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water system
Nuclear Engineering and Design 177 (1997) 339 – 349
FCI experiments in the corium/water system I. Huhtiniemi *, H. Hohmann, D. Magallon European Commission, Joint Research Centre, Safety Technology Institute, I-21020 Ispra (Va), Italy
Abstract The KROTOS fuel coolant interaction (FCI) tests are aimed at providing benchmark data to examine the effect of fuel/coolant initial conditions and mixing on explosion energetics. Experiments, fundamental in nature, are performed in well-controlled geometries and are complementary to the FARO large scale tests. Recently, a test series was performed using 3 kg of prototypical corium (80 w/o UO2, 20 w/o ZrO2) which was poured into a water column of 5 1.25 m in height (95 and 200 mm in diameter) under 0.1 MPa ambient pressure. Four tests were performed in the test section of 95 mm in diameter (ID) with different subcooling levels (10 – 80 K) and with and without an external trigger. Additionally, one test has been performed with a test section of 200 mm in diameter (ID) and with an external trigger. No spontaneous or triggered energetic FCIs (steam explosions) were observed in these corium tests. This is in sharp contrast with the steam explosions observed in the previously reported alumina (Al2O3) test series which had the same initial conditions of ambient pressure and subcooling. The post-test analysis of the corium experiments indicated that strong vaporisation at the melt/water contact led to a partial expulsion of the melt from the test section into the pressure vessel. In order to avoid this and to obtain a good penetration and premixing of the corium melt, an additional test was performed with a larger diameter test section. In all the corium tests an efficient quenching process (0.8–1.0 MW kg-melt − 1) with total fuel fragmentation (mass mean diameter 1.4 – 2.5 mm) was observed. Results from alumina tests under the same initial conditions are also given to highlight the differences in behaviour between corium and alumina melts during the melt/water mixing. © 1997 Elsevier Science S.A.
1. Introduction In the event of a severe reactor accident a significant fraction of the core may melt and pour down into the lower plenum of the reactor pressure vessel. Unless outside cooling of the vessel wall is provided, the vessel integrity might be threatened and eventually core material could enter into the reactor cavity. In many countries the accident management strategies for such situa* Corresponding author. Fax: +39 33 2785412; e-mail: [email protected]
tions are based on having water in the cavity to quench the melt and ultimately to obtain a coolable configuration of debris on the basemat. However, such a strategy relies on the fact that no highly energetic fuel coolant interactions (FCI) will take place or that the cavity structures are sufficiently strong to cope with loads due to such an explosion. Active modelling work is currently underway in several research centres to investigate FCIs and, in particular, different stages of it, pre-mixing, triggering, propagation and expansion (in the case of an explosion), which determines the energetics
0029-5493/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 2 9 - 5 4 9 3 ( 9 7 ) 0 0 2 0 2 - 1
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and structural loading (Murphy and Corradini, 1994; Theofanous and Yuen, 1994). To support these modelling efforts, fundamental experimental investigations are being performed in the KROTOS facility at JRC-Ispra. These experiments are aimed at providing benchmark data to examine the effect of fuel-coolant initial conditions and mixing on explosion energetics. Detailed data is needed where mixing and explosion processes occur under controlled conditions. With such data one can also validate in small scale the fuel coolant mixing models such as IFCI [Sandia National Laboratory (Davis and Young, 1994)], TEXAS [University of Wisconsin (Tang, 1993)], PM-ALPHA [University of California (Angelini et al., 1995)] and COMETA [JRC Ispra (Annunziato and Addabbo, 1994)] and explosion models such as ESPROSE.m [Univ. of California (Theofanous and Yuen, 1994)], IDEMO [IKE-Stuttgart (Bu¨rger et al., 1994)] and TEXAS. These models are currently actively in use to analyse KROTOS test results with an aim to improve their predictive capabilities so that reliable extrapolation of results to the reactor scale could be achieved. The KROTOS facility is used for FCI studies in the molten corium/water system in conjunction with the largescale FARO facility (Magallon and Hohmann, 1995). The objectives of the current tests are to investigate in 1-D and 2-D geometries the premixing of molten fuel jets with nearly saturated and subcooled water and subsequently, the potential for an energetic interaction. During every experiment the dynamic pressurisation of the test section and pressure vessel, as well as the water and cover gas temperatures, and water level swell are measured. Post-test debris analyses provide additional qualitative information about explosion efficiency.
ous test series with molten alumina. At high temperatures (up to 3273 K), involved in the UO2 melting, the use of graphite heaters caused material problems due to unexpected chemical reactions. Therefore, the graphite heater elements had to be replaced by tungsten heater elements. The pre-tests also demonstrated that helium was better suited as furnace cover gas than argon. Furthermore, due to the high temperatures in these tests only tungsten could be used as the material for the melt crucible and the puncher (the device for perforating the crucible bottom), see Fig. 1b. Extensive work had to be done to refine the sophisticated fabrication techniques to machine the bottom membrane of the tungsten crucibles according to the required dimensions (0.2–0.3 mm thickness).
