experimental investigations of the quenching phenomena for … · 2009-12-03 · pool quenching...

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International Communications in Heat and Mass Transfer 34 (2007) 28–36
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Experimental investigations of the quenching phenomena forhemispherical downward facing convex surfaces with narrow gaps☆

Kwang Soon Ha ⁎, Rae Joon Park, Sang Baik Kim, Hee Dong Kim

Thermal-Hydraulic Safety Research Division, Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong, Daejeon,305-600, Republic of Korea

Available online 2 November 2006

Abstract

To investigate a gap cooling mechanism between a corium and a reactor pressure vessel, quenching phenomena ofhemispherical downward facing surfaces with narrow gaps with respect to wall heating conditions have been investigatedexperimentally. The experiments were performed with a hemispherical gap thickness of 1 and 2 mm, and the wall heatingconditions, that is, with and without an external wall heating. That is, the internal vessel (or both internal and external vessels) washeated up to 450 °C, and distilled water was supplied to the hemispherical gap region, and then the temporal histories of the internaland the external vessels were measured by the thermocouples embedded in the internal and the external vessels. From theinterpretations of the wall temperature and the heat flux history, it was found that the quenching process was changed by the wallheating conditions. If the external wall was not heated, then the internal vessel wall was stably quenched from the bottom to the topregion because the coolant water was supplied to the gap region through the cold external wall. However, if both the internal walland the external wall were heated, then the quenching phenomena were more complicated and slower as the gap thicknessdecreased, because the coolant supply to the gap was restricted.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Quenching process; Hemispherical downward facing convex surface; Narrow gap; Two phase flow

1. Introduction

In the Three Mile Island Unit 2 (TMI-2) accident, a nuclear fuel was melted and relocated to the bottom of a pressurevessel. All the well-known safety codes had predicted that the pressure vessel would fail and corium (a mixture ofmolten fuel and structure material) would be released if this kind of severe accident happened. However, the pressurevessel was not damaged and the molten corium was kept inside the pressure vessel and cooled down with a steepcooling rate, such as 10–100 °C/min [1]. In order to explain the safe cool-down of the relocated corium, three coolingmechanisms have been suggested and a gap cooling mechanism is considered to be the most plausible one whichplayed a major role in the cooling of the corium [2]. Such a mechanism works like this: when molten corium relocatesto the bottom of a pressure vessel filled with water, it is shaped like a hemispherical pool and its surface forms a crust.

☆ Communicated by W.J. Minkowycz.⁎ Corresponding author.E-mail address: [email protected] (K.S. Ha).

0735-1933/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.icheatmasstransfer.2006.09.009

mailto:[email protected]

http://dx.doi.org/10.1016/j.icheatmasstransfer.2006.09.009

29K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) 28–36

As the corium is solidified, it shrinks and the pressure vessel experiences a creep, causing a gap to develop between thecrust and the pressure vessel. The water penetrates the gap and the corium is cooled down. In order for a gap cooling tobe effective, the contact area through a water penetration should be quenched and water should be continuouslysupplied through the gap so that a boiling heat transfer is maintained. As the gap cooling concept has only recently beenconsidered to be of importance, there has not been a great deal of research related to this concept. Some of the researchhas focused on the gap formation [3,4] and some upon the heat transfer through the spherical gap [5,6].

By performing the SONATA-IV (Simulation Of Naturally Arrested Thermal Attack In-Vessel) program [7] atKAERI (Korea Atomic Energy Research Institute), a gap formation was experimentally verified (1 to 3 mm thickness)between the oxidic molten pool and the lower pressure vessel under a cooling process [3], and the total removable heatpower in this gap range [8,9] under a steady-state condition was also quantified.

In this study, the quenching phenomena of hemispherical downward facing convex surfaces with narrow gaps havebeen investigated experimentally. Experiments employed test sections with a 1 and 2 mm gap thickness and a 1 atmsystem pressure. Though many researches have been performed on the quenching phenomena [10–12], there havebeen very few quenching studies of a hemispherical downward facing surface with a narrow gap. Compared with thepool quenching condition, the quenching phenomena of a hemispherical downward facing surface with a narrow gaphas many different behaviors such as a restriction of the coolant supply by a counter current flow limitation.

