PSA Level 2 Source Term Approach in the Loviisa NPP

1
PSA Level 2 Source Term Approach in the Loviisa
NPP

• T. Routamo, S. Siltanen, P. Lundström
• Fortum Nuclear Services, Finland
• OECD/CSNI International Workshop on Level 2 PSA
  and Severe Accident Management
• Köln, Germany, 29-31 March 2004

2
Outline

• SAM approach of the Loviisa NPP
• Tool for source term calculation in PSA 2 (SaTu)
• Sample results
• Uncertainty calculation
• Conclusions

3
Loviisa NPP

• Specific features
• Ice condenser containment (volume 58 000 m3,
  design pressure of 1.7 bar)
• Horizontal SGs
• Low decay power level
• Narrow reactor cavity
• No penetration in the RPV lower head

4
SAM Mitigation in the Loviisa NPP

• Successful containment isolation
• Primary system depressurisation
• Mitigation of hydrogen combustion
• Reactor pressure vessel lower head coolability
  and melt retention
• Long-term containment cooling

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SaTu - Introduction

• Update of Loviisa PSA level 2 for internal events
  in at-power states due in 2004 - SaTu as a new
  source term model
• Initially built as a support system for radiation
  experts (two modes radiation level mode and PSA
  2 ST mode)
• Calculations are based on user input and fixed
  parameters modelling the processes and phenomena
  during severe accidents
• Approach to describe the release from core and
  transport with simple models (implemented in
  Excel)

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SaTu - Transport solution (basic idea)

• Constant source (S) and removal rate (k)
• Source formed as a combination of the release
  from the core and fission product flows from
  consecutive compartments, and averaged in each
  time step
• Calculation of one compartment at a time
• Mass loss in the containment loop flow avoided by
  special treatment of the returning mass flow
• Mathematical solution considered adequate

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SaTu - System Description (Flow Chart)
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SaTu - System Description (1)

• Modelled processes
• FP and hydrogen release from the core
• Effect of ECCS injection recovery
• Deposition in the primary and secondary circuits
  and in the auxiliary system piping
• Transport along with gas and water flows
• Deposition in the containment
• diffusiophoresis
• sedimentation
• effect of ice condensers
• containment spray system
• containment external spray

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SaTu - System Description (2)

• Modelled processes (cont'd)
• Hydrogen recombination
• Iodine chemistry (formation of gaseous and
  organic iodine)
• Leakage out of the containment
• Transport and deposition in the areas outside the
  containment
• Release to the environment
• Parameters affecting the fission product
  behaviour are based on experimental data and code
  calculations.

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SaTu - Input Data

• Essential input information
• LOCA type (used for defining the leak location)
• Openings through the main deck (containment loop
  flow mode)
• Time of core uncovery (FP release from degrading
  core)
• SAM measures (depressurisation, IC doors,
  containment external spray)
• Possible time of ECCS injection recovery
• Emergency water resources and sump water level
  (amount of ice in ICs)
• Containment internal spray flow rate (FP washout)
• Containment pressure and leakages (FP release out
  of the containment)
• Ventilation system flow rates (FP transport in
  areas outside the cont.)
• Status of stack filters (FP retention from stack
  release)

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SaTu - User Interface

• Built on several workbooks
• PSA 2 ST calculation mode
• User input and FP transport calculation
• Limited amount of input information
• Input mainly based on plant measurements
• Results in a graphical form
• Additional workbooks in the radiation level mode
• Activity of radionuclides
• Coefficients for activity conversion to radiation
  levels
• Dose rates at control centres and at some
  additional areas
• Dose rates at a selected time in plant layout
  drawings
• Pre-calculated accident sequences

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SaTu - User Interface (Sample Output)
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SaTu - User Interface (Sample Output)
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SaTu - Uncertainty Environment

• Parameters of the FP modelling not accurately
  known
• Uncertainty calculation environment was
  implemented
• Monte Carlo simulation used to study the overall
  effect of the fixed parameter variations
  (sequence uncertainties e.g. timing of events not
  included)
• A single case runs in SaTu in some seconds
  (immediate result)
• A thousand case simulation in less than two hours
• Results shown as percentiles of the FP release
  behaviour
• Possibility to find out the modelling
  deficiencies

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SaTu - Uncertainty Calculation Example
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SaTu - Conclusions

• Capabilities of SaTu seem adequate, some models
  still need revising though.
• Issues specifically important (e.g. SAM measures)
  for the Loviisa NPP are included in the system.
• SaTu is fast-running and easy to use compared
  with integral codes.
• Uncertainty environment help in justification of
  the reliability of the results.