MAIN OUTCOMES ON DEBRIS BED COOLING FROM … 2.8... · MAIN OUTCOMES ON DEBRIS BED COOLING FROM...

6th European Review Meeting on Severe Accident Research (ERMSAR-2013)Avignon (France), October 02-04, 2013

Session 2 : In-Vessel Corium and Debris Coolability : paper 2.8 1/14

MAIN OUTCOMES ON DEBRIS BED COOLING FROM PRELUDEEXPERIMENTS

G. Repetto, N. Chikhi, F. Fichot

Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Cadarache, France

[email protected], [email protected], [email protected]

ABSTRACT

Debris bed coolability is an important issue for stopping or at least slowing down a severe accidentevolution. One of the major concerns of safety studies is to evaluate the consequences of waterreflooding of a severely damaged reactor core, where a large part of the core has collapsed andformed a debris bed. Many experiments have been conducted to study two-phase flow and heattransfer in porous medium configuration. The “Institut de Radioprotection et de Sûreté Nucléaire”,has launched an experimental program PRELUDE, to validate simulation tools, as ASTEC, to getbetter confidence in safety studies results. Main physical parameters have been investigated: theeffects of the initial temperature of the debris bed, the injected water flow rate, the specific powermaintained during the reflooding, the particle size (monodispersed bed of stainless steel particles orpolydispersed debris), the effect of the presence of a bypass around the debris and finally theconfiguration of the water supply (bottom or top injection) to be more representative of the reactorsituations. These results will contribute to assess the possibility to stop the progression of in-vesselcore degradation in a Nuclear Power Plant and to the optimization of the Severe AccidentManagement strategy.

1 INTRODUCTION

Debris bed coolability [1] is an important issue for stopping or at least slowing down a severeaccident evolution. One of the major concerns of safety studies is to evaluate the consequences ofwater reflooding of a severely damaged reactor core, where a large part of the core has collapsed andformed a debris bed. Many experiments have been conducted to investigate two-phase flow and heattransfer in porous medium configuration. The “Institut de Radioprotection et de Sûreté Nucléaire”,has launched an experimental program [2] [3], supported by EDF and by the European Commission inthe frame of the network of excellence on severe accident research (SARNET), to validate simulationtools, as ASTEC.The PRELUDE experiments involving different sizes (Ø170 and Ø290mm, 200 to 250mm height) atatmospheric pressure, allowed getting very useful information on debris bed cooling ([4] [5] [8] [11])for the modeling and the validation ([6] [7] [9][10]) of simulation tool (ICARE/CATHARE, moduleof ASTEC).Main physical parameters have been investigated: the effects of the initial temperature of the debrisbed, the injected water flow rate, the specific power maintained during the reflooding, the particle size(monodispersed bed of stainless steel Ø4, Ø2 and Ø1 mm particles or polydispersed debris), the effectof the presence of a bypass around the debris and finally the configuration of the water supply(bottom or top injection).This paper presents the main outcomes of those experiments regarding relevant phenomena(temperature evolution, steam production, quench front propagation and reflooding duration)occurring during debris bed reflooding.

2 PRELUDE FACILITY AND INSTRUMENTATION

PRELUDE facility (Fig. 1) includes several components:



- A pressurized water tank connected to the test section for reflooding with water flowmeasurement,

- A quartz test section varying from Ø110mm to Ø290mm containing a debris bed instrumented withthermocouples and pressure sensors (Fig. 2a),

- An induction furnace (coil around the test device),- A downstream heated vertical tube to remove steam from test section, including wall and fluid

temperature and flow rate measurements.

In the PRELUDE experiments, the major physical variables for model qualification are as follows:- Injected water and steam mass flow rate generated during reflooding,- Temperature of the steam at different points outside the debris bed,- Temperature inside the debris (thermocouples at different elevations and radial positions : centre,

mid radius and periphery – Fig. 2b),- Pressure at different points in the debris and at the boundaries.

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In PRELUDE experiments, specific efforts have been done to measure steam production and pressuredrop in the debris bed in order to characterize the flow regimes.The total mass of debris were respectively, 21, 24, 41 and 55 kg for the different beds (Table I).The characterisations of the physical properties of the debris bed (porosity, permeability, passability)are obtained from specific experiments (pressure drop measurements with air flow in CALIDE facility[12]). The porosity of the debris is from 0.37 to 0.4 according to the bed construction process.Some experiments have been performed with the presence of a bypass around the debris made ofquartz balls (Ø8 mm) for the so-called PRELUDE 2D.

