1. JSC Atomenergoproekt, Moscow, RF
Combined Use of Probabilistic and Deterministic Methods in External events PSA
Vladimir Morozov, Head of Safety Analysis Division
Gennady Tokmachev, Chief Engineer of Safety Analysis Division,
Igor Kalinkin, Head of EE Department in Safety Analysis Division
JSC Atomenergoproekt, Moscow, RF

2. About
JSC Atomenergoproekt is an engineering company, General Designer of nuclear power plants. The company developed the designs of the majority of NPPs in Russia, Eastern Europe and CIS countries. JSC Atomenergoproekt is the General Designer of the Bushehr NPP in Iran, Kudankulam NPP in India, Akkuyu NPP in Turkey. Atomenergoproekt is the General Designer and General Contractor of the Novovoronezh-2 NPP 2

3. About
PSA division (bureau) has over 25 year experience in developing PSA for units with WWER power reactors. Operation conditions include power operation and shutdown states. At present time the scope of development include internal level 1, level 2 and external events PSA. 3

4. Internal events PSA aids design and operation
Aid to justification of design basis
Initiating events selection and categorization
Provide input to categorisation of SSC
Aid to optimisation of system configurations
Helps to provide justification of conditions for safe operation (STI & AOT)
Justification of design basis – It is supplement to deterministic analysis. Assessment may include investigation of variants and exploratory design options, sufficiency in systems’ redundancy and diversity, effectiveness in emergency and accident management measures, as well as development of reliability and availability targets for SSCs to meet safety goals, if set
IE selection and categorization - probabilistic methods are used for categorization of initiating events within SAR development in terms of their frequency, one of the inputs to initiating event selection is a list of deterministic TH analyses, Depending on initiating event frequency acceptance criteria are defined in terms of fuel rod damage
Input to categorization –
Aid optimization
Provide input to categorisation of SSC
Helps to provide justification of conditions for safe operation (surveillance test intervals and average outage intervals) - Conditions for safe operation (limiting conditions for operation) specified in the technical specifications define equipment operability requirements and allowed outage times. The PSA is used to develop quantitative bases and justify limits on equipment allowed outage times, surveillance test intervals, and testing strategies. 4

5. Requirements of regulatory documents
“It should strive to have PSA estimated value for severe BDBA frequencies less than /year” (NP General provisions to ensuring NPP safety) for the whole scope of events
“PSA for external events must be developed to obtain NPP accident risk assessment due to external events” (NP Analysis of effects of natural and man-induced external events on nuclear energy objects.)
Justification of design basis – It is supplement to deterministic analysis. Assessment may include investigation of variants and exploratory design options, sufficiency in systems’ redundancy and diversity, effectiveness in emergency and accident management measures, as well as development of reliability and availability targets for SSCs to meet safety goals, if set
IE selection and categorization - probabilistic methods are used for categorization of initiating events within SAR development in terms of their frequency, one of the inputs to initiating event selection is a list of deterministic TH analyses, Depending on initiating event frequency acceptance criteria are defined in terms of fuel rod damage
Input to categorization –
Aid optimization
Provide input to categorisation of SSC
Helps to provide justification of conditions for safe operation (surveillance test intervals and average outage intervals) - Conditions for safe operation (limiting conditions for operation) specified in the technical specifications define equipment operability requirements and allowed outage times. The PSA is used to develop quantitative bases and justify limits on equipment allowed outage times, surveillance test intervals, and testing strategies. 5

6. The scope of external events analysis
Extreme natural events
Seismic events
High winds and tornadoes
Extreme air and water temperatures
External floodings, fires
Extreme precipitation …
External man-induced events
Air, road, water transportation incidents
Industrial facility incidents 6

7. General procedure for external events analysis
Is defined by national safety guide RB “Evaluation of reactor severe core damage due to external events” (at present time under revision)
It include:
Events selection and screening
Hazard analysis
Plant walkdowns
Plant response and fragility analysis (if required)
Bounding or detail analysis of accident sequences based on modified level 1 models, core damage estimation. 7

8. Event selection and screening
Development of preliminary generic list of external events
Analysis of site characteristics, preliminary screening using RB criteria A (distance – event cannot occur close to the site) ,B (inclusion – event is included in definition of another event which already have been analyzed) ,C (time – event is slow developing and plant operating personnel have enough time to prevent its consequences)
At preliminary stage only deterministic criteria are used 8

9. Hazard analysis (seismic)
Design base estimation is deterministic aimed to conservative assessment of maximum design earthquake (MRZ – once per years) and design earthquake (PZ – once per 1000 years) according to NP (Norms for design of seismic resistant NPP)
Seismic hazard curves are developed according classical PSHA approach
Along with seismic hazard response spectra is also estimated
Global seismic hazard program results ( and local seismological, tectonic and geological data are used to develop models for analysis 9

10. Hazard analysis (seismic)
Aleatory uncertainties include
Attenuation law variabilities
Seismic source characteristics (magnitude-length)
Source location inside source zone
Epistemic uncertainties (require development of logic tree)
Source characteristic (active/not, linear or area source, maximum magnitude determination)
Selection of attenuation law type
Selection of parameters for source magnitude – occurrence rate law
Others 10

11. Example of logic tree for seismic hazard
Source mechanism
Maximum magnitude estimate
Attenuation law
Thrust, 0,2
Normal, 0,5
Strike-slip, 0,3
Max, 0,2
Ave, 0,6
Min, 0,2
...
Campbell Bozorgnia, 0,33
Boore&Atkinson, 0,33
Model А, 0,33 11

12. Hazard curves after logic tree quantification12

13. Example of percentile and mean hazard curves13

14. Example hazard curve for maximum air temperatures (Gumbel)14

15. Uncertainty in hazard curve for minimum air temperatures (Weibull)15

16. Plant walkdown and seismic margin evaluation (deterministic procedures)
plant walkdowns in according to GIP-WWER procedures (Balakovo unit 1)
Seismic margin evaluation using CDFM (conservative definition of failure margins) using EPRI NP-6041 SL1 16

17. Conditional failure probability (fragility)
For seismic PSA – using hybrid approach
IAEA TECDOC Annex 1
Kennedy, R.P., Overview of Methods for Seismic PRA and Margin Assessments Including Recent Innovations, Proceedings of the OECD-NEA Workshop on Seismic Risk, Tokyo, Japan, August 1999
Load – strength model or FORM (first order reliability methods) for others 17

18. Some results for EE PSA (Balakovo unit1 Novovoronezh-2 unit 1)18

19. Thank you for attention !
ATOMENERGOPROEKT
Thank you for attention ! 19