NSD-NRC-96-4691, Submits Summary of 960229 Meeting Re AP600 Thermal-Hydraulic Uncertainty & MAAP4 Benchmarking Plan
| ML20107C939 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 04/12/1996 |
| From: | Mcintyre B WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| DCP-NRC0497, DCP-NRC497, NSD-NRC-96-4691, NUDOCS 9604180072 | |
| Download: ML20107C939 (20) | |
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Westinghouse Energy Systems Box 355 Electric Corporation Pittsburgh PennsyNania 15230-0355 NSD-NRC-96-4691 -
DCP/NRC0497 Docket No.: STN-52-003 April 12,1996 Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555 ATTENTION:
T.R QUAY
SUBJECT:
SUMMARY
OF AP600 THERMAIrHYDRAULIC UNCERTAINTY AND MAAP4 BENCHMARKING PLAN AS DISCUSSED AT FEBRUARY 29, 1996 MEETING
Dear Mr. Quay:
A meeting was held on February 29,1996 between Westinghouse and NRC staff to discuss the AP600 Thermal-Hydraulic Uncertainty and MAAP4 Benchmarking plan that was submitted by Westinghouse j
on December 8,1995 and the staff's response to the plan in a letter dated January 18,1996. As explained in the Westinghouse December 8 cover letter and at the February meeting, the plan continues to evolve in order to accommodate NRC staff comments presented to us during several meetings and provided in NRC correspondence. At the February 29 meeting, the staff committed to provide feedback to Westinghouse of what was presented at the meeting.
1 As requested by the staff at the March 19,1996 Westinghouse /NRC senior management meeting, Westinghouse agreed to provide the staff with a written summary of what was presented at the February 29 meeting. Enclosed please find the Westinghouse summary of the February 29,1996 Thermal-Hydraulic Uncertainty and MAAP4 Benchmarking meeting. The enclosed material fulfills j
the action Westinghouse took at the March 19,1996 senior management meeting.
l The staff was informed at both the February 29 and the March 19 meetings, that Westinghouse is currently working on the activities as they were spelled out in February meeting. Feedback from the staff is essential to appropriately expend resources and maintain momentum on the path to resolving this issrc.
Westinghouse would like to mea with the staff the week of April 22 to continue discussions on the T H uncertainty and MAAP4 benchmarking issues. Specifically, we would like to discuss the process Westinghouse is currently pursing to resolving these issues and to discuss key technical issues to be addressed in benchmarking. Prior to this meeting the staff should review the enclosed material.
Cindy Haag will contact Mr. Bill Huffman to set up the proposed meeting.
180020
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9604180072 960412 PDR ADOCK 05200003 g
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NSD-NRC-96-4691 DCP/NRC0497 April 12,1996 Please contact Cynthia L. Haag on (412) 374-4277 if you have any questions concerning this transmittal.
6a AM Brian A. McIntyre, Manager Advanced Plant Safety and Licensing
/nja Enclosure cc:
B. Huffman, NRC D. Jackson, NRC N. J. Liparulo, Westinghouse (w/o Enclosure) 2743A
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Enclosure I to Westinghouse Letter NSD-NRC-96-4691 April 12,1996 t
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SUMMARY
OF AP600 THERMAL HYDRAULIC UNCERTAINTY AND MAAP4 BENCHMARKING PLAN AS DISCUSSED AT FEBRUARY 29,1996 MEETING i
A meeting was held between Westinghouse and the NRC on February 29,1996 to discuss the MAAP4 Benchmarking and T&H Uncertainty resolution plans. This document describes the plans as they were described at the meeting, with annotations regarding issues that were discussed or issues that require further discussion.
1.0 HACKGROUND 1.1 Summary of Meetings and Proposed Plans The mission statement describing the tasks to be accomplished is:
To provide a higher level of comfort that AP600 success criteria have been defined " robustly."
so that PRA results are not significantly impacted by:
T/H uncertainty in the behavior of the passive systems MAAP4's simplified models
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t This mission statement was first formulated for the July 27,1995 meeting, and remains unchanged.