3 PARAMETERS INVESTIGATED

Different parameters have been investigated during these PRELUDE experiments:

1. initial temperature of the debris bed,2. injected water flow rate,3. specific power maintained during the reflooding to simulate residual power,4. particle size (monodisperse bed of stainless steel particles or polydisperse debris),5. the effect of the presence of a bypass around the debris,6. the configuration of the water supply (bottom or top injection).

Thermocouples

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The variability of these experimental parameters is summarized in Table II below.

TABLE I

Parameters investigated during PRELUDE experiments

Parameters Variability1 200 , 300, 400, 550, 700 and 900°C2 2 , 5, 7.5, 10, 15 , 20 , 30 and 50 m/h3 0, 150, 200 and 300 W/kg4 Ø4, Ø2, Ø1 mm and mixture of Ø4-3-2-1 mm

5without bypass - test section : Ø110, Ø174 and Ø170mm, height : 200 mmwith bypass (2 sizes : 22,5 and 42,5 mm) - test section : Ø290mm, height : 250 mm

6 Bottom and Top water injection

The water was injected at the “room temperature” from 5 to 25°C at atmospheric pressure. The effectof the system pressure and of the subcooling will be studied in PEARL program.

4 EXPERIMENTAL CAMPAIGNS

Many experimental campaigns (with roughly 150 tests) have performed during the four year ofSARNET2 period, with different configurations and test sections.

TABLE II

Synthesis of the PRELUDE experiments

Number of test Debris bed configuration (height 10, 20 and 25cm)1st campaign in 2009 6 Ø110 mm test section (particle size Ø4 mm)2nd campaign in 2010 11 Ø174 mm test section (particle size Ø4 mm)3rd campaign in 2010 15 Ø174 mm test section (particle size Ø4 mm)4th campaign in 2010 10 Ø174 mm test section (particle size Ø2 mm)5th campaign in 2010 8 Ø174 mm test section (particle size Ø1 mm)6th campaign in 2011 12 Ø170 mm test section (particle size Ø2 mm)7th campaign in 2011 24+7 (Top) Ø170 mm test section (particle size Ø4 mm)8th campaign in 2011 6+5 (Top) Ø170 mm test section (particle size Ø4-3-2-1 mm)9th campaign in 2012 13+7 (Top) Ø290 mm test section (particle size Ø4 mm)10th campaign in 2012 15+8 (Top) Ø290 mm test section (particle size Ø4 mm)

The main configuration for which PRELUDE was constructed is for bottom injection, neverthelesssome adaptations have be done to perform few experiments with Top injection.

This paper is focused on the main physical outcomes on debris bed reflooding.

5 MAIN OUTCOMES ON DEBRIS BED REFLOODING

The main physical phenomena analysed are the temperature evolution during the reflooding, the steamproduction (source for further metallic structure oxidation), the quench front progression and theduration or efficiency (duration related to injected water flow velocity) of the reflooding.

5.1 Thermal behaviour

The bed is heated by an induction furnace before and during the reflooding. The heating homogeneityhas been assessed with the thermocouples inside the bed, either for 1D or 2D (i.e. with a by-pass)configuration of the bed. The heat losses at the quartz shroud have been partially compensated by apromoted induction heating at the peripheral region of the bed.



Figure 3 illustrates typical results obtained from thermocouples inside the debris bed. Themeasurements of temperature are accurate enough to derive the heat fluxes in the different regimesduring the reflooding phase for modelling purpose. Four regimes have been observed splitting the bedinto four zones.Nucleate boiling region: upstream the quench front, the walls are permanently wetted and thetemperature is equal to the saturation temperature Tsat; the bubble nucleation is stopped when thetemperature gets under Tsat.Transition boiling region : just above the quench front, when temperature exceeds the CHFtemperature, non-stable two-phase flow establishes at the wall side, with intermittent wetting of thewalls; a very large axial temperature gradient exists just downstream of the quench front, driving theprogress of the quench front (location where the wall is permanently wetted).The film boiling regime with droplet entrained by steam: well above the quench front, the gaseousphase is predominant (intermediate or high quality), possibly entraining droplets and/or water slugs.The gaseous convection: the steam generated at the quench front is dried out and get out of the bedby the top or through the by-pass (the steam is attracted by the low permeability region). Therefore itis observed heat transfers from the centre of the bed to the top and to the by-pass.

