EXPERIMENTAL INVESTIGATION ON … and counter-current liquid-vapour flow and respective heat...

6th European Review meeting on Severe Accident Research (ERMSAR-2013) Avignon (France), Palais des Papes, 2-4 October, 2013

Session “In-vessel corium and debris coolability”, paper S2-7 1/16 pages

EXPERIMENTAL INVESTIGATION ON REFLOODING OF DEBRIS BEDS

S. LEININGER1, R. KULENOVIC

1, S. RAHMAN1, G. REPETTO

2 AND E. LAURIEN

1

1 Institute of Nuclear Technology and Energy Systems (IKE), University of Stuttgart, Germany

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

[email protected], [email protected]

ABSTRACT

In case of a severe accident, continuous unavailability of cooling water to the core may result in over heating of the fuel elements and the loss of core integrity. Under such conditions a structure of heated particles of different sizes and shapes (debris) may be formed by fragmentation of core material inside the reactor pressure vessel (RPV). To avoid any damage to the RPV the reflooding is of great importance in order to establish long-term coolability.

In the framework of the r&d project (Ref. [1]), monitored by GRS (Cologne), and the SARNET2 network (Ref. [2]) specific experimental investigations on the coolability of debris beds with different bed contents (e. g. mono-/polydispersed bed of stainless steel balls with 6/3/2 mm in diameter, irregularly shape particles from PREMIX experiments - KIT) and various initial bed temperatures at ambient pressure (1 bar) were carried out at IKE using the DEBRIS test facility and a second test set-up. In both experimental configurations the particles are volumetrically heated by an electromagnetic induction coil to predefined temperatures and then flooded with subcooled water from top (top-flooding) or from bottom (bottom-flooding). Depending on the flooding situation the cooling down (quenching) behaviour of the beds varies significantly due to the change of co- and counter-current liquid-vapour flow and respective heat transfer between solid and generated two-phase flow.

This paper presents mainly the experimental results of systematic quenching studies at IKE. Furthermore, some representative results of benchmark experiments performed by IKE and IRSN (PRELUDE test facility) in the frame of the SARNET2 joint work are demonstrated. Finally, exemplary comparisons between experimental data and numerical results of simulations with IKE’s code MEWA-2D (Ref. [3]) are shown.

Keywords: Quenching, coolability, debris bed, reactor safety

1 INTRODUCTION

During a severe accident in a light water reactor, the core can melt and can be relocated to the lower plenum of the reactor pressure vessel (RPV). Due to the possible presence of water the melt can form a particle debris bed (in-vessel scenario). An insufficient heat removal of decay heat in the debris bed may lead to a re-melting of the debris and to a failure of the RPV. Hence, the molten core can relocate to the reactor cavity and form again a debris bed due to the possible presence of water (ex-vessel scenario). Therefore, addressing the issue of coolability the behaviour of heat generating particle debris beds is of prime importance in the framework of severe accident management strategies, particularly in the case of above mentioned late phase accident scenarios.

In case of quenching a hot and dry debris bed is flooded with coolant water either from bottom (bottom-flooding) or from top (top-flooding). Hence, a two-phase co- or counter-current flow establishes inside the bed. Due to the large surface area of porous media, the



coolability of particle beds is normally not limited by the heat transfer from the particle to the coolant. In case of top-flooding the uprising vapour can block the penetrating water from an overlaying water pool, so that not sufficient water can enter the bed with the effect of a reduction of the bed’s coolability.

The installation of a downcomer (DC) in the centre of the bed can offer a low resistance flow path for water. In case of top-flooding with installed downcomer, water can flow through the downcomer, which yields a combination of top- and bottom-flooding. According to Bürger et al. (Ref. [4]) these multidimensional effects can significantly increase the coolability.

In previous studies at IKE top- and bottom-flooding experiments were carried out by Groll et al. (Ref. [5]) and Rashid et al. (Ref. [6]). The focus was on the phenomenology of quenching with variation of initial bed temperatures (T0) and particle compositions (mono- and polydispersed bed, PREMIX particles (Ref. [7])). Rashid et al. (Ref. [8]) already employed a downcomer (10 mm ID) for boiling and dryout experiments for system pressures up to 5 bar. They found that a downcomer can increase the dryout heat flux (DHF) by a factor of 2. A perforation of the downcomer was expected to improve further the coolability, because water can also penetrate laterally into the bed. The DHF was surprisingly not significantly higher than for pure top-flooding. This may be due to vapour, which penetrates into the downcomer and therefore is decreasing the flow cross-section of down-flowing water.

