ML20206B168

Summary of ACRS Advanced Reactor Designs Subcommittee 880622 Meeting in Washington,Dc to Review Draft SER for Modular High Temp Gas Cooled Reactor

ML20206B168
Person / Time
Issue date:	07/13/1988
From:	Advisory Committee on Reactor Safeguards
To:	Advisory Committee on Reactor Safeguards
References
ACRS-2588, NUDOCS 8811150357
Download: ML20206B168 (15)

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BERTIFE DATE ISSUED: 7/13/88 ACRS MEETING MINUTES /

SUMMARY

OF THE ADVANCED REACTOR DESIGNS SUBCOMMITTEE JUNE 22, 1988 WASHINGTON, D.C.

PURPOSE The purpose of the meeting was to review the NRC Staff's draft safety evaluation report (SER) for the modular high teniperature gas-cooled reactor (MHTGR).

ATTENDEES:

ACRS NRC D. Ward, Chairman T. King, RES W. Kerr, Member P. Williams, RES F. Remick, Member J. Wilson, RES P. Shewmon, Member J. Flack, RES C. Siess, Member R. Johnson, RES C. Wylie, Member R. Landry, RES J. Lee, Consultart M. Spangler, NRR M. El-Zeftawy, Staff S. Long, Fellow Others S. Ball, ORNL A. Neylon, GA D. Moses, ORNL S. Gray, EPRI P. Kroeger, ORNL S. Sharron, Serch .

. C. Allen, IAIC L. Walker, SWEC C. hoffmann, CE M. Long, DOE D. Ayers, CE 0. Hedden, CE

, R. Ng, CE 5. Jadrnak, Student l R. Raymond, CE A. Pope, Student M. Basol, CC R. Stark, NUS J. Kendan, GCRA F. Homan, ORNL L. Mears, GCRA R. Mills, PDC 0 F. Silady, GA A. Millunzi, DOE MEETING HIGHLIGHTS, AGREEMENTS, AND REQUESTS

1. Mr. Ward, Subcomittee Chairman, stated the purpose of the Subcom-mittee meeting and introduced the other present ACRS members and l

l consultants.

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Advanced Reactor Mtg Minutes June 22, 1988 Mr. Ward comented that the SER is incomplete, but much of the key material is present. He comended the NRC staff on a well written document and stated that it is readable and understandable. Mr.

Ward, also stated that due to the incompleteness of the SER, it is questionable if the full Comittee would consider its review of the conceptual design without the benefit of the full SER.

2. Mr. T. King, Chief / Advanced Reactors and Generic Issues Branch /RES, presented a sumary of the HTGR concept as follows:

• Designers - GA Technologies /S&W/Bechtel/CE/GE  ;

• 350MWt(137MWe),modulardesign,withreactorvesseland stesm generator located below grade

• Steel reactor vessel

• External steam generator

• TRISO coated fuel particles similar to FSV

' Annular core design with prismatic fuel blocks similar to FSV

• One loop per module l

• One turbine-generator per 2 modules

• No conventional containment

• Passive decay heat removal and shutdown systems

• None-safety gride BOP

• 40 year module life time Mr. King indicated that there are four chapters needed to be completed as follows:

• Chapter 3 - criteria

• Chapter 11 - Radionuclide control

' Advanced Reactor Mtg Hinutes June 22, 1988

• Chapter 12 - Occupational radiation protection

• Chapter 14 - Plant and performance testing The Staff expects to complete its review by mid-July 1988, and submit the final SER to the Commission by late August 1988.

The purpose of the Staff's review is to provide preliminary guid-ance early in the design process on the licenairig requirements for the MHTGR and also to make an assessment of the adequacy of the DOE proposed supporting R&D programs. The review process is consistent with the Connission's Advanced Reactor Policy Statement and NUREG-1226. The approach is to establish licensing framework to ensure at least the same degree of protection to the public as is required for LWRs.

Mr. King stated that the existing rules and regulations for LWRs were used as the starting point for the development of guidance for the MHTGR, with the following exceptions:

' Use source term based upon mechanistic analysis in lieu of TID-14844 Type source term.

• Containment function may be performed in a different fashion by showing fission product re ention capability to be at least equivalent to LWRs for the same class of events.

• Emergency planning may be modified to reflect plant safety charac. eristics.

• Maintaining and ensuring "defense-in-depth" philosophy in l

l performing key safety functions via deteminitically re-giring: (1) two diverse, independent means of reactor f

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i Advanced Reactor Mtg Minutes June 22, 1988 shutdown, and (2) two diverse, independent means of decay heat removal (assuming a single active failure).

• Demonstrate via a prototype test prior to design certifica- I tion, the ability to maintain and control of radionuclide release.