The July 27 meeting described an integrated process by which the MAAP4 benchmarking issue and T/H uncertainty issue would be resolved. The plan identified four accident scenarios that were chosen because they are the only ones in the PRA success criteria with core uncovery. The cases represented a range of i
j break sizes (0.5", 2.0",4.0" and 8.75"), break locations (hot leg, cold leg, DVI line), CMT or accumulator actuation, and the most important sequence in respect to core damage frequency (CDF). The plan identified that the NOTRUMP analyses would be performed with the DBA-like assumptions, including:
Appendix K decay heat (1971 ANS +20%)
102% in:Wil Power Initial wate temperatures at Tech Spec maximums ADS minim im valve area IRWST lin. maximum resistances
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Maximum Tech Spec peaking factors I
The MAAP4 analysis assumptions would match the NOTRUMP assumptions in the extremely important boundary condition of decay heat and the initial power level. Other MAAP4 analysis assumptions would remain at previously analyzed (usukily nominal) conditions. The MAAP4 input changes were limited so that applicability to previous MAAP4 analyses performed for the PRA would be clear.
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ewpup600\\2,27% txt Page 1
The July 27 integrated MAAP4 benchmarking and T&H uncenainty plan identitled the closure process for each of the issues:
The T/H uncertainty issue will be closed by showing that NOTRUMP (and LOCTA) calculation of PCT meets the 2200*F criterion for all four cases.
'Ihe MAAP4 benchmarking effort consists of comparing system responses from the NOTRUMP and MAAP4 analyses. All differences in the following system responses will be investigated and explained:
RCS Pressure Break Flowrate CMT Flowrate CMT Level Accumulator Flowrate IRWST Flowrate RCS Inventory Mass Core Mixture Level Peak Clad Temperature ADS Flowrate
. NRC comments on the July 27 plan were expressed in a letter dated August 14, 1995,. and in meetings held between Westinghouse and the NRC in August, September and October. The major comments were:
The NRC cannot concur on the sufficiency of the number of cases or the selection of cases without the identification of the important phenomena.
Westinghouse needs to provide basis for why MAAP4 is good enough to have chosen the most limiting cases.
PRA sensitivities that support the resolution of T&H uncertainty need to consider the focused PRA rather than the baseline PRA.
Comparison of MAAP4 and NOTRUMP should be done with exactly the same set of analysis assumptions.
MAAP4 needs to be compared to data from tests.
c Awpsap600\\2_27% ist Page 2
On December 8,1995, Westinghouse submitted a written plan to the NRC for the resolution of MAAP4 benchmarking and T/H uncertainty issues. De plan was based on the July 27 presentation, with j
modifications to incorporate NRC comments to date. He major differences in the December 8 plan from the July 27 plan were:
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Key models for the PRA scenarios were defined. The definition of key models was done in a j
table that includes the reason of importance, potential reasons for concerns, and the parameters that will be used to confirm the key models. In addition, the plan described three types of core j
uncovery that occur in the PRA scenarios. Many of the key models were discussed in context of how they are important for these types of cases.
2)
De cases for comparison were modified. His was done to better exhibit the key models.
3)
An OSU assessment was added.
4)
A final set of success criteria analyses will be done, based on the benchmarked parameter file and the final AP600 design.
5)
The pathway for T&H uncertainty resolution changed. In the July 27 plan, there were three curves that would be compared for each parameter:
i Nominal MAAP4 run MAAP4 + Higher Decay Heat run DBA-like NOTRUMP run In the December 8 plan, the curves would be:
Nominal MAAP4 run Nominal NOTRUMP run Sensitivities that encompass the DB A-like assumptions from the July plan (performed with MAAP4 and LOCTA)
De T&H uncertainty resolution process is one that requires further discussion between Westinghouse and the NRC. De NRC has stated a number of different concerns that sometimes appear to Westinghouse to be contradictory in nature. On one hand the July 27 plan was " approved," yet the caveats to that approval seem major. There is more discussion within this document on the T&H uncertainty resolution plan, but this issue remains one that is in need of clarification of NRC concerns. Comments on this 4
subject that were transmitted in August 1995 and January 1996 include:
" Westinghouse has assumed that. Appendix K inputs provide sufficient margin to bound the
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effects of passive system thermal hydraulic uncertainties on the PRA success criteria..