Nayak et al. (Ref. [9]) performed quenching experiments of a radially stratified bed employing downcomers. They installed a large downcomer (54 mm ID) in the bed centre and six smaller downcomers (9.5 mm ID) in the periphery. In their experiments different combinations of these downcomers were tested in order to study the effect of their location and size on quenching. It was found, that the accumulated flow area of the downcomers has a significant effect on the total quenching time.

The downcomer topic is also related to the topic of inhomogeneities in a porous debris bed. Spencer et al. (Ref. [10]) and Karbojian et al. (Ref. [11]) have shown, that porosities up to 50 resp. 70 % (including internal porosities) can be expected. Such high porosity regions can also serve as a downcomer, which was numerically investigated by Ma and Dinh (Ref. [12]) using the WABE-2D code.

At IRSN quench experiments at different initial bed temperatures (T0 = 200 °C up to 900 °C) were carried out by Repetto et al. (Ref. [13]) using the PRELUDE set-up. Water was injected from the bottom at a fixed flow rate. In the new PEARL programme it is foreseen on the one hand to extend the dimensions of the set-up and on the other hand to simulate a bypass at the periphery by a non-heated part with higher porosity.

The focus of this paper is on the quenching behaviour of dry hot debris under several inflow conditions. The particles will be quenched both from top and from bottom driven by gravity. In order to improve the access of water into the lower part of the bed two central downcomers are tested. Additionally, the results of the DEBRIS / PRELUDE benchmark tests in cooperation with IRSN are presented. For validation purpose several experiments were simulated using IKE’s MEWA-2D code.

2 SMALL DEBRIS TEST SET-UP

The small DEBRIS test set-up at IKE (Figure 1) allows to carry out quench experiments with a bed height of 200 mm. Inside the particle bed 34 thermocouples are installed (Figure 2). Most of them are arranged on three main levels at an elevation of 20, 100 and 180 mm. In contrast to the test section of the DEBRIS test facility most of the thermocouples are located close to the wall. Additionally, one thermocouple is installed below and above the bed.



Figure 1: Small DEBRIS test set-up

220 T36

180 T27-34 T25 T26 Level 3

(increment 45°)

140 T24 Debris

100 T16-23 T15 T13 T14 Level 2

60 T12

20 T4-T11 T2 T3 Level 1

-20 T1

Ceramic Particles

75 0 75

r / mm

200 T35

H /

mm

0

-130

Figure 2: Positions of thermocouples in the small DEBRIS test set-up

Cooling water at ambient temperature can be discharged to the bed either from top by a water pool or from bottom. In case of top-flooding the initial height of the water level in the pool is 800 mm from the top of the particle bed. A fine wire mesh breaks up the water jet and distributes the water uniformly over the bed’s cross section. In the later phase of top-flooding a constant water level is maintained by an overflow. In case of bottom-



flooding water can be discharged either by a fixed flow rate or gravity-driven by a lateral water column with a constant height of 530 mm. If water is discharged by a lateral water column the flow rate depends on the pressure difference between the levels of uprising water in the bed and lateral water column and on the minimum cross section and the pressure loss coefficient of the water supply, which are d = 10 mm and ζ = 10, respectively. For bottom-flooding the water flow rate can be measured by a flow meter. To ensure homogeneous inflow conditions from the bottom the lower part of the test crucible is filled with non-heated ceramic particles. In case of top-flooding three downcomer configurations (closed, open and perforated) installed in the centre of the bed were investigated. All downcomers have an inner diameter of 25 mm, and the perforated downcomer is uniformly drilled with 2 mm holes, which yields a porosity of 68 %. The particle bed consists of 3 mm stainless steel spheres, the bed porosity is 39 %.

The particle bed is heated by a water-cooled induction coil, which is connected to a RF-generator. The RF-generator operates at a maximum frequency of 300 kHz and has a nominal electrical output power of 40 kW.

