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Analysis of Representative DEC Events of the ETDR with RELAP5

LEADER Project: Task 5.5. Analysis of Representative DEC Events of the ETDR with RELAP5. G. Bandini - ENEA/Bologna LEADER 5 th WP5 Meeting JRC-IET , Petten , 26 February 2013. Outline. Analysed DEC transients at EOC Transient r esults Conclusions. Analyzed DEC transients at EOC.

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Analysis of Representative DEC Events of the ETDR with RELAP5

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  1. LEADER Project: Task 5.5 Analysis of Representative DEC Events of the ETDR with RELAP5 G. Bandini - ENEA/Bologna LEADER 5th WP5 Meeting JRC-IET, Petten, 26 February 2013

  2. Outline • Analysed DEC transients at EOC • Transient results • Conclusions

  3. Analyzed DEC transients at EOC Main events and reactor scram threshold UNPROTECTED PROTECTED

  4. TR-4: Reactivity insertion (UTOP) (1/2) • ASSUMPTIONS: • Insertion of 250 pcm in 10 s without reactor scram • No feedwater control on secondary side • Fuel-clad linked effect  fuel expansion according to clad temperature (closed gap) Total reactivity and feedbacks Core and MHX powers • The reactivity insertion is mainly counterbalanced by Doppler effect  initial core power rise up to 680 MW 4

  5. TR-4: Reactivity insertion (UTOP) (2/2) Core temperatures Core temperatures • MAIN RESULTS: • Maximum clad temperature remains below 650 °C • Maximum fuel temperature of 2930 °C at t = 57 s (hottest FA, middle core plane, fuel pellet centre) exceeds the MOX melting point (~2670 °C)  only local fuel melting  no extended core melting 5

  6. T-DEC1: ULOF transient (1/2) • ASSUMPTIONS: • All primary pumps coastdownwithout reactor scram • No feedwater control on secondary side • Fuel-clad not-linked effect  fuel expansion according to fuel temperature (open gap) Active core flowrate Core and MHX powers • Natural circulation in the primary circuit stabilizes at 23% of nominal value • Core power reduces down to about 200 MW due to negative reactivity feedbacks 6

  7. T-DEC1: ULOF transient (2/2) Core temperatures Total reactivity and feedbacks Core temperatures • MAIN RESULTS: • Initial clad peak temperature of 764 °C • Max clad temperature stabilizes below 650 °C • No clad failure is expected in the short and long term • No vessel wall temperature increase 7

  8. T-DEC3: ULOHS transient (1/2) • ASSUMPTIONS: • Loss of feedwater to all MHXs without reactor scram • Startup of DHR-1 (3 out of 4 IC loops ofin service) • No heat losses for the external vessel wall surface Core and MHX powers Core and vessel temperatures • Core power progressively reduces down towards decay level • Maximum clad and vessel temperatures rise up to 700 °C after about one hour 8

  9. T-DEC3: ULOHS (2/2) Total reactivity and feedbacks Core temperatures • MAIN RESULTS: • No fuel rod clad rupture is expected in the medium term • No vessel failure is expected in the medium term (to be verified) • Enough grace time is left to the operator to take the opportune corrective actions and bring the plant in safe conditions in the medium and long term 9

  10. T-DEC4: ULOHS+ULOF transient (1/2) • ASSUMPTIONS: • Loss of feedwater to all MHXs and all primary pumps without reactor scram • Startup of DHR-1 (3 out of 4 IC loops of in service) • No heat losses from the external vessel wall surface Active core flowrate Core and MHX powers • Natural circulation in primary circuit reduces down to very low value (around 1%) • Core power progressively reduces down towards decay level 10

  11. T-DEC4: ULOHS+ULOF transient (2/2) Core temperatures Total reactivity and feedbacks • MAIN RESULTS: • Max T-clad rises up to 800 °C after about 15 minutes and stabilizes around 825 °C  no fuel rod clad rupture is expected in the short and medium term (to be verified) • No vessel failure is expected in the medium and long term • Enough grace time is left to the operator to take the opportune corrective actions and bring the plant in safe conditions in the medium and long term 11

  12. TO-3: FW temp. reduction (1/2) Active core flowrate (short term) • ASSUMPTIONS: • Loss of one preheater (FW temperature from 335 °C down to 300 °C in 1 s) + all primary pumps coastdown • Reactor scram at t = 2 s on low primary pump speed signal • Startup of DHR-1 (4 IC loops in service) Core temperatures Active core flowrate (long term) 12

  13. TO-3: FW temp. reduction (2/2) Core decay and MHX powers Primary lead temperatures • MAIN RESULTS: • Power removal by 4 IC loops of DHR-1 system is about 7 MW • No risk of lead freezing after DHR-1 startup • Lead freezing at MHX outlet is reached after about 2 hours (cold lead at the MHX outlet flows to the core inlet without mixing with the hotter lead of the cold pool surrounding the MHXs) 13

  14. TO-6: FW flowrate + 20% (1/2) Active core flowrate (short term) • ASSUMPTIONS: • FW flowrate increase of 20% + all primary pumps coastdown • Reactor scram at t = 2 s on low primary pump speed signal • Startup of DHR-1 (4 IC loops in service) Core temperatures Active core flowrate (long term) 14

  15. TO-6: FW flowrate + 20% (2/2) Core decay and MHX powers Primary lead temperatures • MAIN RESULTS: • Power removal by 4 IC loops of DHR-1 system is about 7 MW • No risk of lead freezing after DHR-1 startup • Lead freezing at MHX outlet is reached after about 2 hours (cold lead at the MHX outlet flows to the core inlet without mixing with the hotter lead of the cold pool surrounding the MHXs) 15

  16. T-DEC6: SCS failure (1/2) Secondary pressure • ASSUMPTIONS: • Depressurization of all secondary circuits at t = 0 s (no availability of the DHR) • Reactor scram at t = 2 s on low secondary pressure Core and MHX powers Primary lead temperatures • Initial MHX power increase up to 850 MW  no risk for lead freezing 16

  17. T-DEC6: SCS failure (2/2) Core decay and MHX powers Core and vessel temperatures • MAIN RESULTS: • No risk for lead freezing in the initial transient phase • Slow primary temperature increase due to large thermal inertia of the primary system (effective mixing in the cold pool surrounding the MHXs)  large grace time for the operator to take opportune corrective actions 17

  18. TDEC-5: Partial FA blockage • ASSUMPTIONS: • Total ΔP over the FA = 1.0 bar • ΔP at FA inlet = 0.22 bar • Partial flow area blockage at FA inlet • No heat exchange with surrounding FAs • MAIN RESULTS: • 75% FA flow area blockage  50% FA flowrate reduction • 85% blockage  T-max clad = 700 °C • No clad melting if area blockage < 95% • Fuel melting if area blockage > 97.5% • 50% inlet flow area blockage can be detected by TCs at FA outlet 18

  19. Conclusions • The analysis of DEC transients with RELAP5 code has highlighted the very good intrinsic safety features of ALFRED design thanks to: • Good natural circulation characteristics, • Large thermal inertia, and • Prevalent negative reactivity feedbacks • In all analyzed transients there is no risk for significant core damage or risk for lead freezing  large grace time is left to the operator to take the opportune corrective actions and bring the plant in safe conditions in the medium and long term • The RELAP5 results for unprotected transients (UTOP, ULOF, ULOHS and ULOHS+ULOF) are confirmed by the results of the analyses performed with the CATHARE code 19

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