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Water boiling on the corium melt surface under VVER severe accident conditions
Nuclear Engineering and Design 195 (2000) 45 – 56 www.elsevier.com/locate/nucengdes
Water boiling on the corium melt surface under VVER severe accident conditions S.V. Bechta a,*, S.A. Vitol a, E.V. Krushinov a, V.S. Granovsky a, A.A. Sulatsky a, V.B. Khabensky a, D.B. Lopukh b, Yu.B. Petrov b, A.Yu. Pechenkov b a
Scientific Research Technological Institute (NITI), Sosno6y Bor, Leningrad region 188537, Russia St. Petersburg Electrotechnical Uni6ersity (SPbEU), Prof. Popo6 st 5 /3, St. Petersburg, Russia
b
Received 27 October 1998; received in revised form 23 June 1999; accepted 7 July 1999
Abstract Experimental results are presented on the interaction of corium melt with water supplied on its surface. The tests were conducted in the ‘Rasplav-2’ experimental facility. Corium melt was generated by induction melting in the cold crucible. The following data were obtained: heat transfer at boiling water-melt surface interaction, gas and aerosol release, post-interaction solidified corium structure. The corium melt charge had the following composition, mass%: 60% UO2 + x –16% ZrO2 –15% Fe2O3 –6% Cr2O3 –3% Ni2O3. The melt surface temperature ranged within 1920 –1970 K. © 2000 Elsevier Science S.A. All rights reserved.
1. Introduction At a beyond-design VVER accident with the core melting, one of the important accident management steps is the melt quenching after its localization in the core catcher. Water supply is necessary for the heat removal, preventing corium overheating and the catcher structures damage. A less obvious, but, under certain circumstances, possible severe accident management method is the water supply to the corium melt pool retained on the vessel bottom accompanied by the outside water cooling of the latter. The * Corresponding author. Tel.: +7-812-696-0675; fax: + 7812-696-1672. E-mail address: [email protected] (S.V. Bechta)
concept of a severe accident localization inside the reactor vessel is used for such reactors as VVER640 (Russia), AP-600 (USA), VVER-440 (Loviisa, Finland). Efficiency of a severe accident management by water supply on the corium melt surface depends very much on the water–corium interaction processes. They are: heat transfer during water boiling, interaction hydrodynamics and possible steam explosions, gas and aerosol releases. In a general case these processes can depend on the way of water supply and its temperature, as well as on the melt composition, properties and superheating, which, in their turn, are stipulated by the reactor and core catcher design parameters, melt formation region, accident stage and scenario.
0029-5493/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 9 9 ) 0 0 1 9 8 - 3
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Certain aspects of the considered problem were investigated in a number of experimental facilities. Unfortunately the systematized information about experimental results was not available for us. The limited information presented in (Green et al., 1986; Spencer et al., 1992; Yamano et al., 1995; Rohde, 1996; Alsmeyer et al., 1998) gives a certain idea about the current state of research in this field.
2. Current state of experimental research Of the carried out experimental studies on the melt–water interaction with the latter poured on the melt surface, the following deserve a special attention: Experimental program on MACE facility at the Argonne National Laboratory (ANL, USA), done in the framework of ACE-MACELACE international program. UO2 –ZrO2 corium was used as the melt material. The test facility has been in operation since 1989. Experimental studies in SWISS facility at the Sandia National Laboratory (SNL, USA). Steel was used as the melt material. The steel melt–concrete interaction accompanied by water supply onto the melt surface was investigated. This facility was operated until 1987. Tests at TEPCO facility, jointly developed by Hitachi and Toshiba companies (Japan). Steel was used as the melt material. Melt spread with its subsequent flooding was studied in one of the experimental series. The facility was in operation until November 1992. Tests at WETCOR facility in Sandia National Laboratory (SNL, USA). Oxide mixture was used as the melt material. Demonstration tests on the melt cooling by water supply on its surface were conducted. The facility was in operation until November 1993. Of all mentioned above, results closest to real conditions were obtained at MACE test facility. In 1989 MO tests were carried out in a box with 0.30×0.30 m2 interaction surface, and in 1991 M1 tests in a box with 0.50×0.50 m2 interaction area. In these experiments the melt-water heat flux immediately after the water supply was 3 MW
m − 2, but dropped dramatically (down to 150 kW m − 2). This effect was due to the melt surface crust formation, it blocked the interaction surface. To eliminate it M3 test was conducted in early 1997 in a 1.2× 1.2 m2 box. Just like in previous tests, the crust grew on the surface. Heat flux into water dropped within a short time interval. Gas gap formed between the crust and the melt and the subsequent cooling was accompanied by the crust breaking and spontaneous geyser gas and melt particles discharges into water. More detailed information about the tests is not available. The melt–water interaction processes has been also studied in the ALPHA program (Yamano et al., 1995). In this research, water was poured onto Fe+Al oxide melt of maximum 31.5 kg. Melt eruptions were observed when the subcooled water was poured through a pipe nozzle, though it was not the case when the water was near the saturation temperature or supplied through a spray nozzle. There is some limited information (Green et al., 1986) that boiling on the molten metal surface was not stable. After a certain time interval from the test start (the delay could be 15–40 s) steam exploded (one or several explosions). The scattering of pre-explosion heat transfer factors determined in separate tests was 9 60%, but their average value was close to that typical of film boiling on a solid surface. The incompleteness and controversy of data on the supplied water–melt surface interaction stimulated developing new research programs to study this problem. The following planned research programs should be mentioned here: Experimental studies on the VOLCANO facility in the Cadarache Laboratory (CEA Cadarache, France) aimed at tuning up the key melt cooling stages in the EPR reactor core catcher. UO2, ZrO2, Zr, Fe… corium is used. The melt is generated in the arc furnace and poured into a container for induction heating. Experimental program on the FARO facility in the Ispra International Center (JCR ISPRA/ CCR, Italy). The planned second stage of the program is to determine the melt thermal load on the reactor bottom and/or on instrumenta-
S.V. Bechta et al. / Nuclear Engineering and Design 195 (2000) 45–56
tion penetrations with a subsequent water supply on the melt surface. The 50 kg melt material comprises UO2, ZrO2, Zr. Tests at the RIT facility (Sweden) to study the melt pool cooling and spreading. The melt composition is PbO, FeO, SiO2 mixture. Experimental studies at COMET facility in the Federal Center in Karlsruhe (FZK, Germany). The subject of the first experimental stage started in 1993 was the melt cooling with water injected from the bottom. Currently the second stage is planned, which involves water supply on the melt surface. The melt material is Fe + Al2O3 mixture. Recently ANL has obtained the first data (Alsmeyer et al., 1998) from tests with a prototype melt (55% UO2 +17% ZrO2 and 22% concrete materials). It is planned to continue tests within MACE program.
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Taking into account incompleteness of experimental data on the melt–water interaction (corium melt in particular), NITI has conducted tests on the ‘Rasplav-2’ facility, some results of which are presented below. 3. Experimental facility Experimental studies were carried out on the ‘Rasplav-2’ facility, which has been in operation since 1988. The uranium-bearing corium melt was produced by induction melting in the cold crucible (IMCC). The solid phase (lining crust) formation between the high temperature melt and the cold crucible enables to superheat the melt, work in the inert and oxidizing atmosphere. Along with other advantages it neither contaminates the melt by crucible material, nor limits the test duration.
