Transcript Nuclear Research Institute Řež plc Nuclear Research Institute Řež plc Level 2 PSA for the VVER 440/213 Dukovany NPP and Its Implications for.

Nuclear Research Institute Řež plc
Nuclear Research Institute Řež plc
Level 2 PSA for the VVER 440/213 Dukovany NPP
and Its Implications for Accident Management
Jiří Dienstbier, Stanislav Husťák
OECD International Workshop on “Level-2 PSA and Severe Accident
Management”, Cologne, 29-31 March 2004
11/6/2015
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Outline
• Plant features
• History of PSA 2
• Methodology used
– Main characteristics
– Containment failure modes
– Large event tree - APET
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PSA 1 – PSA 2 interface
Main part of APET
Hydrogen model
Fission product release – source term to the environment
Results
Sensitivity studies
Accident management
Conclusions and plans for near future
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Plant features
•4 units in 2 twin-units, twin units in common building, each unit has its own containment
•Mostly rectangular leak tight rooms, pressure suppression system … bubble condenser
•Recirculation sump is not at the lowest level, possibility to lose ECC coolant to ventilation
•Reactor cavity is the containment boundary including double steel cavity door
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History
PSA 2 for unit 1
First (Revision 0) Limited scope Level 2 PSA
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From 1995 to April 1998 as US AID project – contractor SAIC (Science Applications International Corporation) with
NRI Řež as subcontractor and with plant support
Based on SAIC-NRI level 1 PSA from 1994
Limited to normal operation at power without ATWS, no shutdown states, no external events
4 fission product groups, point estimates of frequencies, uncertainties treated by sensitivity study
Large event tree (APET) method (program EVNTRE)
MELCOR 1.8.3 physical analyses
Knowledge transfer to NRI specialists was a part of the project
Revision 1
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Autumn 1998 (SAM proposals updated in autumn 1999) by NRI Řež
Using NRI Řež living PSA 1 from 1998 (partially including new EOP), much different from the PSA 1 in rev.0
Extended to fires and internal floods
Large modification of the event tree – about ½ of questions changed keeping their order
Only small modification of basic events
Revision 2
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End of 2002, living PSA 1 2001 used, fully taking into account new EOPs, including ATWS sequences (did not
propagate into PSA 2)
Revision of the AICC hydrogen burn model
Containment failure (leak type) due to slow pressurisation by steam and non-condensable gases added
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Main characteristics
Main characteristics
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Limited scope Level 2 PSA
Similar to IPE for US power plants
Limited to normal operation at power including internal events - fires, floods
Not included: External events like earthquake, low power and shutdown states
4 fission product groups – Cs, Te, Ba, noble gases, only Cs+Ba used for sorting the
results to release categories
Large event tree (APET) method, the resulting tree has 100 nodes (usually more than
2 states in each node):
– 12 nodes PSA 1 – PSA 2 interface (PDS vectors)
– Nodes 13 to 85 accident progression
– Nodes 86 to 100 related to fission product release to the environment – source
term
Program EVNTRE (developed by SNL)
The results are probabilities of 12 release categories + results of binning and sorting
About 90 basic events and several physical parameters
Revision 0 only
MELCOR 1.8.3 physical analyses of selected sequences (5 basic sequences + their
variations), results used to specify some parameters and basic events
Other activities – plant walkdown, containment feature notebook
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Containment failure modes
Table 1 Containment failure modes
Failure mode
Early bypass rupture
Early or late rupture
Assumed effective leak size
Early leak
Late leak
Intact containment
0.01 m2
0.01 m2
natural leak
1 m2
Caused by phenomena
Bypass sequences – SGCB (single SG tube added to early leak)
Containment isolation failure*, pressurization due to hydrogen burn,
hydrogen detonation, steam explosion, vessel rocket, cavity or cavity
door failure
Cavity door loss of tightness, SGTR
Cavity basemat penetration, containment failure by slow pressurization
12.5 % / day used, it is about 9 % at present
* The fact that containment isolation failure starts very early is taken into account for source term.
