experimental and numerical mixing studies inside a reactor …

Proceedings of FORUM ON THE FLUID MECHANICS OF MIXING PHENOMENA III: FUNDAMENTALS AND INDUSTRIAL APPLICATIONS

4th ASME/JSME Joint Fluids Engineering Conference July 2003, Hawaii, USA

FEDSM2003-45294

EXPERIMENTAL AND NUMERICAL MIXING STUDIES INSIDE A REACTOR PRESSURE VESSEL

T. Höhne, S. Kliem, H.-M. Prasser, U. Rohde Research Center Rossendorf (FZR)

Institute of Safety Research P.O. Box 510119

D - 01314 Dresden / Germany

ABSTRACT The work was aimed at the experimental investigation and numerical simulation of coolant mixing in the downcomer and the lower plenum of pressurized water reactors (PWR). For the investigation of the relevant mixing phenomena, the Rossendorf test facility ROCOM has been designed. ROCOM is a 1:5 scaled Plexiglas model of a German PWR allowing conductivity measurements by wire mesh sensors and velocity measurements by LDA technique. The CFD calculations were carried out with the CFD-code CFX-4. For the design of the facility, calculations were performed to analyze the scaling of the model. It was found, that the scaling of 1:5 to the prototype meets both: physical and economical demands. Flow measurements and the corresponding CFD calculations in the ROCOM downcomer under steady state conditions showed a Re number independency at nominal flow rates. The flow field is dominated by recirculation areas below the inlet nozzles. Transient flow measurements with high performance LDA-technique showed in agreement with CFX-4 results, that in the case of the start up of a pump after a laminar stage large vortices dominate the flow. In the case of stationary mixing, the maximum value of the averaged mixing scalar at the core inlet was found in the sector below the inlet nozzle, where the tracer was injected. At the start-up case of one pump due to a strong impulse driven flow at the inlet nozzle the horizontal part of the flow dominates in the downcomer. The injection is distributed into two main jets, the maximum of the tracer concentration at the core inlet appears at the opposite part of the loop where the tracer was injected. For turbulent flows the CFD-Code CFX-4 was validated and can be used in reactor safety analysis. Due to the good agreement between measured results and the corresponding CFD-calculation efficient modules for the coupling of thermal hydraulic computer codes with three-dimensional neutron-kinetic models using the results of this work can be developed. A better description of the mixing processes inside the RPV is the basis of a more realistic safety assessment.

INTRODUCTION During so called boron dilution or cold water transients at pressurized water reactors too weakly borated water or too cold water might enter the reactor core. This results in the insertion of positive reactivity and possibly leads to a power excursion. If the source of deborated or subcooled water does not affect all loops to the same extent, the concentration resp. the temperature may differ between the loops. In this case the amount of reactivity insertion depends on the coolant mixing in the downcomer and lower plenum of the reactor pressure vessel (RPV). Such asymmetric disturbances of the coolant temperature or boron concentration might e.g. be the result of a failure of the chemical and volume control system (CVCS) or of a main steam line break (MSLB) that does only affect selected loops [Kl99]. For the analysis of boron dilution or MSLB accidents, coupled neutron kinetics (DYN3D) / thermo-hydraulic (ATHLET) system codes have been used [Kl97]. To take into account coolant mixing phenomena in these codes in a realistic manner, analytical mixing models might be included. These models must be simple and fast running on the one hand, but must well describe the real mixing conditions on the other hand. One possibility is to use pre-determined mixing matrices mapping the contribution of each cold leg to each fuel assembly at the reactor core inlet. The coolant mixing in the downcomer and lower plenum depends significantly on the construction of the RPV and on the instantaneous flow conditions. The models and assumptions for coolant mixing description to be used in the coupled codes must be validated against experimental data and detailed computational fluid dynamics (CFD) calculations. Therefore the Institute of Safety Research of Forschungszentrum Rossendorf has constructed a 1:5 mixing test facility ROCOM (Rossendorf Coolant Mixing Model) representing the geometry of the German Konvoi type pressurized water reactor. Existing facilities (BORA-BORA [Al92] , Vattenfall [Ala95] and Gidropress [Me01]) represent other reactor types with significant differences in the geometry. Furthermore, the

1 Copyright © 2003 by ASME

ROCOM facility disposes of four complete loops each with controllable pumps. This allows to study a wider range of different mixing scenarios, as this can be done at the existing facilities, which are due to their construction aimed only at pump start-up tests. In comparison to the early air-operated models of the Konvoi type PWR [Ul83] and the Russian type WWER-440 [Dr87] the influence of the compressibility of air is eliminated. The use of water makes it possible to study scenarios with unsteady flow.