• Requirc enh:nced QA, surveillance, in-service inspection as necessary.

3. Mr. P. Williams, NRC/RES, described DOE's design criteria approach for the MHTGR. DOE's philosophy is to produce a safe and econom-ical plant that meets the NRC and user requirements by pursuing the following four goals:

i) maintain plant operation. f

11) maintain plant protection.

iii) maintain control of radionuclide release l iv) maintain emergency preparedness.

To achieve goals (i) and (ii), reasures are to be taken to minimize j defects in the fuel and to purify the prinary system of any radio- ,

nuclides. DOE's requirements for goals (iii) and (iv) ere to control radionuclide release so reliably such that emergency i planning is not required.

DOE s process begins with the quantification of top-level criteria pertaining to how well each of the four goals is to be achieved.

I Next, an integrated systems engineering approach is systematically applied to develop the functions, requirements, and specific design selections to achieve the top-level criteria and user requirements.

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Advanced Reactor Mtg Minstes June 22, 1988 Mr. Williams indicated that DOE's goal is to minimize safety related equipment and confine safety functions to nuclear island.

The NRC Staff believes that BOP can be non-safety grade, except safety classifications of plant and service water systems are open items. At a later review stage DOE is expected to nropose adminis-trative and qualification procedures to assure industrial grade (IG) equipment meets suitable standards of construction quality.

l The standard site characteristics assumed by DOE are representative of about 85 percent of the potential sites in the United States (SSE = 0.3G, OBE = 0.15G population distribution of 500 persons persquaremile,and425metersradiusofbothLPZandEAB). The site and supporting material presented is a typical of a rural relatively low population area. Metropolitan or industrial park  ;

type siting has not been considered in the Staff's review.

Mr. Williams stated that the MHTGR uses a low-enriched uranium /

thoriu:n (LEU /Th) fuel cycle which has an initial cycle length of 1.9 years. Subsequent burnup cycles are 3.3 years with one-half of the active core being replaced each 1.65 years. The major differ-ences from Ft. St. Vrain are

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• UC0 Kennel rather than UC 2 i

• LEU rather than HEU l

• much more exacting manufacturing product specification The fuel safety objectives for the NHTGR are more demanding because

! of the fuel particle coatings are considered to be the primary fission product containment barrier. Both the fertile material and

! fissile fuels are in the fonn of separate dense microsphere that are mixed within fuel compacts. The fissile fuel is formed into i

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kernels of two component mixtures of about 20% enriched uranium dioxide and uranium dicarbide. The fertile material is similarly formed into kernels of thorium dioxide. These kernels are coated from inside to outsiae with four :,uccessive protective type coating j shells know as TRISO.

The NRC Staff reviewed the fuel failure models that is being 4 proposed by DOE and its contractors. These failure models depend on temperature, time-at-temperature, operating history, kernel composition and density, fast fluence, burnup, internal and external chemical attacks and interactions, manufacturing deficiencies, and time-to-failure.

t The Staff believes that the fuel design and quality can be devel-oped to meet the DOE-proposed performance objectives, provided a successful outcome of a research program that includes additional work on fuel integrity and fission product transport (plate-out, lift-off, holdup,etc.). The NRC Staff noted that the actual fuel performance at Fort St. Vrain and in the FRG reactors, together with reported laboratory and in-pile tests offer some helpful data i

base.

i Mr. Williams comented that the nuclear design of the MHTGR con-sists of a long, annular, LEU core with inner and outer reflector ,

i control and it is uniquely different from previous HTGR reactor physics designs. The Staff found the conceptual nuclear design is acceptable subject to resolution of safety issues such as:

l i ' Calculational oncertainties (rod worths, reactivity feedbacks, j etc.)

• Reactor shutdown 1

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• Steam / water ingress

• Core and structural graphite

• Reactor vessel fluence ,  ;

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• Decay heat i

• Control materials l

• Safety classification

• Qualification and stertup tests

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00E has comitted to improve methodology and validation through cooperative programs with Gennany and Japan 6nd startup testing.

l The reactor vessel is similar in size, material and duty to a BWR vessel, but normally operates at 400*F with proposed elevated temperatures limited duty for conduction cooldown events to 800'F (pressurized),upto1000'F(depressurized). Other issues that would be considered for resolution later on are: probability of l

gross failure, potential for pneumatic failure mode, leakage detection, thermal stress, strains and creep-fatigue interaction.