3 EXPERIMENTAL RESULTS WITH DIFFERENT INFLOW CONDITIONS

3.1 Bottom-flooding with lateral water column

In case of bottom-flooding an elevated water storage tank is interconnected to the bottom of the bed. After a valve is opened the water is fed into the bed with a constant hydrostatic head of 530 mm.

The observed quench front progression is in principle one-dimensional. The markers in Figure 3 represent the “quenching time” tq (reaching and remaining at Tsat = 100 °C) for each thermocouple along the bed height H at three different radial positions. It can be seen that the quench front progression in the near wall region is faster than in the centre of the bed. This can be explained by lower temperatures and higher porosities (wall effect) in this region. The inhomogeneous temperature distribution can be attributed to heat losses and restrictions of the inductive heating system. This also affects the quench front velocity at the wall, which is not uniform.

Figure 3: Quench front progression, bottom-flooding with lateral water column,T0 = 500 °C



In previous studies at IKE quenching experiments for monodispersed beds (6 mm spheres) with a height of 630 mm were carried out (e. g. Schäfer et al. (Ref. [14])). He observed a violent quenching process for maximum initial bed temperatures up to 430 °C, which led to a partial ejection of the bed contents out of the crucible. This effect or any other kind of fluidisation was not observed in the present study. Due to the reduced height of the lateral water column and the pressure losses in the piping system the current flow rate is lower than in the previous study. The maximum measured inflow velocity at the beginning of the water injection was 4.3 mm/s. This flow velocity rapidly decresases to 2.5 mm/s when the quenching process starts and a counter pressure inside the bed establishes. According to Repetto et al. (Ref. [15]) for this flow rate and bed temperatures no fluidisation can be expected.

3.2 Top-Flooding with closed downomer

In case of the closed downcomer the downcomer is blocked, so that water can only penetrate into the bed from top. In Figure 5 it can be seen, that after injection of water the upper part of the bed is partly quenched. Then water preferably penetrates downwards in the near wall region, where temperatures are lower and porosity is higher. Later, a water pool establishes in the lower part of the bed, which is supplied by the flow paths in the bed. The water level rises and the quench front propagates upwards. As already observed in the bottom-flooding case, the quench front close to the wall is faster than in the centre. In general for pure top-flooding the quenching process can be devided in two stages (Figure 4). In the first stage water penetrates downwards and establishes a water pool in the lower part of the bed, which is supplied by flow paths in the bed. In the second stage the water pool rises and quenches the remaining hot parts of the bed.

Depending on the initial bed temperature there are little differences in the quench behaviour. It was observed, that for low initial bed temperatures water preferably penetrates in the centre, whereas for high initial bed temperatures water tends to penetrate in the near wall region. This can be explained by higher porosity not only at the outer wall but also at the downcomer. For high temperatures the temperature gradients inside the bed are higher, which offers the water a low resistance flow path at the outer wall.

Figure 4: Quench front progression, top-flooding with closed downcomer, T0 = 500 °C

Stage 2 Stage 1



Figure 5: Temperature profiles, top-flooding with closed downcomer, T0 = 500 °C

3.3 Top-Flooding with open downcomer

For top-flooding with open downcomer, the downcomer provides a low resistance flow path from the top to the bottom of the bed. Hence, the bed can be quenched from top and from bottom. From Figures 6 and 7 it can be seen that water only partly penetrates inside the bed from top. Most of the water flows through the downcomer. From the beginning of water injection it takes only a few seconds to quench the lower part of the bed. After that, the water level rises in the bed as it was also observed for the closed downcomer. Previously quenched regions on the top level are re-heated by uprising steam. This re-heating was more prominent for higher initial bed temperatures due to higher thermal load stored inside the bed.