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Figure 3 : Evolution of temperature during the reflooding for different axial levels and radial positions

These thermal results allowed the determination of the quench front propagation for which moredetails are given in section 5.3.

5.2 The steam production

The measurements of the injected water flow, as well as the steam flow rate generated duringreflooding were accurately obtained by means of specific sensors to reach a very satisfactory massbalance (roughly 2-3 %) [5]. The accuracy of the steam measurement is better than +/- 10% [11].Figure 4 illustrates the effect of the initial temperature of the debris bed, on the steam production fortwo configurations (Ø2mm particles without bypass and Ø4mm particles with bypass, as examples).According to these curves, the reflooding phase can be divided in two stages. First, there is animportant vaporization leading to a peak. It can be seen that the initial temperature of the bed has agreat impact on the peak. The peak value increases with the initial temperature until a limit valuewhich corresponds to the quasi total conversion of injected water into steam. The second stage ofreflood is nearly independent of the initial temperature [8] and is strongly linked to the residual powermaintained during the transient.



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Figure 4 : Effect of temperature on steam production

For tests including a bypass (so-called PRELUDE 2D), the general behaviour is similar to thatobtained in the PRELUDE 1D experiments. It is confirmed that the initial temperature plays animportant role in the steam production and that the power affect only the end of the transient.

In case of bottom injection (Fig 5 example at 700°C), the effect of water injection velocity has alsobeen studied. It shows an important sensitivity of the steam peak amplitude and duration with thisparameter. At the end of the transient, an energy balance can be made by checking that the total steamamount is consistent with the energy stored (i.e. the initial temperature of the bed) [11].

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In the presence of a bypass, the peak steam is also linked to the water flow rate for a given initialtemperature, however the total steam production measured at the end of the transient is no moreconsistent with the energy stored and decrease as function of the increase of the water flow. This isdue to the accumulation of water in the upper plenum when water is bypassing the debris bed. Thenthe steam produced during the reflooding is condensed when passing the upper part.

For Top injection, the steam flow rate measured in the outlet line, at the contrary, is lower when theflow rate increase and independent of the debris temperature (rather constant).

The production of steam in the outlet line is very strong for bottom reflooding tests (Fig. 6a). For topreflooding tests, the steam release in the circuit is smaller (Fig. 6b) and also due to the condensationof the steam when passing through the water collected in the upper plenum.These phenomena (water accumulation and steam condensation) were enhanced for Top injection onpolydispersed debris bed (Fig. 7) probably due to the lower porosity of the bed (0.36 versus 0.4).



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Figure 6 : Effect of water injection configuration on steam production

For this case (Top reflood), the two successive stops of the water injection lead to differentrevaporization sequences which ended the transient to a total steam production consistent to whatobtained for bottom injection.

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Revaporization occurs when the water injection is stopped during the reflooding, which is a necessaryoperation when the upper plenum is filled with saturated water. In that configuration, no more watercould be injected. This was the case for:

high flow rate (20m/h) in the presence of a bypass (Fig. 5b), Top injection and at high temperature (900°C -Fig 6b) or polydisperse debris (Fig. 7) when

water has more difficulty to penetrate inside the debris, due to Counter Current FlowLimitation (CCFL).

Figure 8 illustrates the steam flow rate generated during the first peak as function of the injectedwater flow rate. While the tendency is rather linear for PRELUDE 1D experiments (without bypass),the slope (i.e. the conversion factor water to steam) seems to decrease above 10 m/h in case ofPRELUDE 2D (i.e. with a bypass).

The effect of particle size (Ø4, Ø2 and Ø1 mm) has also an impact on the evolution steam production[5]. In the same thermal hydraulic conditions, the steam peak magnitude and the vaporization durationincrease when the particle size decreases.