2. Experimental apparatus and method

Fig. 1 shows a schematic diagram of the quenching test facility. The facility has been constructed to obtain thetemporal quenching data for hemispherical narrow gaps. It consists of a test vessel, a coolant injection system, a closedcoolant circulation system, a heat exchanger, a secondary coolant system of the heat exchanger, and a data acquisitionsystem. The test vessel consists of an internal heating vessel and an external hemispherical vessel. The internal heatingvessel consists of an inner brass mold and an outer copper shell. Rod heaters were put inside a hemispherical brassmold, which provided a maximum average heat flux of 350 kW/m2 at the outer surface. A copper shell was selected asan inner vessel to obtain a uniform heat flux distribution. The thickness and outer diameter of the copper shell are 19and 238 mm, respectively. Two units of stainless steel external vessels were manufactured to provide gap sizes of 1.0and 2.0 mm between the copper shell and the external vessel itself. A 5.1 kW band heater was installed on the outersurface of the external vessel to maintain the external vessel at a set temperature.

Fig. 1. Schematic diagram of the quenching test facility (unit: mm).

30 K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) 28–36

The closed coolant circulation system played the role of a system pressure regulator. That is, the steam generatedfrom the heating vessel was condensed into water by passing it through the heat exchanger. If the secondary flow of theheat exchanger was turned on, the steam passing through the primary tube of the heat exchanger was condensed intowater, and the condensed water was returned to the test vessel. As a result, the pressure of the test vessel was decreased.So the pressure level was constantly maintained by controlling the secondary flow of the heat exchanger. As theexperimental facility constituted a closed loop, the first step, necessary to carry out the experiments, was to vent the airaccumulated in the loop. If air were to remain in the loop, it would obstruct the heat transfer in the heat exchanger so the

Fig. 2. Thermocouple locations on the internal and external vessels (unit: mm).


working fluid would not circulate. Initially the heater power was maintained at a low level and the pressure controlsystem was set at a pre-determined value in the test matrix. The pressure was controlled to within ±5% of the set valueduring the entire test.

The temporal variations of the internal heating and the external vessels were observed by monitoring the K-typethermocouple readings. The thermocouples were embedded in a copper shell, as shown in Fig. 2. Pairs ofthermocouples were installed at different depth positions to calculate the surface temperature and heat flux at eachlocation. The measured temperatures were processed by a Hewlett Packard data acquisition system and a HP-VEEprogram. The temperature was maintained to within ±2% of the average temperature.

The experiments were performed by the following procedure. First, the heating vessel was heated up to 350 −450 °C.Then the heater power was turned off, and distilled water with a normal temperature was filled up to the 140mm from thetop of the heater vessel. The quenching process was monitored and recorded by 68 thermocouples embedded in theinternal vessel and the external vessel. The pressure of the test vessel was kept at 1 bar during the entire quenchingexperiment.

The experiments were performed with a hemispherical gap thickness of 1 and 2 mm, and the wall heatingconditions, that is, with and without an external wall heating.

3. Experimental results

Fig. 3 shows the temporal quenching history of the internal and external vessels wall along with the samelongitudinal positions under the 1 mm gap thickness and no external wall heating condition. Only the internal wall washeated at 355 °C. The distilled water supply for cooling the internal vessel wall was completed at about 30 s, so therewas no influence of the external effect after 30 s. As shown in Fig. 3, the internal vessel wall was slowly quenched atabout a 20 °C/min cooling rate till 510 s, after this time, it was abruptly quenched at about 220 °C/min. This largecooling rate was sequentially propagated from the lower vessel wall to the upper wall. Though the external vessel wallwas not heated directly, the external wall quenching process was started from 117–176 °C because of an internal wallheating. The temperature of the external vessel wall also decreased from the lower part to the upper part. From theinitial cooling of the external vessel wall and the stable quenching phenomena of the internal vessel wall, it could besuggested that the coolant water should be supplied to the gap region through the cold external wall. Though thecoolant was fully supplied to the gap region, the internal wall might maintain a film boiling condition, and then, after510 s, it might start the transition to a nucleated boiling. The transition temperature from the film to the nucleate boilingwas 155 °C, which is similar to the Leidenfrost temperature of the upper plate under the pool quenching conditions. Ifthe nucleate boiling started at the lower internal wall and the film was broken at that point, the heat transfer abruptlyincreased at the lower internal wall, and finally the temperature of the upper vessel wall also decreased because the heatin the upper region was transmitted by a conduction mechanism. This is why the internal wall was made of copper with

Fig. 3. Typical temperature history of the internal and external walls along with same longitude (gap thickness: 1 mm, no external wall heating).


a high conduction coefficient. That is, the local nucleate boiling phenomena had an effect on the overall temperature ofthe internal vessel wall due to the high conduction coefficient.

