ERMSAR 2012, Cologne March 21 – 23, 2012 MELCOR Severe Accident Simulation for a “CAREM-like” Integral Reactor M. Caputo, J. M. García, M. Giménez, S. Sánchez Nuclear Safety Group Comisión Nacional de Energía Atómica (CNEA) Centro Atómico Bariloche - Argentina

ERMSAR 2012, Cologne March 21 – 23, 2012 Contents “CAREM-like” reactor description MELCOR input deck description LOHS Modelling hypotheses LOHS description Conclusions 2

ERMSAR 2012, Cologne March 21 – 23, 2012 “CAREM-like” reactor description: RPV Integrated primary cooling system by natural circulation Large volume/power ratio 12 vertical “once through” helical-coil type SGs Self-pressurization Safety systems relying on passive features. Pressure: MPa “Hot leg” temperature: 326ºC (saturation) “Cold leg” temperature: 285ºC Core mass flow: 410 kg/s 3

ERMSAR 2012, Cologne March 21 – 23, 2012 MELCOR input deck: Primary system Model: MELCOR Simplified input deck: 8 CV SG heat removal was represented as an external energy sink (secondary system was not modeled) Fuel elements, absorbers and other core components were modeled with COR package 4

ERMSAR 2012, Cologne March 21 – 23, 2012 MELCOR input deck: COR package 5 Axial nodalization Radial nodalization 19 levels 14 levels corresponds to active zone 6 rings 5 inner rings corresponds to active core

ERMSAR 2012, Cologne March 21 – 23, Passive Residual Heat Removal System (PRHRS) pools Second Shutdown System and Ventilation room (RSSEyV) Upper Dry well (RSS) Peripherical Dry well (RSP) Central Dry well (RSC) Lower Dry well (RSI) Suppresion pool (PS) “CAREM-like” reactor description: Containment

ERMSAR 2012, Cologne March 21 – 23, 2012 “CAREM-like” MELCOR input deck: Containment 7

ERMSAR 2012, Cologne March 21 – 23, 2012 Modelling hypotheses: Total Loss Of Heat Sink 8 Initial reactor condition: Normal operation, 100 MW th Safety Systems: First Shutdown System success and failure of core cooling and injection safety systems. Hydrogen mitigation systems: They have not been modeled. Heat transfer between RPV and containment: RPV heat structures have been modeled as adiabatic. Heat transfer between containment and atmosphere: The containment has been considered adiabatic. Initiating event: Total loss of heat removal through SG in t = 0. RPV Safety Valves (discharge inside Suppresion Pool): Only 1 of the 2 has been modeled. The valve opens at 14.0 MPa and closes at 13.3 MPa. No valve failure is assumed. Postulated instrumentation tube break: This tubes are connected to the dome of the RPV. The break has been modeled at different times due to the uncertainty related with this event. In this case, the break was postulated at 26:24 h.

ERMSAR 2012, Cologne March 21 – 23, RPV Pressure and temperature 0:27 h First safety valve opening (P = 14 MPa). 0:00 h Loss of cooling by SG. RPV temperature homogenization LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, Containment pressure and temperature 0:27 h First safety valve opening LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, RPV level and flow mass 0:37 h Primary system saturation 2:01 h Dome emptying 0:27 h First safety valve opening 7:00 h Core uncovery LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, K/s RPV and cladding temperature 7:00 h Core uncovery 9:30 h T = 1000 K LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, RPV level and flow mass 9:40 h Water level in the bottom of the core barrel 11:10 h Establishment of steam natural circ. 9:40 h Beginning of steam natural circulation 14:05 h RPV emtpying LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, K/s 7.2 kg/h 2.2 kg/h Cladding temperature and hydrogen mass 19:10 h Clad temp = 1100 K 14:05 h RPV emtpying 19:10 h Clad temp = 1100 K Temperature homogenization LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, RPV Pressure and temperature 26:24 h Instrumentation tube break LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, K/s 60 kg/h Cladding temperature and hydrogen mass 26:24 h Instrumentation tube break 27:10 h Begginning of core relocalization – End of simulation LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, Containment pressure and temperature 26:29 – 26:36 h Two deflagrations in Central Dry Well 26:24 h Instrumentation tube break 26:29 – 26:36 h Two deflagrations in Central Dry Well Max. temp.: approx 1350 K LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, RSCyP ternary diagram LOHS sequence description

ERMSAR 2012, Cologne March 21 – 23, 2012 Conclusions 19 The core uncovery begins in 7 h, a time period greater than that another reactors, because the high water inventory/power ratio. The steam natural circulation is very important because it can delays the oxidation process. For this reason, to understand and model properly this phenomenon is needed in CAREM-like reactors. The Suppression pool has an adequate safety margin respect of saturation at the end of simulation. The combustion in the dry-well does not generate a dangerous peak pressure. In a next stage, to complement these results with parametric and sensitivity studies and particular phenomena detailed analysis (lower head failure, suppression pool behavior, hydrogen stratification, radionuclides distribution, radiation heat transfer, etc., is needed.

ERMSAR 2012, Cologne March 21 – 23, 2012 Thank you for your attention! 20