Interpretation of Containment Chemistry Results from Phebus Test FPT2

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• Interpretation of Containment Chemistry Results
  from Phebus Test FPT2

Shirley Dickinson
Nathalie Girault
Luis Herranz
Philippe Raison
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Introduction

• Phebus-FP tests are an important source of data
  on fission product release and transport under
  severe accident conditions
• New insights into iodine transport in primary
  circuit and chemistry in containment
• Focus for model development / validation /
  comparison
• FPT2 iodine chemistry in containment
• Summary of main results
• Interpretation and understanding
• Modelling studies

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Phebus-FP facility
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Phebus-FP facility
FPT2 containment 10 m3 steel vessel 100 dm3
aqueous sump, pH 9 Tatmosphere 90C Tsump
90C (aerosol phase), 120C (chemistry
phase) Sump evaporation atmosphere painted
condensers sump
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Experimental observations

• 55 of initial iodine inventory released to
  containment
• Mainly detected in containment as aerosol
• Small gaseous fraction during fuel degradation /
  release transient
• No evidence for gaseous iodine in circuit
• Iodine concentration evolution

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Iodine evolution during degradation / aerosol
phase
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Iodine evolution during chemistry phase
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Experimental observations (2)

• Final gaseous iodine concentration is lt 0.01 of
  containment inventory
• Predominantly inorganic
• Iodine in sump is mainly soluble
• Contrast FPT0/1 where mainly insoluble (AgI)
• Solubility decreased at end of test sump
  cooling
• Decrease in iodine activity on vertical
  containment walls in late aerosol chemistry
  phase
• Slight increase in condenser activity in late
  aerosol phase

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Modelling studies

• ASTEC-IODE, IMPAIR-JRC, INSPECT, MELCOR iodine
  module, IODAIR
• Dissolution of iodide aerosol in sump I- ions
• Radiolytic oxidation of I- I2 (volatile I)
• Reactions of I- and/or I2 with silver AgI
  (insoluble I)
• Partition of I2 containment atmosphere (gaseous
  I)
• Reaction of I2 with surfaces deposited I
• Reaction with organic impurities organic I
• Radiolytic oxidation of gaseous I2 or RI
  involatile I
• Input data experimental measurements, boundary
  conditions, geometry, estimated values

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Results overall iodine distribution

• Gross iodine behaviour is generally well
  reproduced by the models
• Low final gaseous concentration
• Mainly in sump or on surfaces during chemistry
  phase
• Surface deposition not reproduced as aerosol
  mechanisms are dominant
• Example

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IODEv5.2 and MELCOR results iodine distribution
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Iodine in the containment atmosphere

• Assumption of 1 gaseous source essential to
  reproduce early behaviour
• Origin? Circuit? Reactions on condenser surface?
• Early peak iodine rapidly removed
• Surface deposition transport to condensers
• Gaseous radiolysis
• Surfaces are not a strong sink for iodine
• Persistent gaseous concentration throughout
  aerosol phase
• Condensation is an efficient removal mechanism
• Transfer of iodine to sump

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IODEv5.2 calculation of gaseous iodine evolution
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Iodine in the containment atmosphere (2)

• Models suggest that organic iodide formation is
  the main source of gaseous iodine at longer times
• I2 volatilisation from sump is insignificant
• Decomposition of surface aerosols not modelled
• Gas-phase radiolysis has a significant impact on
  iodine behaviour
• Products are solid iodine oxides
• Uncertainties in reaction mechanism airborne
  speciation
• Aerosol formation and deposition modelling

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INSPECT / IODAIR calculations of airborne iodine
speciation
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Iodine in the sump

• Measurement show mainly soluble iodine
• Solubility decrease during washing cool-down
• Model predictions of solubility depend on Ag
  aerosol characteristics
• AgI formation rate depends on surface area,
  degree of oxidation both very uncertain
• Solubility changes suggest decomposition of AgI
  not generally modelled
• Iodine volatility from sump remains low as high
  pH and temperature suppress I2 formation

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IODEv5.2 calculations of iodine speciation in the
sump
Deposited
Dissolved
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Iodine release from surfaces

• Results appear to show loss of iodine from
  containment wall deposits
• 30 of deposited iodine re-released in late
  aerosol and chemistry phases
• Large wall deposition in FPT2 significant
  additional gaseous iodine source
• Possible contributor to concentration increase
• Not included in models
• Mechanism unknown, possible decomposition of
  iodide aerosol in containment atmosphere
• Potential impact in reactor calculations?

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Conclusions

• Modelling of iodine behaviour in FPT2 containment
  
• Detailed mechanistic models integral code
  modules
• Reasonable success in reproducing the main
  aspects
• Relative importance of phenomena varies between
  codes
• Consensus reached in many areas
• Potentially important uncertainties remain
• Radiolytic oxidation of gaseous species identity
  and behaviour of products

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Conclusions (2)

• Early gaseous iodine peak cannot be accounted for
  by reactions in sump
• Radiolysis of gaseous species involatile
  species
• Important influence on airborne iodine behaviour
• Iodine chemistry in sump not dominated by AgI
• Volatility suppressed by high pH, T
• Details of solubility behaviour not understood
• Evaporating/condensing conditions low
  steady-state I concentration in the containment
  atmosphere