1NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Lessons Learned from InterimVolunteer-Plant (VP) ECCS

Vulnerability Assessment

Bruce LetellierDesign Safety and Risk Analysis Group

Los Alamos National Laboratory

NEI PWR Sump Performance WorkshopBaltimore, MDJuly 30-31, 2003

2NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Overview

• Objectives of Volunteer-Plant Analysis• Plant Description• Sump-Screen Head Loss• Pool Transport• Blow Down/Wash Down Debris Transport• Debris Generation• Break Location• Current Insights

3NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Objectives of Study

• Volunteer Plant study integrates all phenomenology infoand analysis methods using best available plant specific data

• Illustrates one possible implementation of the Reg Guide• Provides NRC a detailed standard of comparison for

reviewing future submittals and NEI ground rules• May provide a template for content of plant assessments but

with exaggerated detail needed for methodology insight• Sets expectations for conservatism and application of data• Will address all major accident scenario components

BUT… will not analyze all industry conditions/configs• ‘Best Available’ info will still require approximation and

engineering judgment. Will improve as conditionassessment and further head-loss analyses are completed

4NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Required Plant Information

• Water balance calculations– Return flow locations and rates– Minimum pool depths for various break scenarios– ECCS flow rates for various break scenarios

• ECCS pool geometry– Flow velocity calculations– Identify dead sumps that can trap debris during fill up– Scope pool dynamics (regimes of fill up, spray return, steady-state)

• Piping layout and insulation applications by type• Sump-screen geometry• Plant cleanliness characterization (Latent Debris)

5NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

VP Geometry Features

• Large dry containment (133 ft diam), four loop, ‘remote’ sumps• Raised steam generator compartments

– Continual falling water sweeps compartment floor– Compartment opposite break cannot accept debris during fill phase– No damping of falling water. Momentum directed to annulus

• Two adjacent roughly equivalent sump cages (260 ft2 total)– Very close to one steam generator compartment outlet

• Sump-screen curb (4 in)– Effective at stopping RMI debris unless severe piling occurs– Reduces effective pool depth

• Nonsubmerged vertical sump screens 4.75 ft above curb– Failure criteria ~1/2 pool depth above curb (ft H2O)

• Fall height from upper level drains ~10 ft• Spray return drains adjacent to vertical sump screens• Reactor cavity access has curb and partial steel-plate cover

6NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Sump Pool Plan View

7NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Sump Cages

8NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Sump-Screen Construction

9NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

SG Compartment Entry

• Door closed during operation byradiation safety procedure

• Intermediate debris trap or waterflow blockage?

• Both compartment doorsmust block to prevent flow yetlarge fraction of total debrismay pass through these doors

• Comparison of compartmentfill up rate may show thatstructural failure loads arereached before switch over

10NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Steam Generator Compartment

RHRpiping

Loop 4roomdoor

Loop 4SG

supportlegs

Loop 4 RCPump

supportleg

11NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Floor Drains and Curbing

12NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Concrete Structures

CAD modelconstructed by

tracing plan view ofeach level

13NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Piping and Equipment Model

14NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Ranges of Sump Flow

0 5000 10000 15000 20000 25000

Total Sump Flow (gpm)

0

1

2

3

4

5Po

ol H

eigh

t (ft)

LLOCA (Brk)

LLOCA (CS+Brk)

SLOCA (Brk)

SLOCA (CS+Brk)

MSLB (Brk)

Spray Flow = ~15,200 gpmwhen running

15NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Head-Loss Vulnerability Assessment

Boxes Represent Available Debris Based on Parametric Evaluation Model

(Min-K Included but Not Latent Fibers)

Plant-Specific‘h’ Curves

Thin-Beds

16NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Containment Pool Flow Analysis

Volume fraction at 90 s and 0.1 m height Velocities typical during fill up

• Fluent fill up calculation with 7400 gpm break in upper left quadrant

• CS return cascades begin to hit pool at about 90s. Difficult to compute

17NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Estimated Spray Return Cascades

325

768

1021

849849

804

58

1345

130

150

700

520Falling Down Annular Gap

Floor Drains from Level 832

Flow Rates in GPM

397 GPM1743 GPM2030 GPM

Level 808 Sprays – 1080 GPM

730

Total Spray Flow = 15220 GPM

1021

Containment Liner FilmFlow of 700 GPM Uniformly Distributed

18NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Observations Regarding CFD

• Good qualitative agreement between CFD models offill/steady-state velocities and Tank Experiments

• Ancillary sources representing containment spray returnpaths can dominate pool activity