Fig. 3 Test facility ROCOM

Recent experiments at ROCOM, together with data on mixing obtained at the Vattenfall test facility, the Russian VVER-1000 mock-up and measurements at the VVER-440 NPP in Paks (Hungary) are integrated into the research project FLOMIX-R within the 5th Framework Programme of EC. The objective of the project is to obtain complementary and confirmatory data on slug mixing using improved measurement techniques with enhanced resolution in space and time. The experimental data will be used to contribute to the validation of CFD codes for the analysis of turbulent mixing problems. A few benchmark problems based on selected experiments will be used justify the application of various turbulence and turbulent mixing models for various flow conditions, to suppress numerical diffusion and to decrease grid, time step and user effects in the CFD analyses.

THE MIXING TEST FACILITY ROCOM

The transparent model of the RPV (Figure 2) is the main component of the test facility (Figure 3). The linear scale is 1:5. It is connected to four circulation loops with pumps, which are individually controlled by frequency transformers. The volume of the loops is kept according to the original reactor. The traveling time of the coolant can be adjusted to the original. The water volumes in the primary sides of the steam generators are represented by special cylindrical vessels, the similarity to the original steam generators is not kept. One of these vessels is used to compensate volume changes. Each loop is equipped with inductive flow meters, used for controlling the flow rate. The flow rates are set constant or can be controlled according to given time functions. The following internals of the reactor pressure vessel are modeled (see Figure 4): core barrel with

lower core support plate and core simulator, perforated drum in the lower plenum, inlet and outlet nozzles. Table 1 Comparison original PWR - 1:5 scaled mixing model ROCOM

Dimension Unit Original Model

diameter of the

pressure vessel

mm 5000 1000

height of the

pressure vessel

mm ~12 000 ~2400

inlet nozzle

diameter.

mm 750 150

downcomer

gap

mm 315 63

general mass

flow of the

coolant

m3/h 92 000 1400

mass flow loop m3/h 23 000 350

speed at inlet

nozzle

m/s 14.5 5.5

speed in the

downcomer

m/s 5.5 2.1

Re inlet nozzle - 8.4⋅107 8.3⋅105

Re downcomer - 2.7⋅107 2.5⋅105

Re Original /

Re Model

- 1 ~100

Fig. 2 RPV Plexiglas Model

For the investigation of the mixing in the downcomer and lower plenum it is not necessary to model the upper plenum. Special


attention was given to constructional details, which may significantly influence the velocity field. So, the geometry of

thrvafdthcRb AsloterRr[trddmcw

Due to the fact, that the mixing is dominated by turbulent mechanisms, it is assumed that both boron concentration and temperature fields can be modeled by the concentration field of a tracer solution. The test facility is operated with de-ionised water. The initial conductivity has to be adjusted to about 10 µS/cm for a proper operation of the inductive flow meters. The disturbance is created by injection of salted water with a conductivity between 100 and 2000 µS/cm into the cold leg in one of the loops (disturbed loop) near the inlet nozzle of the reactor. For that purpose, a mixing device is installed, which distributes the tracer over the cross section in the circulation line. The length of the slug is adjusted by the help of magnetic valves and a separate injection pump. The salt significantly changes the conductivity of the water what can be measured by conductance methods. In the facility, so called wire mesh sensors are applied. The working principle of the mesh sensors is explained in [Pr98]. The sensors are shown in detail in Figure 5.