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Classification of events timing:
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Early … before reactor vessel bottom failure (and about 2 hours later for fission products)
Late … after this time
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Failure locations in the containment (several possible) and cavity (or cavity door)
Retention in walls or auxiliary building surrounding containment neglected
Containment fragility curve (after DOE/NE-0086, 1989)
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Containment – normal distribution, m = 400 kPa overpressure, s = 80.9 kPa
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Cavity – normal distribution, m = 2420 kPa overpressure, s = 460 kPa
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Possible containment isolation failure
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Ventilation lines P-2 (TL-40), O-2 (TL-70)
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Drainage, neglected in revision 2
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PSA 1 – PSA 2 Interface
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PDS (plant damage state) vectors representing first 12 nodes of PSA 2 event
tree and characterizing the plant systems at the onset of core damage
Respecting US NRC IPE and IAEA recommendations to reflect PSA 1 results
PDS description
First node representing initiating event
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13 events, ATWS, ILOCA (interfacing LOCA other than through SG) screened out because of
low frequency in PSA 1
initiating events specific for PSA 2, especially RPV-PTS …reactor vessel rupture due to
thermal shock
Other 12 events
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Different size LOCA – S-LOCA, MS-LOCA, M-LOCA, LG-LOCA
LOCA leading to water loss outside main sump – IL/RCP, IL/POOL
SGCB … SG collector break and lift off, SGTR … SG tube rupture
SB-OUT … steamline break outside containment, SB-IN … steamline break inside containment
TRANS … transient – very similar PDS vectors to SB-OUT, total loss of feedwater in both
SBO … station blackout – failure of electric power supply including category 2
Flood included as SBO 34
Fires in some of the TRANS and IL/RCP initiators
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PSA 1 – PSA 2 Interface
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Following 11 nodes
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HPI
... state of HP ECC injection and recirculation
LPI
... state of the LP ECC injection and recirculation
Sprays
... state of containment sprays
SHR
... secondary heat removal (mainly feedwater availability)
SecDP
... secondary system depressurisation (important only for SHR OK)
PrimDP
... primary system depressurisation by the operator
ECCS_Inv ... location of (decisive part) of ECC water inventory
VE_Cat2 ... state of category 2 electric power (diesels)
VE_CI
... Two events combined:
• containment isolation (CI)
• recirculation sump isolation against water loss (fSumpI = sump isolation
failed)
– VE_CHR
... containment heat removal system status (not including water
and electricity availability)
– BC_Drain ... location of bubble condenser water:
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These nodes have 2 to 4 attributes
Result – 34 PDS vectors (table 2 in the paper), only 5 of them with
frequency > 10-6/y
– RPV-PTS, SB-OUT, TRANS, IL/RCP, blackout
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PSA 1 – PSA 2 Interface
Figure 1 Analysis of CDF
Loss of ECC water
Complete loss of all electric power
including batteries
Hardware or control problem
difficult to solve (switch over to
recirculation)
1%
22%
Complete loss of electric power up
to category 2
6%
Error in procedure including human
error (primary depressurisation)
1%
1%
69%
Very limited core damage
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APET
Nodes (questions) 13 to 85
Development of APET - Main event tree as framework including:
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primary pressure before vessel failure, ECCs water location, early recirculation, vessel failure
containment failure early
late recirculation
containment status late
Phenomenology
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The same as for PWR reactor (importance often different, e.g. in-vessel hydrogen)
Special connected with cavity design and its function as containment boundary
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Technical systems complicated the event tree and required repeating of some questions:
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category 2 electric power early and late
primary system depressurisation
sprays early and late
late phase - water in cavity / cavity door status (to avoid feedback)
Quantification of basic events and physical parameters (quantification tables for probability)
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HPME and cavity failure by gases or steam overpressure
Cavity door failure by debris jet impingement
Containment failure by gases transfer from the cavity
