Idaho National Engineering and Environmental Laboratory Assessment of Margin for In-Vessel Retention in Higher Power Reactors 2004 RELAP5 International
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Idaho National Engineering and Environmental Laboratory Assessment of Margin for In-Vessel Retention in Higher Power Reactors 2004 RELAP5 International Users Seminar Sun Valley, Idaho, USA August 25-27, 2004 D. L. Knudson and J. L. Rempe
Idaho National Engineering and Environmental Laboratory
Assessment of Margin for In-Vessel Retention in Higher Power Reactors
2004 RELAP5 International Users SeminarSun Valley, Idaho, USA
August 25-27, 2004
D. L. Knudson and J. L. Rempe
Joy L. Rempe
Good afternoon. As indicated by on my first slide, the title of my presentation is "Assessment of Margin for In-Vessel Retention in Higher Power Reactors ". The research that I'll be discussing is part of an INERI between S. Korea and the U.S. Organizations participating in this collaboration include Seoul National University, Korea Atomic Energy Research Institute, Penn State University, and INEEL. The objective of this research is to evaluate various options for enhancing in-vessel retention. This presentation focusses on the status of S/R5-3D calculations being performed to evaluate various IVR options considered in this INERI. I also want to acknowledge that the S/R5-3d calculations that I'm discussing today are being performed by Mr. Darrell Knudson, who was unable to attend this meeting. So, I hope you'll understand if I defer any difficult questions to him.
Idaho National Engineering and Environmental Laboratory
My presentation is organized as shown in this viewgraph. First, I'll be discussing some general background information about our INERI project. Second, I'll discuss the approach that we are using to evaluate various IVR options with S/R5-3D. Then, I'll present some initial results that illustrate the capabilities of the code and in particular, the 2D COUPLE model. I'll next discuss our plans for completing this task, and close with some conclusions about the status of our work.
Idaho National Engineering and Environmental Laboratory
304-GA50005-30
K-INERI Program ObjectiveUse systematic approach to develop specific recommendations to improve the margin for in-vessel retention of relocated materials during a severe accident in high-power reactors (up to 1500 MWe).
– Combine state-of-the-art analytical tools and key U.S. and Korean experimental facilities
– Focus on modifications to enhance external reactor vessel cooling and in-vessel core catcher performance
– Focus on APR1400, but methodologies developed so that they can easily be applied to other reactor designs
Background
Joy L. Rempe
The objective of our INERI is to use a systematic approach to ...To accomplish this objective, we are combining state-of-the-art analytical tools and key U.S. and Korean experimental facilitieswe are focussing on modifications to enhance ERVC and in-vessel core catcher performance.and, we are focussing on the advanced Korean PWR APR1400 design, although our methodologies are being develoepd so that they can easiloy be appolied to other reactor designs.
Idaho National Engineering and Environmental Laboratory
404-GA50005-30
K-INERI Applies State-of-the-Art Analytical Tools and Key Experimental Facilities
Background
Joy L. Rempe
As noted in the last VG, our research is foccusing on two strategies to enhance IVR -- an in-vessel core cathcer and ERVC.For each strategy, we're employing several existing (and new) experimental facilities and analytical tools.We have several facilities at SNU, KAERI, PSU, and INEEL to help us obtain key data for predicting heat loads to and from the reactor vessel with and without a core catcher.
Idaho National Engineering and Environmental Laboratory
504-GA50005-30
SBLB Used to Evaluate Proposed Coatings and Insulation/Vessel Configuration
Background
Subscale Boundary Layer Boiling Facility
Proposed coatings and enhanced insulation/vessel configuration significantly increase CHF
Insulation modified to enhance ERVC flow
0.0
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Angular location (degrees)
q" C
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Enhanced insulation/vessel configuration
Coated vessel
Joy L. Rempe
In the case of ERVC, two methods are being evaluated... an enhanced vessel/insulation configuration (which prevents choking of steam that is vented) and the use of microporous coatings on the external surface of the reactor vessel. The Subscale Boundary Layer Boiling facility at Penn State University is uniquely suited for evaluating both of these options for a scaled APR1400 reactor vessel and enhanced insulation geometry. Data obtained from the SBLB illustrate that CHF is significantly increased with either options (by over a factor of two at many locations). Tests are underway at PSU to assess the combined effect of these ERVC enhancements.
