AN INTEGRATED APPROACH TO AN INTEGRATED APPROACH TO LIVING LEVEL 2 PSALIVING LEVEL 2 PSA
R. Himanen and H. SjövallTeollisuuden Voima Oy, FIN-27160 Olkiluoto, Finland
Presented at:
INTERNATIONAL WORKSHOPONLEVEL 2 PSA AND SEVERE ACCIDENT MANAGEMENT
COLOGNE, GERMANY
29TH TO THE 31ST OF MARCH 2004
Severe Accident Severe Accident Management in Olkiluoto Management in Olkiluoto
1 and 2 NPP1 and 2 NPP
• Asea-Atom BWR• Reactor thermal power 2500 MW • Net electric power 840 MW• Reactor pressure 7 MPa • Safety systems 4x50 %• Automatic liquid boron system for
ATWS and ATWC• Pressure suppression containment• Containment inerted with nitrogen
during normal operation• Drywell gas volume 4300 m3 • Wetwell gas volume 3000 m3 • Condensation pool volume
2700 m3
Severe Accident Severe Accident Management in Management in
Olkiluoto 1 and 2 NPPOlkiluoto 1 and 2 NPP• Containment design pressure
0.47 MPa • Containment ultimate capacity
1.01 MPa at 100 oC (95 % non-exceedance probability)
• Primary containment surrounded by reactor building acting as secondary containment
•Severe accidents were not included in the original design.•The provisions for severe accident management were installed in Olkiluoto 1 and 2 BWRs during the SAM project, which was finished in 1989.•The SAM approach is hardware oriented.•Plant modifications in order to prevent/withstand severe accident loads and minimize environmental consequences.
• Emergency Operating Procedure for severe accidents
The Emergency Operating Procedure for severe accidents contains instructions for severe accident management and covers all phases of severe accident including a full core melt:
- Primary system depressurisation- Flooding of lower drywell- Containment water filling- Procedures for filtered containment venting- Instructions to recover active core and containment cooling systems
Severe Accident Severe Accident Management in Management in
Olkiluoto 1 and 2 NPPOlkiluoto 1 and 2 NPP
PDS 10-6/ry Description
CBP 0.41 Containment by-pass (refuelling only)
RCO 1.3 Reactivity control lost.
ROP 0.13 Very early reactor overpressurization
COP 0.0072 Very early containment overpressurization
HPL 0.045 LOCA initiated core melt begins early at high pressure
HPT 3.6 Transient initiated core melt at high pressure
LPL 0.61 LOCA initiated core melt at low pressure
LPT 8.5 Transient initiated core melt at low pressure
RHL 0.22 LOCA initiated late core melt due to loss of RHR
RHT 2.2 Transient initiated late core melt due to loss of RHR
VLL 0.00005 Unsuccessful RHR using containment venting
VEN (51.) Successful RHR using containment venting (no CD)
FCF (11.) Fuel cladding failure (no CD)
CM 17. Total core damage frequency
Plant damage states and their frequencesPlant damage states and their frequences (Jan 2004)(Jan 2004)
Severe accident phenomena Severe accident phenomena studied in level 2 PSAstudied in level 2 PSA
In-vessel issues: Steam explosion and other in-vessel fuel-coolant interactionsRecriticalityHydrogen generationModes of vessel failure
Ex-vessel issues:
Direct containment heatingSteam explosion and other ex-vessel fuel-coolant interactionsGeneration of noncondensible gasesDebris coolability in the lower drywellCore-concrete interaction
Containment issues:
Non-inert containment during start-upDirect containment bypassContainment venting, leakage and failureBasemat penetration
Integrated simulation of physical and Integrated simulation of physical and probabilistic modelsprobabilistic models
• Simple graphical presentation of CET– ”if–then–else” –statements inside the branching points
• Physical parameters transferred and modified in accident sequences
• Simulation of the phenomenon at branching point– as a function of the input parameter set – production of the output parameter set for next b.p.
