SARNET WP5 - Level 2 PSA Comparison between classical and dynamic reliability methods. Specification and results of a benchmark exercise on consequences of hydrogen combustion during in-vessel core degradation E. Raimond, T. Durin IRSN, BP 17

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SARNET WP5 - Level 2 PSA Comparison between
classical and dynamic reliability methods.
Specification and results of a benchmark exercise
on consequences of hydrogen combustion during
in-vessel core degradationE. Raimond, T.
DurinIRSN, BP 17 92265 Fontenay-aux-Roses
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Background

• SARNET WP 5.3 Dynamic methods for level 2 PSA
• Stage 1 Review of existing approaches (cf. P.E
  Labeau ERMSAR-05)
• Stage 2 Practical application in a benchmark
  exercise
• Benchmark specification End 2005 / Mid 2006
• Solutions proposal by participants 2006
• Synthesis and additional contributions Mid-2007

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Background

• THE  CHALLENGE 
• TO PROVIDE A  SIMPLE  EXAMPLE
• THAT DEMONSTRATE THE LIMITATION
• OF CLASSICAL EVENT TREE METHODS
• AND GAIN OBTAINED BY DYNAMIC METHODS

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• PRESENTATION OF THE BENCHMARK EXERCISE

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Benchmark description

• A basic transient
• A French 900 MWe PWR (3 loops, with Passive
  Autocatalytic Recombiners PAR) operating at
  nominal power before the initiating event
• Loss of coolant accident (LOCA) after a 3 break
  size on cold leg of RCS,
• Failure of all water injection system and spray
  system
• An ASTEC calculation provides basic information
  on
• The kinetic of core degradation process
• The kinetic of hydrogen and vapor releases in
  containment
• The delay before vessel rupture
• The pressure evolution in containment (and
  atmosphere composition)

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INFORMATION FROM ASTEC
Maximum Hydrogen mass (100 Zr oxydation) May
be increased by steel oxydation
Pressure evolution in containment (between 2 or
3 bar)
Hydrogen mass released in containment
Time for core degradation beginning
Vessel Rupture time
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Benchmark description
For the basic transient (no water injection, no
spray) the containment atmosphere is not
flammable no combustion
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The issue

• After reparation, water injection and spray
  system are available after beginning of core
  degradation
• QUESTION what is the dominant risk of
  containment failure
• Three independent events are considered
• Water injection
• Spray system start
• Ignition of H2-H20-Air mixture by recombiners
• No chronological link between the events is
  assumed
• Consequences of each event is described by
  analytical models

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Water injection

• For the benchmark, consequence of water injection
  is only hydrogen production. An analytical model
  has been proposed

Maximum Hydrogen mass (100 Zr oxydation) May
be increased by steel oxydation
New hydrogen source term as a function of time
reflooding
Hydrogen mass released in containment without
reflooding
Vessel Rupture time
Time for core degradation beginning
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Spray system effect

• In the benchmark, consequence of spray system
  start is atmosphere containment depressurization,
  cooling and composition modification
• An analytical model has been proposed based on
  ASTEC results

Pressure evolution in containment (between 2 or
3 bar)
1 bar
New evolution of pressure in containment
Time for core degradation beginning
Vessel Rupture time
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Ignition (by recombiners or other)

•  Classical  physical criteria have been used to
  precise if combustion is possible and probable
• Combustion can be total or local only
• Multiple combustions can also occur
• Delay before combustion is unknown
• Pressure peak in containment due to combustion
  are evaluated by PAICC model

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List of used physical information

• A representative ASTEC transient without spray
  and reflooding
• Beginning of core degradation Vessel Rupture
• Hydrogen mass released in containment
• Containment Pressure as a function of time
• A simple law that allows to predict pressure
  evolution as a function of time after spray
  system start
• A simple law that allows to predict H2 release
  after core reflooding
• A simple law that allows to predict recombiners
  efficiency in function of H2 and H20
  concentrations
• Criteria for hydrogen combustion Shapiro,
  ignition by recombiners
• The probability of containment failure as a
  function of pressure peak

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Stochastic events

• Water injection
• Probability 0.5 to have water injection before
  vessel rupture
• uniform probability distribution
• Spray system start
• Probability 0.5 to have water injection before
  vessel rupture
• uniform probability distribution
• Ignition of hydrogen combustion
• Atmosphere flammability is defined with a
  Shapiro diagram and a criteria for ignition by
  recombiners
• (the atmosphere ignition within a short delay is
  very problable if the recombiners ignition
  criteria is achieved for average H2
  concentration)
• Local ignition (partial) have been taken into
  account

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• ??
• CAN THIS SIMPLE QUESTION BE SOLVED WITH A
  CLASSICAL EVENT TREE METHOD
• ??