• Quantitative flow maps provide access to an approximate,yet tractable estimate of transport fraction

– Logic maps and engineering judgment will be needed to considerfractions and characteristics of debris returned to the pool viavarious paths

• Uncertainties in location and timing of debris entering poollimit the need for a high fidelity model of debris transport

Area > threshold velocity proportional to degradation andtransport for initial uniform distribution of fill-up phase debris

19NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Transitional Pool Flow SequenceEvent

Break Occurs

Sprays Trip

Max Spray Return

Lower Sumps Full

Characteristics

Jet impingement, steam expansion, water to bare floor withsheet flow directed away from break. Highesttransport velocities. Initial deposition pattern in deadareas and sumps.

Spray runoff accumulates and washdown begins, Sheetcover complete. Sumps fill via directed flow.Deposition pattern modified by splash zones

Maximum energy in minimum pool depth (~inches). Nodirected flow. Pool begins to fill. Max degradation.Pseudostable deposition pattern develops.

Directed flow begins to develop. Deposition patternmodified in vicinity of sump. Suspended debriscollected very quickly. Steady-state flow patternestablished.

Dead sump sheltering is only significant sequester

20NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Key Transport Test Observations

• Cal-Sil and fiber were able to form a thin bed on a ¼-inmesh vertical screen at nominal approach velocity

• Fiber flocks that enter turbulent splash zones are effectivelyshredded to transportable sizes

• Individual fibers are suspended and continue to collect formany hours

• Shear forces between higher and lower pool velocity zonesmay be capable of slowly degrading piles of fiber flocks

21NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Containment Airborne/WashdownDebris Transport Logic Chart

Break Debris Size Volume Region Surface Orientation Exposure Primary Capture Secondary

CaptureDebris Sump

Pool

Fines

Other

Capture

Condensate CaptureTransported

Transported Small Debris

CaptureRegion j

CaptureSmall Floor Spray Transported

Transported Small DebrisZOI Debris

Eroded Transported Fine Debris

Capture

Drainage CaptureTransported

Transported Small Debris

Region j+1

Large

Intact

22NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Two-Phase Debris Generation

OPG Test Conditions•Tank Capacity = 2.2 m3

•Nozzle Size = 2.87 in.

•Initial Pressure = 1400 psia

•Initial Temp = 311oC

23NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Survey of Break Locations

24NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Integrated Vulnerability Assessment

DebrisGeneration

DebrisTransport

DebrisAccumulation

SumpPerformance

ZOI CADSimulations

CFD Simulation and Testing

Head-LossAnalysis

Systems Analysisand PRA

Plant-SpecificConditions andConfiguration

T/H models

AccidentConditions

25NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Required Skill Set

• Familiarity with containment– Visual understanding of spray and floor water flow paths

• Understanding of water levels and pump-flow rates asrelated to EOPs

• Competent application of BLOCKAGE or otherimplementation of NUREG/CR-6224 head-loss correlation

– All plants should start by understanding current sump vulnerability• Understanding of ZOD correlations to scope break locations• Knowledge of applied insulation types and ability to

query/manipulate electronic spatial information– CAD models desirable, but not critical

• Awareness of debris generation and head-loss data– Identify unique materials and plan for characterization

26NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Methodology Insights

• Design/adapt screens to defeat thin-bed formation– Transverse bulk and inlet flows to sweep or ‘self-clean’ surface

• Stacked disks, crenulated plates, etc.– Complex filter surface to fragment fiber layer

• Can mitigate to protect against large debris volumes– Reducing insulation volume– Increase screen area with compact high surface modifications– Intermediate gates at pool level– Divert fill-up flow towards dead sumps/cavities

• Always maximize pool depth– Especially important for nonsubmerged screens– Run sprays for breaks of all sizes?

• Special attention to cleanliness at pool level for small break/no spray• Fill-up retention in dead sumps is perhaps the only important pool-

transport reduction factor

27NEI PWR Sump Performance WorkshopJuly 30 – 31, 2003

Mitigation Strategies

• Submerge screens without compromising area– Utilizes full NPSH margin of mechanical pumps

• Avoid horizontal screens below grade• Test and approve back-flush/throttle cycles to dislodge compacted

debris• Midstream debris screens to intercept steady-state flow channels• Plant cleanliness programs• Modification of insulation types

– With due care not to increase resident loading• Active mechanical sweep and collect concepts• Innovative porous media designs on top of existing screens• Multiple inclined screen surfaces that fall away to expose new area

Preserve Integrated Safety Plan!