ERMSAR 2012, Cologne March 21 – 23, 2012 ESTIMATION OF THERMAL-HYDRAULIC LOADING FOR VVER-1000 UNDER SEVERE ACCIDENT SCENARIO Barun Chatterjee 1, Deb Mukhopadhyay 1, Hemant G. Lele 1, Pavlin Groudev 2 1 BARC, Mumbai (India) 2 INRNE, Sofia (Bulgaria)

ERMSAR 2012, Cologne March 21 – 23, 2012 Study Objective 2 Structural components like SG tubes, RCP seals, hot leg, Pressuriser Surge line and RPV, Different valves of VVER1000 are likely to experience high temperature and pressure under postulated severe accident conditions arising from SBO ( a high pressure event among severe accident scenarios) These loading may fail the components These failure estimation will alter the course of the accident * System depressurisation followed by less inventory/steam available for hydrogen generation * converting the possibility of high pressure melt ejection into a low pressure melt ejection scenario in the Containment. The change in scenario will alter the load on the ultimate barrier As severe accident analysis are based on realistic models, hence a realistic predictions will help to plan SAMG for Core and containment in a better way

ERMSAR 2012, Cologne March 21 – 23, 2012 VVER-1000 (V320) RCS Layout 3 Hot leg Surge Line Pump Seal SG tubes RPV

ERMSAR 2012, Cologne March 21 – 23, 2012 Station Black Out (SBO) ANALYSIS 4 Following assumptions have been made for SBO simulation 1.Transient is initiated with Station Blackout 2.MCPs and turbine trip at 0.0 s due to SBO 3.Reactor trips at 1.6 s. due to loss of power 4. Feed pumps trip at 5 s due to turbine trip. 5.Pumped Safety Injection systems are assumed to be not available due to SBO

ERMSAR 2012, Cologne March 21 – 23, 2012 COMPUTER CODE ASTECV1.3rev3p1 AND VVER1000 -PLANT MODEL 5 ParametersDesign Value Steady State value Reactor Power (MW) RCS Pressure (Mpa) SG Pressure (MPa) Coolant Temperature at reactor inlet (K) Coolant Temperature at reactor outlet (K) Coolant Flow per loop (kg/s) Feed flow per SG (kg/s) Initial Inventory RCS Inventory (t.)245.5 UO2 inventory in core (t.)73.12 Zr inventory in core (t.)21.6 B4C inventory in core (kg)369.8 Total Steel Inventory (t.)263.0 Decay HeatEnd of life

ERMSAR 2012, Cologne March 21 – 23, 2012 REACTOR BEHAVIOUR UNDER SBO EVENTTime (s) SBO0.0 MCP #1,2,3,4 trip0.0 Turbine trips0.0 Reactor trips1.6 Feed pumps stop5.0 Beginning of oxidation 14,114.0 Start of FPs release17,660.0 Total core uncovery25,116.0 First corium slump in vessel lower head 25,595.0 Lower head vessel failure 48, SG Boil off SG Inventory depletion Loss of SG as a Heat Sink RCS Pressurisation Opening of PRZ relief valve RCS Boil off RCS Inventory Loss Core Heat up and Degradaton Vessel Failure

ERMSAR 2012, Cologne March 21 – 23, 2012 ASSESSMENT OF RCS AND SURGE LINE PIPE INTEGRITY 7 Material of construction of Reactor Coolant System and Surge line for VVER-1000 (V320) is 10 GN2 MFA steel (Dn-350) High Temperature creep model for this material is not available in Open Literature, Hence high temperature creep model for SS316 [R. M. Goldhoff ] for temperature range of K has been used for assessment Ref. 1. F. R. Larsen and J. Miller, “a time temperature relationship for rupture and creep stress, Transaction of the ASME, July 1952, pp R. M. Goldhoff, “A Comparison of Parameter Methods for Extrapolating High Temperature Data”, ASME Journal of Basic Engineering, 1959, pp

ERMSAR 2012, Cologne March 21 – 23, The creep rupture time correlation for SS316 material is given as follows ASSESSMENT OF RCS AND SURGE LINE PIPE INTEGRITY

ERMSAR 2012, Cologne March 21 – 23,

ASSESSMENT OF RCS AND SURGE LINE INTEGRITY 10 Time Required to Rupture Once the Component remains at a sustained high temperature : Hot Leg : hrs [850 mm, 22 ksi (153 MPa)] Surge Line : 0.11 hrs. (360 s) [400 mm, 14.8 ksi (108 MPa)] Surge line fails after 360 s once it attains and remained at a Temperature higher than 1089 K : Hot Leg

ERMSAR 2012, Cologne March 21 – 23, 2012 Conclusion 11 Analysis shows failure of Surge line (35,360 s) prior to Reactor Vessel rupture from lower plenum creep (48,678 s) SG tubes, hot leg, pump seal are unlikely to fail by thermal creep as they remain at a lower temperature Surge line rupture prior to vessel rupture at a high pressure event will turn the event into a low pressure event This situation will cause less severity to the containment as high pressure melt ejection from lower head will not take place

ERMSAR 2012, Cologne March 21 – 23, 2012 Comparison between ASTECV1 and ASTECV2 EVENTTime (s) ASTECV1.3rev3p1 Time (s) ASTECV2.0r2p1 SBO0.0 MCP #1,2,3,4 trip0.0 Turbine, Reactor, Feed pumps trip0.0, 1.6 s, 5.0 Beginning of oxidation14, ,058.5 Start of FPs release17, ,679.6 Total core uncovery25, ,617.9 First corium slump in vessel lower head25, ,516.5 Vessel failure48, , Large Discrepancy In code (V1 –V2) predictions CORE DEGRADATIONASTECV1ASTECV2 Corium mass in the lower head (te) H2 mass produced during the in-vessel phase (kg) Aerosols mass produced during the in-vessel phase (kg) Aerosols mass in containment at vessel failure (kg)

ERMSAR 2012, Cologne March 21 – 23, 2012 Comparison between ASTECV1 and ASTECV2 13 ASTECV1 ASTECV2

ERMSAR 2012, Cologne March 21 – 23, 2012 Comparison between ASTECV1 and ASTECV2 14 ASTECV1ASTECV2

ERMSAR 2012, Cologne March 21 – 23, 2012 Comparison between ASTECV1 and ASTECV2 15 ASTECV1 ASTECV2 Voided core No radial spread radial spread (Magma model )

ERMSAR 2012, Cologne March 21 – 23, 2012 Comparison between ASTECV1 and ASTECV2 16 ASTECV1 ASTECV2 Creep Correlation Range

ERMSAR 2012, Cologne March 21 – 23, 2012 Comparison between ASTECV1 and ASTECV2 17 Surge Line ASTECV1 ASTECV2 Surge Line Failure Time : 0.11 hrs Surge Line Failure Time : 80 hrs

ERMSAR 2012, Cologne March 21 – 23, 2012 Conclusion 18 In both versions of ASTEC, loss of RCS and SG inventory are comparable. Core heat up and material relocation is faster in case of ASTECV2, hence a large extent of anomalies are observed in core degradations parameters like hydrogen, corium relocated mass and aerosol generation AGAINST V1 predictions In case of ASTECV1, The surge line failure was predicted before vessel failure. But in case of ASTECV2, th ere was no surge line failure prior to vessel failure. Suggestion to SARNET: Benchmark exercise among SARNET partners is strongly suggested for VVER-1000 SBO case to eliminate user effect, as the SA analyses are used for SAMG verification and Level-2 calculations.

ERMSAR 2012, Cologne March 21 – 23, 2012 Thank You 19