Transcript No Slide Title

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
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Outline
SAM approach of the Loviisa NPP
Tool for source term calculation in PSA 2 (SaTu)
Sample results
Uncertainty calculation
Conclusions
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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
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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 atpower 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)
dN1 dt  S1  k1 N1

dN2 dt  f1, 2 k1 N1  k2 N 2
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)
Outer annulus *
Dome
Stack
Sampling filters
Stack filters
Environment
Reactor hall
*
Lower compartment
Material lock *
Material corr. *
Auxiliary building *
Gas sampling
Secondary circuit
Primary circuit
Water pool possible
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Leakage room in
the aux. building *
Leakage room in
the basement *
Auxiliary system to
the aux. building
Water sampling
* Ventilation to the stack
Deposition on the floor
Annular room *
Core
Auxiliary system
to the basement
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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)
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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
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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
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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 - 2.08.3
Tomi Routamo
Noble gases
1E+0
1E-1
1E-2
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
0h
5h
10 h
Core
Aux. system to the aux. build.
Reactor hall
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15 h
Primary circuit
Lower compartment
Reactor hall floor
20 h
25 h
Secondary circuit
Sump water
Environment
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30 h
35 h
40 h
Aux. system to the basement
Dome
SaTu - User Interface (Sample Output)
SaTu - 2.08.3
Tomi Routamo
Iodine
1E+0
Iodine volatilisation from water pools is taken into account
Formation of organic iodine is taken into account
Revaporisation of deposits in the pipe systems is taken into account
1E-1
1E-2
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
0h
5h
10 h
Core
Aux. system to the aux. build.
Reactor hall
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15 h
Primary circuit
Lower compartment
Reactor hall floor
20 h
25 h
Secondary circuit
Sump water
Environment
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30 h
35 h
40 h
Aux. system to the basement
Dome
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
Iodine release into environment
1E+0
1E-1
1E-2
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
0h
10 h
5%
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20 h
25 %
50 %
30 h
75 %
16
95 %
40 h
99.5 %
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.
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