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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. Outline. SAM approach of the Loviisa NPP

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psa level 2 source term approach in the loviisa npp

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

Fortum Nuclear Services

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

Fortum Nuclear Services

loviisa npp
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

Fortum Nuclear Services

sam mitigation in the loviisa npp
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

Fortum Nuclear Services

satu introduction
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)

Fortum Nuclear Services

satu transport solution basic idea
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

Fortum Nuclear Services

satu system description 1
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

Fortum Nuclear Services

satu system description 2
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.

Fortum Nuclear Services

satu input data
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)

Fortum Nuclear Services

satu user interface
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

Fortum Nuclear Services

satu uncertainty environment
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

Fortum Nuclear Services

satu conclusions
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.

Fortum Nuclear Services

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