Fig. 1. ‘Rasplav-2’ facility arrangement. 1, Protective box; 2, metal bottom; 3, corium melt; 4, inductor; 5, cold crucible; 6, quartz tube; 7, cooling water in and out; 8, water cooled lid; 9, pyrometer access port; 10, gas in; 11, gas cylinder; 12, gas pressurizers; 13, 14, 15, 16, valves; 17, drive; 18, funnel; 19, spectral ratio pyrometer; 20, infrared pyrometer; 21, main aerosol line; 22, port lids; 23, main filter; 24, high frequency generator; 25, ventilator.
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Fig. 2. Experimental cell diagram. 1, Crucible sections; 2, bottom; 3, corium melt; 4, quartz tube; 5, weir; 6, tray with level sensor; 7, inductor.
Fig. 1 shows schematics of the facility. Induction furnace 4 is mounted in air-tight glove box 1 and equipped with crucible vertical shift drive 17 against the stationary inductor. The diagram of the experimental cell is shown in Fig. 2. The crucible is a 0.4 m high cylindrical vessel, having the inner diameter of 72× 10 − 3 m. The crucible side walls are composed of copper tubes, arranged around uncooled metal bottom 2 with gaps between them. Through them cooling water is pumped. To eliminate water leaks from the crucible cavity during its interaction with corium melt, the thermally treated (573 K, 1 h) crucible has the outer alumina coating. To control a possible crucible water leakage in the interaction zone it is mounted on metal tray 6 where water level is measured. To reduce the condensate formation on cold sections during water boiling, in the second series of experiments the height of the crucible top zone was reduced by its bottom elevation. With the same purpose, and so as to prevent water spillage over the crucible upper edge during its interaction with corium, 0.30-m quartz tube 4 was inserted into the crucible top zone. The distance between the quartz tube lower edge and the melt surface
was 5× 10 − 2 m. From a graduated vessel, placed on the protective box lid the distilled water flowed down through quartz weir 5, equipped with a control valve, onto the melt surface. Distance between the vessel and the melt surface was 1 m. The water temperature maintained in the vessel was 368– 373 K. The molten uranium dioxide mass was 1 kg, the corium charge having the following composition, mass%: 60% UO2 + x –16% ZrO2 –15% Fe2O3 –6% Cr2O3 –3% Ni2O3. Liquidus temperature corresponding to this corium composition was about 1820 K. The corium melt temperature was maintained at 1920–1970 K, that is, a 100–150 K melt superheating was provided. At the above-mentioned melt temperature calibration tests gave the following values: heat fluxes from the melt into the crucible side wall qside 1.2 MW m − 2 heat fluxes from the melt into the crucible bottom qbottom 0.4 MW m − 2 Measuring devices were prepared and calibrated to monitor the melting process and measure the following parameters: Tm, melt surface temperature; GCC, Gind, the cold crucible and inductor cooling water flow rate; T0, TCC, Tind, water temperature at the crucible and inductor inlet and outlet, electric parameters of the high frequency generator modes. Total evaporation time of water on the melt was measured. During the tests steam-gas mixture from the crucible top and above-melt water were periodically sampled. In the first experimental series a corium ingot was cooled under water. Glass burettes were prepared and filled with vacuum to take steam-gas mixture at the instant of their contact with the crucible top atmosphere. A quartz tube sampler was used to take the above melt water. The 84-channel data acquisition system (DAS) was used for continuous data logging and processing. The corium, water and steam–gas mixture samples were subjected to the post-test analysis.
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4. Test procedure and results After the 1970 K corium melt pool formation and homogenization the tests were started with a staged increase of water supply to the melt surface. Initially hot water was added dropwise. Water drops moved chaotically in the steam layer above the melt surface. At the moment of evaporation they were replaced by corium crusts, which instantly melted or solved. The crusts were probably formed by aerosol and other minute melt particles
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captured and cooled by water drops. As volume and flow rate of the added water increased, the interaction pattern did not change basically until the time when water started to cover the melt surface (Fig. 3). It was accompanied by a turbulent boiling with heated corium particles ejection. When the volume of supplied water was increased to maintain its layer at 30 mm, the corium crust was observed to grow from the crucible walls to the center and form a solid corium layer in water above the melt level. As the layer was forming, water, steam and corium particle dis-
Fig. 3. Melt surface overview. a, Water supply start; b, water layer on the melt; c, water boiling; d, water boiling out; e, after boiling out.
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Fig. 4. Melt surface with crust layer.
charges from the crucible became more intensive. After water supply termination and its complete evaporation from the melt surface, as a result of radiation the layer melted and streamed down into the molten pool. The layer (Fig. 4) was likely to be formed by melt droplets, discharged from the corium pool surface at its interaction with water. They were brought by the steam flow up to the cold crucible walls, where they cooled down and crystallized. The water temperature reduction did not result in any changes in the processes observed. In all tests when water covered the melt surface, the melt temperature measuring by the spectral ratio pyrometer was not feasible due to the partial spectrum absorption by water. Attempts to measure the melt temperature through water by W/Re thermocouples immersion were not successful due
to their prompt failure. The second experimental series results are presented in Table 1. The measured heat flux from the melt surface into water is the average value, derived from the water mass and boiling time. Here some experimental results can be listed: steam explosions were not observed during the water supply to the uranium-bearing oxide melt. water boiling had the film type, i.e. a steam layer was formed between water and corium melt, no crust on the corium melt surface was formed during the tests; melt surface was extensively disturbed during its interaction with water; steam and boiling water captured corium melt particles; while maintaining constant water level above the melt a corium crust layer was formed in it, growing from the crucible walls to the center; there was no distinct difference in evaporation period between hot and cold water. After experiments corium ingots were taken out of the crucible for post-test studies. Physico chemical examination of corium, as well as of water and steam-gas mixture taken during the test contributed into a more comprehensive understanding of the corium melt–water interaction. Some results of the studies, as well as the analysis of heat transfer at water boiling on the melt surface are presented below.
Table 1 Experimental results Test no.
Water supply rate, Vsup, 10−3 kg s−1
Evaporation rate, Vev, 10−3 kg s−1
Specific evaporation rate, V%ev, kg (s m2)−1
Specific heat flux, q, MW m−2
2/1 2/2 2/3 2/4 2/5 2/6 2/7 2/8 2/9
13.39 – 15.00 25.00 25.00 25.00 – – 16.70
1.49 1.50 1.32 1.97 1.68 1.75 1.39 1.07 1.95
0.35 0.31 0.31 0.46 0.39 0.41 0.32 0.25 0.45
0.791 0.701 0.701 1.040 0.881 0.927 0.723 0.565 1.017
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Fig. 5. Top view of corium ingots. a, 1st experimental series; b, 2nd experimental series. Table 2 The elementary analysis results No.