Inlet Nozzle Sensor

Core Inlet Sensor

Pos. 2

Pos. 3

Pos. 4

2 downcomer sensors

sensor core inlet(193 points)

sensor inlet nozzle

Pos. 1

Fig. 4 Positions of the wire mesh sensors in the

plexiglasmodel

e inlet nozzles with their diffusor segment and the curvature adius of the inner wall at the junction with the reactor pressure essel was modeled in detail. A special feature of the Konvoi is diffusor in the downcomer close below the inlet nozzle. It is ormed by a decrease of the thickness of the vessel wall in flow irection, which leads to an increase of the inner diameter of e vessel, while the outer diameter of the core barrel remains

onstant. This detail was also accurately modeled. The eynolds numbers (Table 1) of the prototype cannot be reached y the model.

t the maximum flow rate of 350 m³/h per loop, they are maller by the factor 100, caused by the linear scale of 1:5, the wer water velocity and the higher viscosity of water at room mperature in comparison to the hot reactor coolant. The

esulting Reynolds numbers Re=8.3⋅105 at the inlet nozzle and e=2.5⋅105 indicate a highly turbulent flow. According to

esults of scaling analysis [Hö98] for Konvoi type reactors and Dr87] for the WWER-440, this is a sufficient condition for a ansferability of the measured flow field and the concentration istributions to the original reactor as long as density ifferences are negligible. In both works the flow field and ixing conditions in the prototype reactor and the

orresponding scaled model were compared, in [Dr87] even ith experimental data from a prototype reactor [Tsi82].

Downcomer Sensor

Fig. 5 Wire mesh sensors in the plexiglas model of ROCOM at different positions There is one sensor at the lower core support structure (Pos. 4), two are in the downcomer (Pos. 2 and 3) and there is another one (Pos. 1) in one of the cold legs (see Figure 4). The extensive instrumentation with these sensors permits a high resolution of the concentration field in the RPV in space and time. The downcomer sensors and those in the cold leg have


16x16 measurement points each. The sensor at the core bottom yields 193 points at the same time. This means, there is one concentration measurement at the bottom of each fuel element. All sensors provide 200 measurements per second and work in the conductivity range of 10-500 µS/cm. In the experiments, a time resolution of 20 measurements per second is sufficient, i.e. 10 individual measurements were averaged. Table 2 Classification of the mixing experiments and CFD calculations Group Experimental Objects

A Mixing under full loop operation with different

mass flow rates

B Mixing under partly operating loop conditions

C Start of coolant circulation

The measured conductivities were transferred to a mixing scalar Θx,y,z(t) representing the contribution of the coolant from the disturbed loop to the mixture at the given position x,y,z. It is calculated from the local instantaneous conductivity σx,y,z(t) by relating it to the amplitude of the conductivity range in the inlet nozzle of the disturbed loop.

01

0,,,,

)()(

σσσσ

−−

=Θt

t zyxzyx (1)

The upper reference value σ1 in (1) was obtained by averaging the distribution measured at the disturbed inlet nozzle over the cross section and over the period, when the tracer slug was fully developed (steady state flow field experiments). The lower reference value σ0 is the initial conductivity of the water in the test facility before the tracer is injected. According to the boron dilution transients and MSLB scenarios the experiments shown in Table 2 were performed at the test facility ROCOM. The mixing measurements in the reactor model are realized by the following steps: First the test facility is filled with low conductivity water (deionat). The wanted flow field is adjusted by controlling the main coolant pumps. After this, the injection pump forwards a slug of salted water continuously or discontinuously to the mixer in one of the cold legs. The concentration profile is measured by the wire mesh sensor in that cold leg. All processes, including the measurement of the mass flow, temperature, pressure, the tracer injection and the water cleaning with ion exchangers run computer controlled. Post test CFD calculations have been performed to all groups of experiments.