Figure 6: Quench front progression, top-flooding with open downcomer, T0 = 500 °C

Figure 7: Temperature profiles, top-flooding with open downcomer, T0 = 500 °C



3.4 Top-Flooding with perforated downcomer

Finally the perforated downcomer was investigated. In addition to top- and bottom-flooding water can also laterally penetrate into the bed. In the experiments strong vibrations of the test set-up occurred for T0 ≥ 400 °C. It was observed that water was pushed out of the downcomer. This can be attributed to steam, which enters the downcomer and blocks the complete cross section of the downcomer. This effect probably caused the vibrations. From the thermocouple readings it can be seen that for T0 = 500 °C almost the complete top level was quenched after a few seconds (Figures 8 and 9). The same quench behaviour as already mentioned for pure top-flooding with closed downcomer was found. Water needs to flow under counter-current conditions through the bed, which is a very ineffective way of flooding. In a pre-study applying a 10 mm perforated downcomer it was also observed, that the downcomer was blocked by steam.

Figure 8: Temperature profiles, top-flooding with perforated downcomer, T0 = 500 °C



Figure 9: Quench front progression, top-flooding with perforated downcomer, T0 = 500 °C

3.5 Comparison of inflow conditions

For all inflow conditions presented in chapter 3.1 - 3.4 a set of experiments was carried out varying the initial bed temperatures (T0 = 300 °C up to 700 °C). The total quenching times ttq for these experiments are depicted by solid lines in Figure 10 in order to characterize the effectiveness of the inflow condition on coolability. The dashed lines represent the time needed to establish a water pool in the lower part of the bed, which was called the first stage of the quenching process. It can be seen, that the open downcomer can significantly reduce the total quenching time compared to the closed downcomer. In case of a closed downcomer it takes much longer to establish a water pool in the lower part of the bed. It is evident that it is less efficient to supply a water pool by flow paths in the hot bed than by a low resistance downcomer. The quenching times for bottom-flooding with lateral water column are in the same range of the open downcomer. Nevertheless, these two inflow conditions cannot be compared directly. The quenching time depends on the flow rate, which should be higher for the downcomer with ID = 25 mm than for the external water supply with a diameter of 10 mm.

The total quenching time of the perforated downcomer is comparable to the closed downcomer. From the dashed lines it can be seen that at least at the very beginning water can enter the downcomer to establish a water pool. Later it seems that the downcomer is completely occupied by steam and so no water can enter the downcomer. Therefore, the total quenching time for the perforated downcomer is in the same range like for the closed downcomer.



Figure 10: Comparison of total quenching times for all inflow conditions

4 DEBRIS / PRELUDE BENCHMARK TESTS

The objective of the DEBRIS / PRELUDE benchmark tests is to perform quench experiments with two different set-ups under comparable conditions. For these benchmark tests the small DEBRIS test set-up at IKE is used because its dimensions are similar to the PRELUDE set-up at IRSN. Both experiments have a bed height of 200 mm, whereas the PRELUDE bed diameter is a little bit larger (174 mm). In both set-ups thermocouples are measuring the fluid temperatures in the beds’ pores. Additionally, in PRELUDE some particles are equipped with thermocouples in order to detect the quench front, not only the water front. Monodispersed particle beds were investigated, 3 mm in DEBRIS and 4 mm in PRELUDE. The slight difference in particle size should not have any significant effect on the phenomenology of quenching or on the total quenching time ttq. In both experiments subcooled water (ambient temperature) was injected from the bottom with constant superficial velocities Jl = 0.56, 1.39 or 2.78 mm/s, respectively. Higher flow rates were not of interest because for higher flow rates fluidisation was expected (Ref. [15]), which was not the aim of this benchmark study. All experiments were carried out at ambient pressure. During flooding the heating was maintained in PRELUDE and switched off in DEBRIS for safety reasons.



Figure 11: Comparison of DEBRIS / PRELUDE total quenching times

In Figure 11 the total quenching times ttq for small DEBRIS and PRELUDE are compared with each other. For a higher initial bed temperature quenching takes longer, and a linear relation can be observed. For higher bed temperatures more water is needed to cool down the bed, and therefore the quench front progresses more slowly. For higher flow rates the quenching time is significantly reduced. Regarding this phenomenology and the total quenching time the DEBRIS and PRELUDE results show a good agreement. The small deviations in quenching time can be explained by slightly different initial bed temperatures in both experiments.

