Psa level 2 source term approach in the loviisa npp
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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

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Satu user interface sample output
SaTu - User Interface (Sample Output)

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Satu user interface sample output1
SaTu - User Interface (Sample Output)

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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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