COMPUTATIONAL MODELING The 3-D computational fluid dynamics (CFD) codes provide an effective tool for mixing calculations. In recent years, the rapid development of both the software and the computers has made

it feasible to study the coolant mixing in sufficient detail and to perform the calculations for transient conditions. The used CFD-Code for mixing studies at the Forschungszentrum Rossendorf (FZR) is CFX-4 [CFX]. CFX-4 is a finite volume program that offers the following options, which can be used in the mixing studies: • Solution of the Navier-Stokes-Equations for steady and

unsteady flows for compressible and incompressible fluids • Applicability for laminar and turbulent flows (different

turbulence models) • Porous media model, implementation of body forces added

to the momentum equation, user defined scalar equations

Turbulence modeling The modeling of the turbulence is important both for the flow field and the concentration field. At the same time turbulence modeling is one of the biggest problems in the field of the CFD. In the boron diluted slug mixing calculations made so far the mostly used turbulence model are the standard K-ε model and it’s variations like RNG K-ε model. Some occasional tests are made also with Reynolds Stress Model (RSM) and even using Large Eddy Simulation (LES) [Be96]. However in spite of the well-known limitations the most common and the most robust models like k-ε seems to be most often used. Figure 6 shows a calculated slug at the end of a simplified downcomer model obtained with different used turbulence models. There are no differences between the two equation models but there are differences compared to the algebraic RSM. However although there are differences in results, it is not easy to discover whether one model is better than the other and how it is better. In principal the more developed models like RSM and LES should be more accurate than older k-ε models.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 0.3 0.6 0.9 1.2 1.5 1.8

t / s

thet

a / -

k-epsilon Model

RNG k-epsilon Model

AlgebraicReynoldsStress Model

Fig. 6 CFD-calculation of a tracer slug at the end of a simplified downcomer model with different turbulence models


Numerical diffusion, nodalization and time step size Numerical error is a combination of many aspects; the grid density, discretization method, time step size and convergence error have all their own effect. When a validation of the computational model is made using a certain experiment, the

separation of different numerical effects is difficult; for example, the numerical diffusion, i.e. a numerical error which acts like an artificial extra diffusion, can affect to the result in the same direction like too large turbulent viscosity used in some turbulence models.

The numerical diffusion can be minimized using denser grids, higher order discretization methods and suitable time step size. Often the computation time puts some limits for these, but anyhow in all CFD computations results should be ensured to be grid and time-step independent, and if not possible, the uncertainties should be quantified. Figure 7 shows CFX-calculations of a slug at the end of a simplified downcomer model with different time steps. If the time steps are too large the influence of numerical diffusion on the results is very high, if the time steps are too small the computational time exceeds.

Generally, it is important to find an optimum between acceptable results and computational time. In the calculated case the optimum is a time step of 0.01 s (Fig. 7).

Model assumptions, geometry preparation and grid generation An incompressible fluid was assumed for the coolant flow in the ROCOM test facility. The turbulence was modeled using the standard (k, ε ) approximation. The inlet boundary conditions (velocity, temperature, boron concentration etc.) were set at the inlet nozzles. The outlet boundary conditions were pressure controlled. Passive scalar fields were used to describe the boron dilution processes. The generated grid contained ca. 450000 nodes. Steady state calculations with at least 1000 iterations last a few hours,

transient calculations a few weeks on a SGI Origin 200 workstation platform. The geometric details of the construction internals have a strong influence on the flow field and therefore on the mixing. Therefore, an exact representation of the inlet region (bend radii etc.), extension of the downcomer below the inlet region and obstruction of the flow by the outlet nozzles cut through the

downcomer (Figure 8). The core support plate, the core and the perforated drum are modeled as a porous region. The porosity value γ for perforated plates is determined by relating the area of orifices to the total area of the sieve. Body forces B are added to the momentum equation (2), to take into account distributed friction losses in the sieve plate. Using the Cartesian coordinate system, the momentum equation is written:

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.2 0.4 0.6 0.8 1 1.2 1.t / s

thet

a / -

4

dt=0.01 s

dt=0.005 s

dt=0.05 s

Fig. 7 CFD-calculation of a tracer slug with different time steps

Fig. 8 Computational grid (RPV ROCOM)

(j=1,2,3)

ρ∂∂

∂∂

∂∂

τUt

UUx

Bx

ji

j

ij

iij+

= − (2)

( )B B R R u uF C F= − + (3)

In the equation (2) U are the components of velocity and j τ

is the shear stress, , and are the coefficients of the body force dependence on the velocity. In the model, only the second order contribution of the body forces according to relation (3) is used being typical for turbulent flow. The corresponding coefficient is obtained from calculated values for the flow resistance coefficient (see Table 3).