At moderate water flow rate and temperature, the analyses of the steam production (Fig 9a) shows thatspecific polydispersed bed (mixture of 4, 3, 2 and 1mm) behaves as a monodispersed debris bed ofaround Ø2 mm [11]. It is confirmed regarding the thermal results and the quench front propagation(see § 5.3). At higher initial temperature or higher water flow injection (Fig. 9b): the 1st peak seemsto be more pronounced due to lower porosity of the polydispersed bed.

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Figure 9 : Effect of particle size (Ø4, Ø2 and Ø1 mm particles and mixture Ø4-3-2-1mm)

This indicates that the data obtained in PRELUDE with monodispersed debris beds can be relevant toevaluate the behaviour of more complex debris beds. Complementary analytical and experimentalanalyses will give information to determine which physical parameters (mean diameter, exchangesurface, porosity, permeability, passability) should be considered for simulations using homogeneousporous medium configurations ([13]).Regarding the conversion factor, the steam peak is mitigated in the presence of a bypass made ofquartz particles. It means that, in the first stage of the reflooding, the cooling is more efficient in 1Dconfiguration. It is confirmed by Fig. 10 which represents the steam conversion factor.

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Figure 10 : Conversion factor (water to steam) Figure 11 : Steam flow map in PRELUDE 2D

5.3 Quench front propagation

Thermocouples inside the debris bed allowed a fine view of the different phases of the transient andare useful to follow the front quench propagation.Figure 12a illustrates an example of the quench front propagation (timing for quenching identifiedwhen thermocouples reached the saturation conditions) for a typical value of water flow velocity(5m/h). The existence of a quasi steady propagation of the quench front is verified for most of thetests in bottom injection configuration. The faster quench front velocity in the periphery than in thecentre outlines the 2D behaviour of the reflooding process (Test 75 as an example) situation enhancedwith debris temperature and water flow rate without bypass (Fig. 12b).Similar behaviour was observed in the DEBRIS experiments for which an experimental benchmarkwith those of PRELUDE has been performed [14].For Top injection (Test 135 as an example), the general behaviour of the internal debris bed is rathersimilar except for the duration of the reflooding time which is longer by a factor of 1.5 (in thisexample). Actually, for top injection, counter current flow occurs as the water goes down while thesteam goes up. This phenomenon implies a limitation of the critical heat flux (see Lipinski model[15]) already observed in a rod bundle geometry.

steam



We observed a downward progression at the periphery of the debris before a standard upwardprogression inside the debris (6.8-7m/h) with a similar quench front velocity (QFV) (7.5-8 m/h) as forbottom injection. There is like a “time” offset between the top and the bottom quenching.The first series of tests (bottom situation with a 2cm bypass) showed that the water does not easilybypass the debris due to steam release through the bypass (Fig. 11) for most of the T/H configurationexcept for the last two tests at high temperature or water flow rate. The flow map has been derivedfrom thermocouple’s analyses in the bypass. So, it has been decided to increase the bypass size (4 cm)to promote the water propagation around the debris bed as it will probably occur in a reactor situation.

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When the thickness of the bypass is efficient enough, and depending on thermal hydraulicsconditions, the liquid water level progresses very fast at the periphery with the non-heated quartz ballsbefore the water penetrates inside the stainless steel debris (Fig 12b).

Whereas 2D progression was observed in PRELUDE 1D tests (Test 74 as an example), more or less1D progression is outlined in PRELUDE 2D experiments (Test 221 as an example), even if the debrisbed is surrounded by water (Fig 12b) and the water flow rate is rather high (20m/h).As we have observed that the quench front velocity is strongly linked to the initial debris bedtemperature, this behavior (2D progression in PRELUDE 1D and 1D progression in PRELUDE 2D)could be due to the different radial temperature profile in the two geometrical configurations. Withoutbypass, the periphery of the debris experienced lower temperature due thermal losses close the wall.

In the presence of a bypass, the quartz balls play the role of an insulator layer, and the fact that theinductive efficiency is slightly increased at the periphery of the stainless steel debris bed, the radialtemperature profile was more or less flatter that the previous configuration. Due to the presence of bigparticles (8mm), the permeability is larger in the bypass than in the stainless steel particles bed.Then, both steam and water are attracted by the bypass (Fig. 11) which generates lower pressure drop.The vaporization is so strong during reflood that the steam flows mainly in the by-pass preventing thewater 2D penetration. Finally, no radial quenching has been observed. A larger by-pass has been buildand water have accessed early in the by-pass. Nevertheless, the stainless steel bed reflood remains 1D.Regarding the QFP, the same behavior is observed for polydispersed debris bed, however, it seemsthat when the porosity is lower, the difference between bottom and top injection is enhanced (Fig13).

