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Insights and lessons learned from Level 2 PSA for Bohunice V2 plant MACIEJ KULIG

Insights and lessons learned from Level 2 PSA for Bohunice V2 plant MACIEJ KULIG ENCONET Consulting, Ges. m. b. H., Auhofstrasse 58, 1130 Vienna, Austria. International Workshop on Level 2 PSA and Severe Accident Management Köln Germany March 29-31, 2004. Outline.

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Insights and lessons learned from Level 2 PSA for Bohunice V2 plant MACIEJ KULIG

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  1. Insights and lessons learned from Level 2 PSA for Bohunice V2 plant MACIEJ KULIG ENCONET Consulting, Ges. m. b. H., Auhofstrasse 58, 1130 Vienna, Austria International Workshop onLevel 2 PSA and Severe Accident ManagementKöln Germany March 29-31, 2004

  2. Outline • Overview of the PSA project • Organisation • Objectives and scope • Selected methodological aspects • PDS definition • Source Term categorization / analysis • Confinement Event Trees • Quantification • Insights from sensitivity analysis • Conclusions

  3. Project Organisation ENCONET Consulting Ges. m. b. H., Vienna, Austria Project leadership Definition of PDS Preparation and quantification of containment ETs Analysis and interpretation of the results VUJE Trnava Inc., Slovakia Identification of containment challenges Source Term Analysis RELKO Ltd., Bratislava, Slovakia Preparation and quantification of PDS ET/FT models Lenkei Consulting Ltd., Pecs, Hungary Structural analysis of the confinement building

  4. Objectives and scope of the PSA project PSA objective • Frequency of LER and risk dominant sequences PSA scope • Complies with the current state-of-the-art practices All typical Level 2 elements/tasks included (Level 1 model extended to quantify selected PDS, containment isolation and damage/CET, Source Term and RC, supporting T/H analyses and containment structural analysis, sensitivity analysis, etc.) • PSA compatible with V2 integrated Level 1 PSA model Reflects 1999 plant status • PSA covers full power and shutdown operating modes

  5. PSA Level 2 Task Overview

  6. Plant Damage States Definition • PDS grouping attributes – parameters of highest influence on the post-CD accident progression • Insights from other studies (NUREG-1150, PH4.2.7.a, PH2.09/95), specific features of V2 taken into account • Confinement status (3 possible parameter values) (Confinement isolated, not isolated, bypassed). • Sequence type (8 possible parameter values) LOCA initiators (4), and transient IEs including ATWS and SLBI (4) Secondary status considered if it affects the SA progression. • Safety injection (4 possible parameter values) Possibility of in-vessel recovery & long term debris cooling (1 LP or HP train operable, recovered early, recovered late, failed) • Cavity water (2 possible parameter values) Water in the cavity below RPV prior to vessel failure (dry or wet) • Confinement spray (4 possible parameter values) Long term pressure in the confinement, inert vs. non-inert CONF (1 CSS train operable, recovered early, recovered late, failed)

  7. Plant Damage States Definition • Assigning CD sequences to the selected PDS – based on a systematic process • PDS grouping logic – Decision Trees with PDS parameters as DT headings • Grouping process repeatable • Non-possible combinations of parameters explicitly identified / excluded • PDS grouping – separate logic for different Power Operational State (POS) groups • PDS set reduced to 69 combinations of attributes Many combinations eliminatedtaking into consideration the dependencies between PDS headings and POS-specific boundary conditions

  8. Plant Damage States Definition • PDS grouping logic – 5 POS groups • G0 - Full power operational states RCS and the confinement normally closed. • G1 - Applicable to POS 1, 11 and 12, Essentially similar to the full power state RCS and the confinement normally closed. • G2 - Applicable to POS 2, 3, 7, 8, 9 and 10, RCS closed but the confinement open. • G3 - Applicable to POS 4, 5S and 6, RCS and confinement open, the fuel in the RPV • G4 - Applicable to POS 5L, The fuel relocated to the refuelling pool

  9. Plant Damage States Definition • Sequence type– important PDS parameter • Influence on RCS pressure and CONT pressure • Categorization based on plant specific thermal hydraulic analyses • Parameter values (full power): • L1 - LOCA 7-20 mm without FW (potential IR + CAV overpressurization) • L2 - LOCA 20-40 mm without FW (no IR, potential CAV overpressurization) • L3 - LOCA 7-40 mm with FW (no IR, potential CAV overpressurization) • L4 – LOCA 7-40 with FW and APR (aggressive SG bleed) and LOCA 40-500 mm (no IR, no CAV overpr., LPSI can inject before VF) • T1 - Reactivity transients / ATWS (potential CAV overpressurization) • T2 - Transients without FW (potential CAV overpressurization) • T3 - Steam line break (SLB) inside confinement (CONT press. higher than T2) • T4 - Transients without FW and SLB with CONT not isolated (CS effect can be neglected)

