'Pu +]u* + 4 oO * - 2[Pu3 +]u* + 3 oo * + % O2 + V" ZrB2 + 5/2 O2 - [Zr 4+]u* + B2O3 2Csl + B 2O3 - 2 "CsBO "2 + 21 + H2 "CsBO 2" may be Cs2 2nB O 3n +1 O
i10
- ~'~ ~~ . , . . . . . .