The document discusses the proceedings from the global nuclear regional workshop on advanced Level 2 probabilistic safety analysis (PSA) held in Sofia, Bulgaria from July 15-19, 2013. It outlines the importance of Level 2 PSA in assessing containment's effectiveness following severe core damage, and describes methodologies for improving safety features and procedures in nuclear plants. Additionally, it highlights the development of filtered venting systems and various mitigation strategies to reduce radioactive releases during severe accidents.

1. WorleyParsonsWorleyParsons
Global NuclearGlobal Nuclear
Regional Workshop on Ad anced Le el 2 Probabilistic SafetRegional Workshop on Advanced Level 2 Probabilistic Safety
Analysis (PSA Level 2)
Bulgaria Sofia 15 19 July 2013Bulgaria, Sofia, 15-19 July, 2013
Alexander Wolski, Director Strategic Projects

2. Industry Leadery
2 19-Sep-13

3. Global ReachGlobal Reach
A combination of extensive global resources, world recognized technicalA combination of extensive global resources, world recognized technical
expertise and deep local knowledge
40,800 personnel |163 offices | 41 countries

4. Stress Test Reports
W l P A l i d R lt
Reviewed and analyzed all EU Progress and Final Stress Tests reports +
id tifi d l t d th d l t i l di IAEA id
WorleyParsons Analysis and Results
identified related methodology reports, including IAEA guidance
• Developed an activity & task-level work breakdown structure (WBS) for performing
stress tests
E t t d ll id tifi d b t i t ( h th l d f d d• Extracted all identified robustness improvements (whether already performed, under
implementation, planned or proposed) and developed categorized Improvements
Database

5. Improvements Database
G i C t i ti d C R f iGrouping, Categorization and Cross-Referencing
SEISMIC
►> 1500 d t i t General
FLOODING
General
Flooding protection engineering features/structures, e.g. dykes
EXTREME WEATHER CONDITIONS
ELECTRICAL SYSTEMS
►> 1500 raw data improvements
►Condensed to 209 across 72 plants
Almaraz ■■
Asco ■■
Emergency diesel generator (EDG) (Primary)
Mobile diesel generator (MDG)
…
Batteries
HEAT REMOVAL SYSTEMS
General
Belene ■■
Borssele ■
Safety injection systems
…
Spent fuel pool
Mobile pumps
ACCIDENT MANAGEMENT
General
…
…
PWR / VVER ■
BWR
General
Staffing
Procedures (Development & updating)
…
Hydrogen analysis and mitigation
Ex-vessel cooling
OTHER / GENERAL
Vandellos II ■
Zaporizhzhya ■■■■■■
PHWR ♦
UNGG / AGR / Magnox ○
RBMK ‡
OTHER / GENERAL
…
RBMK ‡

6. On-Going Efforts
P d t / S l ti D l t
Hardened Vent Design
• Finalized for Excelon & PPL plants
Product / Solution Development
• Finalized for Excelon & PPL plants
• Dry filter system developed
Passive Spent Fuel Cooling
• VVER Pools and internals modeled
• Residual Heat calculations finished
• Mitigation strategies defined, engineered
solutions under development
Hydrogen mitigationHydrogen mitigation
• WP MELCOR containment model for VVER-
1000/1200
• Cooperation with Bulgarian Academy of Science
Alternative Battery Systems
• Identified high-capacity FePO4-battery systems
• Commercial dedication on-going
Alternative EPS using GTG
0.4
0.45
CVH-X.4.14
CVH-X.4.15
CVH-X.4.22
Alternative EPS using GTG
• Relationship with Kawasaki and Siemens
• Joint definition of design & testing requirements
Steam-driven Aux F/W pumps0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
-2000 0 2000 4000 6000 8000 10000 12000
Steam driven Aux F/W pumps
• Transfer US concept to VVER
time, sec

7. IAEA policy on PSAIAEA policy on PSA
PSA is recognized to provide important safety insights in
addition to those provided by deterministic safety analysis.
IAEA pays a lot of attention and provides broad spectrum of support
in PSA development and assessments.S p
• Level 1 PSAs have now been carried out for almost all NPP worldwide.
• Level 2 PSAs have been, or are being, carried out for most NPP, g,
worldwide.
• Level 3 PSAs have been carried out for some NPP in some States.

8. PSA level 2PSA level 2
Level 2 PSA deals with assessment of the last physical barrier in DID concept –
Containment.Containment.
In Level 2 PSA, the chronological progression of core damage sequences
identified in Level 1 PSA is evaluated, including a quantitative assessment of, g q
phenomena arising from severe core damage to reactor fuel.
Level 2 PSA identifies ways in which associated releases of radioactive
material from fuel can result in releases to the environment.
Containment event tree analysis models the accident progression and identifies
the accident sequences that could challenge the containment and release
radioactive material to the environment.