At very low water injection flow rate (2m/h), the reflooding process is strongly delayed, in case ofTop reflooding, even in the presence of a bypass, due to the immediate vaporization of the waterinjected on the debris (Fig. 14): some early quench with immediate re-heating was observed,nevertheless, the total duration of the quenching, in that configuration, was not significantlyincreased.



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Figure 13 – Quench front progression Figure 14 – Quench front progressionfor polydispersed debris bed for Top reflooding and bypass

Figure 15a gives the quench front velocity as function of the water flow rate in case bottom injection.The quench front velocity outlined a quasi linear increase versus water flow rate velocity withoutbypass, at 400 and 700°C. At higher water flow rate (30m/h for 400°C and 20 m/for 700°C) thetendency is less linear, due to less efficiency of the cooling in the center of the debris.

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a) Tests without bypass (Ø4 and Ø2 mm) b) Tests without and with bypass (42.5mm)Figure 15 : Quench front velocity as function of water flow rate

Figure 15a gives the quench front velocity as function of the water flow rate in case bottom injection.The quench front velocity outlined a quasi linear increase versus water flow rate velocity withoutbypass, at 400 and 700°C. At higher water flow rate (30m/h for 400°C and 20 m/for 700°C) thetendency is less linear, due to less efficiency of the cooling in the center of the debris.

With the presence of a bypass (Fig. 15b), the quench front velocity seems to be saturated above 10m/h even at low temperature (400°C), indicating a maximum of the efficiency of the refloodingbetween 5 and 10 m/h. This is an important outcome of PRELUDE experiments and will be detailedin the following section.

5.4 Duration and efficiency of the reflooding

The reflood duration is defined as the time needed to cool all the bed at the saturation temperaturestarting from the initiation of the reflooding.Figure 16 presents the reflooding time versus the water flow velocity for several initial temperatures(400, 700 and 900°C) and configurations (without and with the presence of a by-pass).

As expected, the reflooding duration increases regularly with the initial temperature. It means that forPRELUDE beds-like, the residual power has no impact (or at least is negligible) on cooling.Regarding the effect of the injection flow rate, the reflood duration decreases while the water flowvelocity increases. There is an important gain in the range 0-5m/s. For velocity higher than 5m/s, the

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gain is more limited in 1D configuration. Indeed, in the 2D configuration, there is nearly no moregain. Actually, the presence of a by-pass limits the pressure drop across the stainless steel bed andalso the water/steam flow rate through this bed, and finally the cooling. It shows the importance of theby-pass. The minimum reflood duration depends mainly on the temperature anyway the flow rateabove a threshold value of the flow rate. It is an important information in order to optimize themanagement of water source during a severe accident, as water can be used either for the refloodingor for another safety system (the containment aspersion for instance).

Tests without bypass Tests with bypass

Figure 16 : Duration of the reflooding as function of water flow velocity

The efficiency of the reflooding is decreasing while the particle size is decreasing; this is observedwhen looking at the quench front propagation versus water flow rate for a given initial debristemperature (Fig. 15a).

5.5 Possible contribution of the fluidization phenomenon during the reflooding

The different campaigns performed in PRELUDE facility allowed characterizing the domain of thethermal hydraulics conditions (Temperature of the debris versus water flow rate) for which thefluidization of the debris occurs.Figure 17a describes this domain for the largest particles (Ø4 mm). Six tests outlined fluidization,with one of them (700°C/20m/h) in a violent manner (strong eruption).

