Level 2 PSA for EBO V2 - Nuclear Energy Agency
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Transcript Level 2 PSA for EBO V2 - Nuclear Energy Agency
International Workshop on
Level 2 PSA and Severe Accident Management
Köln Germany March 29-31, 2004
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
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
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
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
PSA Level 2 Task Overview
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)
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 eliminated taking into consideration the
dependencies between PDS headings and POS-specific boundary
conditions
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
Plant Damage States Definition
type – important PDS parameter
Influence on RCS pressure and CONT pressure
Categorization based on plant specific thermal
hydraulic analyses
Parameter values (full power):
Sequence
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)
Plant Damage States Definition
Sequence type - summary features of LOCA SA
scenarios, definition of L1 – L4 parameter values
No secondary side
cooling
Secondary side
available
Secondary side
depressurised
with FW (APR)
LOCA <20mm
IR
No LP
(L1)
HP
CAV
No IR
No LP
(L3)
HP
CAV
No IR
LP
(L4)
HP
No CAV
IR – Induced RCS rupture is a possibility
CAV – Cavity overpressure may occur
LOCA 20-40 mm
No IR
No LP
(L2)
HP
CAV
No IR
No LP
(L3)
HP
CAV
No IR
LP
(L4)
HP
No CAV
LOCA >40 mm
No IR
LP
(L4)
HP
No CAV
No IR
LP
(L4)
HP
No CAV
No IR
LP
(L4)
HP
No CAV
LP – LPSI can inject before vessel failure
HP –HPSI can inject before vessel failure
Note: In L3 due to availability of FW the cavity pressure rise following VF is expected
lower as compared to L2 case (without FW)
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)
Example Source Term – LB LOCA case
100,00%
- Gap release only
- Partial CD, core cooled in RPV
- Full CD with MCCI
RC
G0-A8
10,00%
RC
G0-B5
1,00%
0,10%
0,01%
0,00%
Conf. isolated,
spray available
Conf. isolated,
no spray
Conf. failed early, Conf. failed early,
spray available
no spray
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
Source Term
Examples of high consequence RCs
25,00%
I2 radionuclide group
Cs radionuclide group
20,00%
15,00%
10,00%
5,00%
0,00%
G3-LPS7,
G4-LPS8
G3-LPS7 –
G0-P-A –
G0-PG-A –
G0-TA8 –
G0-A8 –
G0-TB8 –
G0-P-A,
G0-P-Aa
RPV open
IS LOCA,
PRISE LOCA,
Transient,
LOCA,
Transient,
G0-PG-A,
G0-PG-Aa
G0-TA8,
G0-TA8a
Full CD
Full CD with MCCI,
Full CD with MCCI,
Full CD with MCCI,
Full CD with MCCI,
Full CD in-vessel,
G0-A8,
G0-A8a
G0-B8
CONT open
CONT bypassed
CONT bypassed
Early CONT failure,
Early CONT failure,
Early CONT failure,
G0-TB8
No sprays,
No sprays,
Spray on,
Containment Event Trees – CET headings
CET Header
Relevant characteristics
Induced RCS rupture
Creep rupture of the hot leg or SG tubes (RCS
depressurization, potential bypass).
Very early H2 burn
H2 burn prior to the in-vessel phase of the
accident (hydrogen depletion).
Very early CONT failure
Confinement failure due to hydrogen combustion
before vessel failure
Reactor vessel failure
Possibility of core cooled in-vessel (subsequent
confinement challenges can be avoided).
Early CONT failure
Confinement fails due to hydrogen combustion
loads at the time of vessel failure
Debris cooled ex-vessel Prevention of molten core concrete interaction
Late CONT failure
CF due to the combustion of hydrogen and/or
combustible gases during the ex-vessel phase
Long term CONT failure
CONT overpressurized by non-condensable gas/
vapour generation or basemat melt-through
Containment Event Trees Example: CET for PDS G0-01 to PDS G0-48
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 –
CORE DAMAGE EXTENT –
SPRAY STATUS –
CONFINEMENT STATUS EXAMPLE:
A
A, T
PART_CD, FULL_CD, CD+MCCI,
SPRAYS_OK, NO_SPRAYS
VEARLYCF, NOT_ISOL, EARLY_CF,
CAV_DOOR, _LATE_CF, __NO_CF,…….
