2nd IAEA Research Coordination Meeting on CRP on Natural Circulation Phenomena, Modelling and Reliab

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2nd IAEA Research Coordination Meeting on CRP
on Natural Circulation Phenomena, Modelling and
Reliability of Passive Safety Systems that
Utilize Natural CirculationOregon State
University, Corvallis, Oregon (USA), 29th August
2nd September 2005

• RELIABILITY EVALUATION OF PASSIVE SYSTEMS THROUGH
  FUNCTIONAL RELIABILITY ASSESSMENT
• L. Burgazzi
• ENEA
• Bologna, Italy

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PRESENTATION OUTLINE

• Introduction
• Passive Systems
• Passive Systems Reliability and Safety
• Functional Reliability Approach
• Implementation of the Probabilistic Approach
• Probabilistic Model Application
• Results and Conclusions

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GENERICS on PASSIVE SYSTEMS

• IAEA definitions
• Passive Component a component which does not
  need any external input to operate
• Passive System either a system which is composed
  entirely of passive components and structures or
  a system which uses active components in a very
  limited way to initiate subsequent passive
  operation
• Passive System Categorization
• A physical barriers and static structures,
• B moving working fluids,
• C moving mechanical parts,
• D external signals and stored energy (passive
  execution/active actuation)

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CLASSIFICATION of PASSIVE SYSTEMS
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PASSIVE SYSTEMS SAFETY and RELIABILITY

• Innovative reactors largely implement passive
  systems
• Applications of passive systems for innovative
  reactors demand high availability and reliability
• PSA analysis
• Still lack of reliabilty estimates for passive
  systems (Reactivity Control, Decay Heat Removal,
  Fission Product Containment)
• In fact current system reliability is evaluated
  considering only the component reliability
  (mechanical, electrical)
• The reliabilty of the implemented natural
  physical phenomena (natural convection,
  conduction, gravity, etc.) under extreme
  boundary conditions, upon which they rely, is
  never considered
• Need for a consistent approach for reliability
  assessments of all passive functions categories

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PASSIVE SYSTEMS SAFETY and RELIABILITY contd

• Probabilistic reliability methods for passive A
  safety functions have been extensively developed
  and applied in structural and fracture mechanics
• For many active systems as well as for several
  passive C and D systems reliability figures may
  be derived from operating experience
• For passive B type functions basing on physical
  principles, there is no agreed approach towards
  their reliability assessment yet
• systems/components reliability
• physical phenomena reliability

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OBJECTIVE

• Methodology for natural circulation cooling
  systems performance evaluation on the probability
  standpoint
• Passive Systems for Decay Heat Removal
  implementing in-pool heat exchangers and
  foreseeing the free convection (e.g. PRHR for AP
  600, IC for SBWR and ESBWR)
• Isolation Condenser
• Core Decay Heat removal from the reactor, by
  natural circulation following an isolation
  transient
• Limit the overpressure in the reactor system at a
  value below the set-point of the safety relief
  valves, preventing unnecessary reactor
  depressurization
• IC is actuated on MSIV position, high reactor
  pressure and low reactor level

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Pool Makeup
                                     
Isolation Condenser
Cooling Pool
Turbine
Steam
Vent Line
Drain Valve
Vent Valves
Feedwater
Liquid
Vessel
Suppression Pool
Scheme of Isolation Condenser for a BWR
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NATURAL CIRCULATION ANALYSIS

• Failure modes e.g.
• non-condensable build-up
• thermal stratification
• physical degradation
• surface oxidation
• heat dissipation
• cracking
• Difficulties in performing meaningful reliability
  analysis and consequently in deriving relevant
  reliability figures
• Introduction of the functional reliability
  approach
• System is required to achieve a given mission

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FUNCTIONAL RELIABILITY ASSESSMENT APPROACH

• The Reliability of Passive Safety System is
  assessed through the Passive Function Reliability
  defined as the probability to fail the requested
  mission to achieve a generic safety function
• The mission, to be fulfilled by the system, is
  defined through a nominal requested time
  dependent evolution, for a set of representative
  plant parameters, and an allowable range is
  allocated around the nominal evolution

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FUNCTIONAL RELIABILITY ASSESSMENT APPROACH contd