2 5 10 20 30 40 50m/h

900

800

700

600

500

400

300

200

Test 58Test 67

Test 57Test 29 / 30Test 46Test 79/80

Test 31

Test 69Test 71Test 75

Test 15Test 16

Test 17 à 26

Test 126

Test 125 (300w/kg)

Test 78


Test 121

Test 123Test 124

Test 81

Test 74Test 76

Test 142

Test 150

Test 149

Test 148

Test 144

Test 143

Test 131-4

Test 127à130

Test 147

FLUIDISATIONdomain

Ø4 mm particles

Temperature (°C)

Inlet fluid velocity (m/h)

0 2 5 10 20 m/h

Ø2 mm particles


Test 95

Test 87Test 88

Test 91 Test 92Test 96

Test 89Test 107

Test 108

Test 109

Test 113

Test 118

Test 112

Test 110

Test 111

Test 114Test 115

Test 117

Test 116

FLUIDISATIONdomain

900

800

700

600

500

400

300

200

Temperature (°C)


1 2 4 5 7,5 10 20 m/h

FLUIDISATIONdomain

Test 106

Test 104Test 100

Test 98

Test 99

Test 102

Test 101

Test 105

900

800

700

600

500

400

300

200

Ø1 mm particles

Temperature (°C)


a) Tests for 4 mm particle size b) tests for 2 mm particle size c) tests for 1 mm particle size

Figure 17 : Fluidization domain in PRELUDE

NB : Tests positioned at the right of the dashed line, experienced fluidisation.

Regarding the Ø2mm particle size (Fig. 17b), fluidisation was observed for very high temperature(900°C) and moderate water flow rate (5 m/h) and from 550°C at higher flow rate (10 m/h). Theresults for the smallest size (Ø1mm – Fig. 17c), demonstrated that the fluidisation was reached verysoon (5 m/h), regarding the water velocity even at low temperature (400 °C) below 10 m/h (i.e.7.5m/h). This is due to the increase of the pressure drop as the particle size is decreasing.

4 cm bypass

2 cm bypass



In any case, fluidisation has been observed when the pressure drop across the porous medium, given

by the classical Ergun law (see equation below), was above the pressure due to the weight of the

debris (i.e. 100 mbar in PRELUDE, confirmed by pressure drop measurement).

g

H

PU

dU

dvpv

p

v

p

1

1150175,132

2

2

3

with pvv et , fluid dynamic viscosity, density of fluid and particles,

pd , particle diameter and , debris bed porosity.

Figure 18a) below illustrates temperature evolution which shows many perturbations onthermocouples signals due to the destabilisation of the debris bed.

0

35

0

850

10600 10900

Tem

pera

ture

(°C

) Fluidization

water B1 mm

steam

0

150

0

100

200

300

400

500

1025 1350

Flo

wra

te:

wate

r-

steam

(g/s

)

Tem

pera

ture

(°C)

Test 161

46 s

water

steamtime (s)

0

150

0

500

1025 1350

Flo

wra

te:

wate

r-

steam

(g/s

)

Tem

pera

ture

(°C

)

265 s

Test 160 water

steam time (s)

Figure 18 : Effect of fluidization on thermal behaviour

As for monodispersed small particles (Ø2 and Ø1mm), for the polydispersed debris bed that we haveexperienced (mixture of Ø4, Ø3, Ø2 and Ø1mm), fluidisation occurs for bottom injection even formoderate temperature (400°C), leading to a very short reflooding time (46s) (Fig. 18b).

For “Top reflooding” , most of the injected water accumulated in the upper plenum due to the countercurrent steam flow limitation (CCFL) which limits the downward progression of the water increasingthe duration of the reflooding up to 265 s (about a factor of 6 more than the previous configuration).This configuration (Top injection) avoids, of course, the fluidisation phenomenon (Fig 18c).

This fluidization phenomenon is well known in the industry field, to increase the heat exchange in aporous medium and it will increase the efficiency of the reflooding, nevertheless we have toinvestigate the condition of appearance in case of nuclear reactor case, in particular the effect of apossible bypass of the core from the periphery.

The series of PRELUDE 2D experiments performed with a bypass have shown that the limit forfluidization could be pushed at very high flow rate (conditions never reached) due to due to the loweramount of water penetrating the debris bed and the steam bypass (see previous section 5.4).

Nevertheless, fluidisation is likely to appear in the Nuclear Reactor case, for very high water flow ratein particular when the Safety Injection occurs at low pressure in the primary circuit, this situation(water velocity above 50 m/h up to 100 m/h) should be studied in further experiments and modeled inSevere Accident codes.

6 MODELING IMPROVMENTS

Progress in the models [6], [7], [9] and [10] of IRSN simulation tool as ICARE/CATHARE, havebeen draw from the described experimental results. The main conclusions of the validation aredescribed below.