Idaho National Engineering and Environmental Laboratory
604-GA50005-30
Unique In-Vessel Layered Design Proposed and Evaluated
Background
1 of 4 segments
Holes for instrument nozzles
Locating p ins
Segm entinterlock
03-GA50047-45
Base m ateria l
Insu la tor coating
Vessel
Possible m eta llic c ladd ing
Joy L. Rempe
In the case of an in-vessel core catcher, an enhanced design was developed and its merit is being evaluated using the LAVA-GAP facility at KAERI and the HTTL at INEEL. The conceptual design of the in-vessel core catcher proposed for the APR1400 is shown in this viewgraph. It is hemispherical and placed in the vessel so that a small gap exists between the vessel and the core cathcer (so its presence only impacts the small amount of coolant flowing in this gap (~2%). It is sized to contain masses of materials expected to relocate during APR1400 severe accidents. The core catcher consists of several interlocking segments. Each segment is perforated to accommodate the 61 lower head penetrations. Each segment is attached to loading pins or clasps in the reactor vessel.Several features of the deisng make it unique:- A layer of insulation is applied to the inner surface of the core cathcer.-Fins may be attached to the lower surface to promote cooling.-Cladding or coatings of paint may be applied to lower surfaces to prevent corrosion and promote cooling. Our evaluations suggest that plasma thermal spraying techniques should be used to apply the insulator coating to the inner surface of the core catcher. Materials interactions tests suggest that the core catcher should be consist of a yttria-stablished zirconia insulator coat over an Inconel 718 bond coat over a stainless steel substrate.
Idaho National Engineering and Environmental Laboratory
704-GA50005-30
KAERI and INEEL Facilities Provide Integral Data for Assessing Core Catcher Performance Using Simulant and Prototypic Materials
INEEL HTTL prototypic tests suggest that proposed coatings protect substrate from molten core materials
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Argon outSS top cover
RS-100 insulation board
Graphite
Carbon steel crucible Core catcher sim ulator
Corium
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Argon inArgon out
Argon inArgon in
M oly lead
C1C2
Joy L. Rempe
The performance of this combination of materials is being evaluated using simulant materials in the KAERI LAVA-GAP facility and using prototypic materials in the INEEL High Temperature Test Facility. As shown in this viewgraph, the 1/10th scale LAVA-GAP facility includes a vessel and a core cathcer. This facility uses iron-aluminate thermite to simulate the core materials that relocate to the vessel. As indicated in this figure, the heat loads to the vessel are significantly reduced when a core catcher is incorporated.The INEEL HTTL was employed to evaluate the protection that proposed coatings provide against relocating core materials. As shown in this viewgraph, a simulated core catcher consisting of the proposed coatings was loaded with prototypic materials, which were heated using a unique resistance heater. Post-test examinations indicate that the proposed coatings offer significant protection to the stainless steel substrate.
Idaho National Engineering and Environmental Laboratory
As I mentioned at the beginning of my presentation, the S/R5-3D code is being used to evaluate the increase in margin for IVR for the various options investigated in this INERI.The SCDAP/RELAP5-3D code was selected for these evaluations because it is capable of simulating all major processes affecting IVR of relocating material:- molten pool formation and heat transfer via convection-- note that the code is now capable of simulating a uniform and a stratified pool with an overlying metallic layer- crust formation and heat transfer via conduction... note that the code is capable of modeling heat transfer from the narrow gap that may form between the crust and the vessel. - heat transfer from the upper crust to the overlying coolant- heat tranfer from vessel through the surrounding insulation and to the containment cavity (both dry and flooded).