1 2 3
Sampled part
Ingot surface crust (1st series tests) Corium under the crust (1st series tests) Ingot center (Test 2/9)
5. Corium, water and steam – gas samples post-test examination
5.1. Corium samples examination During the post-test observation of the first series corium ingot (Fig. 5) a readily separated crust was found on top. It had a convex surface with rounded corium particles stuck to it. After the 2×10 − 3 – 4 × 10 − 3 m thick crust removal a fragmented 5 ×10 − 3 m thick corium layer was uncovered. On removing this layer a wavy ingot surface was observed. There were many pores of various sizes in the remaining ingot part, which makes it different from dense ingots of the same composition molten without interaction with water. To estimate corium porosity, which characterizes crystallization and gas discharge during ingot cooling, its apparent density was determined by the immersion methodology
Element content, mass% U
Zr
Ni
Fe
Cr
54.0 54.3 60.3
9.5 9.6 7.5
1.2 1.1 1.1
6.4 6.1 6.4
2.4 2.2 2.4
with the sample submersion into a saturating liquid (GOST 2409-80). The average apparent density of the ingot was (7.09 0.2) 103 kg m − 3. For the sake of comparison it can be noted that apparent density of molten corium of the same composition, but resulting from tests without water was (7.490.2) 103 kg m − 3. The apparent density reduction testifies to porosity. The elementary analysis of corium samples was made by X-ray fluorescence on the short wave X-ray spectrometer. Table 2 presents the analysis results of samples taken from different ingot sections. Data listed in Table 2 prove the uranium content reduction in the top part of the ingot. The phase composition of corium samples was measured by an X-ray diffractometer. Resulting diffractograms were processed by the PDOS software package provided with the reference base of powder diffractograms JCPDS 1980.
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Comparison of the first series diffractograms of the sampled crust, under crust region and ingot bulk (Fig. 6a) did not reveal differences in the phase structure across the ingot (solid solution (U, Zr)O2 + X, FeCr2O4). Comparison of the second series diffractograms of the sampled upper, middle and bottom parts (Fig. 6b–d) revealed the following: U3O8 phase presence was detected. It crystallized in the middle and upper parts of the ingot due to a more active interaction of the top corium layers with oxidizing medium, i.e. steam film, and probably because of the free oxygen displacement from the lower layers during crystallization.
5.2. Water, gas and aerosol sample examination Melt–water interaction products were gathered and filtered. The filter cake underwent X-ray fluorescence analysis; the solution was chemically analyzed. The elementary composition of aerosol products was determined directly from the analytical filter and precipitates of water sampled above the melt. The elementary analysis results are shown in Table 3. The data of Table 3 show that aerosols are chiefly composed of uranium oxides and practically free from zirconium oxides. Aerosol concentration in steam–gas–aerosol
Fig. 6. Corium sample diffractograms. (a) 1st experimental series phase-(UZr)O2 + x phase-FeCr2O4; (b) 2nd experimental series phase FeCr2O4; (c) 2nd experimental series phase U3O8; (d) 2nd experimental series phase (UZr)O2 + x with different lattice parameters. Table 3 Results of elementary composition analysis Sampled part
Solution precipitate (1st series) Aerosol filter (test 2/9)
Content of elements, mass% U
Zr
Ni
Fe
Cr
66.3 76.4
– –
0.4 0.7
2.0 2.3
1.4 3.3
S.V. Bechta et al. / Nuclear Engineering and Design 195 (2000) 45–56 Table 4 Above melt atmosphere composition
0.2% vol.; nitrogen, 0.4% vol. Analyses results are presented in Table 4. It follows from the water thermal dissociation equation that at thermolysis the hydrogen volumetric concentration should exceed that of oxygen by a factor of two. But, as it is evident from Table 4, experimentally obtained hydrogen/oxygen ratio is 0.032/1.475, or the concentration of oxygen is 46 times higher. One of the possible reasons for this can be redox reactions taking place at corium melt–water interaction.
Sample no. Analyzed components concentration, % vol.
2/1 2/2 2/3 2/9 Average
Nitrogen
Hydrogen
Oxygen
D of oxygen*
77.6 74.2 76.0 75.0 75.7
0.018 0.050 0.030 0.029 0.032
21.5 22.2 22.3 23.7 22.425
0.55 1.25 1.35 2.75 1.475
* D of oxygen, the oxygen concentration increase in the sample as compared to its concentration in air under normal conditions, calculated against nitrogen concentration in the mixture.
interaction products was evaluated. For that, a specified volume of gas – aerosol mixture was pumped through the preliminary weighed analytical filter. The following equation was used to calculate aerosol concentration: Ca =
G2 − G1 , Wt
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(1)
Experimentally determined aerosol concentration was 1× 10 − 3 kg m − 3, which corresponds to the aerosol generation rate per the melt surface area unit 44 ×10 − 6 kg (m2 s) − 1. This value is considerably lower than in tests without water atop the melt, which, at all other conditions being similar, amounted to 550 × 10 − 6 kg (m2 s) − 1. The results show that the content of elements in the sampled solution precipitates correlates to that of aerosol samples. From here it can be inferred that aerosols from the melt surface enter the steam film and bubble through the boiling water layer in the steam. At that, most of aerosols are brought into water and their concentration in the off-steam decreases. Four gas samples were taken during tests 2/1–2/ 9 in order to determine the atmosphere composition. They were gathered by a sampler into vacuum burettes above the flooded melt. Hydrogen, oxygen and nitrogen content analyses were performed by chromatography. Absolute concentration measurement error for hydrogen, 0.002% vol.; oxygen,
6. Thermohydrodynamic processes at the above-melt water boiling A high corium melt temperature resulted in the film type boiling on its surface. There is a considerable amount of experimental data concerning film boiling on a solid surface and verified calculated correlation of the heat transfer factor estimation under these conditions. But the same cannot be said about the heat transfer during boiling on the melt surface. The only available data on heat transfer in water boiling on the molten metal can hardly be considered as adequate for the corium melt, as: (a) the data obtained (Green et al., 1986) had a considerable scattering (9 60%); (b) surface temperature did not exceed 873 K; (c) thermophysical characteristics of liquid metal do not correspond to those of the corium melt. That is why the unscattered heat transfer data of water boiling on the uranium-bearing corium melt, having surface temperature of 1970 K, seem to be very reliable. Their analysis is given in (Bechta et al., 1997). Here some of the results will be summarized. In accordance with Table 1 the average heat flux density from the melt into the boiling water (qexp) was 0.82 90.12 MW m − 2, with 0.95 probability. To compare the experimentally obtained heat flux value with heat transfer at boiling on a solid surface, a correlation for film boiling on a horizontal plane, put forward in (Granovsky et al., 1995) was used. It takes into consideration heat transfer by radiation and generalizes over 350 available experimental data, with a 9% average error, including tests with water boiling on a 1270 K surface.
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Taking into account the radiant heat transfer, heat flux in (Granovsky et al., 1995) is determined on the basis of the following equations: qS =qfb +qr
(2)
qfb =afbU
(3)
afb =0.031llg3,5(A)8 2/3/L
(4)
A= Ar*/[(K%/Pr)2 +(K%/Pr)]
(5)
8 =qfb/qS
(6)
qr = 5.67× 10 − 8oef[T 4c −T 4o]
(7)
Experimental data on the emissivity of oxidic corium melt, with composition given in Section 3 are not known. Available though are those for liquid steel (Kymalainen et al., 1997), its emissivity is within oc =0.40 – 0.45 and it can increase if impurities and slugs are present. In some calculations oxidic corium emissivity is assumed as rather high, e.g. in study (Azarian et al., 1998) for UO2 +ZrO2 melt oc =0.8. Therefore oxidic corium emissivity was experimentally evaluated in separate tests. The measurements were made by calorimetry. The radiant heat flux from the melt surface was measured independently by: heat fluxes into the water-cooled calorimeter, set close to the melt surface; changes in the integral heat flux into the cold crucible after the melt surface was covered by the corium powder.
