1. No category

Experience of Nuclear Power Plant NEK Modeling in Computer
Code APROS
Samo Fürst
GEN energija
Vrbina 17
8270, Krško, Slovenia
[email protected]
dr. Luka Štrubelj
GEN energija
Vrbina 17
8270, Krško, Slovenia
[email protected]
Jure Jazbinšek
ZEL-EN
Vrbina 17
8270, Krško, Slovenia
[email protected]
Ivica Bašić
APOSS
Repovec 23B,
49210 Zabok, Croatia
[email protected]
dr. Tomaž Žagar
Fakulteta za energetiko
Univerza v Mariboru
Hočevarjev trg 1
8270 Krško, Slovenija
ABSTRACT
GEN energija, as an interested investor in a new nuclear power plant (NPP) at Krško
site, is developing its own engineering capacity for deterministic safety analyses (DSA) and
other technical analyses of NPP. APROS is a multifunctional software for modelling and
dynamic simulation of various physical processes of different types of power plants, including
PWRs. APROS allows us to build complete models of whole plants, with a complete thermalhydraulic, electrical and regulation systems including reactor kinetics. Such models could be
used in several stages, with its use for new NPP design verification, possible optimization of
NPP systems, for training purposes as an engineering simulation tool and also as a model for
full scope simulator. The purpose work presented in the paper is to get experience with
APROS and is small step in building model of primary circuit of Krško Nuclear Power Plant.
The mode of water accumulator pressurised with nitrogen is modelled with computer
program APROS. The accumulator of Nuclear Power Plant Krško with corresponding piping
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is developed and results are compared with the test performed in Nuclear Power Plant. The
results of simulation, namely the accumulator level and pressure are in line with the
measurement and are inside measurement error.
1
INTRODUCTION
It is known that the verification and validation should be performed on all computer
codes used for the deterministic safety analysis of NPPs. The purpose of validation (also
referred as code qualification or code assessment) is to provide confidence in the ability of a
software package to realistically or conservatively predict the values of the safety parameters.
It should also quantify the accuracy with which the values of parameters can be calculated.
This paper describes the partial validation of the APROS model of the NEK low pressure
accumulator safety injection taking into observation the real plant test data.
2
APROS SIMULATION SOFTWARE
APROS [2] is a simulation program intended for full-scale modelling and dynamic
simulation of industrial processes of nuclear and combustion power plants. A model in
APROS can be built as a complete model of a whole plant, with a complete thermalhydraulic, electrical and regulation systems including reactor kinetics.
The Apros simulation engine contains versatile solvers and model libraries. The
APROS design user interface (Apros Modeller Interface) is built on the Simantics platform,
and it provides a user friendly on-line access for configuring and running simulation models.
Users have access to a set of predefined process component models that are
conceptually one-to-one analogous with actual components (pumps, valves, tanks etc.), and
have no access to solution algorithms. Users can also select appropriate process components
from model libraries, connect them together, and enter the process related input data.
Apros database structure supports hierarchical model description. Users usually operate
on the component level using predefined process components such as pipes, valves, heat
exchangers, tanks, etc., which automatically generate the calculation level objects
(constructed from vast array of nodes and branches). Tested parts of processes can then be
stored in libraries for re-use in other models.
The simulation model is composed graphically through Apros Modeller Interface, a
CAD-like user interface, by drawing diagrams and filling in dialogs of component properties.
Model libraries of Apros environment offer a comprehensive set of plant components such as:
pipes, valves, heat exchangers, adders, measurements, generators, transformers etc.
Apros Modeller Interface includes a diagram editor view and additional views to edit
parameter values, define trend charts etc. It is used together with property view for definition
and simulation of specific processes and their modifications. There is another view for
simulation control. Models can be divided into several diagrams with the use of connection
flags. Re-usable model configurations can be packaged into structural components. The
number of diagrams in a process model is not limited.
All configuration changes are passed to the simulation engine on-line, and the
simulation can continue straight after the change. Various states can be saved at any time as
different Initial Condition (IC) and, therefore users can revert to an IC previously saved.