Idaho National Engineering and Environmental Laboratory
• Compare results with and without core catcher and ERVC
Approach
Joy L. Rempe
The approach that we're following to complete this assessment is outlined in this slide.First, a "bounding" transient was selected after completing several full-plant analyses. The "bounding" analysis was selected that maximized the masses of materials that may relocate to the lower head. Mr. Knudson reported results from these analyses at last year's user's group meeting.Second, the FIDAP code and plant dimensions were used to construct a lower head model, which is used for the COUPLE model in S/R5-3D.Third, a base case analyses was completed w/o assuming any core catcher or ERVC enhancements.Fourth, as experimental data become available, the code will be modified. For example, as complete boiling curves become available from PSU for assessing the impact of microporous coatings and the enhanced insulation /vessel configuration, the correlations in the code for a flooded cavity will be modified. Likewise, boiling curves are needed to reflect the impact of the cooling in the narrow gap between the core catcher and the vessel. Fifth, the plant response will be evaluated for each of the options considered in this program.and last, code results for the cases without and with enhancements will be compared to assess the increase in margin due to the proposed modifications.
Idaho National Engineering and Environmental Laboratory
SCDAP (3 channels, 15 axial nodes, with radial crossflows)
Approach
Joy L. Rempe
This slide illustrates the model used to simulate the APR1400 full plant transients. It was actually sent to us from KHNP, but INEEL verified the dimensions used in this model. It is fairly complex ... with 250 RELAP volumes, connected with 316 junctions, and 284 heat structures. The reactor core is simulated with 3 channels and 15 axial nodes that consider radial cross-flow. The model includes a COUPLE mesh to simulate the reactor vessel lower head.
Idaho National Engineering and Environmental Laboratory
1104-GA50005-30
Simplified RELAP5 Lower Head Model
• Lower head dimensions consistent with plant model
• Time-dependent downcomer and core
• TH boundary and initial conditions from plant analysis
• Simplified downcomer / lower head connection
• Suitable for base case calculations
V174
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accumulated corium
vessel
TDV173TDV225sink
coolant
source (downcomer)
vessel cavity
V190
V100
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CL
Approach
Joy L. Rempe
A "simplified" RELAP5-3D model is used for these margin assessments. In these "simplified" models, the lower head dimensions are consistent with values assumed in the full plant transient. Time-dependent downcomer and core volumes are used that interface with the volumes in contact with the COUPLE mesh. Simplified downcome and lower head connections are assumed. The TH and plant transient conditions from the full-plant transient analysis (at the time of relocation) are assumed as initial conditions. Note that this model is only being used for the "base" case analysis and for the cases evaluating ERVC w/o an in-vessel core catcher.
Idaho National Engineering and Environmental Laboratory
• Adjustment of external boundary conditions consistent with ERVC
• Flow paths for engineered (vessel-to-catcher) gap
Approach
Joy L. Rempe
Modifications are needed for cases with an in-vessel core catcher to allow simulation of the narrow gap between the core catcher and the vessel. Furthermore, the ERVC correlations must be modified for cases with microporous coatings on the vessel external surface and the enhanced vessel/insulation configuration, this model can be applied. However, S/R5-3D is an ideal tool for considering these proposed options.
Idaho National Engineering and Environmental Laboratory
This viewgraph contains a diagram of the 2D COUPLE mesh used for the calculations w/o an in-vessel core catcher. It has 588 nodes and 540 elements. The region above the hemisphere (where it becomes cylindrical) was ignored because full-plant transients indicate that relocated materials will remain below this point. In the base case analysis w/o ERVC, calculations assumed an external convective boundary conditions (h=70 w/m2C and Tinf=390 K) and a contact resistance between the crust and the vessel of 500 W/m2C (e.g., narrow gap cooling was neglected). The relocated materials were assumed to form a homogeneous pool.
Idaho National Engineering and Environmental Laboratory
1404-GA50005-30
LOCA-1 Selected as Bounding Lower Head Transient
TransientTime of
relocation
(s)
Relocated constituents (kg) Corium characteristics at relocation
a. Assuming hemispherical configuration, without sensible heat effects, for quasi-steady conditions, with estimated heat loss from upper corium surface at 10 and 90% of total.