FULL_CD
NO_SPRAYS
CAV_DOOR
Containment Event Trees
CET sub-models - example “Very early CT failure -VECF0”
Confinement structural analysis
1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
bubbler tower
cavity door
0,
25
0,
40
1
0,
44
1
0,
48
8
0,
50
3
0,
56
2
0,
59
4
MPa
Best estimate failure pressure
Probability distribution:
bubbler tower 0.426 MPa (abs);
cavity door - 0.594 MPa (abs)
Log-normal (SD = 16%, NUREG-1150)
CONF structures checked: hermetic doors, reactor dome, lock
covers, tube penetrations, SG room, and barbotage tower (liner
and concrete wall)
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)
Overview of results
PDS frequencies
The first dominant PDS in the group
The second dominant PDS in the group
Other
5,00E-04
4,50E-04
4,00E-04
3,50E-04
3,00E-04
2,50E-04
2,00E-04
1,50E-04
1,00E-04
5,00E-05
0,00E+00
PDS G0-40,
PDS G0-01
PDS G1-68, PDS G2-03,
PDS G1-40 PDS G2-01
PDS G3-01,
PDS G3-03
PDS G4-01
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.
Overview of results
Small/delayed release (confinement intact or failed late)
Large release (confinement failed early, bypassed or open)
5,0E-04
4,5E-04
4,0E-04
3,5E-04
3,0E-04
2,5E-04
2,0E-04
1,5E-04
1,0E-04
5,0E-05
0,0E+00
G0
G1
G2
Full power operational states (G0) -
G3
G4
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)
Early/large release ~ 100%
~6.10E-04/yr,
Overview of results
Full power - Small release end-states groups
(< 1% of core inventory)
RC
RC1
RC characteristics
Total RC
frequency
Dominant end-states
End-state description
Frequency/ STC
4.045E-07
A-PART_CD-SPRAYS_OK-___NO_CF
4.02E-07/ G0-B1
RC2
Core recovered in-vessel,
partial CD, no CF.
No confinement failure
1.572E-06
RC3A
Late CF, No MCCI
1.066E-07
RC3B
Late CF,+ MCCI
1.779E-7
RC4
Basemat penetration
(Meltthrough)
1.773E-5
T-FULL_CD-SPRAYS_OK-___NO_CF
A-FULL_CD-SPRAYS_OK-___NO_CF
A-PART_CD-SPRAYS_OK-_LATE_CF
T-FULL_CD-SPRAYS_OK-_LATE_CF
T-FULL_CD-NO_SPRAYS-VLATE_CF
A-FULL_CD-SPRAYS_OK-_LATE_CF
T-CD+MCCI-NO_SPRAYS-_LATE_CF
T-CD+MCCI-SPRAYS_OK-_LATE_CF
T-CD+MCCI-NO_SPRAYS-MELTTHRU
T-CD+MCCI-SPRAYS_OK-MELTTHRU
1.37E-06/ G0-TA1
1.97E-07/ G0-A1a
7.99E-08/ G0-B3
1.38E-08/ G0-TA3a
9.03E-09/ G0-TA6a
1.97E-09/ G0-A3a
1.73E-07/ G0-TA7
3.75E-09/ G0-TA3
1.72E-05/ G0-TA2
3.74E-07/ G0-TA2
Overview of results
Full power - Large release end-states groups
(> 1% of core inventory)
RC
RC characteristics
Total RC
frequency
Dominant end-states
End-state description
Frequency/ STC
RC5A
Early CF, No MCCI,
Cavity door failure
3.566E-05
T-FULL_CD-NO_SPRAYS-CAV_DOOR
T-FULL_CD-SPRAYS_OK-CAV_DOOR
T-FULL_CD-SPRAYS_OK-EARLY_CF
1.81E-05/ G0-TA8
1.53E-05/ G0-TA4
2.07E-06/ G0-TA4a
RC5B
Early CF, + MCCI
7.248E-06
T-CD+MCCI-NO_SPRAYS-EARLY_CF
T-CD+MCCI-SPRAYS_OK-EARLY_CF
6.65E-06/ G0-TA8
5.63E-07/ G0-TA4
RC6
Very early confinement
failure
3.278E-05
T-CD+MCCI-NO_SPRAYS-VEARLYCF
T-FULL_CD-SPRAYS_OK-VEARLYCF
T-CD+MCCI-SPRAYS_OK-VEARLYCF
