Recent Issues in Reactor Thermalhydraulics and Safety

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1 Recent Issues in Reactor Thermalhydraulics and Safety Sanjoy Banerjee Distinguished Professor and Director, The Energy Institute The City College of New York, New York June 2012, Japan

2 Some Current Issues (those discussed in red) Thermalhydraulic-Related Flow/phase distribution development Debris bed and particulate flows Fluid-structure interactions Safety-Related Power Uprates Extended Power Uprate Operation Flow-Induced Vibrations (Dryers and SGs) Generic Long Term Cooling (Debris Effects) High Burnup Thermal Conductivity Degradation Containment Accident Pressure Credit New Reactor Issues (one issue discussed) Hydrogen distribution, control and combustion Containment venting and vent filtration

3 Flow Regime Development

4 PWR LOCA/ECCS: SHORT FLOW DEVELOPMENT LENGTHS

5 Issues and Approaches Short flow and phase distribution lengths Flow regime maps are for very long lengths Effects could be handled by incorporating multiple fields or interfacial area evolution This requires more detailed closures in some cases Insufficient database in on interfacial area evolution, e.g. due to spacers in reflood TRACE peer review recommended four field model incorporation

6 Four field model development - a four-field model, consisting of a continuous liquid layer containing gas bubbles and of a continuous gas layer containing liquid droplets, - closures not adjusted for flow pattern transitions: no use of flow pattern maps - model naturally evolves interfacial area as dispersed area (for bubbles and drops) and continuous interfacial area - drop entrainment/deposition rates and bubble entrainment/disengagement rates are key closures Reference: Bonnizzi et al, IJMF Vol 35, pp 34-46, 2009

7 Typical closure laws

8 Transition prediction: stratified to bubbly flow 9cm pipe: at inlet 1 m/s air velocity,10m/s water velocity, Liquid fraction 0.2, atmospheric pressure. Boundary condition: stratified flow at inlet A portion of the line near the inlet remains in stratified flow, while the rest transitions to bubbly flow. The bubble fraction at 10 s from start of flow is shown on the RHS ordinate. Holdup means fraction of layer consisting of continuous liquid containing entrained bubbles

9 Four field model prediction: flow pattern map

10 Debris Bed Flow Simulation

11 Potential Effect of LOCA Debris on Long-Term Cooling Phenomena affecting screen performance. Blowdown: debris formation. Debris transport and dropout from blowdown and containment spray. Debris accumulation on screens. Effect on residual heat removal and containment spray pumps. Downstream effects. 11

12 The Problem LOCAs lead to jets that erode insulation and coatings within zone of influence (ZOI). Debris formed together with latent debris transport by flow into sumps. During recirculation strainers cover (fully or partially) with fragmented insulation, latent debris, paint chips, erosion/corrosion products. Debris may interact with chemicals in containment. Debris & chemicals accumulation may lead to strainer head loss beyond design basis. May cause recirculation pumps to cavitate, reduce coolant flow and challenge adequate core cooling. Penetration of debris through strainers may affect LTC, e.g. causing flow blockages in fuel assemblies.

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16 Debris Bed Calculation 2-D calculation (actually 3-D code using periodic BC in 3 rd direction Mesh opening 2mm across; blocked section 1.2mm across Particles of radius 0.05mm and 0.1mm Fibres of radius 10 micron at various orientations Velocity 0.2m/s

17 Image Showing Close-Up of Debris Bed

18 Geometry used in Simulation

19 Velocity Vectors

20 Pressure Field

21 Vorticity

22 Streamwise Velocity

23 HEAD LOSS DEBRIS DISTRIBUTION DEPENDENCE Case 1 A Case 1 B Case 2 A Case 2 B Case 2 C Case 4 A Case 4 B Case 4 C Head Loss (in H2O) Case 4 - CalSil added followed by Nukon Case 1 Nukon bed formed then CalSil added Screen Approach Velocity (ft/s) Case 2 - Premixed Nukon/CalSil Constituent debris debris loading loading sequence sequence affects pressure affects drop pressure drop.