  10. Plant Damage States Definition • Sequence type - summary features of LOCA SA scenarios, definition of L1 – L4 parameter values

  11. Source Term • Source Term grouping parameters • Extent of core damage Gap release, Full CD in-vessel, Full CD ex-vessel with MCCI • RCS status LB LOCA, Transient or SB LOCA, IS LOCA, Open reactor/pool • Containment spray CSS available or recovered (1 train), CSS failed • Containment isolation CONT isolated, failed after 20 hrs, failed after 7 hrs, failed early • ST estimated based on plant specific thermal hydraulic analyses for representative SA scenarios • SA codes used - MARCH3, CORCON, TRAP-MELT3, VANESSA, MELCOR (open vessel)

  12. Example Source Term – LB LOCA case The amount of iodine (I2) released from the confinement (% of the initial core inventory) for sequences initiated by LB LOCA. The most relevant factors that affect the radiological release: ▪ Extent of core damage, ▪ Availability of sprays, ▪ CONT isolation status RC G0-A8 RC G0-B5

  13. Source TermExamples of high consequence RCs • G3-LPS7 – RPV open Full CD CONT open • G0-P-A – IS LOCA, Full CD with MCCI, CONT bypassed • G0-PG-A – PRISE LOCA, Full CD with MCCI, CONT bypassed • G0-TA8 – Transient, Full CD with MCCI, Early CONT failure, No sprays, • G0-A8 – LOCA, Full CD with MCCI, Early CONT failure, No sprays, • G0-TB8 – Transient, Full CD in-vessel, Early CONT failure, Spray on, The amount of iodine and caesium (I2 and Cs) released from the confinement (expressed as % of the initial core inventory) for selected RCs.

  14. Containment Event Trees – CET headings

  15. Containment Event Trees - Example: CET for PDS G0-01 to PDS G0-48

  16. Containment Event Trees • CET sub-models • Each CET header uses several different decomposition event tree (DET) models depending on different possible boundary conditions (PDS and sequence characteristics) • Each CET heading is decomposed into several more detailed questions easier to answer or quantify, suitable DET is selected based on the answers • DET provides graphical representation of different possibilities under each CET header (probabilities assigned) • End states • Defined as a string with the following information: LOCA/TRANSIENT – A, T CORE DAMAGE EXTENT – PART_CD, FULL_CD, CD+MCCI, SPRAY STATUS – SPRAYS_OK, NO_SPRAYS CONFINEMENT STATUS - VEARLYCF, NOT_ISOL, EARLY_CF, CAV_DOOR, _LATE_CF, __NO_CF,……. EXAMPLE:

  17. Containment Event Trees CET sub-models - example “Very early CT failure -VECF0”

  18. Confinement structural analysis CONF structures checked: hermetic doors, reactor dome, lock covers, tube penetrations, SG room, and barbotage tower (liner and concrete wall) Best estimate failure pressure bubbler tower 0.426 MPa (abs); cavity door - 0.594 MPa (abs) Probability distribution: Log-normal (SD = 16%, NUREG-1150)

  19. Model quantification • Quantification of PDS using Risk Spectrum code (each PDS sequence analysed separately) • PDS frequencies calculated as a sum of sequence analysis results (in MS Excel code after export of Risk Spectrum results) • PDS inputs to each CET is identified • CET is quantified using the code in which they are developed • Quantification conducted for each PDS using sub-models (DET) which correspond to that PDS (logical rules are provided to select DET sub-model) • End states are assigned automatically to each CET sequence, textual identifiers serve to provide link with RCs. • Grouping of sequences using a short script (written in Phyton)

  20. Overview of resultsPDS frequencies PDSs with the highest frequencies: • PDS G3-01 – man induced LOCA in POS6 as a dominant IE, (SI not initiated due to HE) • PDS G3-03 – man induced LOCA in POS6 as a dominant IE, (SI not available) • PDS G2-03 – cold overpressurisation in POS7 as a dominant IE, • PDS G0-40 – medium LOCA in full power as a dominant IE, • PDS G4-01 – loss of non-vital operational 6 kV bus in POS5L as a dominant IE.

  21. Overview of results • Full power operational states (G0) -All PDS - 9.99E-05/yr. Early/large release ~75% dominated by cavity door failure at VF and CONT failure due to hydrogen burn during in-vessel phase • POS 1, 11, 12 (G1) -All PDS - 2.05 E-05/yr, Early/large release ~80.5% - similar to G0 • Other shutdown states (G2, G3, G4)~6.10E-04/yr, Early/large release ~ 100%

  22. Overview of resultsFull power - Small release end-states groups(< 1% of core inventory)

  23. Overview of resultsFull power - Large release end-states groups(> 1% of core inventory)

  24. Overview of resultsSensitivity analysis - Case S1: ‘hot leg rupture probability’ • Assumption investigated: Hot leg rupture probability increased from base case (e.g. ~ 0.33 for L1 group) to 0.7 (i.e. the value used in NUREG-1150 for HP PDS) • Impact on RC frequency profile: • ‘Cavity door failure’ contribution reduced • ‘Intact containment’ and ‘intact vessel’ contributions increased

