HWR Moderator Sub-cooling Requirements to Demonstrate Back-up Capabilities of Moderator During Accidents HEMANT KALRA NPCIL IAEA International Collaboration Standard Problem (ICSP) 1 st Workshop November, 19-21,2012 Ottawa CANADA
Working Team S. Hajela, Chief Participant Hemant Kalra V.V. Reddy
CONTENTS INTRODUCTION SALIENT FEATURE OF IPHWR ACCIDENT SCENARIO : LOCA & ECCS FAILURE CASE STUDY SUMMARY ICSP INPUT REQUIREMENTS
INTRODUCTION Nuclear Power Corporation of India Limited is a Public Sector Enterprise Under the administrative control of the Department of Atomic Energy (DAE), Government of India. NPCIL is responsible for design, construction, commissioning and operation of nuclear power reactors. NPCIL is presently operating 20 nuclear power reactors with an installed capacity of 4780 MWe. The reactor fleet comprises of 18 Pressurised Heavy Water Reactors (PHWRs) 7 units are under construction including four units of 700 MWe PHWRs
TYPICAL IPHWR 540MW(e) 7
Salient Features of IPHWR with respect to severe accident 392 Pressure tubes Horizontal orientation On power refueling Two independent diverse fast acting shutdown systems 8
Salient Features of IPHWR with respect to severe accident Heavy water coolant Fuel bundles lie inside the pressure tube Calandria tube separates the pressure tube from the cold moderator A gas gap between the pressure tube and the calandria tube provides thermal insulation Short bundles (0.5 meters) 9
LOCA AND ECCS FAILURE During accidents such as LOCA and ECCS Failure, the Pressure tube will Balloon/Sag or strain into contact with the calandria tube. The initial contact between hot pressure tube and cold calandria tube would result in a sudden increase in the heat flux to the moderator. The magnitude of peak heat flux combined with the temperature of moderator surrounding the calandria tube determines the boiling regime on the calandria tube surface and thus the rate of heat removal.
LOCA AND ECCS FAILURE If nucleate boiling is predicted to occur, the stored energy of the pressure tube is quickly removed and the fuel radiates to a low temperature pressure tube. If film boiling is the predicted mode of heat transfer, the stored energy in the pressure tube is not removed quickly. Heat generated in the fuel radiates to higher temperature pressure tube until the calandria tube surface rewets and the situation returns to nucleate boiling condition. NPCIL In-House developed Computer Code CONTACT simulates the Thermo-Mechanical Channel Behavior under such accident conditions.
LOCA AND ECCS FAILURE VOIDING IN CHANNEL FUEL TEMPERATURE INCREASES HEAT TRANSFER FUEL ELEMENT FUEL to PT to CT CT to MODERATOR BY CONDUCTION BY RADIATION BY CONVECTION HIGH TEMPERATURE METAL WATER REACTION D2 GENERATION CREEP BALLOON/SAG PT CT CONTACT HEAT FLUX MODERATOR ACTS AS A HEAT SINK
LOCA AND ECCS FAILURE FUEL CHANNEL MODEL UPPER SECTOR TOP BOTTOM LOWER SECTOR
LOCA AND ECCS FAILURE
HEAT TRANSFER MODELS Conduction Heat Transfer in Fuel Elements Radiation Heat Transfer Between Fuel Pressure Tube (PT) and PT-CT Gas Conduction between PT-CT Contact Conductance Between PT-CT Convective Heat Transfer between CT and Moderator BALLOONING & SAGGING MODEL Thermal Creep models for Sagging & Ballooning
16 HEAT TRANSFER MODELS S.No. Heat transfer Mode Boundaries 1. Natural convention TW <Tsat 2. Pool boiling TSat<TW<T 3. Transition boiling TCHF<TW<Tmin 4. Film boiling TW>Tmin LOCA AND ECCS FAILURE t T c q T k r r r T k r r p δ δ ρ θ δ θ δ δ δ δ δ δ δ = + + " 1 1 2 Conduction in Fuel Elements with Heat Flux at Surface as Boundary Condition Heat Transfer Between Calandria Tube to Moderator
Pressure Tube Deformation Ballooning Incase of LOCA where pressure tube sees high temperature and pressure especially in critical break LOCA Sagging In case of LOCA+ECC failure where the system pressure is low after blowdown 17
Modeling Concept Pressure tube Sagging Calandria Tube Roll Joint Garter spring Pressure Tube Initial Segment Deflection Axial Deformation Strained segment Note: There is a local deformation of pressure tube around each Garter spring 18
LOCA AND ECCS FAILURE METAL WATER REACTION Zr +2D 2 O ZrO 2 + 2D 2 +ENERGY Kinetics of oxidation dφ φ = dt K φ 2 Where K φ = A exp(-q/rt) where φ is mass of Zr, oxidized per unit area, [mg/cm2] Kφ is parabolic rate constant [ mg/cm2]2 S-1 The kinetic data of zircaloy steam reaction is considered for three corresponding temperature ranges 19
Effect of Moderator Temperature The type of deformation and rate of heat up determine the temperature at which the pressure tube contacts the calandria tube. The parameter which determines the boiling regime on the calandria tube when contact occurs is the moderator subcooling. This is because CHF increases with increase in degree of subcooling. The degree of sub cooling affects the time period during which dry out exists. Period of dry-out is one of the detrimental factor for PT-CT failure.