Fig. 7. Comparison of calculated and experimental data on heat transfer. , experiment: —, calculation from Eqs. (2) – (7) at o = 0.475.
The results coincided within measurement errors. The measured integral heat flux gave the integral emissivity range of 0.4–0.55. Besides, the monochromatic melt emissivity was evaluated at 0.659 0.01 mm. Color and monochromatic pyrometer working at the above mentioned wave length were used for the purpose. If it is assumed that the corium melt radiation spectrum corresponds to ‘the grey body’ (which is true for many multi-component melts studied) and the color temperature is close to thermodynamical, then o0.65 = 0.599 0.13. In this way, the experimentally determined integral emissivity of the melt surface at Tsurf = 1970– 2120 K was oc = 0.47590.075. It changed insignificantly within the temperature range specified. The methodologies and results of corium melt surface emissivity estimation will be presented in more detail in a separate paper. Comparison of Eqs. (2)–(7)) calculations at oc = 0.475 with experimental results listed in Table 1 is given in Fig. 7. It is evident from Fig. 7, that calculations of heat transfer at film boiling on a solid surface give a smaller value of heat flux as compared to the experimental data with the corresponding emissivity. In order to correlate calculations and experimental data the heat transfer intensity should be increased approximately by 35%. Such an intensified heat transfer can be regarded as a product of instability at the melt–steam interface, which results both in the melt drops ejection and in their penetration under the melt surface resulting in a larger heat removal area and increased heat transfer. The Kelvin–Helmholtz instability analysis (Bechta et al., 1997) enabled the steam critical velocity estimation (along the melt surface), stipulating the generation of instability. Calculations made in (Bechta et al., 1997) demonstrated that values of both critical velocity and steam velocity calculated for experimental conditions, are close and amount to several dozen meters per second. It testifies to a high probability of instability generated on the melt surface. It is indirectly confirmed by the melt particles observed in the boiling water and voids in corium ingots.
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In conclusion it is also reasonable to point to the difference in the corium crust formation mechanism in MACE tests and in the ones described here. Evidently, in MACE experiments the crust formed on the melt surface due to a less considerable start-up melt superheating and was likely to be influenced by the melt – concrete interaction processes. In the tests presented here the equilibrium of input and output power was reached when the melt temperature exceeded the liquidus temperature. Therefore, surface crust did not form. It did form in water at a visible distance from the melt surface. It could be distinctly observed after water evaporation and the crust layer melting caused by the radiation from the melt surface. Similarly to MACE experiments when the opening in the crust was growing in, a geyser gas, steam–water mixture and corium particles were observed to discharge from the crucible. A further experimental work with a reduced melt superheating temperature, at which the crust will be formed on the melt surface, as well as with modifications in corium composition and simultaneous corium interaction with concrete or refractory material is planned for a more detailed water–corium melt interaction study. 7. Conclusions Experimental studies of water boiling on the oxide corium surface enabled to determine the following: In the test conditions provided water boiling was not accompanied by steam explosions. Corium crust formation at 1 ×10 − 2 m distance from the melt surface was observed, it started on the crucible side walls and with time propagated to the center. The increased ingot porosity as compared to the porosity of the melt crystallized without water on top testifies to the steam (water) penetration under the melt surface and the melt–steam interface instability. The integral emissivity value of corium melt (60% UO2 9 x – 16% ZrO2 – 15% Fe2O3 –6% Cr2O3 –3% Ni2O3.) within 1970 – 2120 K, was colorimetrically evaluated as being within 0.40–0.55 range.
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Heat flux densities at film boiling on the melt surface were approximately 35% higher than the same for a solid surface. The hypothesis that the melt–steam interface instability promotes heat transfer was advanced. Presence of water on the melt considerably reduced aerosol release. Chemical interaction between water and melt components is confirmed by changes in the ingot phase composition as compared to the same typical of melting in the absence of water and by the difference in relative O2 and H2 content in the gas samples from their ratio stipulated by the H2O thermolysis. To specify the quantitative characteristics obtained, it is necessary to continue investigations of melt–water interaction processes under similar conditions, but with modifications in corium composition, degree of its superheating and at increased dimensions of the test facility.
Acknowledgements The contributions of Mrs Tatiana Pautova, Mr V. Bulygin, Mrs. Helena Kaljago, Mr A. Lisenko, Dr A. Lubomirov, Mr A Martinov, Dr A Gorsckov to the experimental part of the program are gratefully acknowledged. The authors are also grateful to Ms Tatiana Talalaeva for the paper translation into English. Appendix A. Nomenclature Vsup Vev V%ev qexp Ca G2 G1 W
is water supply rate on the melt, kg s−1 water evaporation rate, kg s−1 specific evaporation rate, kg (m2 s)−1 heat flux from the corium melt into water (= rV%ev), MW m−2 aerosol concentration in the abovemelt atmosphere, kg m−3 aerosol filter mass after the pumping, kg aerosol filter mass before the pumping, kg volumetric flow rate of air pumped through the filters, m3 s−1
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t qS qfb qr afb u l Ar* r%, r¦ Dr g m K% r* Pr Tc To oef oc ow L
pumping time, s total heat flux, W m−2 film boiling heat flux, W m−2 radiation heat flux, W m−2 heat transfer ratio at film boiling, W (m2 K)−1 temperature difference (=Tc−To), K steam thermal conductivity, W (m K)−1 Archimedes modified number (= r¦ DrgL3/m 2) saturated water and vapour density, kg m−3 density difference, (=r% =r¦), kg m−3 gravity acceleration, m s−2 vapour dynamic viscosity, Pa s undimentional steam superheating (= Cp u/r*) modified vaporisation heat, J kg−1 Prandtl number corium temperature, K water temperature, K effective emissivity (=1/[(1/oc)+(1/ ow)−1]) melt surface emissivity steam–water interface emissivity Laplas constant, m
References Alsmeyer, H., Farmer, M., Felderer, F., et. al. The
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COMET-Concept for Cooling of Ex-Vessel Corium Melts. 6th International Conference on Nuclear Engineering ICONE-6086, May 10 – 14, 1998. Azarian, G., Gandrille, P., Dumontet, A., et. al. GAREC analysis in Support of In-Vessel Retention Concept, OECD/CSNI Workshop on in-vessel core debris retention and coolability, Garshing, Germany, 3 – 6 March, 1998. Bechta, S.V., Granovsky, V.S., Sulatsky, A.A. Water boiling o corium melt surface. Proceedings of the International Symposium on the Physics of Heat Transfer in Boiling and Condensation May 21 – 24, 1997, Moscow, Russia, pp. 235 – 237. GOST 2409-80 (standards SEV 980-78). Refractory materials and items. Methods for identifying water saturation, apparent density, open and general porosity (In Russian). Granovsky, V.S., Sulatsky, A.A., Khabensky, V.B., et al., 1995. Theoretical and experimental studies of film boiling on a horizontal surface. TBT 33 (5), 765 – 772 In Russian. Green, G.A., Finfrock, C., Burson, S.B., 1986. The effect of water in film boiling over liquid metal melts, Trans. Am. Nuclear Soc. 53, 360 – 362. Kymalainen, O., Tuomisto, H., Theofanos, T.G., 1997. Invessel retention of corium at the Loviisa plant. Nucl. Eng. Des. 169, 109 – 130. Rohde, J. Survey on R&D-Programs to support the Design and Validation of New Reactor Concepts. -Mitigation Strategies and Concepts. Russian-German Seminar on: R&D Programs on Severe Accidents to support the Design and Safety Assessment of the Reactor Concept WWER-640/407, Moscow, 16 Sept., 1996. Spencer, B., et al. Results of MACE MO and MI. Second CSNI Specialist Meeting on Core Debris-Concrete Interactions, Karlsruhe, Germany, 1 – 3 April, 1992. Yamano, N., Maruyama, Y., Kudo, T., et al., 1995. Phemenological Studies on Melt-Coolant interactions in the ALPHA program. Nucl. Eng. Des. 155, 369 – 389.