24 Extended Power Uprate Operation: BWRs

25 Implemented and Proposed Changes Changes: EPU; Expanded Domains; Cycle Length Extension (24 month) Outcome: Changes in core design, 24% higher steam flow, with unchanged steam and bypass capacity

26 Some Changes: BWR EPU & MELLLA + Core Design Core Operating Parameter Changes Higher Batch Fraction (>40%) Higher Bundle Powers More Maximum Powered Bundles (flatter core) Higher Core Average Void (from 40% to 70%) Maximum Channel Exit Void in 90%range Plant Condition Higher Steam Flow (Fixed SRV and Bypass Capacity) Higher Decay Heat Higher feedwater flow Reduced flow window, without expanded domain Operation at higher rodline (expanded domain)

27 Impacts of Uprate and Expanded Domain Anticipated Plant Response Changes Thermal Limits Margin ATWS Limiting Vessel Overpressure Transient (ASME) ECCS-LOCA Safety System Performance (COP) addressed later Instability (Expanded Domain) Other Significant Impacts Steam Dryer

28 EPU & MELLLA + brings plant closer to stability boundary: AOOs and ATWS instability issues 120% MELLLA+ Line 100% Stability Boundary MELLLA Line OLTP Line Core Power (% OLTP) 80% 60% 40% MELLLA+ MELLLA OLTP 20% Operating Point Following a Two-Pump RPT 0% 0% 20% 40% 60% 80% 100% Core FLow (%rated)

29 Flow Induced Vibrations: BWR Steam Dryer Performance in Extended Power Uprates

30 Image 1: Steam Dryer:

31 Typical Example of Core Parameters Changes BWR EPU Parameter OLTP EPU Thermal power (MW) Vessel Steam Flow (Mlb/hr) Full Power Core Flow Range Mlb/hr 36.0 to to 51.4 Full Power Core Fow Range % Rated 75.0 to to Max.Operating Dome Pressure (psia) Max. Operating Dome Temperature(F) Pressure at upstream side of Turbine Stop Valve (psia) Full Power Feedwater Flow (Mlb/hr) Core Flow Temperature (F) Core let Enthalpy (Btu/lbm)

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33 Steam Dryer Performance Steam and FW flow increases with EPU (~20%) Steam dryer fatigue failure due to flow induced vibration Quad Cities Failure Low frequency, fatigue loads Due to flow induced vibration of ERV actuators and SRV standpipe Dryer Acoustic loads Plant-unique configuration and response Installation of strain gages on main steamlines Installation of instrumentation on some dryers

34 Steam Dryer Performance Challenges Dryer acoustic loads extrapolated from main steamline strain gage data Newly developed acoustic models need benchmarking for wide range of plant configuration-unique responses Models rely on Quad Cities configuration-unique data and small scale testing data (SMT) Acoustic loads at the uprate conditions projected, using the SMT model (bump up loads) Reliance on Careful and stepwise power ascension testing Available margins to calculated loads Increased uncertainties and biases

35 Containment Accident Pressure (CAP) Credit

36 Schematic : OVERVIEW: BWR/4 MARK I Design ECCS & RHR (LOCA Scenario) core spray LPCI drywell wetwell Strainer 23

37 NPSH and CAP Credit

38 CAP CREDIT ISSUES Reliance on CAP credit impacts defense-in-depth Importance of CAP issue merits renewed scrutiny of calculation tools, key input parameters, conservatisms and scenarios assumed as most limiting Need for generic methodology for calculating CAP for all scenarios, with uncertainty treatment.

39 NEW REACTOR: NO HIGH HEAD SAFETY INJECTION

40 Rapid secondary side depress. issue SG acts as significant heat sink to depressurize the RCS for SBLOCA Delayed high head injection or lack of high head injection Reflux condensation (perhaps for extended period) CCFL during refluxing still requires better understanding, particularly at hot leg bend Of interest for new reactors with changes in high head injection strategy (also for PWR EPUs: checks required as to whether EPU hot leg steam flows increase to conditions close to CCFL limits)

41 Liquid is held up in the riser ΔH1 ΔH2 Reflux Condensation: Clarifies how liquid holdup on riser side of SG can depress core level

42 PWR: THE REFLUX CONDENSATION HL FLOODING POTENTIAL (Vallee et al. 2008, FZK Dresden)