CRP on Natural Circulation Phenomena, Modelling and Reliability

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• CRP on Natural Circulation Phenomena, Modelling
  and Reliability
• of Passive Safety Systems that Utilize Natural
  Circulation
• Oregon State University, Corvallis, USA, 29
  August - 2 September 2005
•    
• Natural circulation and use of passive heat
  removal principles in
• VVER-440/V213 reactors
•  
•  P. Matejovic, M. Bachraty

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• VVER-440/V213 reactors
• 2nd generation of Russian PWRs
• specific features horizontal SG, six loop
  arrangement, etc.
• large primary and secondary side coolant
  inventories
• low core power density
• huge T-H margin allows to use benefit in BDBAs
  or (after
• certain modifications) even in severe
  accidents

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Two examples of decay heat removal via natural
circulation presented here 1)  transition from
forced to natural circulation in the case of ATWS
with trip of all RCPs   2)  in vessel corium
retention via reactor cavity flooding One
example of gravity driven cooling 3)  passive
gravity driven feeding of SGs in emergency case
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• 1)  ATWS with trip of all RCPs during nominal
  operation
• Smooth transition from forced to natural
  circulation (high RCP inertia)
• The reduction of core power occurs only due to
  neutron kinetic feedback from increased moderator
  and fuel temp.
• RELAP5-3D code used
• Two cases analyzed BOL and EOL (main difference
  in moderator coefficient due to high boron
  concentration)
• No operator intervention considered

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• Results
• new stable state reached after transition
  period
• BOL
• - power stabilized 23 Nnom
• - PRZ RV opening and the coolant leak lasted 11
  min.
• - saturation state at the core exit, steam
  content 11
• EOL
• - power stabilized 16 Nnom
• - only negligible coolant leak through PRZ RV
• - subcooled water at the core exit

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• 2) In vessel corium retention via reactor cavity
  flooding
• Accident management measure for severe accidents
  of PWRs of small and medium power
• Main goal limiting the consequences of severe
  accidents to in-vessel phenomena
• Heat is removed from corium through RPV wall to
  coolant in flooded cavity and hence via natural
  circulation to confinement

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• Assumptions of VVER-440/V213 design for
  implementation of IVCR
• - low thermal power
• - exiting flow paths for cavity flooding
• - sufficient water resources for cavity flooding
• Contrary two new advanced designs, there are 2
  main obstacles in VVER-440/V213 cavity
• - exit restriction on the elevation of RPV
  support structures
• - thermal/biological shield of lower RPV head
• gt certain hardware plant modifications are
  necessary

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In vessel corium retention for VVER 440/V 213
variant with simple modification of thermal
shield
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Ventilation ducts for reactor cavity flooding
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Thermal and biological shield of RPV lower head
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Hydraulic cylinder used for lowering of
thermal/biological shield at Loviisa NPP
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• RELAP5-3D analysis
• performed in order to study natural circulation
  and heat transfer in closed loop
  pool-downcomer-cavity-riser-pool
• RPV modelled as thermal structure with
• - defined heat flux profile on internal surface
• - convective boundary condition on external
  surface
• Results
• Oscillatory flow behaviour predicted
• Even during short periods of flow stagnation heat
  transfer crisis not predicted

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Temperature distribution in the RPV wall
(RELAP5-3D) (elliptical bottom is spread)
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• 3) Gravity driven SG feeding in the case of plant
  blackout
• Plant blackout total loss of AC power, RCP trip,
  loss of FW, gradual decreasing of SG water level
• Time margin to start of core heat-up
• Basic idea of passive SG feeding
• gt elevation of 2 FW tanks gt elevation of SGs
• gt depressurization of SGs below pressure in FW
  tanks
• gt passive once-through bleed and feed on the
  secondary
• side

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• Modelling of horizontal SG
• Water level changes on secondary side of SG
• Special refined nodalization necessary to model
  changes of flooded heat transfer area

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VVER-440 horizontal SG
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• Results
• Initial depressurisation of secondary system with
  defined trend 30 C/hour
• FW injected into SGs in several pulses
• Time margin to core damage gt 20 hours

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Energy balance in primary system
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