Experimental investigations of the quenching phenomena for hemispherical downward facing convex surfaces with narrow gaps

Transcription

1 International Communications in Heat and Mass Transfer 34 (2007) Experimental investigations of the quenching phenomena for hemispherical 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, , 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 of hemispherical downward facing surfaces with narrow gaps with respect to wall heating conditions have been investigated experimentally. The experiments were performed with a hemispherical gap thickness of 1 and 2 mm, and the wall heating conditions, that is, with and without an external wall heating. That is, the internal vessel (or both internal and external vessels) was heated up to 450 C, and distilled water was supplied to the hemispherical gap region, and then the temporal histories of the internal and the external vessels were measured by the thermocouples embedded in the internal and the external vessels. From the interpretations of the wall temperature and the heat flux history, it was found that the quenching process was changed by the wall heating conditions. If the external wall was not heated, then the internal vessel wall was stably quenched from the bottom to the top region because the coolant water was supplied to the gap region through the cold external wall. However, if both the internal wall and the external wall were heated, then the quenching phenomena were more complicated and slower as the gap thickness decreased, because the coolant supply to the gap was restricted 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 pressure vessel. All the well-known safety codes had predicted that the pressure vessel would fail and corium (a mixture of molten fuel and structure material) would be released if this kind of severe accident happened. However, the pressure vessel was not damaged and the molten corium was kept inside the pressure vessel and cooled down with a steep cooling rate, such as C/min [1]. In order to explain the safe cool-down of the relocated corium, three cooling mechanisms have been suggested and a gap cooling mechanism is considered to be the most plausible one which played a major role in the cooling of the corium [2]. Such a mechanism works like this: when molten corium relocates to 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. address: tomo@kaeri.re.kr (K.S. Ha) /$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi: /j.icheatmasstransfer

2 K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) As the corium is solidified, it shrinks and the pressure vessel experiences a creep, causing a gap to develop between the crust and the pressure vessel. The water penetrates the gap and the corium is cooled down. In order for a gap cooling to be effective, the contact area through a water penetration should be quenched and water should be continuously supplied through the gap so that a boiling heat transfer is maintained. As the gap cooling concept has only recently been considered to be of importance, there has not been a great deal of research related to this concept. Some of the research has 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] at KAERI (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 heat power 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 have been investigated experimentally. Experiments employed test sections with a 1 and 2 mm gap thickness and a 1 atm system pressure. Though many researches have been performed on the quenching phenomena [10 12], there have been very few quenching studies of a hemispherical downward facing surface with a narrow gap. Compared with the pool quenching condition, the quenching phenomena of a hemispherical downward facing surface with a narrow gap has 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 the temporal quenching data for hemispherical narrow gaps. It consists of a test vessel, a coolant injection system, a closed coolant circulation system, a heat exchanger, a secondary coolant system of the heat exchanger, and a data acquisition system. The test vessel consists of an internal heating vessel and an external hemispherical vessel. The internal heating vessel consists of an inner brass mold and an outer copper shell. Rod heaters were put inside a hemispherical brass mold, which provided a maximum average heat flux of 350 kw/m 2 at the outer surface. A copper shell was selected as an inner vessel to obtain a uniform heat flux distribution. The thickness and outer diameter of the copper shell are 19 and 238 mm, respectively. Two units of stainless steel external vessels were manufactured to provide gap sizes of 1.0 and 2.0 mm between the copper shell and the external vessel itself. A 5.1 kw band heater was installed on the outer surface of the external vessel to maintain the external vessel at a set temperature. Fig. 1. Schematic diagram of the quenching test facility (unit: mm).

3 30 K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) The closed coolant circulation system played the role of a system pressure regulator. That is, the steam generated from the heating vessel was condensed into water by passing it through the heat exchanger. If the secondary flow of the heat exchanger was turned on, the steam passing through the primary tube of the heat exchanger was condensed into water, 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 the experimental facility constituted a closed loop, the first step, necessary to carry out the experiments, was to vent the air accumulated 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).