2.48E-05/ G0-TA8
5.69E-06/ G0-TA4a
1.55E-06/ G0-TA4
RC7A
Confinement isolation
failure, No MCCI
1.230E-06
T-FULL_CD-NO_SPRAYS-NOT_ISOL
A-PART_CD-NO_SPRAYS-NOT_ISOL
1.18E-06/ G0-TA8a
2.81E-08/ G0-B8
RC7B
Confinement isolation
failure, + MCCI
1.479E-06
T-CD+MCCI-NO_SPRAYS-NOT_ISOL
A-CD+MCCI-NO_SPRAYS-NOT_ISOL
1.40E-06/ G0-TA8
7.59E-08/ G0-A8
RC8A
Bypass, scrubbed
0.311E-06
_-_______-_________-SG/V_WET
_-_______-_________-SGTR_RVC
7.96E-07/ G0-P-Aa
5.15E-07/ G0-PG-Aa
RC8B
Bypass, unscrubbed
1.61E-07
_-_______-_________-SGTR_UNM
1.61E-07/ G0-PG-Aa
RC9
Vessel fails in ROCKET
mode
4.852E-08
T-VROCKET-NO_SPRAYS-_VROCKET
T-VROCKET-SPRAYS_OK-_VROCKET
3.49E-08/ G0-PG-Aa
1.35E-08/ G0-PG-Aa
Overview of results
Sensitivity analysis - Case S1:
‘hot leg rupture probability’
Late and long term confinment failures
1,0E-04
Confinement failures no VF
9,0E-05
No confinement failure
8,0E-05
Confinment bypass due to SGTR or IFSL
7,0E-05
6,0E-05
Accidents - no confinment failure (VF)
5,0E-05
Accidents with confinment isol. failure (VF)
4,0E-05
Confinment fails due to hydrogen burn at VF
3,0E-05
Long term basemat melthrough
2,0E-05
Confinment fails during in-vessel phase (VF)
1,0E-05
Cavity door fails at VF phase
0,0E+00
Base case
Case S1
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
Overview of results
Sensitivity analysis - Case S2:
‘hydrogen burning before vessel failure’
Late and long term confinment failures
1,0E-04
Confinement failures no VF
9,0E-05
No confinement failure
8,0E-05
Confinment bypass due to SGTR or IFSL
7,0E-05
6,0E-05
Accidents - no confinment failure (VF)
5,0E-05
Accidents with confinment isol. failure (VF)
4,0E-05
Confinment fails due to hydrogen burn at VF
3,0E-05
Long term basemat melthrough
2,0E-05
Confinment fails during in-vessel phase (VF)
1,0E-05
Cavity door fails at VF phase
0,0E+00
Base case
Case S2
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
Overview of results
Sensitivity analysis - Case S3:
‘hydrogen combustion in the cavity’
Late and long term confinment failures
1,0E-04
Confinement failures no VF
9,0E-05
No confinement failure
8,0E-05
Confinment bypass due to SGTR or IFSL
7,0E-05
6,0E-05
Accidents - no confinment failure (VF)
5,0E-05
Accidents with confinment isol. failure (VF)
4,0E-05
Confinment fails due to hydrogen burn at VF
3,0E-05
Long term basemat melthrough
2,0E-05
Confinment fails during in-vessel phase (VF)
1,0E-05
Cavity door fails at VF phase
0,0E+00
Base case
Case S3
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)
Overview of results
Sensitivity analysis - Case S4:
‘Ex-vessel cooling analysis’
Late and long term confinment failures
1,0E-04
Confinement failures no VF
9,0E-05
No confinement failure
8,0E-05
Confinment bypass due to SGTR or IFSL
7,0E-05
6,0E-05
Accidents - no confinment failure (VF)
5,0E-05
Accidents with confinment isol. failure (VF)
4,0E-05