The complete model, or parts of a model, may be exported or imported. This enables the
re-use of models between projects and the storing of models in libraries for future use.
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Figure 1: structural modelling levels of APROS
A brand new feature of the program is the possibility of saving multiple initial
conditions files within full models (containing diagrams, initial conditions and simulation
histories), that can be later used for running different scenarios on one model. Version 6
offers simple, fast and interactive access to specific data, even when the simulation is running
and is much more stable than previous version of program (APROS 5).
All model configurations are exported with a model including module structure, initial
conditions, simulation history and created to Simantics database. Components of database can
be moved between different workspaces with Export and Import features. Changes in
configuration of the model are continuously newly created components such as component
types, typical diagrams and template diagrams.
3
ACCUMULATOR DISCHARGE TEST CALCULATION
Each High Pressure Safety Injection (HPSI) pump discharge line has two branch pipes.
One branch pipe is connected to the low pressure injection line via accumulator discharge
lines into cold leg of the Reactor Coolant System (RCS). Another branch pipe is connected to
the low pressure injection line, then to the hot leg of the RCS system. A mini-flow
recirculation line valve is provided between HPSI pumps discharge line and the test reflux
line of safety injection. The isolation valves are closed by the operators when the injection
phase is switched to the recirculation phase. During normal operation of the plant, this miniflow recirculation line may be used for testing of HPSI pumps. Accumulator discharge test
during the outage is performed to the reactor cavity where the increase of water level can be
measured.
3.1
Test Description
The accumulator system test is performed to verify the operability of the accumulator
valve and to confirm that the total pipe resistance is within design range [3]. The test
sequence is as follows:
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3.2
•
The reactor head should be removed, reactor defueled (NEK Plant Shutdown State
H9) and upper internals removed.
•
Accumulator pressure before test is equal to Pi (all pressures are presented in
relative to initial pressure Pi).
•
Accumulator is filled to 62.13 % of its full level (LF).
•
The valve is opened from fully closed to fully open position.
•
Accumulator injection is observed from 62.13 % LF to less than 11 % LF; after that
the valve is fully closed.
•
The selected variables are measured.
Test Nodalization Description
In order to have one-to-one correspondence, APROS nodalization parameters were
based on NEK Engineering Handbook for RELAP5 computer code [1] for the NEK SI
respective loop as shown in Figure 2. The accumulator, pipe, point and valve components are
used. The boundary condition in reactor cavity is defined in point with red bracket, simply by
defining the pressure and defining that the values are fixed and not calculated.
Figure 2: Nodalization of the NEK SI Loop-A with APROS
NEK SI Loop-A is modelled as a two-fluid system (mass, momentum and energy
conservation equation for each phase). Accumulator is filled with borated water and has
nitrogen atmosphere. Two-fluid model is the most common selection, where it is assumed that
fluid is a mixture of liquid and gas. It includes solutions of thermohydraulic pressures, flows
and enthalpies. APROS can simulate heat transfer between fluid and heat structures, however
it was not used for the presented case, due to minor temperature changes.
3.3
Results of Accumulator Discharge Test APROS Simulation
The APROS user interface is presented in Figure 3 while the characteristic data between
two observed times are presented in Table 1. It should be noted that plant test starts with an
accumulator pressure of Pi (initial pressure) and liquid level at 62.13 % of full level (LF). The
observed measurement interval is to the liquid level of 11 %LF. Table 1 summarizes APROS
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calculated characteristic values at the same time frame as it was observed on the plant test.
The flow-rate from the accumulator was not measured directly in the actual physical test, but
obtained indirectly from the level increase in the Rx cavity. Since we did not model the
cavity, the comparison in the change of this level was not performed.