9. Codes for SA Analysis
Several thermal hydraulic codes are used for assessment and evaluation of
Severe Accidents.
MAAP (EPRI) d MELCOR (S di NL f NRC) i USAMAAP (EPRI) and MELCOR (Sandia NL for NRC) in USA
ASTEC (IRSN and GSR) in Germany and France
Country specific codes for other countries Canada Japan RussiaCountry specific codes for other countries – Canada, Japan, Russia
MELCOR is the most used for SA evaluation for VVER reactors outside Russia.
MELCOR is extensively validated against experimental data. Adopted by a
worldwide group of users in regulatory, research and utility organizations.
Modularly structured in interchangeable code packages with well-defined
interfaces.

10. Containment Filtered Venting
R ti l
S id t lt i d t t
Rationale
Severe accidents can result in pressure and temperature
increase which may lead to containment failure and
uncontrolled release of radioactive products to thep
environment
Stress Test Results - “Containment venting must be
id d i th filt d i d f id tconsidered via the filters designed for severe accident
conditions, such as to ensure a sufficiently long venting
time”e
• Prevents over-pressurization
• Minimizes the radioactive releases into the environment and
decrease the off site dosesdecrease the off-site doses
• Decreases the land contamination
• H2 and other non-condensable gases concentration reduction

11. Containment Filtered Venting
D i C id ti
Maintain Defense in Depth, the containment is the last Barrier
Design Considerations
p ,
(retention of aerosols inside containment)
Aerosol retention better than 99.9%
A l i b l th 0 5 iAerosol size may be lower than 0.5 microns
Ability to handle decay heat
No catastrophic failure scenarioNo catastrophic failure scenario
Passive actuation and operation
Proven components with operating experience
Minimum impact on operation and maintenance of the plant
“Install and Forget”
No additional “concrete” for installationNo additional concrete for installation
Flexible dimensioning for every possible location
Minimum weight for seismic qualificationg q

12. Sizing of the Filtered Venting SystemSizing of the Filtered Venting System
5x10
5 Simulation of severe accidents
3x10
5
4x10
5
MPa
on VVER-1000, V-320 Model
P and T in one different cavity
geometries
1x10
5
2x10
5
P,
O C it
geometries
• Containment design pressure = 5
bar
0 20 40 60 80
1x10
Time, h
One Cavity
Two Cavities
600
• LB LOCA + Loss of all AC power
supply sources
• Core Damage
500
550
• Vessel Failure + melt ejection
• MCCI
• Generation of steam and non
350
400
450
T,K
• Generation of steam and non-
condensable gases
• P and T increase
Diff t ti i f t t f th
0 20 40 60 80
300
350
Time, h
One Cavity
Two Cavities
• Different timing for startup of the
filtered venting

13. Sizing of the Filtered Venting SystemSizing of the Filtered Venting System
2,5x10
5
1 4 10
5
1,6x10
5
1,5x10
5
2,0x10
5
kg
8 0 10
4
1,0x10
5
1,2x10
5
1,4x10
5
kg
5,0x10
4
1,0x10
5
Mass,k
2 0x10
4
4,0x10
4
6,0x10
4
8,0x10
4
Mass,
0 2 4 6 8 10
-5,0x10
4
0,0
Cav 1
Cav 2
0 1 2 3 4
-2,0x10
4
0,0
2,0x10
Melt ejected
• About 150 tons of melt transited to the cavity
Time, hTime, h
y
• Steel door between the cavity and other containment
compartment (2nd cavity)
Door failure and corium spreading• Door failure and corium spreading

14. Sizing of the Filtered Venting SystemSizing of the Filtered Venting System
5x10
5
520
4x10
5
5x10
a
440
460
480
500
K
2 10
5
3x10
5
P,MPa
VVER M 360
380
400
420
Temp,
VVER M
0 10 20 30 40 50
1x10
5
2x10
5
VVER M
VVER L
Limestone M
Limestone L
0 10 20 30 40 50
320
340
360 VVER M
VVER L
Limestone M
Limestone L
Comparison of P and T in VVER and Limestone
concrete cases
Time, h Time, h
• Two cavities
• Different structure of the molten pool
Mechanistic Mixture (available since MELCOR Version 1 8 3B)− Mechanistic Mixture (available since MELCOR Version 1.8.3B)
− Stratified corium layers

15. Sizing of the Filtered Venting SystemSizing of the Filtered Venting System
Mass of Aerosols
captured on filter
Total Decay Heat
inside Containment
Decay Heat from
Aerosols capturedAerosols captured
on filter

16. Sizing of the Filtered Venting Systemg g y
P t WPNS Uk i B l iParameter WPNS
(typical)
Ukraine Bulgaria
Start of venting, h 50.5 5.8 (37.2) 26.9
Temperature, ºC 223.3 138 (220) 139
Steam, % 53.3 70.1 66.2
H2, % 13.6 12.0 4.1
O2, % 4.8 3.7 4.9
Mass flow through the filter, kg/s 5.5 4.1 8.1
Mass median diameter, µm 1.44 / 0.95 1.48 / ---
Mass of filtered aerosols of the
aerosols, kg
3.4 (229*) 11.0 (324*) --- (331*)
Residual heat on the filter, kW 8.1 (80*) 14.6 (---) --- (236*)Residual heat on the filter, kW 8.1 (80 ) 14.6 ( ) (236 )
* Total released radioactive substances (solid and gaseous)

