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.
ENCONET Consulting, Ges. m. b. H.,
Auhofstrasse 58, 1130 Vienna, Austria
ENCONET Consulting Ges. m. b. H., Vienna, Austria
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
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.)
Reflects 1999 plant status
(Confinement isolated, not isolated, bypassed).
LOCA initiators (4), and transient IEs including ATWS and SLBI (4)
Secondary status considered if it affects the SA progression.
Possibility of in-vessel recovery & long term debris cooling
(1 LP or HP train operable, recovered early, recovered late, failed)
Water in the cavity below RPV prior to vessel failure (dry or wet)
Long term pressure in the confinement, inert vs. non-inert CONF (1 CSS train operable, recovered early, recovered late, failed)
Many combinations eliminatedtaking into consideration the dependencies between PDS headings and POS-specific boundary conditions
RCS and the confinement normally closed.
Essentially similar to the full power state
RCS and the confinement normally closed.
RCS closed but the confinement open.
RCS and confinement open, the fuel in the RPV
The fuel relocated to the refuelling pool
LOCA 40-500 mm (no IR, no CAV overpr., LPSI can inject before VF)
(CS effect can be neglected)
Gap release, Full CD in-vessel, Full CD ex-vessel with MCCI
LB LOCA, Transient or SB LOCA, IS LOCA, Open reactor/pool
CSS available or recovered (1 train), CSS failed
CONT isolated, failed after 20 hrs, failed after 7 hrs, failed early
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
The amount of iodine and caesium (I2 and Cs) released from the confinement (expressed as % of the initial core inventory) for selected RCs.
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,…….
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)
Log-normal (SD = 16%, NUREG-1150)
PDSs with the highest frequencies:
Early/large release ~75%
dominated by cavity door failure at VF and CONT failure due to hydrogen burn during in-vessel phase
Early/large release ~80.5% - similar to G0
Early/large release ~ 100%
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)
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)
‘Very early confinement failure’ end-states contribution significantly increased
Hydrogen combustion does not occur in the cavity concurrent with blow-down loading (assumed very likely in the base case)
Ex-vessel cooling analysis assumes failure of ex-vessel cooling (with probability of 0.9 )
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);
Actions aimed at reducing RCS pressure (after unsuccessful recovery of safety injection) using the PRZ safety/relief valves
Adding a cavity flooding system (actuated manually)
Adding a cavity flooding system (automatically actuated)
Adding hydrogen burners and independent spray system