l Westinghouse must justify why the use of Appendix K inputs and models is sufficient to bound i
the thermal hydraulic uncertainties for all AP600 PRA sequences." (August 14, 1995)
)
l "In an August 14, 1995, letter from the NRC to Westinghouse, the staff approved the l
Westinghouse bounding approach provided five concerns could be satisfied. The plan approval l
received high level review and concurrence within the NRC." (January 18, 1996)
(
i "Since passive systems rely on natural forces such as gravity and stored energy to perform their
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functions, the net driving forces are small compared to active systems and are subject to large uncertainties - especially when considering multiple system failure scenarios contained in the PRA." (Jan 18)
"The MAAP4 sensitivity study of the few parameters indicated in the report, including the sensitivity study using LOCTA to show the effect of varying the core peaking factors, appear to be too limited in scope, and do not necessarily cover the T/H uncertainty." (Jan.18)
Core peaking factors Minimum and maximum accuinulator flowrate Minimum and maximum IRWST flowrate Maximum ADS flowrate 1971 ANS + 20% Decay Heat 1.2 Major Issues Based on the preceding history of p!ans, meetings and NRC comments, there are three areas of discussion on which to focus.
Need more than four MAAP4 benchmarking cases.
T&H uncertainty issues cannot be resolved solely with MAAP4.
December 8 proposed OSU assessment is not acceptable.
1.3 PRA, Success Criteria, and MAAP4 Analyses Because the mission of this work is to focus on the PRA impact, the scope must be viewed in context of the PRA purpose. The purpose of a PRA is to quantify the core-damage frequency (CDF) and large-j release frequency (LRF), while gaining insights into any risk significant vulnerabilities of the plant. One l
of the elements in performing a PRA is to define success criteria, which refer to a minimum set of c wpupmn2_2796 ixi Page 4
)
l equipment needed to prevent core damage.
Safety analyses performed for Chapter 15 of the SAR are done in a conservative manner to justify the safety of the plant design. The resulting accident analyses can sometimes deviate from reality, due to the conservatisms that are stacked upon one another. For the PRA, definitions of success criteria are done considering the nominal performance of the phnt. Conservatism in success criteria (requiring more i
equipment) can potentially' mask risk-significant irsights in the plant. Overly conservative success criteria are not desirable when trying to gain PRA insights.
There is a need for additional discussion on this topic between Westinghouse and the NRC Westinghouse believes that understanding the role of PRA will be fundamental to resolution of the outstanding issues.
The MAAP4 code enters the picture because it was used to support some of the AP600 success criteria definitions. Accident scenarios that require ADS actuation as part of the successful sequence of events were considered in MAAP4 analyses. Scenarios were grouped, and " baseline" cases were defined as the most limiting cases for a range of accidents. The baseline cases include the most restrictive set of hardware assumptions and the most restrictive break location and size.
Appendix A of the PRA documents MAAP4 analyses that support the ADS success criteria. This documentation was submitted to the NRC in January of 1995. Since then, the AP600 success criteria i
definitions have continued to evolve. Through work being performed within Westinghouse, and discussions with the NRC, issues have been raised that have caused the success criteria to be 1) more conservative (more equipment is required),2) simpler, because the same equipment is required across a broad spectrum of events, and 3) there is more T&H margin. In addition, success sequences with PRHR are not based on MAAP4 analyses.
Because of the ADS success criteria evolution described above, there are now two basic accident scenarios.
Automatic ADS Manual ADS Initiating Event Initiating Event No startup feedwater No startup feedwater No PRHR No PRHR No accumulators I accumulator ICMT No CMTs 2 stage 4 ADS 2 stage 4 ADS (on 1010 CMT level)
(operator action) 1 IRWST line 1 IRWST line Containment isolation failure Containment isolation failure enwpph2.27% m Page 5
.. - _ - =. -
Both scenarios involve a complete loss of heat removal capability except for the break and ADS. These l
accideat scenarios have been studied for a range of break sizes and initiating events. The minimum RCS inveatory over a spectrum of break sizes is shown in Figure i from the automatic ADS cases.