, Mr. Williams highlighted the major safety issues and sumarized the

! chapters included in the draft SER.

l The NRC Staff considers the MHTGR design to have a potentially high level of safety, thes meets the safety enhancement objectives of advanced reactor policy statement. The fundamental characteristics that's similar to other HTGR are:

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Advanced Reactor Mtg Minutes June 22, 1988

• Slow response to core heat-up

• High temperature fuel

• Metals vulnerable Mr. Williams indicated that the Staff reserves the final judgement .

on the acceptance of the design dependent on the outcome of a research program that must be augmented beyond that presently described. For example, candidates for regulatory technology plan that could be considered are:

• Flow modeling tests

• Core support structures, seismic forces and rapid depres-surizat!nn effects

• Rotating machinery perfonnance

• Human factors engineering plan

4. Mr. J. Glynn, NRC/RES (severe accident issues branch); presented an overview for the MHTGR's PRA analysis. He stated that the NRC's j severe accident policy statement requires the completion of a PRA l to assist ist its certification of new nuclear power plant designs.

Some objectives of the MHTGR's PRA are:

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• Provide means for characterizing the safety of the plant

' Provide basis to develop DBA and BDBA sequences

' Evaluate compliatice with PAGs l

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. Advanced Reactor Mtg Minutes June 22, 1988

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• Evaluate against the safety goal criteria, i.e., risk of early/ latent fatality.

Mr. Glynn presented the MHTGR's PRA results as follows:

• Defined 11 DBA sequences (frequencies y x 10'4/yr.) that met Part 100

• Defined 7 accident initiators and 58 sequences between 1 x 10~4 and 5 x 10-7/yr., none of which were predicted to suffer fuel failure.

t The NRC Staff's findings are as follows: i

• Insufficient information to confirm MHTGR meets PAGs and Safety goals

• Safety goal criteria may not be sufficient by itself to judge adequacy of plant's level of defense-in-depth

• Identification of accident sequences is questionable.

S. Mr. A. Millunzi, DOE, presented an overview for the safaty philoso- [

phy of the MHTGR. He stated that it is important to have a firm and rational basis valid throughout the licensing process. He  !

expressed some concern that some of the NRC's bounding event sequences are too low in frequency (& 10"7/yr)tobeusedfor containment and em rgency planning adequacy. Mr. Millunzi indi-cated that the MHTGR evolutionary design uses extensive technology l base such as: 58 gas-cooled reactors worldwide, fundamental safety i characteristics and passive features demonstrated at operating i reactors, and extensive enveloping out-of-reactor fuel testing. I i

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P Advanced Reactor Mtg Minutes June 22, 1988 Mr. Millunzi connented that only equipment relied on to perform the functions required to meet 10 CFR 100 is tenned "safety-related" in their documents. The conceptual design has placed special focus en ,

equipment relied on to meet 10 CFR 100 in order to set configura-tion.

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6. Mr. S. Ball, ORNL, presented a sunnary for an independent accident analysis that was performed by ORNL. The accidents that were analyzed are'  ;

• Core heatup

• Reactivity transients l

• Water ingress y

) For the core heatup accident studies, the major conclusions are: ,

l (a) good agreement with GA and BNL, (b) fuel failure not expected, I (c) possible vessel overheating, (d) most crucial factors are RCCS ['

heat removal capacity and fuel / reflector thermal conductivity.

For the reactivity transients ORNL verified the results versus Ft.

St. Vrain and AVR. In addition, ATWS accidents analyzed

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• Loss of circulation "

• Moisture ingress

• Control rod group withdrawal  !

1 j ' Rod group ejections t j I i The results showed all mild effects, but it is dependent on the I assumed parameters.

L For the water ingress accident, Mr. Ball indicated that independent checks are needed on the following: >

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• Reactivity worth of steam / water

• Changes of rod worth with moisture in cort:

• Ingress rates  ;

• Credibility of multiple tube / header ruptures

7. Mr. P. Kroeger, BNL, presented a sunnary of an independent safety [

analysis for the MHTGR conceptual design. Such accidents are:

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(a) Depressurized core heatup (with functioning RCCS)

(b) Depressurized core heatup (without functioning RCCS)

(c) Air ingress  ;

(d) Depressurized core heatup accidents without scram t (e) Reactor Cavity gas radiation The overall cenclusions are:

• Except in extended no-scram scenario, fuel temperature remain below 1600*C

• Peak vessel temperatures exceed code limit of 480*C in some severe accident scenarios  ;

• With massive vessel failures, air ingress and graphite oxida-tion remain limited by in-core friction pressure drop.

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8. Mr. A. Neylan, GA technologies, following the conclusion of the Subcobrnittee meeting, urged the Subconnitteo Members to approve the  !

conceptual design of the MHTGR based on the technical aspects l rather than the political ones and waive the policy matters to a later date. The Subconnittee Members thanked him for his point of i vitu.