Water boiling on the corium melt surface under VVER severe accident conditions S.V. Bechta a,*, S.A. Vitol a, E.V. Krushinov a, V.S. Granovsky a, A.A. Sulatsky a, V.B. Khabensky a, D.B. Lopukh b, Yu.B. Petrov b, A.Yu. Pechenkov b a
Scientific Research Technological Institute (NITI), Sosno6y Bor, Leningrad region 188537, Russia St. Petersburg Electrotechnical Uni6ersity (SPbEU), Prof. Popo6 st 5 /3, St. Petersburg, Russia
b
Received 27 October 1998; received in revised form 23 June 1999; accepted 7 July 1999
Abstract Experimental results are presented on the interaction of corium melt with water supplied on its surface. The tests were conducted in the ‘Rasplav-2’ experimental facility. Corium melt was generated by induction melting in the cold crucible. The following data were obtained: heat transfer at boiling water-melt surface interaction, gas and aerosol release, post-interaction solidified corium structure. The corium melt charge had the following composition, mass%: 60% UO2 + x –16% ZrO2 –15% Fe2O3 –6% Cr2O3 –3% Ni2O3. The melt surface temperature ranged within 1920 –1970 K. © 2000 Elsevier Science S.A. All rights reserved.
1. Introduction At a beyond-design VVER accident with the core melting, one of the important accident management steps is the melt quenching after its localization in the core catcher. Water supply is necessary for the heat removal, preventing corium overheating and the catcher structures damage. A less obvious, but, under certain circumstances, possible severe accident management method is the water supply to the corium melt pool retained on the vessel bottom accompanied by the outside water cooling of the latter. The * Corresponding author. Tel.: +7-812-696-0675; fax: + 7812-696-1672. E-mail address: [email protected] (S.V. Bechta)
concept of a severe accident localization inside the reactor vessel is used for such reactors as VVER640 (Russia), AP-600 (USA), VVER-440 (Loviisa, Finland). Efficiency of a severe accident management by water supply on the corium melt surface depends very much on the water–corium interaction processes. They are: heat transfer during water boiling, interaction hydrodynamics and possible steam explosions, gas and aerosol releases. In a general case these processes can depend on the way of water supply and its temperature, as well as on the melt composition, properties and superheating, which, in their turn, are stipulated by the reactor and core catcher design parameters, melt formation region, accident stage and scenario.
0029-5493/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 9 9 ) 0 0 1 9 8 - 3
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Certain aspects of the considered problem were investigated in a number of experimental facilities. Unfortunately the systematized information about experimental results was not available for us. The limited information presented in (Green et al., 1986; Spencer et al., 1992; Yamano et al., 1995; Rohde, 1996; Alsmeyer et al., 1998) gives a certain idea about the current state of research in this field.
2. Current state of experimental research Of the carried out experimental studies on the melt–water interaction with the latter poured on the melt surface, the following deserve a special attention: Experimental program on MACE facility at the Argonne National Laboratory (ANL, USA), done in the framework of ACE-MACELACE international program. UO2 –ZrO2 corium was used as the melt material. The test facility has been in operation since 1989. Experimental studies in SWISS facility at the Sandia National Laboratory (SNL, USA). Steel was used as the melt material. The steel melt–concrete interaction accompanied by water supply onto the melt surface was investigated. This facility was operated until 1987. Tests at TEPCO facility, jointly developed by Hitachi and Toshiba companies (Japan). Steel was used as the melt material. Melt spread with its subsequent flooding was studied in one of the experimental series. The facility was in operation until November 1992. Tests at WETCOR facility in Sandia National Laboratory (SNL, USA). Oxide mixture was used as the melt material. Demonstration tests on the melt cooling by water supply on its surface were conducted. The facility was in operation until November 1993. Of all mentioned above, results closest to real conditions were obtained at MACE test facility. In 1989 MO tests were carried out in a box with 0.30×0.30 m2 interaction surface, and in 1991 M1 tests in a box with 0.50×0.50 m2 interaction area. In these experiments the melt-water heat flux immediately after the water supply was 3 MW
m − 2, but dropped dramatically (down to 150 kW m − 2). This effect was due to the melt surface crust formation, it blocked the interaction surface. To eliminate it M3 test was conducted in early 1997 in a 1.2× 1.2 m2 box. Just like in previous tests, the crust grew on the surface. Heat flux into water dropped within a short time interval. Gas gap formed between the crust and the melt and the subsequent cooling was accompanied by the crust breaking and spontaneous geyser gas and melt particles discharges into water. More detailed information about the tests is not available. The melt–water interaction processes has been also studied in the ALPHA program (Yamano et al., 1995). In this research, water was poured onto Fe+Al oxide melt of maximum 31.5 kg. Melt eruptions were observed when the subcooled water was poured through a pipe nozzle, though it was not the case when the water was near the saturation temperature or supplied through a spray nozzle. There is some limited information (Green et al., 1986) that boiling on the molten metal surface was not stable. After a certain time interval from the test start (the delay could be 15–40 s) steam exploded (one or several explosions). The scattering of pre-explosion heat transfer factors determined in separate tests was 9 60%, but their average value was close to that typical of film boiling on a solid surface. The incompleteness and controversy of data on the supplied water–melt surface interaction stimulated developing new research programs to study this problem. The following planned research programs should be mentioned here: Experimental studies on the VOLCANO facility in the Cadarache Laboratory (CEA Cadarache, France) aimed at tuning up the key melt cooling stages in the EPR reactor core catcher. UO2, ZrO2, Zr, Fe… corium is used. The melt is generated in the arc furnace and poured into a container for induction heating. Experimental program on the FARO facility in the Ispra International Center (JCR ISPRA/ CCR, Italy). The planned second stage of the program is to determine the melt thermal load on the reactor bottom and/or on instrumenta-
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tion penetrations with a subsequent water supply on the melt surface. The 50 kg melt material comprises UO2, ZrO2, Zr. Tests at the RIT facility (Sweden) to study the melt pool cooling and spreading. The melt composition is PbO, FeO, SiO2 mixture. Experimental studies at COMET facility in the Federal Center in Karlsruhe (FZK, Germany). The subject of the first experimental stage started in 1993 was the melt cooling with water injected from the bottom. Currently the second stage is planned, which involves water supply on the melt surface. The melt material is Fe + Al2O3 mixture. Recently ANL has obtained the first data (Alsmeyer et al., 1998) from tests with a prototype melt (55% UO2 +17% ZrO2 and 22% concrete materials). It is planned to continue tests within MACE program.
