Symposium on Risk Integrated Engineering January 21, 2019, Takeda Hall, The Univ. Tokyo Researches on Severe Accident and Risk Engineering

Transcription

1 Symposium on Risk Integrated Engineering January 21, 2019, Takeda Hall, The Univ. Tokyo Researches on Severe Accident and Risk Engineering Koji Okamoto The University of Tokyo

2 Levels of Risk Analysis LEVEL 1 PRA The assessment of plant failures leading to core damage and the determination of core damage frequency(cdf). LEVEL 2 PRA The assessment of containment response leading, together with the results of Level 1 analysis, to the determination of release magnitudes and frequencies. LEVEL 3 PRA The assessment of off-site consequencesleading, together with the results of Level 2 analysis, to estimates of risk to the public. assessment/iv 3_3 Overview of Level 2 PSA(coment1).pdf 2

3 Severe Accident Phenomena Direct containment heating (DCH) RPV Hydrogen Explosion Non-condensable gas Steam Explosion Over temperature leakage Melt attack to CV Steam Explosion Molten corium concrete Interaction (MCCI)

4 March 2, 2011V&V Workshop in Japan Uncertainty Nuclear Power Plant Assessment (Extrapolation) Large Experiment(E) High-accurate Measurement Small Exp. (E) Modeling & Simulation (M&S) Theory / Physics Advanced, 3D

5 Fukushima-Daiichi Severe Accident 5

6 Current status of SA code for Fukushima Huge uncertainty on the simulation Uncertainty for Boundary conditions Amount of water/sea-water supply, depressurization process, heat removal by Tsunami flooding, etc. Uncertainty for Modeling Fuel melting/solidification model Mass/Energy transfer between fuel and structure and so on. Uncertainty for Simulation Model Mesh dependency, Multi-phase/dimensions, Radiation, etc. Uncertainty for Debris Locations Uncertainty for Source Term 6

7 PIRT Project Objectives are Develop Severe Accident Management Program Develop Probabilistic Risk Assessment Tools Understand the 1F accident precisely Estimate the debris distributionin 1F Phenomena Identification and its screening is most important process Ranking Table process depends on the target International Collaboration would be helpful for understanding the phenomena in comprehensive viewpoints

8 1F-PIRT Objective 1 Estimation of the melted corestatusin the three Fukushima-Daiichi NPPs 2 Source term estimation improvement considering Fukushima-Daiichi NPP Accident. Phenomena Identification Based on the knowledge of Severe Accident Researches, the phenomena had been extracted. Discussion by experts, the key phenomena and/or the phenomena with large uncertainty had been selected. Organizer Severe Accident Investigation Committee the Atomic Energy Society of Japan ( )

9 Fukushima-Daiichi PIRT papers N. Sakai, H. Horie, H. Yanagisawa, T. Fujii, S. Mizokami& K. Okamoto, Validation of MAAP model enhancement for Fukushima Dai-ichiaccident analysis with Phenomena Identification and Ranking Table (PIRT), J. Nucl. Sci. Technol., Vol.51, pp (2014) Sakai et al., JNST (2014) S. Suehiro, J. Sugimoto, A. Hidaka, H. Okada, S. Mizokami& K. Okamoto, Development of the source term PIRT based on findings during Fukushima Daiichi NPPs accident, Nuclear Engrg. and Design, Vol.286, pp (2015) Suehiro et al., NED (2015)

10 Sakai et al., JNST (2014)

11 Phenomena H M L K P U H &(P/U) Core Shroud Head Standpipe & separator Dryer Upper Head Main Steam Line Upper Downcomer Lower Dowmcomer Lower Head Recirculation Loop Pedestal Cavity Drywell Drywell head Vent to Wetwell Wetwell Isolaton Condenser R/B Compartments SGTS Operation floor Blowout panel Spent Fuel Pool Equipment Pool TOTAL Sakai et al., JNST (2014)

12 MAAP Improvements Sakai et al., JNST (2014)

13 High Ranked Phenomena for Source Term Radionuclides release from molten fuel MCCI (Concrete erosion) Effects of B4C Chemical form Hydrogen generation Iodine release from R/B contaminated water Migration of radioactive material by the injection water into the reactor Effects of seawater Chemical form Suehiro et al., NED (2015)

14 High Ranked Phenomena Corium relocation Core, downcomer, orifice, lower plenum, penetration,.. MCCI Ablation, Chemical reaction, Multi-phase, Effects of B4C Chemical form, Hydrogen generation Effects of seawater Long-term corrosion, chemical form,. RI behavior in C/V and R/B Deposition, Chemical reaction, Cs Ball, Suehiro et al., NED (2015) Sakai et al., JNST (2014)

15 Core melt and relocation Metallic melt relocation and blockage Various compositions Debris could relocate and freezing in cold zones (observed in TMI-2, LOFT, PBF SFD, Phébus, and CORA experiment, ) located at lower spacer grids or near the water level formation of a crust and even molten pool it can dissolve fuel rods inside the crust and molten pool change the flow pattern in the core coolant flow in region w/o blockage 15