Results Sample case; IPHWR-540MW(e) LOCA + ECC failure + Moderator as a heat sink Using Computer code CONTACT Objective To prove moderator is an effective heat sink Fuel temperature is maintained below melting temperature Channel integrity is maintained so as to prevent loss of cooling/moderator inventory NOTE; During LOCA only one loop i.e. half the core is affected 21
LOCA AND ECCS FAILURE Fuel & PT Temperature In Upper Sector Temperature(c) 1800.00 1600.00 1400.00 1200.00 1000.00 800.00 600.00 CONTACT:SEVERE ACCIDENT ANALYSIS FOR TAPP3&4 LONG-TERM SYSTEM RESPONSE FOR 100% BREAK IN ROH WITH ECCS UNAVAILABLE Max.Rated Bundle of 5500Kw Channel Legend Title Center Bottom Center Top Second Bottom Second Top Third Bottom, Third Top Fourth Bottom Fourth Top Pr.TubeTop Cal. Tube Top 400.00 200.00 0.00 0.00 600.00 1200.00 1800.00 2400.00 3000.00 3600.00 Time(sec.) Figure F.21 a:sheath,pr. Tube and Cal. Tube Temperature Upward Sector HK,RSA 22
LOCA AND ECCS FAILURE Fuel & PT Temperature In Lower Sector Temperature(c) 1800.00 1600.00 1400.00 1200.00 1000.00 800.00 600.00 CONTACT:SEVERE ACCIDENT ANALYSIS FOR TAPP3&4 LONG-TERM SYSTEM RESPONSE FOR 100% BREAK IN ROH WITH ECCS UNAVAILABLE Max.Rated Bundle of 5500Kw Channel Legend Title Center Bottom Center Top Second Bottom Second Top Third Bottom, Third Top Fourth Bottom Fourth Top Pr.Tube Bottom Cal. Tube Bottom 400.00 200.00 0.00 0.00 600.00 1200.00 1800.00 2400.00 3000.00 3600.00 Time(sec.) Figure F.21 b:sheath, Pr.Tube and Cal. Tube Temperature Downward Sector HK,RSA 23
Dry Out Period
Summary Acceptance criteria for LOCA and ECCS failure LOCA AND ECCS FAILURE Channel integrity is to be maintained Channel integrity is maintained if the calandria tube remains intact after pressure tube contacts. This criterion is satisfied if the calandria tube outer surface does not go into prolonged film boiling. Analysis shows that dry-out occurs at few local points of calandria tube for a short period. 25
Requirements to be looked at LOCA AND ECCS FAILURE Relation between Channel integrity & dry out and its time period Note; An experiment is planned at CMRI, India using gas burner as heaters Gap conductance between PT-CT and its time dependency CHF on calandria tube and its effect on dry out period Note; Experiments are planned at IIT, Roorkee to create a data base of CHF on various modified surfaces Slumping of fuel and its effect on conduction 26
ICSP INPUT Requirements 1. Geometric Details 2. Loading Conditions 3. Heat Load 4. Thermal & Physical Properties of Materials 5. Creep parameters 6. Contact Conductance 7. Heat Transfer parameters