Fig. 4 shows the contour plots of the temperatures for the internal vessel and the external vessel according to theelapsed time when both the inner and the outer vessels were quenched from 350 °C under the 1 mm gap condition.Those are projections of the hemispherical surface of the inner and the outer vessels. The boundary and the center of thecircular area refer to the top end of the gap and the lowest bottom part of the vessel wall, respectively. Small circlesshown in the radial directions represent the thermocouple locations. The readings from these thermocouples areinterpolated to provide isothermal lines. As shown in Fig. 4, the initial temperature showed an axi-symmetric behavior,however, at 700 s, the symmetry was broken due to an abrupt increase of the temperature in the right middle region.These phenomena were found in both the internal and the external wall. From these temperature behaviors, it was foundthat the water had penetrated through the local region. However, this water penetration did not continue and it stoppedafter a few seconds. Therefore, the axi-symmetric temperature contours were restored after 900 s. The axi-symmetricbehavior of the temperature profile was continued until 1300 s, and then broken due to the water penetration. Thequenching phenomena were propagated from the bottom to the top region of the internal wall and the external wall.

Fig. 5 shows the typical temperature histories of the internal wall and the external wall along with the samelongitudinal positions under the 1 mm gap thickness and both the internal and the external wall heating conditions. Thatis, Fig. 5 represents the same experimental results as Fig. 4. Both the internal and external vessels wall were heated at350 °C. The distilled water supply, for cooling the internal vessel wall, was completed at about 30 s, so there was noinfluence of the external effect after 30 s. As shown in Fig. 5, the cooling rate of the internal vessel bottom wall wasabout 11 °C/min at the initial stage and 40 °C/min at the latter stage. This cooling rate was smaller than that of Fig. 3which was obtained under only the internal wall heating condition without an external wall heating. In a comparison

Fig. 4. Contour plots of the temperatures on the internal and external vessels (gap thickness: 1 mm, both internal and external walls heating).

Fig. 5. Typical temperature history of the internal and external walls along with the same longitude (gap thickness: 1 mm, both internal and externalwalls heating).

Fig. 6. Typical heat flux history along with the same longitude (gap thickness: 1 mm, both internal and external walls heating).



with the internal wall, the temperature behavior of the external wall was very complicated. The external walltemperature at the 40° latitudinal position (lower middle region) decreased slowly with a 4.5 °C/min cooling rate at theinitial stage. After 500 s, the cooling rate was considerably largely increased at 70 °C/min. And then, at about 670 s, aninstantaneous temperature rise was observed. The temperature rises at the 40° latitudinal position were observed twiceafter 670 s, and the increasing temperature time at the 40° latitudinal position coincided with the decreasingtemperature time at the 60° latitudinal position. It might be thought that these temperature increase and decreasephenomena are due to a variation of the coolant contact condition on the external wall. That is, though the cooling rateincreased at 500 s, the temperature on the external wall was still as high as 270 °C, so the external wall was under thefilm boiling region. However, the steam film was not stable, so the temperature of the external vessel wall was rapidlyincreasing or decreasing. The conduction coefficient of the external vessel wall made of stainless steel was smaller thanthat of the internal vessel wall made of copper. Therefore, in the case of the external vessel wall, the local phenomenahave less of an influence on the other region. This is why an abrupt temperature variation at the 40° latitudinal positiondid not propagate toward the other latitudinal positions. On the basis of the instantaneous temperature increases at the40° latitudinal position, it was found that the steam film of the 40° latitudinal position was unstable. The steam film wasmore stable at the top region (80° latitudinal position), so the cooling rate was almost the same as 5.9 °C/min. Theabrupt temperature decreases were observed at the 40° (lower middle region), 20° (bottom region), 60° (upper middleregion), and 80° (top region) latitudinal positions in a temporal sequence.