• Simulation of the probability at branching point– conditional probability of the branch – as a function of the result of the simulation of the physical
model
Integration of accident progression and Integration of accident progression and nuclide transportation models (1)nuclide transportation models (1)
• The analysis of source term and transportation of radio nuclides integrated into the simulation of each accident sequence
• No need for binning the CET sequences for this analysis
Integration of accident progression and Integration of accident progression and nuclide transportation models (2)nuclide transportation models (2)
•Time dependent transportation model
•Four dynamically sized control volumes– LDW, UDW, WW gas volume, and reactor building
•Time dependent gas flow between volumes – input parameters from MAAP
•Decontamination factors with uncertainty distributions
– pools – filter– containment spray– deposition on surfaces
The strength of containment weak The strength of containment weak pointspoints
Figure 8.5.4.1-1b: Containment weak points' break pressure as a function of temperature (median and confidence limits)
0,00
0,50
1,00
1,50
2,00
2,50
0 50 100 150 200 250 300 350 400
Temperature oC
Pre
ssu
re [
MP
a]
5% Door
Door
95% Door
5%Eq
Equipm
95%Eq
5%Ho
Hoop
95%Ho
5%DoBY
DomeBoltY
95%DoBY
5%DoSI
DomeSeInst
95%DoSI
5%DoS70
DomeSe70h
95%DoS70
5% 361
361
95% 361
5% 362
362
95% 362
Level 2 PSA showed that the containment may break due to sum pressure of steam and noncondensible gas
Modification in procedures:- Venting line isolation valve to be
left open after initiating event.- Possibility to fast automatic
venting through the rupture disk line
Severe Accident Severe Accident Management in Management in
Olkiluoto 1 and 2 NPPOlkiluoto 1 and 2 NPP
No modifications
Air lockstrengthened
Flooding trainedcurrent status
Inert start-up
Early - aggressivephenomena
Early - no flooding
Not inert - Early
Not Inert - Very early
Refueling
Early Venting
Figure 5: Impact of modifications, summaryFigure 5: Impact of modifications, summary
Figure 1: Venting line to be left open after Figure 1: Venting line to be left open after IE(1997). IE(1997).
Total LERF 7.9E‑6/ry, unfiltered 7.0E‑6/ry (89%)Total LERF 7.9E‑6/ry, unfiltered 7.0E‑6/ry (89%)
E_NFL 55%
L_CF 0%
REFUEL 5%
L_VENT_U 0%
E_VENT_U 11%
E_CF 8%
VE_VB_O2 2%
E__VB_O2 18%
VE_UD_ 0%VE_CBP 0%E_CF 8%E_NFL 55%E__VB_O2 18%VE_VB_O2 2%L_CF 0%REFUEL 5%L_VENT_U 0%E_VENT_U 11%L_VENT_W 0%
Severe Accident ManagementSevere Accident Management in Olkiluoto 1 and 2in Olkiluoto 1 and 2 MODE MODE
PROJECTPROJECT
• Energetic ex-vessel fuel coolant interactions
The range of the dynamic loading of steam explosions is estimated to be 10 to 30 kPas.
Regarding steam explosion loads the concrete structures are relatively stiff, particularly during the short period when the pressure waves are reflected.
Severe Accident ManagementSevere Accident Management in Olkiluoto 1 and 2in Olkiluoto 1 and 2 MODE MODE
PROJECTPROJECT
The median ultimate load impulse for the containment concrete structures, i.e. for the liner in the lowermost drywell wall sections corresponds to a rigid wall impulse of 54 kPas. The median ultimate load impulse for the personnel access lock was 6.3 kPas.
The lower drywell access lock of Olkiluoto 1 was modified in 2001 and Olkiluoto 2 in 2002 so that it will sustain a steam explosion of 54 kPas. The personnel lock tube is fixed to the concrete wall so that the connection can resist a steam explosion.
0.00
E+00
1.00
E-06
2.00
E-06
3.00
E-06
4.00
E-06
5.00
E-06
6.00
E-06
7.00
E-06
8.00
E-06
No modifications
Air lockstrengthened
Flooding trainedcurrent status
Inert start-up
Early - aggressivephenomena
Early - no flooding
Not inert - Early
Not Inert - Very early
Refueling
Early Venting
Figure 5: Impact of modifications, summaryFigure 5: Impact of modifications, summary
Figure 2: Figure 2: Lower containment air lock strenghtened (2001). Lower containment air lock strenghtened (2001).