4 K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) working fluid would not circulate. Initially the heater power was maintained at a low level and the pressure control system was set at a pre-determined value in the test matrix. The pressure was controlled to within ±5% of the set value during the entire test. The temporal variations of the internal heating and the external vessels were observed by monitoring the K-type thermocouple readings. The thermocouples were embedded in a copper shell, as shown in Fig. 2. Pairs of thermocouples were installed at different depth positions to calculate the surface temperature and heat flux at each location. The measured temperatures were processed by a Hewlett Packard data acquisition system and a HP-VEE program. 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 C. Then the heater power was turned off, and distilled water with a normal temperature was filled up to the 140 mm from the top of the heater vessel. The quenching process was monitored and recorded by 68 thermocouples embedded in the internal vessel and the external vessel. The pressure of the test vessel was kept at 1 bar during the entire quenching experiment. The experiments were performed with a hemispherical gap thickness of 1 and 2 mm, and the wall heating conditions, 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 same longitudinal positions under the 1 mm gap thickness and no external wall heating condition. Only the internal wall was heated at 355 C. The distilled water supply for cooling the internal vessel wall was completed at about 30 s, so there was no influence of the external effect after 30 s. As shown in Fig. 3, the internal vessel wall was slowly quenched at about a 20 C/min cooling rate till 510 s, after this time, it was abruptly quenched at about 220 C/min. This large cooling rate was sequentially propagated from the lower vessel wall to the upper wall. Though the external vessel wall was not heated directly, the external wall quenching process was started from C because of an internal wall heating. The temperature of the external vessel wall also decreased from the lower part to the upper part. From the initial cooling of the external vessel wall and the stable quenching phenomena of the internal vessel wall, it could be suggested that the coolant water should be supplied to the gap region through the cold external wall. Though the coolant was fully supplied to the gap region, the internal wall might maintain a film boiling condition, and then, after 510 s, it might start the transition to a nucleated boiling. The transition temperature from the film to the nucleate boiling was 155 C, which is similar to the Leidenfrost temperature of the upper plate under the pool quenching conditions. If the nucleate boiling started at the lower internal wall and the film was broken at that point, the heat transfer abruptly increased at the lower internal wall, and finally the temperature of the upper vessel wall also decreased because the heat in 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).

5 32 K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) a high conduction coefficient. That is, the local nucleate boiling phenomena had an effect on the overall temperature of the 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 the elapsed 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 the circular area refer to the top end of the gap and the lowest bottom part of the vessel wall, respectively. Small circles shown in the radial directions represent the thermocouple locations. The readings from these thermocouples are interpolated 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 found that the water had penetrated through the local region. However, this water penetration did not continue and it stopped after a few seconds. Therefore, the axi-symmetric temperature contours were restored after 900 s. The axi-symmetric behavior of the temperature profile was continued until 1300 s, and then broken due to the water penetration. The quenching 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 same longitudinal positions under the 1 mm gap thickness and both the internal and the external wall heating conditions. That is, Fig. 5 represents the same experimental results as Fig. 4. Both the internal and external vessels wall were heated at 350 C. The distilled water supply, for cooling the internal vessel wall, was completed at about 30 s, so there was no influence of the external effect after 30 s. As shown in Fig. 5, the cooling rate of the internal vessel bottom wall was about 11 C/min at the initial stage and 40 C/min at the latter stage. This cooling rate was smaller than that of Fig. 3 which 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).

6 K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) Fig. 5. Typical temperature history of the internal and external walls along with the same longitude (gap thickness: 1 mm, both internal and external walls heating). Fig. 6. Typical heat flux history along with the same longitude (gap thickness: 1 mm, both internal and external walls heating).

