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
• 11-15 September, 2006, CEA, Cadarache, France   
• Modelling of natural circulation phenomena
• in VVER-440 reactors
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•  P. Matejovic, M. Bachratý

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VVER-440/V213 design

• 2nd generation of soviet design PWRs of medium
  power
• incorporated most of design requirements of PWRs
  built at the same time (e.g. 3 x 100 ECCS)
• constructed in Russia (2), Ukraine (2), CEC
  (122), Finland (2)
• 2 units still under construction (Mochovce 34
  NPP, frozen state)
• rather inefficient, but robust and conservative
  design with large T-H safety margin
• Loviisa NPP upgraded for severe accidents (IVR,
  PARs)

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VVER-440/V213 equipped with bubbler condenser
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VVER-440 geometry of primary system

• Natural circulation is influenced by
• horizontal SG gt driving head for the natural
  circulation is rather small
• six loops configuration
• loop seals in both, hot and cold legs
• large primary and secondary side coolant
  inventories

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PACTEL facility (old configuration) Scaled-down
model of Loviisa NPP

• Volumetric scale 1305
• Basic elevations preserved in full scale
• Reduced number of loops (3 instead of 6)
• Limited power (22 of scaled full power)
• Limited maximum primary pressure (8 MPa)
• Full length SG tubes (gt low SG)

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PACTEL facility, new
configuration

• RCPs added
• Shorter SG tubes gt higher SG
• Accumulators and LP ECCS added
• Widely used for LOCAs, SG boil-off , etc.

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SG tubing
VVER-440 SG horizontal cross-section
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Natural circulation with stepwise reduced
primary inventory (ISP-33)     Main Goals of
ISP-33        to study experimentally natural
circulation in VVER-440 over a range of primary
side inventory levels   to test the ability of
thermal-hydraulic computer codes to analyse this
kind of phenomena
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• Expected periods during the experiment
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•    single-phase natural circulation
• two-phase natural circulation with continuous
  liquid flow
•      natural circulation of reflux boiling type
•      cooling of the core with partially
  superheated steam

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Basic characteristics of ISP-33
experiment    core power corresponds to decay
heat (3.7 of the scaled nominal
power)      stepwise reduced primary mass
inventory (7 drainings totally, 9.5 of initial
mass drained in each step)      secondary side
heat sink preserved      start of core heat-up
gt depressurisation of SGs and switch-off core
heaters
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RELAP5-3D analysis of ISP-33 experiment and its
counterpart for full scale Q may we extrapolate
our analytical approach from reduced to full
scale? Differences between PACTEL and
VVER-440/V213      Reduced volumetric scale
(1305)      Higher importance of heat losses
and heat capacities      Reduced number of
loops (3 instead of 6)      Lower height of SG
tubing      Certain different in elevations,
shape of cold leg loops seals, etc.
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RELAP5-3D nodalizations of PACTEL and
VVER-440/V213   - only 1-D capabilities of
RELAP-3D used for both - 3-loop and 6-loop
nodalization - recirculation model of the
SG Validation of input decks PACTEL
characterising tests, (LOCAs, SG boil-off)
VVER-440 transients recorded during plant
operation VVER-440 input deck artificially
adjusted to ISP-33 secenario
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RELAP5 Nodalization of PACTEL
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Visualization of RELAP5-3D nodalization of PACTEL
using Graphical User Interface
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Results of PACTEL ISP-33 analysis and its
counterpart for full-scale VVER-440/V213 case
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• Conclusions
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• For primary mass inventories gt 50 of nominal
  value the overall behaviour agree well with
  experiment
• Transition from single to two-phase NC higher
  number of pressure spikes in full-scale
  VVER-440/V213 case, but the magnitude is lower
• The start of core heat-up is predicted for
  VVER-440/V213 one drainage earlier than in
  experiment (differences in geometrical
  arrangement of cold leg loop seals)
• in typical LOCAs the coolant is lost in the form
  of steam-water mixture from loops whereas in the
  ISP-33 scenario the coolant was drained in water
  phase from RPV bottom.

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• Mass-flow-rate through downcomer versus primary
  inventory