Nuclear Research Institute Řež plc...Nuclear Research Institute Řež plc Nuclear Research...

3/19/041

Nuclear Research Institute Ře� plc



Level 2 PSA for the VVER 440/213 DukovanyNuclear Power Plant

Jiří Dienstbier, Stanislav HusťákOECD International Workshop on �Level-2 PSA and Severe Accident

Management�, Cologne, 29-31 March 2004



19.3.2004 2

Outline• Plant features• History• Methodology used

– Main characteristics– Containment failure modes– Large event tree - APET

• 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



19.3.2004 3

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



19.3.2004 4

HistoryPSA 2 for unit 1

First (Revision 0) Limited scope Level 2 PSA• From 1995 to April 1998 as US AID project – contractor SAIC (Science Applications International Corporation) with

NRI Ře� 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 projectRevision 1• Autumn 1998 (SAM proposals updated in autumn 1999) by NRI Ře�• Using NRI Ře� living PSA 1 from 1998 (partially including new EOP), much different from the PSA 1 in rev.0• Extended to fires and floods• Large modification of the event tree – about ½ of questions changed keeping their order• Only small modification of basic eventsRevision 2• 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



19.3.2004 5

Main characteristics

Main characteristics • 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 release

categories• Large event tree (APET) method, the resulting tree has 100 nodes (usually more than

2 states):– 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



19.3.2004 6

Containment failure modes

Table 1 Containment failure modes

Failure mode Assumed effective leak size Caused by phenomena Early bypass rupture Bypass sequences – SGCB (single SG tube added to early leak) Early or late rupture 1 m2 Containment isolation failure*, pressurization due to hydrogen burn,

hydrogen detonation, steam explosion, vessel rocket, cavity or cavity door failure

Early leak 0.01 m2 Cavity door loss of tightness, SGTR Late leak 0.01 m2 Cavity basemat penetration, containment failure by slow pressurization

Intact containment natural leak 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.

• Classification of events timing:• Early � before reactor vessel bottom failure (and about 2 hours later for fission products)• Late � after this time

• Failure locations in the containment (several possible) and cavity (or cavity door)• Retention in walls or auxiliary building surrounding containment neglected• Containment strength curve (after DOE/NE-0086, 1989)

1) Containment � normal distribution, m = 400 kPa overpressure, s = 80.9 kPa2) Cavity � normal distribution, m = 2420 kPa overpressure, s = 460 kPa

• Possible containment isolation failure• Ventilation lines P-2 (TL-40), O-2 (TL-70)• Drainage, neglected in revision 2



19.3.2004 7

PSA 1 � PSA 2 Interface• 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

– 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• 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



19.3.2004 8

PSA 1 � PSA 2 Interface• Following 11 nodes

– HPI ... state of HP injection and recirculation– LPI ... state of the LP 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– 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:

• 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



19.3.2004 9

PSA 1 � PSA 2 Interface

Figure 1 Analysis of CDF

69%1%

1%

6%

22%

1%

Los s of ECC water

Complete los s of all electric powerincluding batteries

Hardware or control problemdifficult to s olve (s witch over torecirculation)Complete los s of electric power upto category 2

Error in procedure including humanerror (primary depres s uris ation)

Very limited core damage



19.3.2004 10

APET• Nodes (questions) 13 to 85• Development of APET - Main event tree as framework including:

– primary pressure before vessel failure, ECCs water location, early recirculation, vessel failure– containment failure early– late recirculation– containment status late

• Phenomenology– 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

• 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]

• Technical systems complicated the event tree and required repeating of some questions:– 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)– MELCOR plant analyses– detailed problems analysed by MELCOR (cavity)– hand calculation, engineering judgement– literature

• Hydrogen– 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



19.3.2004 11

Fission product release to the environment - source term

• Nodes 86 to 100– 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

• The result of 100 is sorted to 12 release categories– 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 failure2. late high – the same with late containment failure3. 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 failure5. very low – less than 0.1% of Cs and 0.01% of Ba

• 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)



19.3.2004 12

Summary results

Figure 2 Release classes and containment failure, case with PTS

fre que ncy [1/ye ar], CDF=2.968E-05

7.4742E-06 7.248E-06

2.296E-07

1.0909E-05

4.768E-084.773E-06

1.739E-05

3.9547E-06

4.3631E-076.9105E-06

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 2

re le as e containme nt s tate

very low noCF

late low CFL_Leak

early low CFL_Rp

late high CFE_Leak

early high CFE_Rp+Byp_Rp



19.3.2004 13

Summary results

F ig u re 3 R elea se c lasses an d co n ta in m en t fa ilu re , case w ith o u t P T S

fre q u e n c y [1 /y e a r], C D F = 1 .3 5 7 E -0 5

2 .573E -06

2 .2 96E -0 7

2 .597 4E -0 6

2 .3 05E -0 8

1 .6 70E -0 6

2 .799 6E -0 6

9 .1616E -0 7

3 .479 6E -0 7

9 .0 75E -0 6

6 .910 7E -0 6

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 2re le a s e c o n ta in m e n t s ta te

v e ry lo w n o C Fla te lo w C F L_ Le a ke a rly lo w C F L_ R pla te h ig h C F E _ Le a ke a rly h ig h C F E _ R p + B yp _ R p



19.3.2004 14

ResultsResults sorted according to• Consequences for PDS vectors

– 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

• Core damage– Limited 17,7% (38.5% w/o RPV-PTS) or Full

• Pressure at vessel bottom head failure– Low (below 0.8 MPa) 91.8% (82.0%)

• Most Important phenomena leading to containment failure% CDF (w/o RPV-PTS)

– E_Byp_Rp � 0.64 ( 1.40)– E_Rp � 23.78 (17.56)

• Hydrogen deflagration or detonation 12.34 ( 7.70)• Cavity failure (mostly steam explosion) 10.47 ( 7.72)

– E_Leak � 0.77 (1.69)• Single SG tube break 0.37 (0.81)

– L_Rp � 0.16 (0.17)– L_Lk � 16.06 (12.31)

• Thermal failure of door sealing 12.54 7.76• Basemat penetration 2.67 1.85

– Intact containment � 58.62 (66.93)



19.3.2004 15

Sensitivity studies• Sensitivity studies are the only method to assess uncertainty here

• Revision 0 PSA 2– 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– Accident management and preventive measures only– Also small event tree changes if needed– Most efficient

• Cavity flooding and external vessel cooling• Primary system depressurisation by operator• Combining depressurisation with other measures



19.3.2004 16

Sensitivity studies• 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%)



19.3.2004 17

Severe accident management• Present situation

– 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– Accident Management can be divided into �levels of defense�

1. Measures to restore cooling shortly after core damage and stop the accident in the vessel2. Measures to prevent containment failure3. 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



19.3.2004 18

Severe accident management• Hydrogen

– 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– 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

• Cavity and cavity door protection– 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:– Reduction of the release in primary to secondary accidents– Improvement of habitability of the control room



19.3.2004 19

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 curve … 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 … ??

Nuclear Research Institute Řež plc...Nuclear Research Institute Řež plc Nuclear Research...

Documents

Transcript of Nuclear Research Institute Řež plc...Nuclear Research Institute Řež plc Nuclear Research...