Cavity door failures by thermal effects [1) large, 2) small=loss of sealing, a) within 2 hours after VF, b) late]
MELCOR plant analyses
detailed problems analysed by MELCOR (cavity)
hand calculation, engineering judgement
literature
Hydrogen
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Early and late, same models but different assumptions
Production according to scenario and core damage (full, limited), concentration calculated
Type of burn: no burn – diffusion burn – deflagration – detonation specified according to concentration and other
Consequences calculated for deflagration using AICC model and comparing the modified peak pressure with containment
strength curve
no burn – diffusion burn no containment failure
detonation always failure
Update of model in revision 2, the strongest effect had the assumption about electric power not a good igniter
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Fission product release to the environment - source term
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Nodes 86 to 100
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The result of 100 is sorted to 12 release categories
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Early and late release of Cs, Te, Ba, Xe+Kr in % of inventory
Decontamination factors (DF) - primary, containment, sprays
Revolatilization of early released and deposited f.p. also assumed
Calculation (using DF) using user functions and sorting of releases
Thresholds 0.1, 1.0, 10.0 % of inventory for Cs group and 1 order less for Ba group
In revision 2, the results sorted to 5 classes:
1. early high – more than 1% of Cs or 0.1% of Ba with early containment failure
2. late high – the same with late containment failure
3. early low– between 0.1% and 1% of Cs and 0.01% and 0.1% of Ba with early containment failure or no
failure
4. late low - the same with late containment failure
5. very low – less than 0.1% of Cs and 0.01% of Ba
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The last class specified according to Swedish and Finnish criteria (0.1% 137Cs)
Noble gases release higher, not used in these classes
We think about adding one more category for LERF (>10% of Cs and I early)
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Summary results
Figure 2 Release classes and containment failure, case with PTS
frequency [1/year], CDF=2.968E-05
100%
90%
80%
6.9105E-06
4.3631E-07
1.739E-05
70%
60%
1.0909E-05
50%
3.9547E-06
4.773E-06
4.768E-08
2.296E-07
7.4742E-06
7.248E-06
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2
20%
10%
0%
release
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noCF
late low
CFL_Leak
early low
CFL_Rp
late high
CFE_Leak
early high CFE_Rp+Byp_Rp
40%
30%
very low
containment state
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Summary results
Figure 3 Release classes and containment failure, case without PTS
frequency [1/year], CDF=1.357E-05
100%
90%
80%
6.9107E-06
70%
9.075E-06
60%
50%
40%
3.4796E-07
2.5974E-06
30%
9.1616E-07
2.305E-08
2.296E-07
20%
10%
1.670E-06
2.7996E-06
2.573E-06
0%
2
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release
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containment state
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noCF
very low
CFL_Leak
late low
early low CFL_Rp
CFE_Leak
late high
early high CFE_Rp+Byp_Rp
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Results
Results sorted according to
• Consequences for PDS vectors
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Core damage
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Limited 17,7% (38.5% w/o RPV-PTS) or Full
Pressure at vessel bottom head failure
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11 “risk vectors” with early or late high release frequency above 10-7/year found
used for scenario analyses recommendations
initiated by RPV-PTS, SB-OUT or TRANS, SBO, IL/RCP, IL/POOL, SGCB
Low (below 0.8 MPa) 91.8% (82.0%)
Most Important phenomena leading to containment failure
% CDF
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E_Byp_Rp
E_Rp
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Single SG tube break
L_Rp
L_Lk
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Hydrogen deflagration or detonation
Cavity failure (mostly steam explosion)
E_Leak
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Thermal failure of door sealing
Basemat penetration
Intact containment
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(w/o RPV-PTS)
0.64
23.78
( 1.40)
(17.56)
12.34
10.47
( 7.70)
( 7.72)
0.77
(1.69)
0.37
(0.81)
0.16
16.06
(0.17)
(12.31)
12.54
2.67
7.76
1.85
58.62
(66.93)
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Sensitivity studies
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Sensitivity studies are the only method to assess uncertainty here
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Revision 0 PSA 2
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23 sensitivity studies
Showing importance of some basic events like steam explosions
Including accident management
Changing only basic events and parameters, no event tree change
Revision 1
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Accident management and preventive measures only