Bottom injection

With fluidisation

Top



In the range of parameters covered by PRELUDE, comparisons of temperature evolutions at differentelevations show that the model is able to predict quenching velocity [6], [7] for different inlet flowrates and initial temperatures. The steam flow rate (Fig. 19) and the pressure difference across thedebris bed (Fig. 20) are also well predicted which indicates that the model behaves consistently.

0

10

20

30

40

50

60

70

80

1600 1700 1800 1900 2000

Time [s]

Ste

am

flow

[g/s

]

Experiment: steam flow

Calculation: steam flow

PRELUDE dp=4 mm, v_inj=1.38 mm/s, T=700°C

0

10

20

30

40

50

60

1600 1700 1800 1900 2000

Time [s]

Pre

ssu

rediff

ere

nce

[mB

ar]

Calculation dP

Experiment DP

PRELUDE dp=4 mm, v_inj=1.38 mm/s, T=700°C

Figure 19 – Steam flow rate Figure 20 – pressure drop.Experiment and calculation Experiment and calculation

Steady state progression of quench front was identified (as observed in experiments).Quench front velocity was found to be proportional to the liquid injection velocity. It also increaseswith initial temperature, as observed in the experiment (Fig.15).

In PRELUDE 1D geometry, 2D quench front progression was calculated in some situations [9], asobserved experimentally. 2D effects increase as the injection liquid flow increase.Sensitivity calculations on bed heterogeneities were performed. The bed heterogeneities (initialtemperature and porosity) enhance the 2D effects but are not the only criterion.In PRELUDE 2D geometry (with lateral by-pass) [10], the quench front velocity is also wellpredicted. 2D progression is predicted in some situations (Fig. 21).

0

50

100

150

200

250

300

-50 0 50 100 150 200 250 300

Time [s]

Ele

vatio

nin

de

bris

[mm

]

Exp Bypass Exp R= 60Exp TG R=0 Exp R=90Exp R=120 Exp TB R=0Calcul Rmax=45 Calcul Rmax=105Calcul Bypass

PRELUDE dp=4 mm, v_inj=1.38 mm/s, T=700°C Tref=105°C

Figure 21 : Quench front propagation in PRELUDE 2D

It is concluded that the model is very satisfactory with respect to the experimental accuracy. Thelargest discrepancies have been observed for small debris (1mm).

The application of the model to a reactor scale debris bed allows evaluating the efficiency ofreflooding, depending on the height and radius of the bed, and the presence of a bypass or not.



7 CONCLUSIONS

The PRELUDE experiments have been carried out at IRSN Cadarache, prior to the PEARL program,providing an important data base on the debris bed coolability.

The effects of relevant parameters have been investigated for the modeling and the safety analysis:

1. initial temperature of the debris bed: the reflooding time and the steam production increaseswith the initial temperature,

2. injected water flow rate: the reflooding time decreases strongly with the injection flow ratefor flow rate lower than 5m/h ; for higher flow rate, the impact of the flow rate are nearlynegligible, indeed with the presence of a by-pass,

3. specific power maintained during the reflooding to simulate residual power : at PRELUDEscale, it has no impact on reflooding efficiency,

4. particle size (bed of monodisperse or polydisperse stainless steel particles): the reflooding ismore difficult (decrease of the quench front velocity and increase of the duration of thereflooding) for small particle,

5. the presence of a bypass around the debris: the cooling is not enhanced by the presence of theby-pass,

6. the configuration of the water supply (bottom and top injection) ; the efficiency of thereflooding is higher for bottom injection compared to top injection due to the counter currentsteam flow limitation (CCFL).

The improvement of the knowledge on physical phenomena provided by those experiments concernsthe quench front propagation, the steam production, the cooling rate and the efficiency of thereflooding, for which quantitative data are now available. The effects of pressure drop on the stabilityof the quench front and on the stability of the debris bed itself (possible fluidization) have beenobserved and constitute new quantitative results in this field.

These results will contribute to assess the possibility to stop the progression of in-vessel coredegradation in a Nuclear Power Plant and to the optimization of the Severe Accident Managementstrategy.

ACKNOWLEDGEMENTS

This work was performed within the framework of the SARNET2 Network of Excellence of the 7th

European Framework Program. The authors gratefully acknowledge Euratom and Electricité deFrance to support these activities.