Fig. 7. Typical temperature and heat flux history along with the same longitude (gap thickness: 2 mm, both internal and external walls heating).


Fig. 6 shows the typical heat flux histories along with the same longitudinal positions on the internal and externalvessels wall in the case of a 1 mm gap thickness and both an internal and external vessels wall heating. The heat fluxvalues were calculated by solving a simple conduction equation through using the two temperature values which weremeasured at different radial positions with a 10 mm interval in the case of the internal vessel and with a 26 mm in thecase of the external vessel as shown in Fig. 2. In these experiments, the heat flux on the surface in contact with thecoolant is high and the heat flux on the surface in contact with the insulator is about zero. The calculated heat flux byusing the two measured temperatures shows an almost average heat flux between the measured points, so the heat fluxin this paper is not exactly the same as the heat flux on the surface in contact with the coolant. To obtain the exact heatflux value on the surface, an inverse heat conduction equation by using the measured temperature data must be solved.As shown in Fig. 6, the maximum heat flux value of the internal vessel wall was 7 times larger than that of the externalvessel wall. The maximum heat flux values were temporally detected from the bottom to the top region of the internalvessel. However, in the case of the external vessel, the maximum heat flux values were detected at 40° (Mid2), 20°(Bottom), 60° (Mid1), and 80° (Top) in a temporal sequence. When the maximum heat flux value was detected, it couldbe suggested that the boiling condition had changed from a film boiling into a nucleate boiling. Therefore, the brokensequence of the steam film was from the bottom to the top region of the internal vessel wall, and the 40° (Mid2), 20°(Bottom), 60° (Mid1), and 80° (Top) latitudinal positions of the external vessel wall. Under the film boiling region(earlier than 500 s), the heat flux values at 40° (Mid2) of the external vessel were the largest, so the film thicknesses at40° (Mid2) of the external vessel were the thinnest. It is possible that the thinner film vanishes more easily, so this 40°(Mid2) latitudinal region of the external vessel changes to a nucleate boiling for the first time.

Fig. 7 shows the temporal variations of the temperatures and heat fluxes of the internal and external vessels wallalong with the same longitudinal positions under the 2 mm gap thickness and both the internal and the external wallheating condition. Both the internal and external vessels wall were heated at 370 °C. The distilled water supply forcooling the internal vessel wall was completed at about 50 s, so there was no influence of the external effect after 50 s.When compared with the 1 mm gap case, both the internal and external vessels wall were quenched monotonically andstably. The walls were quenched from the bottom and towards the top regions in a sequence. The external wall wasquenched faster than the internal wall. This is why the coolant water penetrated through the external vessel wall. Thissituation is the same as the 1 mm gap and only an internal wall heating condition as shown in Fig. 3.

4. Conclusions

In this paper, to evaluate the gap cooling mechanism between the corium and the reactor pressure vessel, thequenching phenomena of hemispherical downward facing surfaces with narrow gaps with respect to the wall heatingconditions were investigated experimentally. The experiments were performed with a hemispherical gap thickness of 1and 2 mm, and the wall heating conditions, that is, with and without an external wall heating. The internalhemispherical heating block consisted of an inner brass mold and an outer copper shell. Electrical rod heaters wereplaced inside the hemispherical brass mold, and used to heat the outer copper shell. The thickness and outer diameter ofthe copper shell were 19 and 238 mm, respectively. The external vessel was fabricated to maintain a uniformhemispherical gap thickness, and it was heated by an electrical band heater. The internal heating block (or both internaland external vessels) was heated up to 350–450 °C, and distilled water was supplied to the gap region, and then thetemporal histories of the internal and the external vessel were measured by 68 K-type thermocouples embedded in thecopper shell and the external vessel.

From the interpretations of the wall temperature and the heat flux history, it was found that the quenching processwas changed by the wall heating conditions. If the external wall was not heated, then the internal vessel wall wasstably quenched from the bottom to the top region because the coolant water was supplied to the gap region throughthe cold external wall. However, if both the internal wall and the external walls were heated, then the quenchingphenomena were more complicated and slower as the gap thickness decreased, because the coolant supply to the gapwas restricted.

Acknowledgement

This study has been performed under the Long-and-Mid-Term Nuclear R&D Program supported by Ministry ofScience and Technology, Republic of Korea.


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