Observations from studying the automatic ADS cases are:
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The CMT is an effective safety feature of the AP600 plant.
i 1 CMT contains a large amount of water that is able to provide make-up for the Transients and LOCAs of interest.
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The CMT level setpoints have been defined to provide ADS actuation in time to get j
IRWST gravity injection to cool the core, even without credit for accemulator injection.
CMTs have a recirculation and a draining phase of injection.
Recirculation of the CMTs occurs for a longer period of time in the smaller breaks.
Most cases do not experience core uncovery.
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Core uncovery can occur in Transients and Small LOCAs.
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De depth and duration of core uncovery are lirdted.
Maximum uncovery of 30% for 500 seconds.
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F Core uncovery occurs at the small end of the NLOCA spectrum.
The depth and duration of core uncovery are limited.
Maximum uncovery of 10% for 100 seconds.
i The minimum RCS inventory over a spectrum of break sizes is shown in Figure 2 from the manual ADS I
cases. Figure 3 also summarizes plant behavior for this accident scenario over a range of break sizes.
Figure 3 illustrates the time that core uncovery starts, the time the accumulator injection starts and depletes, and how the interplay of these items changes over the break spectrum. Although operator action time is not explicitly shewn, Figure 3 allows one to obtain information of when operator action would be needed to prevent core uncovery. Observations from these cases are:
Because there is no CMT, the response of the plant is dependent on the deoressurization due to the break and operator action time to actuate ADS.
ehpw600\\2.27% txt Page 6
Transients and SLOCAs are slow-acting, and have small to no inventory loss before the operator action is anticipated.
For NLOCAs, the core uncovers before accumulators can inject.
1 RCS pressure decreases after core uncovery, allowing accumulator injection i
Accumulator injection is relatively slow j
Accumulator injection limits the depth of uncovery i
i Duration of core uncovery is a function of operator action time For MLOCAs, the accumulators inject and empty before the core uncovers.
i 3
Depth and dura:ica of core uncovery is a function of operator action time Hot leg break location is a significant factor at the largest breaks l
2.0 MAAP4 BENCHMARKING There is a need to benchmark MAAP4 to provide a higher level of comfort that the success criteria in the PRA are valid.
i i
2.1 Purpose i
l The purpose of MAAP4 benchmarking is to support the baseline PRA success criteria. The benchmarking of MAAP4 will be done against NOTRUMP analysis results, performed with nominal assumptions. The benchmarking will focus on MAAP4's ability to track inventory losses and gains, and to predict the system depressurization. The goal of the benchmarking is to demonstvate an understanding of the behavior
]
of the AP600 plant:
1 as break size changes, as break location changes, and as another tank (CMT or accumulator) is credited.
4 l
As discussed in the Febmary 29,1996 meeting, the MAAP4 benchmarking is not just an exercise of comparing codes, but it is to demonstrate the AP600 plart response to different accident scenarios.
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2.2 Key Models Before benchmarking cases can be defined, the key behaviors ofinterest must be identified. Based on the plant response discussed in Section 1.3, a set of key models were defined. These key models are from the December 8,1995 proposed plan, and are listed in Table 1.
Table I also identifies the parameters of interest that will be used to compare the system response as predicted by MAAP4 and NOTRUMP. Differences in the responses will be investigated and explained.
Within this effon, there is no plan to tune MAAP4. Identical responses are not expected nor necessary to suppon MAAP4 as a scoping tool for PRA. However, if a MAAP4 parameter is changed, it will be done in a systematic manner that either changes the value for all cases, or is based on phenomena that are specific to a set of cases.
The applicability of NOTRUMP to the PRA scenarios is an outstanding issue to be discussed later.