BF RC RF


Table 3 Pressure loss coefficient and corresponding RF at the modeled internals (ROCOM) porosityγ

[-] flow resistance coefficient Cζ [-]

body force

4mkginRF

perforated drum

0.208 13.1 3.2⋅105

core support plate

0.229 10.5 4.9⋅104

RESULTS

Investigation of Steady State Flow and Mixing at ROCOM To simulate the mixing of the coolant under MSLB scenarios with operating main coolant pumps, generic experiments at the ROCOM test facility and CFD-calculations were made at

nominal conditions, i.e. all main coolant pumps were operating. The velocity field, determined in a blind CFD calculation before running the facility (Figure 10), shows a qualitatively good agreement with experimental results (air operated model of Ulrych and Weber [Ul83] and LDA measurements at ROCOM).

Fig. 11 Stream lines (CFX)

Fig. 9 Numerical modeling of the cylindrical sieve in the lower plenum of the RPV ROCOM

The calculations especially confirm location of minimum flow velocities below the inlet nozzles found in earlier experiments [Ul83]. A maximum velocity exists at azimuthal positions between the two inlet resp. the two outlet nozzles. In the investigated flow rate turbulent flow is fully developed. In Figure 11 the flow condition is demonstrated with the help of stream lines in one quarter of the RPV. The other 3 quarters look similar. Mixing test experiments were carried out under steady state flow field conditions at different mass flow conditions. To have a comparison between the CFD calculation with measurements, an experiment at 185 m³/h per loop with steady flow conditions was chosen and a large slug of salted water was injected.

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

0 45 90 135 180 225 270 315 360

azimuthal position / °

c(ve

rtic

al) /

m/s

LDACFX

Outlet Nozzles hatched185 m³/h 185 m³/h 185 m³/h 185 m³/h

Fig. 10 Flow field in the downcomer of ROCOM at nominal conditions (LDA – measurements) in comparison with CFX-4 results

At the inlet nozzle (Figure 12a) the same conditions as in the experiments (conductivity per time step at 0.05 s in one loop, velocities in all loops) were set as inlet boundary conditions for the CFD calculation. In Figure 12b the instantaneous maximum of the mixing scalar at the core inlet independently of the actual fuel element position is plotted against the time. The maximum mixing scalar at the core inlet rapidly increases to the maximum. There is a good agreement between the measurement and the CFD calculation, esp. in the averaged global mixing scalar at the core inlet (Figure 12c). However in the experiments the maximum mixing scalar gives lower values at the peak than the CFD calculation shows.


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0 3 6 9 12t / s

thet

a / -

15

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0.60

0.80

1.00

0 4 8 12t / s

thet

a / -

16

Exp.CFX

a) Time dependent mixing scalar at the reactor inlet b) Time dependent global maximum of the mixing scalar at the core inlet

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0 4 8 12t / s

thet

a / -

16

Exp.CFX

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1.00

0 4 8 12t / s

thet

a / -

16

Exp.CFX

KS 2

KS 4KS 3

KS 1

c) Time dependent global averaged mixing scalar at the core inlet

d) Time dependent local mixing scalar at the core inlet, position near the wall

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1.00

0 4 8 12 16t / s

thet

a / -

Exp.CFXKS 2

KS 4KS 3

KS 1

e) Time dependent local mixing scalar at the core inlet, position near the wall

f) Time dependent local mixing scalar at the core inlet, position near the wall

0.00

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1.00

0 45 90 135 180 225 270 315 360azimuthal position / °

thet

a / -

MessungCFX

t=14.0 s

185 m³/h

CL-1 CL-2 CL-3 CL-4

185 m³/h 185 m³/h 185 m³/h

g) Mixing scalar at azimuthal positions of the core inlet near the wall, (t=9.5 s)

h) Mixing scalar at azimuthal positions of the core inlet near the wall, (t=14.0 s)