Confinment fails due to hydrogen burn at VF
3,0E-05
Long term basemat melthrough
2,0E-05
Confinment fails during in-vessel phase (VF)
1,0E-05
Cavity door fails at VF phase
0,0E+00
Base case
Case S4
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);
Overview of results
Sensitivity analysis - Case M1:
‘Benefit of implementation SB EOP Fr.C-1’
Late and long term confinment failures
1,0E-04
Confinement failures no VF
9,0E-05
No confinement failure
8,0E-05
Confinment bypass due to SGTR or IFSL
7,0E-05
6,0E-05
Accidents - no confinment failure (VF)
5,0E-05
Accidents with confinment isol. failure (VF)
4,0E-05
Confinment fails due to hydrogen burn at VF
3,0E-05
Long term basemat melthrough
2,0E-05
Confinment fails during in-vessel phase (VF)
1,0E-05
Cavity door fails at VF phase
0,0E+00
Base case
-
Case M1
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.
Overview of results
Sensitivity analysis - Case M2:
‘In-vessel retention strategy - manual actuation’
Late and long term confinment failures
1,0E-04
Confinement failures no VF
9,0E-05
No confinement failure
8,0E-05
Confinment bypass due to SGTR or IFSL
7,0E-05
6,0E-05
Accidents - no confinment failure (VF)
5,0E-05
Accidents with confinment isol. failure (VF)
4,0E-05
Confinment fails due to hydrogen burn at VF
3,0E-05
Long term basemat melthrough
2,0E-05
Confinment fails during in-vessel phase (VF)
1,0E-05
Cavity door fails at VF phase
0,0E+00
Base case
-
Case M2
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
Overview of results
Sensitivity analysis - Case M3:
‘In-vessel retention strategy - automatic actuation’
Late and long term confinment failures
1,0E-04
Confinement failures no VF
9,0E-05
No confinement failure
8,0E-05
Confinment bypass due to SGTR or IFSL
7,0E-05
6,0E-05
Accidents - no confinment failure (VF)
5,0E-05
Accidents with confinment isol. failure (VF)
4,0E-05
Confinment fails due to hydrogen burn at VF
3,0E-05
Long term basemat melthrough
2,0E-05
Confinment fails during in-vessel phase (VF)
1,0E-05
Cavity door fails at VF phase
0,0E+00
Base case
-
Case M3
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
Overview of results
Sensitivity analysis - Case M4:
‘Adding hydrogen burners and independent spray system’
Late and long term confinment failures
1,0E-04
Confinement failures no VF
9,0E-05
No confinement failure
8,0E-05
Confinment bypass due to SGTR or IFSL
7,0E-05
6,0E-05
Accidents - no confinment failure (VF)
5,0E-05
Accidents with confinment isol. failure (VF)
4,0E-05
Confinment fails due to hydrogen burn at VF
3,0E-05
Long term basemat melthrough
2,0E-05
Confinment fails during in-vessel phase (VF)
1,0E-05
Cavity door fails at VF phase
0,0E+00
Base case
-
-
Case M4
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
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
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
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