Nathlia N. Arajo, Maritza R. Gual & Marcos C. Maturana Brazil - PowerPoint PPT Presentation

Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in Spent Fuel Pools 2-5 September 2019, Vienna, Austria Nathália N. Araújo, Maritza R. Gual & Marcos C. Maturana Brazil

Sum umma mary ry • PROBABILISTIC SAFETY ASSESSMENT (PSA) • DESCRIPTION OF THE SFP COOLING SYSTEMS (SFPCS) • EVENT SEQUENCES • SPF MODELING WITH MELCOR • RESULTS OF THE SIMULATIONS • CONCLUSIONS AND FUTURE WORK

Probabilistic Safety Assessment (PSA) • Selection of the most frequent accident sequences through PSA level 1 • Preparation of an SFP model in MELCOR • Support the calculation of frequency and release of radioactive material – PSA level 2 – with the accident simulation results PSA Levels At each level the risk associated with the indicated event is analyzed.

SFP COOLING SYSTEMS (SFPCS) Cleaning and Safety Water Purification Cooling System System (CPS) (SWCS) V40 V73 V74 FUEL BUILDING (FB) V4 V5 V56 B1 B2 Compartment T1 V17 FSP-I V11 V7 V9 FE V46 T2 V14 V10 V64 V70 V45 V75 V57 Compartment V71 V52 Channel (TC) P1 Transfer FSP-II V34 V51 V47 V37 V72 V22 V23 V38 Compartment V35 V65 V36 V62 FSP-III B3 B4 FE T3 V25 V27 V31 V63 V28 V53 V44 V76 S2 Sampling Drainage System (SS) System (DS) S3 P2 S1 CONTROL ROOM (CR)

SFPCS OPERATIONAL MODES Operational Cooled Description Mode Compartments Normal operation. Circulation by pump B3. B Cooling by the heat exchanger T2. Degraded operation. Circulation by pump B3. B.1 B. Cooling by the heat exchanger T3. FSP-II Degraded operation. Circulation by pump B4. B. B.2 Cooling by the heat exchanger T2. Degraded operation. Circulation by pump B4. B. B.3 Cooling by the heat exchanger T3.

EVENT SEQUENCES TO BE CONSIDERED IN PSA LEVEL 2 PSA Level 1 results: • SHUTDOWN PSA LEVEL 1 TOP 3 SFP FUEL DAMAGE FREQUENCY CUTSETS – Failure to open valves for Cutset Event Frequency Event Description Number Probability (yr) redundant circuit: first three Plant is shutdown based on 8.22E-2 cases; annual availability Shutdown Phase II, III, IV 7.33E-1 duration – CCF of B3 and B4 to start: 1 Loss of SFPCS 1.00 8.56E-6 fourth case (3.43E-6 / yr); Flag: Spent fuel elements in 1.00 FSP-II. – One pump unavailable due Manual Valve _036_ Failure to 1.42E-4 Open to testing and maintenance Plant is shutdown based on 8.22E-2 annual availability and standby pump failure to Shutdown Phase II, III, IV 7.33E-1 duration start: fifth and sixth cases 2 Loss of SFPCS 1.00 8.56E-6 (5.00E-7 and 4.70E-7 / yr); Flag: Spent fuel elements in 1.00 FSP-II. Manual Valve _034_ Failure to 1.42E-4 Open Plant is shutdown based on The set of abnormal conditions • 8.22E-2 annual availability Shutdown Phase II, III, IV identified in the PSA Level 1 will 7.33E-1 duration be considered for the spent fuel 3 Loss of SFPCS 1.00 3.66E-6 Flag: Spent fuel elements in pool accidents in PSA Level 2. 1.00 FSP-II. Check Valve _032_ Failure to 6.07E-5 Open

MELCOR SFP MODELING • MELCOR 2.2.9541 – SFP – PWR – 3 rings × 12 axial levels (6 for active fuel) – 21 fuel assemblies with UO2 ~5% fuel rods – 48 MW (NPP) ~ time decay heat of 5 days

MELCOR SFP MODELING

MELCOR RESULTS

MELCOR RESULTS

MELCOR RESULTS

CON ONCL CLUSIONS USIONS & FU FUTURE URE WOR ORK • The Shutdown PSA Level 1 of a reference NPP was analysed and a set of accident sequences was selected; • A MELCOR model for SFP was obtained; • A better understand of the LOFA physical phenomena was reached; • The SFP accidents evaluation will be based on the results of these simulations; • Improvement of MELCOR model of the SFP; • The resulting MELCOR model will be useful for the calculation Large Early Release Frequency (LERF) of the reference NPP .

Thank You!