2.3 Benchmarking Cases The benchmarking cases are chosen to cover the key models listed in Section 2.2, to address the system behavior illustrated in Section 1.1, and to cover a spectrum of conditions. The cases that were identified in the February 29,1996 meeting are:
Automatic ADS Actuation 0.5" hot leg break 2.0" hot leg break 5.0" hot leg break 8.75" hot leg break 0.5" hot leg break with I CMT,1 Accumulator 2.0" hot leg break with 2 CMTs 2.0" hot leg break with delayed ADS Manual ADS Actuation 3.0" hot leg break 6.0" hot leg break 8.75" hot leg break 8.75" cold leg break DVI line break An additional issue that was raised at the February meeting is to address whether the inventory loss from cSwpup60m2_27% txt Page 8
more ADS valves could outweigh the benefit of the faster depressurization that is achieved.
.A benchmarking case will be added to address this issue.
Westinghouse is proceeding with the benchmarking of these cases. NRC comments on the adequacy of these cases are needed immediately.
2.4 Comparison to Test Data The NRC August 14, 1995 letter stated that MAAP4 comparison to test data should be performed.
Westinghouse has expressed concerns about the value added. The concerns include:
j Establishing values for MAAP4 OSU parameter file would not show validity of MAAP4 AP600 parameter file.
OSU "PRA" test scenarios are counted as failure in the PRA.
Although two OSU tests experience core uncovery, they do not necessarily exercise the phenomena that are of interest.
Data from "PRA" test scenarios were not documented in the OSU Test Analysis Report.
Nevertheless, in the December 8,1995 plan Westinghouse proposed an "OSU assessment" to compare MAAP4 AP600 results with OSU test data. The OSU assessment would focus on a few parameters (primarily mass flow rate predictions) and would provide a hinher level of comfort that MAAP4 predicts similar trends. Westinghouse had performed a blind feasibility study prior to the proposal of the OSU assessment, and Figure 4 shows a sampling of some of the results. The MAAP4 data is scaled from a full-scale AP600 analysis that is run for the same accident scenario (equipment failures) modelled in the OSU test.
In the January 18,1996 NRC comments, the proposed "OSU assessment" was rejected due to:
Distortions in loop response due to the reduced size of the test facility.
Appropriate scaling ratios change as a function of time.
Response of OSU fuel rod simulators does not represent AP600 fuel rods.
Initial conditions cannot be scaled.
Test scenarios should not be expected to provide global coverage of phenomena that might be encountered in the multiple failure PRA sequences.
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i Westinghouse is no longer planning to do a comparison between MAAP4 and test data because it is not expected to improve the understanding of plant response for the PRA scenarios.
j i
3.0 T&H UNCERTAINTY The resolution of T&H uncertainty issues needs to be performed as a separate effort, after MAAP4 benchmarking is completed. The plan that is proposed in this section establishes a structure for the resolution, more discussion between Westinghouse and the NRC is needed. NRC T&H uncertainty concerns need to be clarified.
l I
3.1 Purpose f
The purpose of the T&H uncertainty resolution plan is to determine whether uncertainty in the T&H performance of passive systems has an acceptable impact on the focused PRA. There are two major components to the plan:
1)
PRA sensitivity to the focused PRA to determine if there are risk-significant, low-margin accident scenarios.
2)
T&H analyses to examine risk-significant, low margin accident scenarios.
3.2 PRA Sensitivity The purpose of an AP600 PRA sensitivity is to determine if the low-margin accident scenarios are risk.
significant to the focused PRA. Applicable event tree paths will be evaluated to further define the 4
j frequency of the low-margin scenarios Number of CMTs and accumulators Break size Break location 4
Operator action time Credit for additional operator ac6ons not considered Determine if focused PRA CDF and LRF goals can be net if low margin sub-sequences are counted as failure. From this sensitivity, determine if the low-maron sequences are risk significant ehpiap600\\2_27% ttt Page 10
3.3 T&H Analyses Supporting T&H Uncertainty if there are risk-significant, low-margin sequences, further T&H analyses will be performed with NOTRUMP, The NOTRUMP analyses will consider the uncertainty associated with the small net driving forces of passive systems. Funher details of the NOTRUMP analyses can only be discussed after it is known which accident scenario will be examined.
I 4
4.0
SUMMARY
The MAAP4 benchmarking and T&H uncertainty issues continue to evolve as Westinghouse and the NRC exchange ideas.