7 34 K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) with the internal wall, the temperature behavior of the external wall was very complicated. The external wall temperature at the 40 latitudinal position (lower middle region) decreased slowly with a 4.5 C/min cooling rate at the initial stage. After 500 s, the cooling rate was considerably largely increased at 70 C/min. And then, at about 670 s, an instantaneous temperature rise was observed. The temperature rises at the 40 latitudinal position were observed twice after 670 s, and the increasing temperature time at the 40 latitudinal position coincided with the decreasing temperature time at the 60 latitudinal position. It might be thought that these temperature increase and decrease phenomena are due to a variation of the coolant contact condition on the external wall. That is, though the cooling rate increased at 500 s, the temperature on the external wall was still as high as 270 C, so the external wall was under the film boiling region. However, the steam film was not stable, so the temperature of the external vessel wall was rapidly increasing or decreasing. The conduction coefficient of the external vessel wall made of stainless steel was smaller than that of the internal vessel wall made of copper. Therefore, in the case of the external vessel wall, the local phenomena have less of an influence on the other region. This is why an abrupt temperature variation at the 40 latitudinal position did not propagate toward the other latitudinal positions. On the basis of the instantaneous temperature increases at the 40 latitudinal position, it was found that the steam film of the 40 latitudinal position was unstable. The steam film was more stable at the top region (80 latitudinal position), so the cooling rate was almost the same as 5.9 C/min. The abrupt temperature decreases were observed at the 40 (lower middle region), 20 (bottom region), 60 (upper middle region), 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).

8 K.S. Ha et al. / International Communications in Heat and Mass Transfer 34 (2007) Fig. 6 shows the typical heat flux histories along with the same longitudinal positions on the internal and external vessels wall in the case of a 1 mm gap thickness and both an internal and external vessels wall heating. The heat flux values were calculated by solving a simple conduction equation through using the two temperature values which were measured at different radial positions with a 10 mm interval in the case of the internal vessel and with a 26 mm in the case of the external vessel as shown in Fig. 2. In these experiments, the heat flux on the surface in contact with the coolant is high and the heat flux on the surface in contact with the insulator is about zero. The calculated heat flux by using the two measured temperatures shows an almost average heat flux between the measured points, so the heat flux in this paper is not exactly the same as the heat flux on the surface in contact with the coolant. To obtain the exact heat flux 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 external vessel wall. The maximum heat flux values were temporally detected from the bottom to the top region of the internal vessel. 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 could be suggested that the boiling condition had changed from a film boiling into a nucleate boiling. Therefore, the broken sequence 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 at 40 (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 wall along with the same longitudinal positions under the 2 mm gap thickness and both the internal and the external wall heating condition. Both the internal and external vessels wall were heated at 370 C. The distilled water supply for cooling 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 and stably. The walls were quenched from the bottom and towards the top regions in a sequence. The external wall was quenched faster than the internal wall. This is why the coolant water penetrated through the external vessel wall. This situation is the same as the 1 mm gap and only an internal wall heating condition as shown in Fig Conclusions In this paper, to evaluate the gap cooling mechanism between the corium and the reactor pressure vessel, the quenching phenomena of hemispherical downward facing surfaces with narrow gaps with respect to the wall heating conditions were investigated experimentally. The experiments were performed with a hemispherical gap thickness of 1 and 2 mm, and the wall heating conditions, that is, with and without an external wall heating. The internal hemispherical heating block consisted of an inner brass mold and an outer copper shell. Electrical rod heaters were placed inside the hemispherical brass mold, and used to heat the outer copper shell. The thickness and outer diameter of the copper shell were 19 and 238 mm, respectively. The external vessel was fabricated to maintain a uniform hemispherical gap thickness, and it was heated by an electrical band heater. The internal heating block (or both internal and external vessels) was heated up to C, and distilled water was supplied to the gap region, and then the temporal histories of the internal and the external vessel were measured by 68 K-type thermocouples embedded in the copper shell and the external vessel. From the interpretations of the wall temperature and the heat flux history, it was found that the quenching process was changed by the wall heating conditions. If the external wall was not heated, then the internal vessel wall was stably quenched from the bottom to the top region because the coolant water was supplied to the gap region through the cold external wall. However, if both the internal wall and the external walls were heated, then the quenching phenomena were more complicated and slower as the gap thickness decreased, because the coolant supply to the gap was restricted. Acknowledgement This study has been performed under the Long-and-Mid-Term Nuclear R&D Program supported by Ministry of Science and Technology, Republic of Korea.

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