16 Sample Simulation results by SAMPSON Debris void fraction distribution in the core for Unit#1 High-pressure case: keep 8MPa and cooling stops (same as Fukushima) Low-pressure case: depressurize first and cooling stops (SAM strategy) High-pressure case Low-pressure case 16

17 Control rod materials in BWR BWR fuels and control blade Control blade Absorber tube B 4 C powder Stainless Steel BWR uses B 4 C as the absorber material B 4 C is filled in the stainless steel tube [1] World Nuclear Association, Nuclear Fuel Fabrication, (accessed ) 17

18 Early liquefaction due to B4C/SS eutectic reaction Eutectic reaction of B 4 C and Stainless Steel starts at around 1000K Melting temperature of eutectic material is lowerthan the original materials When the temperature exceed 1500K, the eutectic material starts to melt suddenly Melting point B 4 C ~2720K Stainless Steel ~1730K B 4 C/SS eutectic ~1500K Relocation of control blade[3] Control rods can melt firstly among other components. 26/01/

19 Our target region in Fe-B binary phase diagram[4] Eutectic composition is about 17 at%. Temperature [ ] 1174 C ~17 Fe Atomic Percent Boron 26/01/ B

20 Research roadmap Experiment Simulation Application Visualization of the B 4 C/SS eutectic melting and relocation behavior Simulation and code development Application of new knowledge to system codes and clarify the Fukushima Accident.

21 Qualitative comparison Experiment Simulation Initial melting interface Time: s New melting interface Time: s Time: s Time: s 21

22 MCCI Sump Shape and Sump drain Cooling characteristics Depth Width Debris height debris C/V steel wall Invaded MCCI boundary Min. distance 1.02m

23 CASE4 With: Active corium solidus temperature Diffusion Chemical reaction Gas generation The ablation behavior before crust breach is similar as Case1 and Case2. The hot corium will invade the domain of liquid concrete when the crevasse is big enough, and results in a rapid erosion. The mixing process is calculated by the diffusion model. corium Liquid concrete concrete crust 23

24 A RAPID EROSION AFTER THE CRUST BREACH concrete corium crust A rapid erosion take place after considering the diffusion process, which confirmed what observed from the experiment. Once the breach is big enough, the cold light liquid concrete particles would going up and the heavier hot particles would invade into the ablated region, which accelerate the erosion. 24

25 Containment sprays Background: Spray mixes the gas atmosphere Condensation of steam increases hydrogen concentration Death regions far away from the spray might pose a danger because of hydrogen accumulation ST3_1(Steam-Helium) ST3_2(Steam-Helium-Air) Erkan et al, Nuclear Eng. Design 241 (9): (2011) Erkan et al, NURETH-16, Chicago (2015) 25

26 Containment sprays ST3_1(Steam-Helium mixture) ST3_2(Steam-Helium-Air mixture) o Removal of steam creates lighter gas mixture. o Helium-rich mixture fills the Vessel-2 from top-to-bottom. o Removal of steam creates heavier gas mixture. o Helium-rich mixture fills the Vessel-2 from bottom-to-top. 26

27 Droplet impingements on cold/hot surfaces Cold Surface: T surf =27 o C,V=0.41m/s,D o =2.12mm Hot Surface: T surf =250 o C,V=0.42m/s, D o =1.95mm 27

28 Velocity and Temperature Measurements inside a Droplet Background Containment spray droplet phase-change Water droplet Evaporation Water droplet Condensation Surrounding gas High temperature Low saturation ratio Surrounding gas High temperature High saturation ratio Mechanisms: Droplets evaporation Steam condensation on droplets 28

29 Velocity and Temperature Measurements inside a Droplet Experimental Setup Phosphorescence light intensity decay Ø2.2 mm * Q. Zhouet al. Meas. SciTech. 28(7),pp (2017). 29

30 Velocity and Temperature Measurements inside a Droplet Droplet internal temperature No overall temperature gradient on vertical direction or on radial direction Droplet temperature increase slightly, at quite low values Downward internal flow, acceleration near the bottom Marangoni convection 30

31 High Ranked Phenomena Corium relocation Core, downcomer, orifice, lower plenum, penetration,.. MCCI Ablation, Chemical reaction, Multi-phase, Effects of B4C Chemical form, Hydrogen generation Effects of seawater Long-term corrosion, chemical form,. RI behavior in C/V and R/B Deposition, Chemical reaction, Cs Ball, Suehiro et al., NED (2015) Sakai et al., JNST (2014)

32 March 2, 2011V&V Workshop in Japan Uncertainty Nuclear Power Plant Assessment (Extrapolation) Large Experiment(E) High-accurate Measurement Small Exp. (E) Modeling & Simulation (M&S) Theory / Physics Advanced, 3D