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 Fortum Nuclear Services

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

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 Fortum Nuclear Services

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 Fortum Nuclear Services

5. 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

6. 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

7. SaTu - System Description (Flow Chart) Fortum Nuclear Services

8. 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

9. 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

10. 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

11. 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

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13. SaTu - User Interface (Sample Output) Fortum Nuclear Services

14. SaTu - User Interface (Sample Output) Fortum Nuclear Services

15. 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

16. SaTu - Uncertainty Calculation Example Fortum Nuclear Services

17. 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