5 MEWA-2D CODE ANALYSIS OF EXPERIMENTAL RESULTS FOR

DEBRIS OPEN DOWNCOMER CONFIGURATION

The MEWA (MElt and WAter) code (Ref. [3]) is being developed at IKE. It describes steam and water flows within a fixed porous matrix, including exchange of heat and evaporation of water, thus dryout and quenching process of particulate or porous debris. Heat transfer and evaporation are in principle based on a thermal non-equilibrium between the phases. Superheated particles, sub-cooled water and superheated steam are taken into account. The key role is played by the heat transfer from solid to the interface of gas and liquid i.e. boiling. Boiling heat transfer at the surface of solid particles is modeled by using the boiling curve model which accounts nucleate and film boiling modes. In MEWA, the Rohsenow correlation (Ref. [16]) is taken for nucleate boiling up to the critical heat flux. A 100 K superheat above saturation is assumed to yield vapour film boiling, for which a correlation of Lienhard (Ref. [17]) is used. In the intermediate range, the heat flux from the particles is calculated by linear interpolation as a function of the temperature difference between particle and saturation temperature.

Concerning the friction, a modified version of the model of Tung and Dhir (Ref. [18]) is presently applied in MEWA-2D code. Especially, the transitions between the flow patterns of bubbly, slug and annular flow have been modified to yield a more rapid transition towards slug and annular flows with smaller particle diameters, due to deficits detected for smaller particles and physical plausibility. Details can be found in (Ref. [19]). A fair amount of validation work for the MEWA model has already been done on the two-phase thermal hydraulics under boil-off and quenching conditions (Ref. [1] and [2]).



Here, the MEWA-2D code has been applied to the DEBRIS experiments in order to verify its applicability as well as to promote better understanding of the experimental results. Experiments with open downcomer configuration are considered in the present contribution. A water pool of about 200 mm height above the bed is always maintained during the test. Thus the bed is flooded both from top and bottom. Quenching behaviour under this kind of flow conditions is really important for realistic multidimensional configurations, where combined top and bottom flooding is provided either via downcomer-like structures or due to the shape (e.g. heap shape) of the bed. Therefore, this experiment gives the opportunity to check the model for quenching of hot debris in such combined flow conditions.

Calculations have been performed for axisymmetric geometry. As in the experiments, axial and radial temperature profile has been chosen with lower bed temperature at the bottom and the outer boundary (near the wall). Figure 12 shows a comparison of measured and calculated position of the quench front versus time for different initial bed temperatures. The experimental data points have been determined from a drop of the temperature measured by thermocouples at different axial location to saturation temperature. It can be seen from both experimental and model data that the quench front progresses mainly from bottom to top, i.e. in an initial phase only parts of the bed directly in contact with the water pool on top get quenched. The quench front at the top then practically stops, while water gets to the bottom of the bed via the downcomer and starts to flood it from below. The timing and the qualitative behaviour of the quench front progression are well captured by the MEWA calculations.

Figure 12: Comparison of measured and calculated position of the quench front (half radius position) versus time in the DEBRIS experiment with quenching via open downcomer

A thin quenching front due to slow progression of water (average water inflow varies from 1 mm/s to 1.5 mm/s) and rapid quenching results from the calculations. With the higher particle temperatures lower quench front velocities are obtained both in the experiments and in the calculations. This is expectable because in case with higher temperature higher thermal energy has to be taken out from the bed.

The temperature development at certain thermocouple locations is reproduced at least qualitatively in the MEWA calculation, as shown in Figure 13. The experimental data points, coming from thermocouples at the same axial location (180 mm) close to the wall, but at different angular positions show some non-uniform behaviour, but similar trends: With the experiments of initial bed temperature 700°C, after some initial drop within



about the first 20 s, the temperatures rise again until the bed is finally quenched after about 150 s. This behaviour is also calculated by MEWA. This can be explained by looking in more detail at the simulation results.