• Failure evaluation by comparing parameters values
  to the allowable ranges

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IMPLEMENTATION OF THE PROBABILISTIC APPROACH

• From structural reliability a structure is
  characterized   by its "resistance" R and its
  "stress" S.
• Two possible outcomes 
• ? the safety margin R-S
• ? the safety factor R/S

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IMPLEMENTATION OF THE PROBABILISTIC APPROACH
contd

• The generalisation of the R-S concept to other
  passive functions or systems can be obtained
  considering
• R functional requirement
• and
• S system state
• R and S represent one parameter characteristic of
  the system performance, referred to the required
  and actual state conditions respectively (e.g.
  flow rate, heat exchanged)  
• In the probabilistic model all possible values r,
  s of R, S are assigned a probability density
  fR(r), fS (s)
• Any possible realisation r ? s contributes to
  the failure probability Pf which is defined as
  the probability that r - s ? 0

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IMPLEMENTATION OF THE PROBABILISTIC APPROACH
contd

• Actually, according to the functional reliability
  model, there are an upper and lower value of the
  critical parameter above and below which,
  respectively, the mission fails, i.e. the system
  based on natural circulation is not able to
  remove the reactor core decay heat
• Failure probability is obtained by combining the
  two failure modes
• Pt 1.0- ((1.0 - P1)(1.0 - P2)
• where Pt overall probability of failure
•  
• P1 through P2 individual probabilities of
  failure

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IMPLEMENTATION OF THE PROBABILISTIC APPROACH
contd

• Assuming as characteristic parameter the
  exchanged heat flux Q
• Qs actual heat flux
• QrH critical heat flux upper value
• QrL critical heat flux lower value

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PROBABILISTIC MODEL APPLICATION

• Water mass flow rate as characteristic parameter
  Z, direct indicator of the passive system
  performance
• Thermal Power exchanged through the IC too as
  suitable characteristic parameter
• Failure criterion
• Zs/ Zn 0,8 Zn is the mission requested nominal
  value for natural circulation
• Zs is the actual value
• Modeling assumptions
• Failure mode for parameter greater than upper
  value is not accounted
• Subtle difference with respect to the R-S model
  reversal of conditions for system failure
  mission failure occurs in case SltR

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PROBABILISTIC MODEL APPLICATION contd

• Probability of failure of the system
• Pf P(Ws-Wrlt0) ? ? f(Ws)f(Wr)dWsdWr
  Ws-Wr0
• Where Ws actual flow rate value
• Wr minimal flow rate value required for natural
  circulation (i.e. WrZn0,8)
• f(Ws)f(Wr) is equivalent to the joint
  probability distribution of the parameters
  Wr,Ws, assumed independent
• Choice of pdfs (range and distribution)
• Only limited knowledge and data
• Engineering judgement
• As a general rule a pivot has been identified and
  range extended to higher and lower values pivot
  represents the nominal conditions, limits exclude
  unrealistic values

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PROBABILISTIC MODEL APPLICATION contd
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RESULTS

• Good agreement among the results
• Different types of distributions don't affect the
  reliability values which lie around 1.0E-1
• Failure probability values relative to discrete
  and continuous distributions are comparable
• Results are not qualified, e.g. pdfs and ranges
  relative to the characteristic parameter W (flow
  rate) have been arbitrarily assigned, due to lack
  of pertinent formulation and a consistent data
  base, basing on engineering judgement 

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CONCLUSIONS

• Failure probability of a natural circulation
  system (Isolation Condenser) through the
  introduction of the functional reliability
  concept related to passive systems
• Implementation of a probabilistic model drawn
  from fracture mechanics
• Selection of joint probability density functions
  of the parameter, indicator of the system
  performance, both for the actual and critical
  values for probabilistic evaluation of system
  stability

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CONCLUSIONS contd

• Results subject to the assumptions taken in the
  model, i.e. either the failure model or the pdfs
  and range assignment
• Results are not qualified, but this is due mainly
  to the exploratory character of this effort
• RD for the future

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REFERENCES

• L. Burgazzi, Reliability Evaluation of Passive
  Systems through Functional Reliability
  Assessment, Nuclear Technology, Vol.144,
  pp.145-151, November 2003