17. Sizing of the Filtered Venting SystemSizing of the Filtered Venting System
The simulations produce substantially different resultsThe simulations produce substantially different results
due to credible modifications of the starting conditions
Many of the phenomena are not yet well understood
(limited experimental data)
• Molten Core Concrete Interaction (including H2 generation)
• Chemical reactions in and above the melt• Chemical reactions in and above the melt
• Behavior of iodine (revolatilization in the sump, etc.)
• Condensation and Settlement of aerosols on “heat surfaces”
• Recombiners (air-mixing, iodine chemistry)
Limitations of the current code
• One dimensional models are applied• One-dimensional models are applied
• Nodalisation with gross transfers between nodes
Mitigation measures must be able toMitigation measures must be able to
handle a wide range of condition

18. Design Considerations
A l Filt R i f i ti S l ti
Dry solution (German) relies on stainless steel filter mesh
Aerosol Filter – Review of existing Solutions
Dry solution (German) relies on stainless steel filter mesh
Dry solution (France) relies on sintered steel filter
cartridges downstream of the gravel bedsg g
Wet solution (Germany) relies on stainless steel filter
mesh downstream of scrubber vessel
Wet solution (Switzerland) relies on “intelligent mixing” in
scrubber vessel
Ultimate reliance on stainless steel filters is the
pre-dominant solution

19. Implementation of Dry Filters
Aerosol FiltersAerosol Filters
Concept proposal: 3 filter modules, each with 85 filter cartridges (1m length)
Cartridge design is extremely flexible (length, diameter, arrangement) and will fit any footprint.
Robustness of design provided by fully welded metallic cartridges
HEPA filter material: Sinterflo® F Metal Fibre

20. Design Considerations
St i l St l filt Old & NStainless Steel filters – Old & New
Filter area each module:
1.32m x 2.00m = 2.64 m2
Filter area each cartridge: 0.54 m2
(length of 1 m, outer diameter 60mm)
Standard module w/ 8 surfaces
8 x 2.64 m2 = 21.12 m2
Module: 6.21 x 1.42 x 2.7 = 23.8 m3
Proposed module w/ 85 cartridges
85 x 0.54 m2 = 45.9 m2
(0.53m)2 x 3.14 x 1.43m = 1.26 m3
0.89 m2 filter / 1 m3 module volume
( )
36.4 m2 filter / 1 m3 module volume

21. Implementation of Dry Filters
Aerosol FiltersAerosol Filters
HEPA filter material: Sinterflo® F Metal Fibre
Ult Hi h ffi i HEPA FiltUltra-High efficiency HEPA Filter
High Permeability - Low Pressure Loss
Pleated - Low Foot Print
Durability SS316 temperature resistant up to 340ºC
Efficiency of Sinterflo® 2F3 Metal Fibre
Durability SS316, temperature resistant up to 340 C
y

22. Design Considerations
St i l St l filt B k dStainless Steel filters - Background

23. Design Considerations
R f M t i l C OH @ 60% idReference Material – CsOH @ 60% void
Densest
packing
of
spheres
26% VOID
LiOH
SiO2
CsOH
CsI
UO2
Tolerance against void fraction uncertainty
CsOH(30%) = 22 mbar / CsOH(90%) = 3 mbarCsOH(30%) = 22 mbar / CsOH(90%) = 3 mbar
LiOH(30/90%) = 455/2 mbar – UO2(30/90%) = 8/1 mbar

24. Design Considerations
St i l St l filt P ti l SiStainless Steel filters – Particle Size
R f P ti lReference Particle:
1 μm
13 mbar Δp- 13 mbar Δp
20 m2 filter will NOTNOT work if20 m filter will NOTNOT work if
average particle smaller
than .54 μm

25. Design Considerations
St i l St l filt L di (T l )Stainless Steel filters – Loading (Tolerance)
11 kg of CsOH11 kg of CsOH
result in
- 50 μm cakeμ
- 13 mbar Δp
(@ 6200 m3/hr)
150 m2 filtration area provide
t t l i t bl kiextreme tolerance against blocking

26. Design Considerations
St i l St l filt Filt ti S fStainless Steel filters – Filtration Surface
11 k f C OH11 kg of CsOH
150 m2 are in a
flat almost linearflat almost linear
range of the
variations.
Option:
2 systems to
increaseincrease
robustness even
further
Surface Heatload Cake Δp
20 m2 750 W/m2 374 μm 725 mbarμ
150 m2 100 W/m2 50 μm 13 mbar
300 m2 50 W/m2 25 μm 3 mbar

27. Implementation of New Dry Filters in V-320p y
Possible configuration - replacement of TL02 Aerosol Filtersg p
EXISTING
AEROSOL
FILTERS
New
AEROSOL
FILTERS

28. Implementation of New Dry Filters in V 320Implementation of New Dry Filters in V-320
Possible installation with the use of the existing penetrations TL42/TL22
A l filt i id t i t ( h f TL02 filt )Aerosol filters inside containment (exchange of TL02 filters)
Iodine filters outside containment in A1022