Figure 13: Comparison of measured and calculated temperatures at different thermocouple locations ( T0 = 700 °C) versus time in the DEBRIS experiment with

quenching via open downcomer

Figure 14: Distribution of liquid saturation at 20 s (left) and 100 s (right) after initiation of quenching, black circles indicate position of temperature gauge in Figure 13



Figure 14 shows the distribution of liquid saturation at 20 s (left) and 100 s (right) after initiation of quenching. The black circles indicate the position of the thermocouples discussed with respect to Figure 13. It can be seen that after quenching starts in the lower bed part (after water has reached it via the downcomer and after filling of the lower, unheated section), water trying to penetrate the upper part of the debris bed is driven out again by steam flowing up. This voids the upper part again, and the previously quenched parts heat up due to flow of hot steam and conduction from unquenched bed parts.

6 CONCLUSIONS

The aim of this study was to compare different inflow conditions for reflooding of hot, dry debris beds and to characterize the effectiveness of coolability by the total quenching time. Therefore, water was injected from bottom and from top driven by gravity. To improve the access of water into the lower part of the bed different downcomer configurations (closed, open and perforated) were applied.

For bottom-flooding a simple one-dimensional quench front progression was observed. For pure top-flooding with closed downcomer the quenching process can be divided in two stages. In the first stage water penetrates into the bed and establishes a water pool in the lower part of the bed. In the second stage the water level increases and quenches the remaining hot parts. The open downcomer reduced the total quenching time significantly. Water partly penetrated into the bed from top, but most of the water went throw the downcomer and flooded the bed from the bottom. The cooling performance of the perforated downcomer was unexpectedly poor. The total quenching time was in the same range as for the closed downcomer. This was due to a blockage of the downcomer by steam, which entered the downcomer through the perforation. In all experiments it was observed that water preferably takes the path of lowest resistance. In case of top-flooding water mainly penetrated into the bed close to the wall where temperatures are lower and porosity is higher, but also in the centre at the wall of the downcomer.

In collaboration with IRSN benchmark tests for bottom-flooding conditions were carried out. Experiments were performed with DEBRIS and PRELUDE test facilities under comparable conditions. In both benchmark test series the effect of initial bed temperature and flow rate on the quenching time was systematically investigated. As a main benchmark result it can be stated that the measured quenching times for same flow and bed temperature conditions were almost the same.

MEWA calculations for DEBRIS quenching experiments with open downcomer configuration were also presented. In such configuration the bed is flooded both from top and bottom which gives the opportunity to check the model for quenching of hot debris in such combined flow conditions. The quench front progression predicted by MEWA agrees well with the experimental results measured in the half radius position of the bed. Qualitative trends, that quench front progresses mainly from bottom, are generally well reproduced. It is shown that downcomer favours the penetration of water to the bottom, with subsequent quenching of the remaining bed from below, which provides an effective mechanism for faster cooling. From analysis it was found that water moves in the debris bed in a slowly propagating front due to high friction, and the quenching is rapid enough to occur in a thin front. This thin quenching front may also be valid for reactor conditions. Then, a detailed modelling of heat transfer regimes is less important since quenching occurs anyway in a small front. Rather an appropriate description for the friction is essential.

7 ACKNOWLEDGEMENTS

This r&d work was performed within the framework of the SARNET2 Network of Excellence project (project no. 231747) of the 7th European Framework Program and in parts within



the nuclear safety research project FKZ 1501312 (BMWi, German Federal Ministry of Economics and Technology) monitored by GRS (Gesellschaft für Anlagen- und Reaktorsicherheit, Cologne).

REFERENCES

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[3] M. Buck, M. Bürger, S. Rahman, G. Pohlner, Validation of the MEWA model for quenching of a severely damaged reactor core, Joint OECD/NEA EC/SARNET2 Workshop on In-Vessel Coolability, Paris, France, 12-14 October 2009.

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[15] G. Repetto, T. Garcin, M. Rashid, R. Kulenovic, W. Ma, L. Li, Investigation of Multidimensional Effects During Debris Cooling, ERMSAR 2012: The 14th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Cologne, Germany, 21-23 March 2012.

[16] W. Rohsenow, A method of correlating heat transfer data for surface boiling of liquids, Trans. ASME 74, pp. 969–976 (1952).

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[19] M. Buck, G. Pohlner, S. Rahman, Documentation of the MEWA code. Report IKE 2 FB 30, University of Stuttgart, August 2009 (unpublished).

EXPERIMENTAL INVESTIGATION ON … and counter-current liquid-vapour flow and respective heat...

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