47
Taking into account incompleteness of experimental data on the melt–water interaction (corium melt in particular), NITI has conducted tests on the ‘Rasplav-2’ facility, some results of which are presented below. 3. Experimental facility Experimental studies were carried out on the ‘Rasplav-2’ facility, which has been in operation since 1988. The uranium-bearing corium melt was produced by induction melting in the cold crucible (IMCC). The solid phase (lining crust) formation between the high temperature melt and the cold crucible enables to superheat the melt, work in the inert and oxidizing atmosphere. Along with other advantages it neither contaminates the melt by crucible material, nor limits the test duration.
Fig. 1. ‘Rasplav-2’ facility arrangement. 1, Protective box; 2, metal bottom; 3, corium melt; 4, inductor; 5, cold crucible; 6, quartz tube; 7, cooling water in and out; 8, water cooled lid; 9, pyrometer access port; 10, gas in; 11, gas cylinder; 12, gas pressurizers; 13, 14, 15, 16, valves; 17, drive; 18, funnel; 19, spectral ratio pyrometer; 20, infrared pyrometer; 21, main aerosol line; 22, port lids; 23, main filter; 24, high frequency generator; 25, ventilator.
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Fig. 2. Experimental cell diagram. 1, Crucible sections; 2, bottom; 3, corium melt; 4, quartz tube; 5, weir; 6, tray with level sensor; 7, inductor.
Fig. 1 shows schematics of the facility. Induction furnace 4 is mounted in air-tight glove box 1 and equipped with crucible vertical shift drive 17 against the stationary inductor. The diagram of the experimental cell is shown in Fig. 2. The crucible is a 0.4 m high cylindrical vessel, having the inner diameter of 72× 10 − 3 m. The crucible side walls are composed of copper tubes, arranged around uncooled metal bottom 2 with gaps between them. Through them cooling water is pumped. To eliminate water leaks from the crucible cavity during its interaction with corium melt, the thermally treated (573 K, 1 h) crucible has the outer alumina coating. To control a possible crucible water leakage in the interaction zone it is mounted on metal tray 6 where water level is measured. To reduce the condensate formation on cold sections during water boiling, in the second series of experiments the height of the crucible top zone was reduced by its bottom elevation. With the same purpose, and so as to prevent water spillage over the crucible upper edge during its interaction with corium, 0.30-m quartz tube 4 was inserted into the crucible top zone. The distance between the quartz tube lower edge and the melt surface
was 5× 10 − 2 m. From a graduated vessel, placed on the protective box lid the distilled water flowed down through quartz weir 5, equipped with a control valve, onto the melt surface. Distance between the vessel and the melt surface was 1 m. The water temperature maintained in the vessel was 368– 373 K. The molten uranium dioxide mass was 1 kg, the corium charge having the following composition, mass%: 60% UO2 + x –16% ZrO2 –15% Fe2O3 –6% Cr2O3 –3% Ni2O3. Liquidus temperature corresponding to this corium composition was about 1820 K. The corium melt temperature was maintained at 1920–1970 K, that is, a 100–150 K melt superheating was provided. At the above-mentioned melt temperature calibration tests gave the following values: heat fluxes from the melt into the crucible side wall qside 1.2 MW m − 2 heat fluxes from the melt into the crucible bottom qbottom 0.4 MW m − 2 Measuring devices were prepared and calibrated to monitor the melting process and measure the following parameters: Tm, melt surface temperature; GCC, Gind, the cold crucible and inductor cooling water flow rate; T0, TCC, Tind, water temperature at the crucible and inductor inlet and outlet, electric parameters of the high frequency generator modes. Total evaporation time of water on the melt was measured. During the tests steam-gas mixture from the crucible top and above-melt water were periodically sampled. In the first experimental series a corium ingot was cooled under water. Glass burettes were prepared and filled with vacuum to take steam-gas mixture at the instant of their contact with the crucible top atmosphere. A quartz tube sampler was used to take the above melt water. The 84-channel data acquisition system (DAS) was used for continuous data logging and processing. The corium, water and steam–gas mixture samples were subjected to the post-test analysis.
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4. Test procedure and results After the 1970 K corium melt pool formation and homogenization the tests were started with a staged increase of water supply to the melt surface. Initially hot water was added dropwise. Water drops moved chaotically in the steam layer above the melt surface. At the moment of evaporation they were replaced by corium crusts, which instantly melted or solved. The crusts were probably formed by aerosol and other minute melt particles
49
captured and cooled by water drops. As volume and flow rate of the added water increased, the interaction pattern did not change basically until the time when water started to cover the melt surface (Fig. 3). It was accompanied by a turbulent boiling with heated corium particles ejection. When the volume of supplied water was increased to maintain its layer at 30 mm, the corium crust was observed to grow from the crucible walls to the center and form a solid corium layer in water above the melt level. As the layer was forming, water, steam and corium particle dis-
Fig. 3. Melt surface overview. a, Water supply start; b, water layer on the melt; c, water boiling; d, water boiling out; e, after boiling out.
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Fig. 4. Melt surface with crust layer.
charges from the crucible became more intensive. After water supply termination and its complete evaporation from the melt surface, as a result of radiation the layer melted and streamed down into the molten pool. The layer (Fig. 4) was likely to be formed by melt droplets, discharged from the corium pool surface at its interaction with water. They were brought by the steam flow up to the cold crucible walls, where they cooled down and crystallized. The water temperature reduction did not result in any changes in the processes observed. In all tests when water covered the melt surface, the melt temperature measuring by the spectral ratio pyrometer was not feasible due to the partial spectrum absorption by water. Attempts to measure the melt temperature through water by W/Re thermocouples immersion were not successful due
to their prompt failure. The second experimental series results are presented in Table 1. The measured heat flux from the melt surface into water is the average value, derived from the water mass and boiling time. Here some experimental results can be listed: steam explosions were not observed during the water supply to the uranium-bearing oxide melt. water boiling had the film type, i.e. a steam layer was formed between water and corium melt, no crust on the corium melt surface was formed during the tests; melt surface was extensively disturbed during its interaction with water; steam and boiling water captured corium melt particles; while maintaining constant water level above the melt a corium crust layer was formed in it, growing from the crucible walls to the center; there was no distinct difference in evaporation period between hot and cold water. After experiments corium ingots were taken out of the crucible for post-test studies. Physico chemical examination of corium, as well as of water and steam-gas mixture taken during the test contributed into a more comprehensive understanding of the corium melt–water interaction. Some results of the studies, as well as the analysis of heat transfer at water boiling on the melt surface are presented below.
Table 1 Experimental results Test no.
