ERMSAR 2012, Cologne March 21 – 23, 2012 Post-test calculations of CERES experiments using ASTEC code Lajos Tarczal 1, Gabor Lajtha 2 1 Paks Nuclear Power Plant 2 NUBIKI Nuclear Safety Research Institute, Budapest

ERMSAR 2012, Cologne March 21 – 23, 2012 Outline Concept of VVER-440/213 reactor vessel external cooling in Paks NPP Introduction of CERES facility ASTEC code, ASTEC model of the CERES facility Post test calculations, comparison with Test 2 results Conclusions 2

ERMSAR 2012, Cologne March 21 – 23, 2012 Background – RV external cooling in VVER Ventillation duct Drain valve Reactor cavity Connection corridor

ERMSAR 2012, Cologne March 21 – 23, 2012 Background – RV external cooling in VVER-440 Natural circulation cooling circuit Problems: Significant uncertainty of heat flux assessment on reactor vessel wall caused by molten pool (surface, heat flux) VVER-440 specific flow channel geometry: The width of the narrowest gap in the flow channel can be 8-10 mm. 4

ERMSAR 2012, Cologne March 21 – 23, 2012 CERES test facility Cooling Effectiveness on Reactor External Surface Aim of experiment: Experimental verification the effectiveness of reactor vessel external cooling Geometry: 1:40 volume ratio of flow channel and elliptic bottom of reactor vessel ( 9° degree section of vessel) Vertical dimensions in 1:1 height rate, because of correct modelling of the natural circulation Main features: Variable minimal gap width in the narrowest cross sections Variable heat flux profile: Varying the amount of heater rods and heater power 5

ERMSAR 2012, Cologne March 21 – 23, 2012 CERES test facility 6

ERMSAR 2012, Cologne March 21 – 23, 2012 CERES test facility Heat flux profile along the reactor vessel wall 7 LOCA 200 mm: Distribution of heat flux at RPV outer surface at different times (x axis represents spread distance from RPV bottom to the top along the surface) Preset heat flux distribution in CERES facility Heat flux profile calculated with ASTEC code Total heat power: 160 kW Heat flux peak occurs right at the elvation of narrowest gap 105 kW (516 kW/m 2 )

ERMSAR 2012, Cologne March 21 – 23, 2012 Summary of the performed experiments 8 1. test2. test3. test4. test Critical gap width [mm]209,7 4,7-9,7 (asymmetric) 3-10,1 (asymmetric) Cross section of critical gap [mm 2 ] Heat power (narrow gap/total) [kW]98/145105/160104/159108/166 Maximum heat flux [kW/m 2 ] Flooding water maximum temperature [C°] Diameter of outlet [mm] test2. test3. test4. test Maximum surface temperature of heater blocks [C°] Permanent surface temperature of heater blocks[C°] Maximum flooding flow rate [liter/hour] The 2. Test is conservative and the most realistic case

ERMSAR 2012, Cologne March 21 – 23, 2012 ASTEC severe accident code 9 CESAR module Modelling the primary and secondary circuits thermal hydraulics 5 equation model Capability of treatment of non- condensable gases (H 2, N 2 ) Basic elements: VOLUME JUNCTION WALL CONNECTI (Boundary conditions) SYSTEMS (system elements)

ERMSAR 2012, Cologne March 21 – 23, 2012 ASTEC model of CERES facility ASTEC v2.0 rev0 5 nodes for elliptic vessel bottom model (V01- V05) 6 nodes for vertical flow channel model (V06- V11) Flooding tank temperature control with heated wall, as user defined boundary condition Electrical heater blocks power defined with „HEAT connection” as boundary condition Modelling the water surface behaviour on atmospheric pressure („open pool”) in the upper region of flow channel and in the flooding tank, the environment node was filled with N 2 gas 10 Narrow gaps

ERMSAR 2012, Cologne March 21 – 23, 2012 Post-test calculations of Test 2 Heater power – boundary condition 11

ERMSAR 2012, Cologne March 21 – 23, 2012 Post-test calculations of Test 2 Inlet flow 12 ~11 min~4 min

ERMSAR 2012, Cologne March 21 – 23, 2012 Post-test calculations of Test 2 Inlet flow 13 Backflow The maximum values of black-flow are higher than in the experiment. Calculated period ~600 s Measured peridod ~500 s

ERMSAR 2012, Cologne March 21 – 23, 2012 Post-test calculations of Test 2 Flooding water temperature – boundary condition 14

ERMSAR 2012, Cologne March 21 – 23, Post-test calculations of Test 2 Water and wall temperatures in bottom part

ERMSAR 2012, Cologne March 21 – 23, 2012 Post-test calculations of Test 2 Water and wall temperatures in bottom part 16

ERMSAR 2012, Cologne March 21 – 23, Post-test calculations of Test 2 Water and wall temperatures in narrowest section The periodic „build up-collapse” of the steam plug shows good agreement with the experiment. Small difference in temperature maximum and minimum values.

ERMSAR 2012, Cologne March 21 – 23, 2012 Post-test calculations of Test 2 Water and wall temperatures in narrowest section 18 The periodic „build up-collapse” of the steam plug shows good agreement with the experiment. Small difference in temperature maximum and minimum values.

ERMSAR 2012, Cologne March 21 – 23, 2012 Post-test calculations of Test 2 Water temperatures in the upper section 19

ERMSAR 2012, Cologne March 21 – 23, 2012 Conclusions The ASTEC calculated values are in a reasonable agreement with the experimental results. The code can correctly model the main physical phenomena in the heated channel; Well predicted tendencies, flow and temperature oscillations; The deviations between the measured and calculated values are in 20% range and can be explained; The VVER-440 reactor vessel wall in the narrowest section can be cooled near stable, °C temperature. 20 Implementation at Paks Unit 1 in 2011:

ERMSAR 2012, Cologne March 21 – 23, 2012 Thank you for your attention! 21