  25. Overview of resultsSensitivity analysis - Case S2: ‘hydrogen burning before vessel failure’ • Assumption investigated: Hydrogen burn during the period before vessel failure would always occur at the highest hydrogen concentration i.e. no burns at low concentration (uniform distribution was assumed in the base case) • Impact on RC frequency profile: ‘Very early confinement failure’ end-states contribution significantly increased

  26. Overview of resultsSensitivity analysis - Case S3: ‘hydrogen combustion in the cavity’ • Assumption investigated: Hydrogen combustion does not occur in the cavity concurrent with blow-down loading (assumed very likely in the base case) • Impact on RC frequency profile: • ‘Cavity door failure’ contribution significantly reduced • ‘Basemat melt-through’ contribution increases (because dominant sequences have all safety injection failed)

  27. Overview of resultsSensitivity analysis - Case S4: ‘Ex-vessel cooling analysis’ • Assumption investigated: Ex-vessel cooling analysis assumes failure of ex-vessel cooling (with probability of 0.9 ) • Impact on RC frequency profile: The results not sensitive to ex-vessel cooling details - only a small increase of ‘basemat melt-through’ contribution (in the base case melt-through is due to PDS with injection unavailable);

  28. Overview of resultsSensitivity analysis - Case M1: ‘Benefit of implementation SB EOP Fr.C-1’ • SAM measure investigated: Actions aimed at reducing RCS pressure (after unsuccessful recovery of safety injection) using the PRZ safety/relief valves • Impact on RC frequency profile: reduced source term • ‘Cavity door failure’ contribution reduced (by a factor of ~2) • ‘Basemat melt-through’ contribution increases • Frequency of ‘confinement intact’ end-states increases.

  29. Overview of resultsSensitivity analysis - Case M2: ‘In-vessel retention strategy - manual actuation’ • SAM measure investigated: Adding a cavity flooding system (actuated manually) • Impact on RC frequency profile: reduced source term • ‘Cavity door failure’, ‘confinement failure during in-vessel phase’ and ‘long term basemat melthrough’ contributions reduced • ‘Confinement intact’ and ‘Confinement failure no VF’ end-states increase

  30. Overview of resultsSensitivity analysis - Case M3: ‘In-vessel retention strategy - automatic actuation’ • SAM measure investigated: Adding a cavity flooding system (automatically actuated) • Impact on RC frequency profile: ST significantly reduced • Similar to the implementation of manual system, but benefit greater • ‘Cavity door failure’ contribution significantly reduced • For the very early confinement failures the sequences without vessel failure become dominant

  31. Overview of resultsSensitivity analysis - Case M4: ‘Adding hydrogen burners and independent spray system’ • SAM measure investigated: Adding hydrogen burners and independent spray system • Impact on RC frequency profile: • The contribution of confinement failures due to hydrogen burn before VF is reduced; improvement to some extent is offset by an increase in cavity door failures. • This measure would be of most benefit when combined with other measures providing protection against cavity overpressure

  32. Conclusions – lessons from modeling • Definition of PDS – “sequence type” found to be useful PDS parameter • Separate set of PDS for each POS group - convenient • Extension and re-quantification of Level 1 PSA model needed • Use of containment performance model with two levels (CET + DET) – model manageable and easily traceable, subjective judgement clearly documented • Definition of End-States sufficiently detailed allows for flexible grouping with regard to source term characteristics • Step-wise process of CET quantification found effective and straightforward

  33. Conclusions – insights from results • Plant risk profile • For power operational states the main concern are hydrogen burn during in-vessel phase and cavity door failure due to loads at vessel failure • For shutdown operational states dominant risk contributors are vessel open / confinement open states • Sensitivity to modelling assumptions • The model is not sensitive to assumptions related to long term confinement overpressure or the detailed modelling of ex-vessel cooling in the cavity • The model is sensitive to the probability of hydrogen burn in the cavity (due to small volume of the cavity) • There are uncertainties in relation to the induced hot leg rupture (during non-LOCA transients) due to uncertainties in creep rupture properties of steel

  34. Conclusions – insights from results • All SAM measures investigated found beneficial • Implementation of RCS depressurization(EOP Fr.C-1) reduces cavity door failure frequency and increases the frequency of low release end states (intact confinement and melt-through confinement failure) • Implementation of cavity flooding with independent system reduces the vulnerability to cavity door failure, by reducing the number of sequences with vessel failure. An automatic actuation more beneficial than manual • Implementation of an effective system for hydrogen control would be beneficial in reducing contribution of very early CF. This measure should be combined with one of the others providing protection against cavity overpressure • These measures are being considered in SAMGs (currently under development for V2 plant) • Preventive SAM measures (EOP level) essential for shutdown POSs

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