Approach
Joy L. Rempe
As I mentioned earlier, relocation masses were taken from a "bounding case" from the full plant transients that Mr. Knudson discussed last year (and the details of this analysis are documented in a May 2004 NED journal article). As shown in this table, a LOCA was selected as the bounding event. Although the masses of UO2 that were predicted to relocate were similar to the SBOs analyzed, the time of the relocation was much earlier. Hence, this case was bounding because its decay power density was higher.
Idaho National Engineering and Environmental Laboratory
1504-GA50005-30
Relocation History Extracted for Bounding Transient
Time (s)Relocated mass (kg) Relocated material
UO2 ZrO2 Zr Steel Absorber Temperature (K) Decay power (MW)
3280
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108,000 5180 3520 95 2350 Relocated mass totals
Lower head structural steel ignored, but will be treated as a sensitivity in COUPLE analyses
Approach
Joy L. Rempe
For the stand-alone COUPLE analyses, the relocation history was extracted from the LOCA1 full plant transient. As shown in this table, absorber materials were first predicted to relocate, followed by structural materials (Zr and SS), and then fuel material and oxidized cladding. As indicated in this viewgraph, lower plenum structural materials were ignored in the base case (to obtain more severe heat loads). However, their impact will be evaluated as a sensitivity study.
Idaho National Engineering and Environmental Laboratory
1604-GA50005-30
Planned Calculations Evaluate Individual and Combined Impact of Proposed IVR Strategies
Calculation
Description
Base case Without in-vessel core catcher and ERVC
CC + ERVC-cEffects of in-vessel core catcher and combined effects of external micro-porous coating and insulation enhancement
CC Effects of in-vessel core catcher
ERVC-cCombined effects of external micro-porous coating and insulation enhancement
ERVC-m Effects of external micro-porous coating
ERVC-i Effects of external insulation enhancement
Approach
Joy L. Rempe
This viewgraph lists the various cases that are planned to be analyzed as part of this task. As indicated in this table, it is planned to perform calculations that assess the individual and combined impact of the proposed options.
Idaho National Engineering and Environmental Laboratory
1704-GA50005-30
Lower Head Fails by Melting Within 15 min in Base Case Analysis Without IVCC or ERVC
2 0 0 0 .0 4 0 0 0 .0 6 0 0 0 .0 8 0 0 0 .0T im e (s)
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M elt tem p era tu re
N o d e 1 6 2 ( in s id e )1 4 11 2 09 97 85 73 6
(o u ts id e)
N o d e N o d e N o d e N o d e N o d e N o d e N o d e 1 5
Initial Results
Joy L. Rempe
The next two viewgraphs shown results for the base case w/o an IVCC or ERVC. As indicated in this plot, the outer surface of the vessel is predicted to reach melting temperatures within 15 minutes.
Idaho National Engineering and Environmental Laboratory
1804-GA50005-30
Lower Head Melting Quick and Extensive in Base Case Analysis
Initial Results
Joy L. Rempe
this is just an extra for the hard copies..
Idaho National Engineering and Environmental Laboratory
1904-GA50005-30
Remaining Evaluations
• Core catcher– Heat transfer correlations for a narrow gap– Countercurrent flow limiting correlations
• ERVC– Heat transfer correlations for a submerged hemisphere with
coatings and enhanced insulation structure
Planned Calculations Require Experimental Data from K-INERI Program
Joy L. Rempe
As I indicated earlier, this is just a status report on this project. The remaining calculations for this task require experimental data from our collaborators. Specifically, we need boiling curves for narrow gap cooling in order to simulate the impact of the IVCC and complete boiling curves for simulating ERVC with microporous coatings and the enhanced insulation and vessel configuration.
Idaho National Engineering and Environmental Laboratory
• Heat transfer correlations (from experimental efforts) needed for simulating core catcher and ERVC.
Joy L. Rempe
So, in summary. We have developed a lower head model for the APR1400 and selected a "bounding transient" for assessing the impact of iVR enhancements.The base case calcualtions are completed. Results indicate that early failure of vessel occurs due to melting and demonstrate that S/R5-3D ideal tool for these assessments.Additional calculations require data from collaborators for simulating narrow gap cooling associated with an IVCC and the heat transfer associated with proposed ERVC enhancements.