Water supply rate, Vsup, 10−3 kg s−1
Evaporation rate, Vev, 10−3 kg s−1
Specific evaporation rate, V%ev, kg (s m2)−1
Specific heat flux, q, MW m−2
2/1 2/2 2/3 2/4 2/5 2/6 2/7 2/8 2/9
13.39 – 15.00 25.00 25.00 25.00 – – 16.70
1.49 1.50 1.32 1.97 1.68 1.75 1.39 1.07 1.95
0.35 0.31 0.31 0.46 0.39 0.41 0.32 0.25 0.45
0.791 0.701 0.701 1.040 0.881 0.927 0.723 0.565 1.017
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Fig. 5. Top view of corium ingots. a, 1st experimental series; b, 2nd experimental series. Table 2 The elementary analysis results No.
1 2 3
Sampled part
Ingot surface crust (1st series tests) Corium under the crust (1st series tests) Ingot center (Test 2/9)
5. Corium, water and steam – gas samples post-test examination
5.1. Corium samples examination During the post-test observation of the first series corium ingot (Fig. 5) a readily separated crust was found on top. It had a convex surface with rounded corium particles stuck to it. After the 2×10 − 3 – 4 × 10 − 3 m thick crust removal a fragmented 5 ×10 − 3 m thick corium layer was uncovered. On removing this layer a wavy ingot surface was observed. There were many pores of various sizes in the remaining ingot part, which makes it different from dense ingots of the same composition molten without interaction with water. To estimate corium porosity, which characterizes crystallization and gas discharge during ingot cooling, its apparent density was determined by the immersion methodology
Element content, mass% U
Zr
Ni
Fe
Cr
54.0 54.3 60.3
9.5 9.6 7.5
1.2 1.1 1.1
6.4 6.1 6.4
2.4 2.2 2.4
with the sample submersion into a saturating liquid (GOST 2409-80). The average apparent density of the ingot was (7.09 0.2) 103 kg m − 3. For the sake of comparison it can be noted that apparent density of molten corium of the same composition, but resulting from tests without water was (7.490.2) 103 kg m − 3. The apparent density reduction testifies to porosity. The elementary analysis of corium samples was made by X-ray fluorescence on the short wave X-ray spectrometer. Table 2 presents the analysis results of samples taken from different ingot sections. Data listed in Table 2 prove the uranium content reduction in the top part of the ingot. The phase composition of corium samples was measured by an X-ray diffractometer. Resulting diffractograms were processed by the PDOS software package provided with the reference base of powder diffractograms JCPDS 1980.
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Comparison of the first series diffractograms of the sampled crust, under crust region and ingot bulk (Fig. 6a) did not reveal differences in the phase structure across the ingot (solid solution (U, Zr)O2 + X, FeCr2O4). Comparison of the second series diffractograms of the sampled upper, middle and bottom parts (Fig. 6b–d) revealed the following: U3O8 phase presence was detected. It crystallized in the middle and upper parts of the ingot due to a more active interaction of the top corium layers with oxidizing medium, i.e. steam film, and probably because of the free oxygen displacement from the lower layers during crystallization.
5.2. Water, gas and aerosol sample examination Melt–water interaction products were gathered and filtered. The filter cake underwent X-ray fluorescence analysis; the solution was chemically analyzed. The elementary composition of aerosol products was determined directly from the analytical filter and precipitates of water sampled above the melt. The elementary analysis results are shown in Table 3. The data of Table 3 show that aerosols are chiefly composed of uranium oxides and practically free from zirconium oxides. Aerosol concentration in steam–gas–aerosol
Fig. 6. Corium sample diffractograms. (a) 1st experimental series phase-(UZr)O2 + x phase-FeCr2O4; (b) 2nd experimental series phase FeCr2O4; (c) 2nd experimental series phase U3O8; (d) 2nd experimental series phase (UZr)O2 + x with different lattice parameters. Table 3 Results of elementary composition analysis Sampled part
Solution precipitate (1st series) Aerosol filter (test 2/9)
Content of elements, mass% U
Zr
Ni
Fe
Cr
66.3 76.4
– –
0.4 0.7
2.0 2.3
1.4 3.3
S.V. Bechta et al. / Nuclear Engineering and Design 195 (2000) 45–56 Table 4 Above melt atmosphere composition
0.2% vol.; nitrogen, 0.4% vol. Analyses results are presented in Table 4. It follows from the water thermal dissociation equation that at thermolysis the hydrogen volumetric concentration should exceed that of oxygen by a factor of two. But, as it is evident from Table 4, experimentally obtained hydrogen/oxygen ratio is 0.032/1.475, or the concentration of oxygen is 46 times higher. One of the possible reasons for this can be redox reactions taking place at corium melt–water interaction.
Sample no. Analyzed components concentration, % vol.
2/1 2/2 2/3 2/9 Average
Nitrogen
Hydrogen
Oxygen
D of oxygen*
77.6 74.2 76.0 75.0 75.7
0.018 0.050 0.030 0.029 0.032
21.5 22.2 22.3 23.7 22.425
0.55 1.25 1.35 2.75 1.475
* D of oxygen, the oxygen concentration increase in the sample as compared to its concentration in air under normal conditions, calculated against nitrogen concentration in the mixture.
interaction products was evaluated. For that, a specified volume of gas – aerosol mixture was pumped through the preliminary weighed analytical filter. The following equation was used to calculate aerosol concentration: Ca =
G2 − G1 , Wt
53
(1)
Experimentally determined aerosol concentration was 1× 10 − 3 kg m − 3, which corresponds to the aerosol generation rate per the melt surface area unit 44 ×10 − 6 kg (m2 s) − 1. This value is considerably lower than in tests without water atop the melt, which, at all other conditions being similar, amounted to 550 × 10 − 6 kg (m2 s) − 1. The results show that the content of elements in the sampled solution precipitates correlates to that of aerosol samples. From here it can be inferred that aerosols from the melt surface enter the steam film and bubble through the boiling water layer in the steam. At that, most of aerosols are brought into water and their concentration in the off-steam decreases. Four gas samples were taken during tests 2/1–2/ 9 in order to determine the atmosphere composition. They were gathered by a sampler into vacuum burettes above the flooded melt. Hydrogen, oxygen and nitrogen content analyses were performed by chromatography. Absolute concentration measurement error for hydrogen, 0.002% vol.; oxygen,
6. Thermohydrodynamic processes at the above-melt water boiling A high corium melt temperature resulted in the film type boiling on its surface. There is a considerable amount of experimental data concerning film boiling on a solid surface and verified calculated correlation of the heat transfer factor estimation under these conditions. But the same cannot be said about the heat transfer during boiling on the melt surface. The only available data on heat transfer in water boiling on the molten metal can hardly be considered as adequate for the corium melt, as: (a) the data obtained (Green et al., 1986) had a considerable scattering (9 60%); (b) surface temperature did not exceed 873 K; (c) thermophysical characteristics of liquid metal do not correspond to those of the corium melt. That is why the unscattered heat transfer data of water boiling on the uranium-bearing corium melt, having surface temperature of 1970 K, seem to be very reliable. Their analysis is given in (Bechta et al., 1997). Here some of the results will be summarized. In accordance with Table 1 the average heat flux density from the melt into the boiling water (qexp) was 0.82 90.12 MW m − 2, with 0.95 probability. To compare the experimentally obtained heat flux value with heat transfer at boiling on a solid surface, a correlation for film boiling on a horizontal plane, put forward in (Granovsky et al., 1995) was used. It takes into consideration heat transfer by radiation and generalizes over 350 available experimental data, with a 9% average error, including tests with water boiling on a 1270 K surface.