Also small event tree changes if needed
Most efficient
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Cavity flooding and external vessel cooling
Primary system depressurisation by operator
Combining depressurisation with other measures
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Sensitivity studies
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Revision 2
– case without RPV-PTS shown before
– case without RPV-PTS and IL/RCP with coolant loss (plant modification)
• CDF decreased to 1.15*10-5 / year LERF decreased to 2.30*10-6 /year
– primary system depressurisation in SAMG
• Low efficiency - mostly low pressure accident and depressurisation in EOP
– higher probability of hydrogen early ignition as in the previous revisions
• Early containment failure due to hydrogen 4%
– higher hydrogen source
• “medium”=50% oxidation, “high”=80% (instead of 35% / 50%)
• LERF = 1.53*10-5, more than 50% of CDF is early containment failure
– lower containment strength
• 300 instead of 400 kPa median, similar results like for higher hydrogen source
– lower containment strength and higher hydrogen source
• Early containment rupture 69% CDF, LERF = 2.06*10-5 / year, hydrogen the only
risk
– lower steam explosion probability in the cavity
• 0.1 (instead of 0.5) for high molten fraction, 0.01 (0.1) for low molten fraction
• containment failure by steam explosion 1.41% CDF (10.43%)
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Severe accident management
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Present situation
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Dukovany concentrated on core damage prevention in the past
CDF decreased considerably, more than one order of magnitude
This was due to plant modification and symptom oriented EOP
Plant modifications not included in the last revision of PSA 2
• modification to eliminate ECC coolant loss from MCP motor deck (IL/RCP) to start soon
• intensive study of RPV-PTS to decrease its probability
• Isolation of cavity drainage
• for eliminating ECC water loss after RPV-PTS also ventilation line isolation would be needed
• using fire pumps for feedwater, filling of SG from tank by gravity – lower blackout CDF
After these modifications, CDF below 10-5/year can be reached
SAMG needed to decrease high early release
WOG generic severe accident management guidelines (SAMG) modified to VVER 440/213
Theory
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Accident Management can be divided into “levels of defense”
1. Measures to restore cooling shortly after core damage and stop the accident in the vessel
2. Measures to prevent containment failure
3. Measure to mitigate release for failed or bypassed containment
Higher level usually less efficient
Good “defense in depth” concept to have all levels
VVER-440 with high natural leak requires level 3 also for intact containment
PSA 2 indicates hydrogen as the highest priority, cavity (door) as the second highest priority
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Severe accident management
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Hydrogen
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Cavity and cavity door protection
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The plant is equipped with PAR for DBA, they are too slow
PHARE 94 2.07 showed that even extension of PAR is a problem – too large area
needed to eliminate risk of DDT
MELCOR 1.8.5 analyses indicate negligible risk for self-ignition at 10% of hydrogen
Caused by large differences in local concentration
Controlled combustion seems the most promising, igniters needed
NRI prepares a project to start in 2005 to analyze their number and location
More complex, the strategy depending on plant modifications – wet or dry cavity
Decision to use in-vessel retention by external cooling not yet taken
If not accepted, we can partially flood the cavity and cool the door
Risk of steam explosion in the cavity must be analyzed
High pressure melt expulsion must be prevented especially for water in the cavity
Existing SAG primary system depressurisation sufficient
Dry cavity strategy … simple thermal protection of cavity door - cheap solution
Other issues can be covered by procedures, except:
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Reduction of the release in primary to secondary accidents
Improvement of habitability of the control room
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Conclusions and plans for near future
• Limited scope PSA 2 proved to be a very good tool especially
when comparing risk importance of individual phenomena
• Extension to shutdown states needed and should start soon
• Before next revision of limited scope PSA 2 for power states (in
2006 ?), some problems have to be solved
• Most of them already included in other project:
– better containment strength calculation … results in 2004
– better scenarios … MELCOR 1.8.5 analyses in 2004 including SAMG
– decreasing conservatism of natural leak from the intact containment
…retention in walls and external building … 2004
– improved knowledge of steam explosions including cavity strength … ??
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