REFERENCES

[1] Bürger, M. Buck, G. Pohlner, S. Rahman, R. Kulenovic (IKE), F. Fichot (IRSN), W.M. Ma(KTH), J. Miettinen, I. Lindholm (VTT) and K. Atkhen (EDF), “Coolability of particulate bedsin severe accidents : Status and remaining uncertainties for In and Ex vessel scenarios, 3rd

European Review meeting on Severe Accident Research (ERMSAR-2008), Nesseber, VigoHotel, Bulgaria, 23-25 September 2008

[2] Stenne N., Fichot F., Van Dorsselaere J.P., “Multi-Dimensional Reflooding Experiments: thePEARL Program, Joint OECD/NEA“ - EC/SARNET2 Workshop “In-Vessel Coolability , Issy-les-Moulineaux, France, Oct 12-14, 2009



[3] Fichot F., Repetto G., Coindreau O. (IRSN), Steibruck M., Hering W. (KIT), Buck M,, , BurgerM. (IKE), “Understanding the effects of reflooding in a reactor core beyond LOCA conditions”,4th European Review meeting on Severe Accident Research (ERMSAR-2010), Bologna, Italy,May 11-12, 2010

[4] Repetto G., Garcin T., and Fichot F., “Experimental program on high temperature debrisreflooding (PEARL) Preliminary results on PRELUDE facility”, MFHT4 Conference, Orléans,France, March 30 – April 01, 2011

[5] Repetto G., Garcin T., Eymery S., March P. and Fichot F., “Experimental program on debrisreflooding (PEARL) – Results on PRELUDE facility”, Proc. of NURETH-14 Conference,Toronto, Canada, September 25-30, 2011

[6] Bachrata A., Fichot F., Repetto G., Quintard M. and Fleurot J., “Reflooding of a severelydamaged reactor core: Experimental analysis and modelling”, Proc. of ICONE-19 Conference,2011

[7] Bachrata A., Fichot F., Repetto G., Quintard M. and Fleurot J., “Quench front progression in asuperheated porous medium: Experimental analysis and model development”, Proc. ofNURETH-14 Conference, Toronto, Canada, Sept 25-30, 2011

[8] Repetto G., Garcin T.(IRSN), Rashid M., Kulenovic R.(IKE), Weimin Ma, Liangxing Li(KTH), “Investigation of Multidimensional Effects during Debris Cooling”, 5th EuropeanReview meeting on Severe Accident Research (ERMSAR-2012), Koln, Germany, March 21-23,2012

[9] Bachrata A., Fichot F., Repetto G, Quintard M., and Fleurot J, “Contribution to modeling of thereflooding of a severely damaged reactor core using PRELUDE experimental results”,Proceedings of ICAPP12 Conference, Chicago, USA, June 24-28, 2012

[10] Fichot F., Bachrata A., Repetto G., Fleurot J. and Quintard M., “Quenching of a highlysuperheated porous medium by injection of water”, Proceedings of Eurotherm 2012Conference, Poitiers, France, Sept 04-07, 2012

[11] Repetto G., Garcin T., Eymery S., “New insights in the Thermal Hydraulics behaviour of a hightemperature debris bed during reflooding”, Proceedings of NURETH-15 Conference, Pisa, Italy,May 12-17, 2013

[12] Chikhi, N. Lenoir B., Garcin T., Reflooding of a damaged core : characterization of a debrisbed, NURETH-15 Conference, Pisa, Italy, May 12-17, 2013

[13] Chikhi N., Coindreau O. (IRSN), Li L.X., Ma W.M. (KTH), Takasuo E. (VTT), Leininger S.,Kulenovic R., Laurie E. (IKE), Evaluation of an effective diameter to study quenching and dry-out of complex debris bed, 6th European Review meeting on Severe Accident Research(ERMSAR-2013), Avignon, France, October 02-04, 2013

[14] Leininger S., Kulenovic R., Rahman (IKE) S., Repetto G. (IRSN) and Laurien E. (IKE),Experimental Investigation on reflooding of debris be, 6th European Review meeting on SevereAccident Research (ERMSAR-2013), Avignon, France, October 02-04, 2013

[15] Lipinski, R.J., “A model for boiling and dryout in particle beds”, Report NUGER/CR-2646,SAND82-0765, Washington D.C, (1982)

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