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Advanced Reactor Mtg Minutes June 22, 1988

9. As a result of the Subcommittee discussion, some of the Subcomit-tee's Members expressed some concerbs in regard to the following:

• Dr. Siess expressed sore concern in regard to the incomplete-ness of the draft SER. He crnmented that the ACRS should not write a letter to the Cemnission based on an incomplete draft of SER.

• Dr. Shewmon questioned the meaning of design basis tornado events for the MHTGR design, since there are no specific sites are chosen yet.

• Dr. Remick expressed some concern in regard to the exact definition and assigned values to the reactivity coefficient for the MHTGR design. He comented that exact values should be determined based on functional components (e.g., fuel temperature, moderator temperature, coolant temperature, void coefficient, power density coefficient, etc.)

• Mr. Ward ex;.ressed some concern in regard to the negativity of l theprompt(Doppler)andnear-prorpttemperaturecoefficient l of reactivity near end-of-cycle when contributions due to

( plutonium-239 btildup are expected to be most positive. Mr.

l Moses (BNL),indfcatedthatthecontributionofthethorium-232 resonance broadening to the total + 'perature coefficients j in HTGR HEU/Th fuel systems is both qui e small (u -3 x10-5 A k/k/C') anc gradually reduces above about 1200'F, and the coefficients appear to become positive above about 1800'F to 2100*F. However, this infomation lacks characterization of l uncertainties, i

• Dr. Lee, ACRS Consultant, expressed some concern in regard to the applicability of physics to the MHTGR, He comented that P

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the available experimental data of cross sections 6nd design  ;

methods is not completely applicable to the MHTGR, because of the change in the fuel cycle from utilizing HEL/Th in the earlier HTGR concepts to LEU /Th.

• Mr. Ward expressed some concern regarding the Staff's review 4

which does not include catastrophic vessel failure that could lead to graphite fire.

• Dr. Shewmon corirented that an emergency planning for evacuat-l ing the public in the event of radiation release is not '

defined in the draft SER.

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• Dr. Kerr questioned that if the external events were included in the PRA studies in a proper manner.

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• Dr. Kerr questinned the decisions and assumptiens that were [

l made by the NRC Staff to account for the defense in-depth philosophy.

• Dr. Remick questioned the various means by which the NRC Staff [

reviewed the neutron control and shutdown margin under various  !

conditions. Mr. Millunzi, DOE, promised tc send tables [

indicating the values assigned for shutdown margins. i l

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• Mr. Ward questioned the role of reactor operator for this (

design. He pointed out that the two major power reactor j accidents that have occurred TMI-2 anc Chernobyl were caused ,

by wrong but purposeful actions by operators. PRA does not de a good job in describing or estimating the likelihood of suct,  ;

events. If the MHTGR is claimed to be safer than LWRs, then there should be some systematic means to analyze its re-  !

sistance to such maloperation. T. King agreed to consider 1

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Advanced Reactor Mtg Minutes June 22, 1988 I

this. Dr. Remick concurred and added other issues such as if the operator perform a safety function, control room is not protected, and the question of staffing, all need to be addressed.

• Dr. Siess questioned the fuel particle failur' "ates assump-tions that were made by the Staff, and added tnat there is no experimental available data to validate those assumptions, i

• Dr. Remick comented that the containment issue need to be discussed further with emphasis on the public perception.

• Dr. Remick requested that additional work should be specified regarding the plant physical security and the sabotage issue.

, Dr. Kerr concurred.

• Dr. Kerr comented that it is not clear that if the conceptual design can deal with the station blackout issue for the longer time duration specified by the Staff, and additional analysis i should be perfonned to validate that assumption. ,

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• Dr. Shewmon comented that the "prototype testing" issue should be looked at more carefully.

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• Mr. Ward comented that the ACRS still has to develop its  !

positiun on the key licensing issues th t were discussed in ,

the Comission Paper, before finalizing the approval of the SER.

l FUTURE ACTIVITIES l The Subcomittee Chainnan is planning to brief the full Comittee in July 1988 regarding the Subcomittee activities. In additi>n, the NRC i

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Ad anced Reactor Mtg Minutes June 22, 1988 l Staff will brief the Committee in July 1988 regarding the draft SER.

DOE and its contractors will also be present. A letter writing is not l expected at the July 1988 full Committee meeting. A Subcommittee  ;

meeting has been scheduled for August 3,1988, to continue discussion of i this subject.

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NOTE: Additional meeting details can be obtained from a transcript of this meeting available in the NRC Public Document Room, l 1717 H Street, N,W., Washington, D.C., or can be purchased fron: Heritage Reporting Corporation,1220 L Street, N.W., i Suite 600, Washington, D.C. 20005,(202)628-4888.  ;

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