Total LERF 7.4E‑6/ry, unfiltered 5.8E‑6/ry (79%)Total LERF 7.4E‑6/ry, unfiltered 5.8E‑6/ry (79%)VE_UD_ 0%
L_CF 0%
REFUEL 6%
L_VENT_U 0%
E_VENT_U 21%
E__VB_O2 19%
VE_VB_O2 2%
E_NFL 47%
E_CF 4%
L_VENT_W 0% VE_CBP 0%
VE_UD_ 0%VE_CBP 0%E_CF 4%E_NFL 47%E__VB_O2 19%VE_VB_O2 2%L_CF 0%REFUEL 6%L_VENT_U 0%E_VENT_U 21%L_VENT_W 0%
Severe Accident ManagementSevere Accident Management in Olkiluoto 1 in Olkiluoto 1 and 2and 2 SIMULATOR TRAINING SIMULATOR TRAINING
•Failure to flood the LDW in time has almost 50% contribution to the LERF.
•Full scope simulator on site•All shifts were trained on the simulator once, and the
flooding seems to succeed in time (2001)•Flooding of LDW trained also to the emergency
organization in full scope emergency exercise (2002)
0.00
E+00
1.00
E-06
2.00
E-06
3.00
E-06
4.00
E-06
5.00
E-06
6.00
E-06
7.00
E-06
8.00
E-06
No modifications
Air lockstrengthened
Flooding trainedcurrent status
Inert start-up
Early - aggressivephenomena
Early - no flooding
Not inert - Early
Not Inert - Very early
Refueling
Early Venting
Figure 5: Impact of modifications, summaryFigure 5: Impact of modifications, summary
Figure 3: LDW flooding – operators trained Figure 3: LDW flooding – operators trained (2001). (2001).
Total LERF 6.6E‑6/ry, unfiltered 3.6E‑6/ry (54%)Total LERF 6.6E‑6/ry, unfiltered 3.6E‑6/ry (54%)VE_UD_ 0%
L_VENT_U 0%
E_CF 9%VE_CBP 0%L_VENT_W 0%
L_CF 0%VE_VB_O2 3%
REFUEL 6%
E_VENT_U 46% E__VB_O2 21%
E_NFL 14%
VE_UD_ 0%VE_CBP 0%E_CF 9%E_NFL 14%E__VB_O2 21%VE_VB_O2 3%L_CF 0%REFUEL 6%L_VENT_U 0%E_VENT_U 46%L_VENT_W 0%
What if?What if?
• Inert start-up from refueling• Several negative effects, like more difficult
leakage check at start-up• Benefit rather small
0.00
E+00
1.00
E-06
2.00
E-06
3.00
E-06
4.00
E-06
5.00
E-06
6.00
E-06
7.00
E-06
8.00
E-06
No modifications
Air lockstrengthened
Flooding trainedcurrent status
Inert start-up
Early - aggressivephenomena
Early - no flooding
Not inert - Early
Not Inert - Very early
Refueling
Early Venting
Figure 5: Impact of modifications, summaryFigure 5: Impact of modifications, summary
Figure 4: Inert cmnt when start-up (option). Figure 4: Inert cmnt when start-up (option). Total LERF 6.4E‑6/ry, unfiltered 2.9E‑6/ry Total LERF 6.4E‑6/ry, unfiltered 2.9E‑6/ry
(46%)(46%)
E_NFL 26%
E_VENT_U 54%
E_CF 11%
REFUEL 6%
VE_VB_O2 0%
E__VB_O2 0%
VE_UD_ 0%VE_CBP 0%E_CF 11%E_NFL 26%E__VB_O2 0%VE_VB_O2 0%L_CF 1%REFUEL 6%L_VENT_U 0%E_VENT_U 54%L_VENT_W 0%
Summary of parts of level 2 PSASummary of parts of level 2 PSA
Structural – analysis of the strength of the containment
– details, strength against static and dynamic loads
– uncertainties before cut off
Physics– thermal hydraulics, phenomena, loads
– sequence specific source terms
– use of several codes, comparison of results
– not to be limited in ”representative” or ”worst” cases
– uncertainties before cut off
Probabilistic– accident sequences
– treatment of uncertainties (not cut off)
– importance ranking
SummarySummary
Structural • Omission of detailed and realistic analyses with
uncertainties may lead to biased risk profile
Physical• Omission of detailed plant and accident
sequence specific analyses with sensitivity studies may lead to misunderstanding of uncertainty and biased risk profile
Probabilistic• Next page
SummarySummary
Probabilistic
• Level 2 PSA in SAM is like map and compass in orienteering
• Without them one can– loose his way in the forest of structures
or – go deep to the endless morass of
physical phenomena
Top Related