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0 4 8 12 16t / s

thet

a / -

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KS 2

KS 4KS 3

KS 1

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0 45 90 135 180 225 270 315 360azimuthal position / °

thet

a / -

MessungCFX

t=9.5 s

185 m³/h

CL-1 CL-2 CL-3 CL-4

185 m³/h 185 m³/h 185 m³/h

Fig. 12 Comparison of the measured and calculated mixing scalar (steady state flow field, 185 m³/h


Experiment CFX Core inlet

Fig. 13 Comparison of the measured and calculated mixing scalar at the core inlet (steady state flow field, time averaged data, 185 m³/h per loop) In the CFD calculations angular oscillations of the concentration distributions at the lower end of the downcomer and at the core inlet were found at the border of the slug (Figure 12e). This phenomenon was also observed in the experiments. The oscillations are caused by flow separations and large vortices in the downcomer below the plane of inlet and outlet nozzles. The coolant from the disturbed inlet nozzle almost completely arrives in the corresponding sector of the core entrance (Figures 12d,f,g,h). On the other hand, areas of the core inlet were not effected at all (Figure 12f). In Figure 13 a comparison of the measured and calculated mixing conditions at the core inlet is shown at steady state conditions (185 m³/h). The experimental data are time averaged. Despite of the flow deflection in the lower plenum and the partial penetration of the perforated barrel, the maximum disturbance observed at the core entrance is 91 % (calculation), 95% (measurement) in comparison to a 100 % concentration change at the inlet nozzle. These results can only be obtained, if a sufficiently large slug of tracer has been injected, to create a tracer plateau shown in Figure 12c. If smaller slugs are injected, steady state conditions at the core inlet are not achieved.

Mixing during start-up of coolant circulation Model experiments for studying mixing of diluted slugs have been performed in Sweden [Ala95], France [Al92], USA [Ga01], Russia [Me01] etc. and at the Mixing Test Facility ROCOM at FZR Rossendorf / Germany. Many of the studies are concentrated on the scenario ‘Start-up of the first pump’. In this scenario the diluted slug is transported from the loop to the core by starting a main circulation pump (MCP). The flow in the starting loop accelerates rapidly. Following boundary conditions were taken from a transient experiment for the numerical simulation of a pump start-up case: • initially all pumps are stopped, no natural circulation

occurs

• the tracer slug is modeled as a scalar impulse corresponding to the experiment (see Fig. 15a), after the beginning of the start-up of the MCP, the length of the slug was chosen to visualize the flow pattern and the mixing conditions in the downcomer and core inlet

• in one loop the MCP starts linearly from 0 to 185 m³/h in 14 s, after 14 s the mass flow rate is constant at 185 m³/h (Fig. 15b), development of counter flows at the other 3 loops

The scalar representing the boron content of the slug is treated in normalized form by setting the original boron concentration deficit of the slug equal to unity (Eq. 1). Figure 14 shows streamlines representing the velocity field in the downcomer and lower plenum (including the perforated

drum and lower support plates) at the pump start-up

scenario calculated with CFX-4. Due to a strong impulse driven flow at the inlet nozzle the horizontal part of the flow dominates in the

downcomer (Figures 14 and 16). The injection is distributed into two main jets, the so called butterfly distribution. In addition several secondary flows are seen in various parts of the

downcomer. Especially strong vortices occur in the areas below the non operating loop nozzles and also below the injection loop.

Here a strong recirculation area occurs, which is controlling the size of other small swirls.

Fig. 14 Flow picture at 15 s after the start-up

The complex flow field promotes strong mixing of the slug in the downcomer. Therefore, the experiment was repeated several times to average over these fluctuations. The results of the single realisations were used to carry out a statistical analysis of the experimental data.