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Difficulties for the classical event tree
approach

• The chronological links between events have to be
  defined
• 12 situations are possible from the chronological
  point of view
• Spray water injection ignition
• Spray water injection no ignition
• Ignition Spray water injection
• Ignition Spray no water injection
• Spray ignition water injection
• Spray ignition no water injection
• Ignition water injection Spray
• Ignition water injection no Spray
• water injection ignition Spray
• water injection ignition no Spray
• water injection Spray ignition
• water injection Spray no ignition

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Difficulties for  classical event tree approach

• The treatment of all chronological issues is
  difficult.
• !!!! The treatment of multiple combustions is
  impossible !!!
• The only way for a classical approach is a
  conservative approach
•  TO CHECK THAT IN THE WORTH CASE, THE
  CONTAINMENT FAILURE RISK IS RESIDUAL 

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2 steps

• STEP 1 First implementation of the problem with
  dynamic or classical method, with simple
  analytical model for the physics
• STEP 2 Second implementation of the problem
  with complements in the analytical models for
  epistemic uncertainties
• ?? Possible STEP 3 Implementation of ASTEC
  modules instead of simple analytic model ??

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• SYNTHESIS OF THE BENCHMARK RESULTS

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10 PARTICIPANTS

• IRSN
• GRS
• CEA
• AREVA
• VEIKI
• ULB
• (CSN)
• LEI
• UJV
• INR

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Five categories of solutions

• Direct calculation
• Classical event tree methods
• Macro-event method with classic tools
• Monte Carlo Dynamic Event Tree (MCDET)
• Stimulus-Driven Theory of Probabilistic Dynamics
  (SDTPD)

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Methods1- Direct calculation (CEA, INR)

• Monte Carlo simulations (C or Fortran program)
  
• time of water injection and spray system
  activation are randomly sampled
• For each sequence
• The evolution of the system is calculated for
  each time step (1 second)
• If the containment atmosphere is flammable, a
  delay before combustion is determined
• Overpressure is calculated
• Containment integrity is determined
• simple method, the validation of the model is
  easy
• - few information for analysis, implementation
  in global event tree ?

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Methods2- Classical event tree method (AREVA,
VEIKI INR)

• Specific calculations of containment composition
  evolution are performed for a few sequences (with
  EXCEL)
• Results are used for quantification of
  probabilities of branching nodes in a containment
  event tree
• Stochastic events become determinist (water
  injection)
• Some assumptions are modified
• easy to implement (because of the modified
  assumptions)
• - relevance of conservative results for
  practical applications ?
• Specifications cannot be fulfilled and
  accordingly results are very different

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Methods2- Classical event tree method (AREVA,
VEIKI INR)
Example (from VEIKI contribution)
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Methods3- Macro-events methods (UJV, IRSN)

• Based on software initially created for accident
  progression event tree (EVNTRE, KANT)
• Division of the simulation time in short time
  intervals (60 seconds)
• Use of the same event tree (macro-event) for each
  interval
• Quite comparable to the direct calculation method
  with Monte-Carlo simulations (for IRSN) or to the
  MCDET-method (for UJV)
• use of classic tools, easy to integrate in a
  global event tree
• -

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Methods3- Macro-events methods (UJV, IRSN)

• IRSN macro-event UJV
  macro-event
• Each macro-event is duplicated in a global event
  tree

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Methods4- MCDET-analysis (GRS)

• Mix of Discrete Dynamic Event Trees (DDET) and
  Monte Carlo Simulation
• Each DDET has the same structure with 22
  sequences, only the random values change (Monte
  Carlo)

allows a lot of sensibility analysis,
epistemic uncertainties and stochastic events are
considered singly
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Methods 5 - Stimulus-Driven Theory of
Probabilistic Dynamics (SDTPD) - (ULB, CSN)

• General methodology used as a basis for a Monte
  Carlo simulation of dynamic reliability problems.
• The SDTPD analysis is based on a particular
  formalism
• the process variables are the variables that
  describe the system evolution (6 variables for
  LEI and 9 variables for ULB)
• the stimuli are the events that can happen
  during the simulation (5 stimuli are defined by
  ULB and 6 by LEI)
• the dynamics are the different regimes of
  evolution of the continuous process variables.

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Methods 5 - Stimulus-Driven Theory of
Probabilistic Dynamics (SDTPD) - (ULB, CSN)

• The evolution of each process variable is defined
  for each dynamics.
• To each stimulus are associated
• a probability of activation and a probability of
  deactivation if needed,
• an activation (and if needed deactivation) delay.
  
• This method allows a more precise modeling of
  combustion if the flammability conditions
  change, combustion can be canceled. This point
  was not clearly specified in the specification of
  the benchmark exercise, but it shows one of the
  interesting capacities of the SDTPD.
• A given number of histories (simulations) are
  performed. For each simulation, the final result
  is the containment integrity (saved or not). This
  allows determining the containment failure
  probability.

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RESULTS STEP 1
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RESULTS STEP 2
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COMMENTS

• The difference between results has not yet been
  fully explained and should not be linked to the
  method but also to benchmark assumptions
  interpretation
• The different contributions shows different
  methodologies with advantages / disadvantages.
• The analysis/comparison of results has shown some
  needs in terms of guidance for the presentation
  of uncertainties

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Some outlook

• These results are an encouragement to continue
  the development of specific methods for dynamic
  reliability problems, including specific
  post-processing of results, especially for
  uncertainties.
• Some participants are interested for a step 3
  with a direct use of a severe accident code like
  ASTEC
• Such application is seen (at IRSN) as an
  interesting way for examination of robustness of
  severe accident guide