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Taking into account the radiant heat transfer, heat flux in (Granovsky et al., 1995) is determined on the basis of the following equations: qS =qfb +qr
(2)
qfb =afbU
(3)
afb =0.031llg3,5(A)8 2/3/L
(4)
A= Ar*/[(K%/Pr)2 +(K%/Pr)]
(5)
8 =qfb/qS
(6)
qr = 5.67× 10 − 8oef[T 4c −T 4o]
(7)
Experimental data on the emissivity of oxidic corium melt, with composition given in Section 3 are not known. Available though are those for liquid steel (Kymalainen et al., 1997), its emissivity is within oc =0.40 – 0.45 and it can increase if impurities and slugs are present. In some calculations oxidic corium emissivity is assumed as rather high, e.g. in study (Azarian et al., 1998) for UO2 +ZrO2 melt oc =0.8. Therefore oxidic corium emissivity was experimentally evaluated in separate tests. The measurements were made by calorimetry. The radiant heat flux from the melt surface was measured independently by: heat fluxes into the water-cooled calorimeter, set close to the melt surface; changes in the integral heat flux into the cold crucible after the melt surface was covered by the corium powder.
Fig. 7. Comparison of calculated and experimental data on heat transfer. , experiment: —, calculation from Eqs. (2) – (7) at o = 0.475.
The results coincided within measurement errors. The measured integral heat flux gave the integral emissivity range of 0.4–0.55. Besides, the monochromatic melt emissivity was evaluated at 0.659 0.01 mm. Color and monochromatic pyrometer working at the above mentioned wave length were used for the purpose. If it is assumed that the corium melt radiation spectrum corresponds to ‘the grey body’ (which is true for many multi-component melts studied) and the color temperature is close to thermodynamical, then o0.65 = 0.599 0.13. In this way, the experimentally determined integral emissivity of the melt surface at Tsurf = 1970– 2120 K was oc = 0.47590.075. It changed insignificantly within the temperature range specified. The methodologies and results of corium melt surface emissivity estimation will be presented in more detail in a separate paper. Comparison of Eqs. (2)–(7)) calculations at oc = 0.475 with experimental results listed in Table 1 is given in Fig. 7. It is evident from Fig. 7, that calculations of heat transfer at film boiling on a solid surface give a smaller value of heat flux as compared to the experimental data with the corresponding emissivity. In order to correlate calculations and experimental data the heat transfer intensity should be increased approximately by 35%. Such an intensified heat transfer can be regarded as a product of instability at the melt–steam interface, which results both in the melt drops ejection and in their penetration under the melt surface resulting in a larger heat removal area and increased heat transfer. The Kelvin–Helmholtz instability analysis (Bechta et al., 1997) enabled the steam critical velocity estimation (along the melt surface), stipulating the generation of instability. Calculations made in (Bechta et al., 1997) demonstrated that values of both critical velocity and steam velocity calculated for experimental conditions, are close and amount to several dozen meters per second. It testifies to a high probability of instability generated on the melt surface. It is indirectly confirmed by the melt particles observed in the boiling water and voids in corium ingots.
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In conclusion it is also reasonable to point to the difference in the corium crust formation mechanism in MACE tests and in the ones described here. Evidently, in MACE experiments the crust formed on the melt surface due to a less considerable start-up melt superheating and was likely to be influenced by the melt – concrete interaction processes. In the tests presented here the equilibrium of input and output power was reached when the melt temperature exceeded the liquidus temperature. Therefore, surface crust did not form. It did form in water at a visible distance from the melt surface. It could be distinctly observed after water evaporation and the crust layer melting caused by the radiation from the melt surface. Similarly to MACE experiments when the opening in the crust was growing in, a geyser gas, steam–water mixture and corium particles were observed to discharge from the crucible. A further experimental work with a reduced melt superheating temperature, at which the crust will be formed on the melt surface, as well as with modifications in corium composition and simultaneous corium interaction with concrete or refractory material is planned for a more detailed water–corium melt interaction study. 7. Conclusions Experimental studies of water boiling on the oxide corium surface enabled to determine the following: In the test conditions provided water boiling was not accompanied by steam explosions. Corium crust formation at 1 ×10 − 2 m distance from the melt surface was observed, it started on the crucible side walls and with time propagated to the center. The increased ingot porosity as compared to the porosity of the melt crystallized without water on top testifies to the steam (water) penetration under the melt surface and the melt–steam interface instability. The integral emissivity value of corium melt (60% UO2 9 x – 16% ZrO2 – 15% Fe2O3 –6% Cr2O3 –3% Ni2O3.) within 1970 – 2120 K, was colorimetrically evaluated as being within 0.40–0.55 range.
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Heat flux densities at film boiling on the melt surface were approximately 35% higher than the same for a solid surface. The hypothesis that the melt–steam interface instability promotes heat transfer was advanced. Presence of water on the melt considerably reduced aerosol release. Chemical interaction between water and melt components is confirmed by changes in the ingot phase composition as compared to the same typical of melting in the absence of water and by the difference in relative O2 and H2 content in the gas samples from their ratio stipulated by the H2O thermolysis. To specify the quantitative characteristics obtained, it is necessary to continue investigations of melt–water interaction processes under similar conditions, but with modifications in corium composition, degree of its superheating and at increased dimensions of the test facility.
Acknowledgements The contributions of Mrs Tatiana Pautova, Mr V. Bulygin, Mrs. Helena Kaljago, Mr A. Lisenko, Dr A. Lubomirov, Mr A Martinov, Dr A Gorsckov to the experimental part of the program are gratefully acknowledged. The authors are also grateful to Ms Tatiana Talalaeva for the paper translation into English. Appendix A. Nomenclature Vsup Vev V%ev qexp Ca G2 G1 W
is water supply rate on the melt, kg s−1 water evaporation rate, kg s−1 specific evaporation rate, kg (m2 s)−1 heat flux from the corium melt into water (= rV%ev), MW m−2 aerosol concentration in the abovemelt atmosphere, kg m−3 aerosol filter mass after the pumping, kg aerosol filter mass before the pumping, kg volumetric flow rate of air pumped through the filters, m3 s−1
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56
t qS qfb qr afb u l Ar* r%, r¦ Dr g m K% r* Pr Tc To oef oc ow L
pumping time, s total heat flux, W m−2 film boiling heat flux, W m−2 radiation heat flux, W m−2 heat transfer ratio at film boiling, W (m2 K)−1 temperature difference (=Tc−To), K steam thermal conductivity, W (m K)−1 Archimedes modified number (= r¦ DrgL3/m 2) saturated water and vapour density, kg m−3 density difference, (=r% =r¦), kg m−3 gravity acceleration, m s−2 vapour dynamic viscosity, Pa s undimentional steam superheating (= Cp u/r*) modified vaporisation heat, J kg−1 Prandtl number corium temperature, K water temperature, K effective emissivity (=1/[(1/oc)+(1/ ow)−1]) melt surface emissivity steam–water interface emissivity Laplas constant, m
References Alsmeyer, H., Farmer, M., Felderer, F., et. al. The
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