0

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1

0 4 8 12 16 20t / s

thet

a / -

a) Time dependent mixing scalar at the reactor inlet

-1.00

0.00

1.00

2.00

3.00

4.00

0 5 10 15 20 25 30

t / s

Velo

city

/ m

/s

Loop 1Loop 2Loop 3Loop 4

b) Inlet velocity loops 1-4, ramp

Fig. 15 Inlet boundary conditions from the transient pump start-up experiment The transient course of the maximum value is shown in Fig. 17a. That maximum value or the minimum boron concentration is an indicator for possible reactivity insertion during a transient. In the experiment as well as in the calculation, the maximum value is determined at each time step over all fuel element positions. Therefore the position can vary, which has also an influence on the width of the confidence-interval of the experimental data. In the calculation, the maximum concentration at the core inlet is very close to the experimental data. The absolute value of the calculation reaches 54%, of the measurements 56% of the loop value (100%). Local time depended mixing scalars at fuel element positions in the center and near the wall of the core inlet are shown in the Figures 17b and 17c. The turbulent fluctuations are significant only in the later part of the transient, when the maximum of the deboration front already passed the corresponding fuel element position. As can be seen, the fluctuations are higher in the outer part of the core. In addition, the mixing scalars at azimuthal positions near the wall of the core inlet are shown in Figure 17d in a later part of the transient flow field, when the slug is almost passed by. About 14 s after switching-on the RCP, the deboration front reaches the core inlet at two positions in the periphery of the core about 120 ° shifted from the azimuthal position of the inlet nozzle of the loop with the starting pump

(Figures 18 and 19). In the CFX-calculation the deboration front reaches the core at the same positions but with a small time delay of less than 0.5 s. The maximum of the deboration front moves over the centre of the core to the side, of the injecting loop (see also Figure 17d).

CONCLUSIONS The temperature and boron concentration fields established by the coolant mixing during nominal and transient flow conditions in the pressure vessel of the PWR Konvoi were investigated. In the case of steady-state conditions with working pumps, the maximum value of the averaged mixing scalar at the core inlet was found to be more than 90% in the sector below the inlet nozzle, where the tracer was injected. At the start-up case of one pump the maximum of the tracer concentration at the core inlet appears at the opposite part of the loop where the tracer was injected, better mixed with the ambient coolant compared to the steady-state case with nominal flow rates. The time-dependent boron concentration field at the core inlet obtained from these investigations can be used as boundary condition for boron dilution transient analysis.

ACKNOWLEGEMENT The research work presented in this paper was partially funded by the German Federal Ministry of Economics and Technology under project number 150 1216.

REFERENCES [Al92] Alvarez, D. et al., Three dimensional calculations and

experimental investigations of the primary coolant flow in a 990 MW PWR vessel, NURETH-5, Salt Lake City, Vol. II, 586-592, 1992

[Ala95] Alavyoon, F., Hemström, B., Andersson, N. G., and

Karlsson, R. I., Experimental and Computational Approach to Investigating Rapid Boron Dilution Transients in PWRs, CSNI Specialist Meeting on Boron Dilution Reactivity Transients, State College, PA, U.S.A., 18th-20th October, 1995

[Be96] Bernard, J.P., Haapalehto, T., Review of Turbulence

Modelling for Numerical Simulation of Nuclear Reactor Thermal-Hydraulics, Research Report, Lapeenranta University of Technology, 1996

[CFX] CFX-4.4 Flow Solver User Guide, AEA Technology, 2001 [Dr87] P. Dräger, Makroskopische Kühlmittelvermischung in Druckwasserreaktoren, Dissertation TH Zittau, 1987 [Ga01] Gavrilas, M. and Kiger, K., ISP-43: Rapid Boron

Dilution Transient Experiment, Comparison Report, NEA/CSNI/R(2000)22, 2001


Time

/ s

3D Downcomer Time

/ s

3D Downcomer Time

/ s

3D Downcomer

8.0

10.0 15.0

Fig. 16 3D Mixing in the downcomer, CFX

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Exp.CFXp1p2

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thet

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Exp.

CFXKS 2

KS 4KS 3

KS 1

a) Time dependent global maximum of the mixing

scalar at the core inlet b) Time dependent local mixing scalar at the core

inlet, position near the wall

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CFXKS 2

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thet

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MessungCFX

t=20.0 s

CL-2 CL-3 CL-4

c) Time dependent local mixing scalar at the core

inlet, position near the wall d) Mixing scalar at azimuthal positions of the core

inlet (t=20.0 s)

CL-1

Fig. 17 Comparison of the transient pump start-up experiment and the corresponding CFX post test calculation


Fig. 18 Experimental results at the core inlet

Fig. 19 Calculation with CFX at the core inlet

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