Figure 3: APROS user interface
Table 1: APROS calculated values at observed time
Δ
Time
(s)
Accumulator
Liquid Level
(% LF)
Accumulator Pressure
(% Pi)
Accumulator discharge flow
(kg/s)
10
49.2 %
68.8 %
750
23
35.1 %
50.0 %
594
13
14.1 %
18.8 %
638*
*calculated average value in selected time interval
3.4
Comparison of results
The Comparison of the described OSP-3.4.509 [2] test values with values obtained from
APROS simulation is presented in Table 2. It should be noted that the measured values
collected through the NEK Process Information System (PIS) data collection, are
measured with significant uncertainties, e.g. wide range (WR) accumulator level
measurement uncertainty is from +6.83 % to -6.08 % [4], while pressure uncertainty is
±2 % [5]. Test data presented in Figures 4 and 5 are scanned test results which
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introduce additional uncertainties, since measurements were not available in electronic
form (ie. ASCI file).
Accumulator Liquid Level
70
Relative Liquid Level [%]
60
50
40
30
20
10
0
0
5
10
15
20
25
30
35
40
45
Time [s]
Test Data
APROS Results
Figure 4: Level PIS test data vs. APROS calculation
Accumulator Pressure
Relative Pressure in Accumulator [%]
100
90
80
70
60
50
40
30
20
0
5
10
15
20
25
30
35
40
45
Time [s]
Test Data
APROS Results
Figure 5: Pressure PIS test data vs. APROS calculation
APROS accumulator simulation is validated through measured intervals of pressure and
liquid level drops, as the transient is stable, therefore pressure and liquid levels are decreasing
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almost linearly. The validation milestone is set to an interval, starting at pressure of 66 % Pi,
which corresponds to an APROS model simulation time of 11.8 s, and ends after 13 seconds
as NEK test time differential.
The pressure drop in APROS simulation is equal to the NEK test and liquid level
deviate from the measurement for approximately 2.8 %. The liquid level measurement
uncertainty is +6.83 %, -6.08 % [4], while pressure measurement uncertainty is ±2 % [5]. The
pressure and level in simulation is inside measurement uncertainty.
Parameter
P1
P2
Δl
Δt
4
Table 2: Comparison of results
Test
Pressure in
accumulator
Pressure in
accumulator
Acc level differential
Time differential
APROS Results
66.0 % Pi(t=10 s)
66.0 % Pi (t=11.8 s)
47.6 % Pi(t=23 s)
47.6 % Pi (t=24.8 s)
14.5 % LF
13s
14.9 % LF
13s
CONCLUSION
The validation simulations of the NEK low pressure accumulator full flow test by the
APROS ver. 6.03.23 have been described and the simulation results are presented. The
purpose of these simulations is to ensure the sound behaviour of accumulator low pressure
injection at surveillance test conditions before the whole Krško NSSS will be coupled
together and exposed to detailed verification and validation.
There were minor differences between APROS simulation results and actual test
measurement, therefore slight adjustments were made on pipe resistance parameters, injected
water compared with increased level in Rx cavity, etc. It should be noted that test data were
scaled from the PIS printed documents which introduced additional uncertainty in the
comparison of results.
The remaining part of pipes to the Rx vessel needs to be added for the full NEK
primary system verification and validation by APROS expected in the near future.
APROS 6 has brand new user interface allowing dynamical modeling with symbols and
their settings, making work clearer and more manageable than older programs using text files
and cards.
Our experience using Apros program shows that models can be developed quite fast,
however some parts of documentation could be more comprehensive.
REFERENCES
[1]
D. Grgić, T. Bajs, S. Špalj, I. A. Jurković, V. Benčik, “NEK RELAP5\MOD3.2.2
Nodalization Notebook (2000 MWt and new SGs)", NEK ESD-TR09/00, 2000
[2]
APROS Help, version 6.03.23, 2013, Fortum
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[3]
N. Djetelic, F. Umek, “SI Accumulator Discharge Check Valves Flow Test”, OSP3.4.509, 2012
[4]
P. Marn, “Surveillance Operational Test/Calibration Procedure NSSS Process
Instrumentation Channel L-950 Accumulator Tank #1 Level Measurement”, SMI-4.029,
2009
[5]
P. Marn, “Surveillance Operational Test/Calibration Procedure NSSS Process
Instrumentation Channel P-960 Accumulator Tank #1 Pressure Measurement”, SMI4.033, 2009