The MAAP4 benchmarking plan has been separated from T&H uncertainty resolution.
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The MAAP4 benchmarking plan is a comprehensive set of cases for comparison to NOTRUMP.
The framework for T&H uncertainty resolution is established; the details will require further discussion.
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Table i Key MAAP4 Models Used in Success Criteria Analysis Model importance / Concerns Parameters of Interest Core Uncovery and The peak core temperature ts used to determine Core mtxture level Heacup whether a sequence is dcIsned as " success" or Peak core temperature
" damage "
Decay heat MAAP4's core model does not simulate the hot pm.
therefore M AAP4's peak temperature predtetton needs to be compared to a more desatled model.
Appronmatdy W o' &."~ ~ent analyses result tn percal core uncovery. They are prtmartly manual ADS scenanos that rely on operator acnon.
Credned m full depressuruanon casca to
. ADS hqutd flow rate ADS Stage 4 depressurue the RCS so that IRWST gravity
. ADS vapor flow rate myocoon can occw. 2 out of 4 sage 4 ADS lines ts RCS pressure a
the success entenon for all full depressuruanon cases.
CMT CMT provides coohng and mventory make-up for CMT impecnon flow raes 1.OCAs
- CMT recarculanon flow rate CMT level deastmines the ame of ADS actuanon
. CMT level a
Tuns CMT rectreulation tramanome to CMT injecnon
. Tuns CMT low level sospoina are reached
!RWST Injecnon
. IRWST injecnon is ths==*a-for long-tenn
. IRWST injecnon flow rate cooling m the full depasswunoon cases
. RCS pressure
. IRWST injecnon recovers the core, or keeps the
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presswa core from uncovenas
. Core amurure level
. IRWST myocoon as sammeve to the AF between canamment and the RCS.
Break Invensory loss through the break determmes I.upud break fbw ram whedner core is covered Vapor break flow rate Syssem depressunsanos deanas beak stas ranges RCS weser inventory for I.OCA categones RCS presswo
/
1.cceaan of break at bessa of her leg was a major consadersoon in deamag success enanna.
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paracularly for larger breaks RCS Nanaral MAAP4's VPSEP model ces have as unpact on:
I.aspud break flow rass Circulanos wheiher the break locance is covered widt Vapor break flow rats Tues CMT recirculanon weaar the end of CMT recurculaman and the sean of treassmens e CMT mjecnon Otr injecoon The acewnulaser ejecean prevents core uncovery Accuandaser mjecnon flow Accumulator (1)
~ for larger (> 6") beats.
ress Tbs accumulaser ejecean plays a role is lassang Com menare level the PCT for breaks areemd 3" to $*
RCS pressure The accustulaser and CMT share tas DVi lias, and uusrecame berween tas asks must be consadored.
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Table 1 '
Key MAAP4 Models Used in Success Criteria Analysis i
1 Model Importance i Concerns Parameters of Interest ADS Stage 1 3 (2)
For rugn pressure scenarios. credited to reduce ADS bquid flow rate
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pressure so that stage 4 ADS can open ADS vapot flow rate l
Credated in partial depressuruanon cases to Pressutuer inventorv a
i depressurue the RCS below RNS shutoff head.
RCS pressure j
Locanon Ls at top of pressuruer, and entrainment j
of water into pressuruer could affect j
depressutuanon capabihty.
Heat transfer to SGs plays a role in Transants and
' SG heat transfer SG Heat Transfer j
SLOCAs. RCS utventory loss starts or tneresses 4
when SGs dry our l
PRHR ADS success criterm widt the PRHR operable are Not Appbcable I
not direcdy supporsed by MAAP4 analyses.
1 i
Notes:
(1) Imersenon between accumulator and CMT wsil not be shown sa MAAP4 / NOTRUMP comparsson. De MAAP4 /
j OSU assesstnent wdl addres Gus issue.
l (2) De MAAP4 / NOTRUMP companson wdl only eassune ADS Sage !
- 3 as a precursor to ADS Stage 4. The behaver of ADS 1 -3. by itself. can be sees thnngh the MAAP4 / OSU assessment.
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