2. KROTOS facility
2.2. Pressure 6essel and test section
Fig. 1a illustrates the main components of the facility: the radiation furnace, release tube, pressure vessel and test section. A pre-test series with corium melts showed that some modifications of the facility were required with respect to the previ-
The lower part of the KROTOS facility consists of a pressure vessel and test section, both made of stainless steel. The pressure vessel is designed for 2.5 MPa at 493 K. It is a cylindrical vessel of 0.4 m inner diameter and 2.21 m in height (freeboard
2.1. Furnace The furnace consists of a cylindrical tungsten heater element which encloses the crucible containing the melt material. The crucible is held in place by means of a pneumatically operated release hook. Eight concentric tungsten, molybdenum and steel radiation shields are radially placed around the heater element. The top and bottom parts of the heated zone are insulated with thermal screens to reduce heat losses to the surroundings. The furnace is covered with a bell-shaped, water-cooled lid designed to withstand 0.25 MPa over-pressure (Ar, He) or vacuum. The 3-phase electric power supply has a maximum voltage of 30 V and a maximum power of 130 kW. Depending on the crucible design, melt masses in the range of about 1–10 kg can be used. Maximum achievable temperatures in the furnace are of the order of 3273 K. The melt temperature is controlled by an optical pyrometer measuring the wall temperature of the crucible.
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Fig. 1. KROTOS test facility.
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volume of 0.290 m3) with a flat bottom plate and flanged flat upper head plate. A number of feedtroughs exist in this vessel for auxiliary gas and water connections and mounting of instrumentation. Two test sections of different diameter, one of which is illustrated in Fig. 1a, are utilised in this test series. The test sections consist of strong stainless steel tubes either of inner diameter 95 mm and outer diameter 135 mm (designated as the narrow test section) or 200 and 240 mm (designated as the wide test section), respectively. Both of them can contain water at variable heights up to about 1.25 m. The bottom of both the test sections can be closed by either a flat plate or with a gas trigger device. In some of the first experiments with tin and alumina melts, spontaneous interactions occurred near the water surface (Hohmann et al., 1995), therefore additional means had to be introduced to improve the penetration of melt into the water and to minimise the risk of an early steam explosion. One of these measures consisted of providing the experimental tube with a 2 mm thick Plexiglas liner, thus avoiding direct contact between the melt and the wall of the test section. At the upper part of the narrow test section, 20 holes (diameter 50 mm) allow the vapour venting into the pressure vessel volume. A steel vessel of 205 mm inner diameter is mounted around the part of the test section with the holes. This vessel fills up with water to the test section water level and level meters are placed there to measure the water level swell during melt-coolant premixing, see Fig. 1c. The level meter vessel is not used with the wide test section. Instead, two level meters are mounted along the inside wall of this test section. In some tests a trigger device is attached at the bottom of the test section. The gas trigger device used in this test series is shown in Fig. 1d. The gas chamber volume of 15 cm3 can be charged to a pressure of up to 20 MPa (argon) and is closed by a 0.1–0.2 mm thick steel membrane. After melt penetration down into the lower region of the test section, the mechanical destruction of the membrane delivers a pressure pulse propagating vertically upwards through the mixture of melt, water and steam. The gas trigger device is activated either
by a thermocouple signal (normally TC 2 or TC 3, see below) or by a backup time delay circuit.
2.3. Instrumentation and data acquisition Pressures, temperatures and water level swell are the main experimental parameters measured in the KROTOS test section during melt-coolant interactions. Up to six piezo-electric pressure transducers (type KISTLER, designated as K0, etc.) with a pressure range up to 100 MPa are used in the test tube to monitor the pressure in the interaction zone and in the gas trigger device. The pressure increase in the cover gas atmosphere can be detected at six positions in the pressure vessel by means of piezoresistive pressure transducers (type KELLER, designated as C1, etc.). All pressure transducers have response frequencies of greater than 10 kHz and the signals are normally recorded on transient recorders with a sampling time of 20 s. The positions of the pressure transducers with respect to the bottom plate upper surface of the test section are given in Table 1. Several thermocouples (K-type) are used to measure the temperatures of the gas atmosphere and the water in the test section, and to detect the position of the melt jet leading edge during its penetration into the water. In order to improve the response times, thermocouples of 0.5 mm diameter are used. The thermocouples installed at the test section axis are positioned at the same elevations as the K-transducers and designated as TC 1, etc. The contact with the melt leads to immediate destruction of the thermocouples therefore no information of the trailing edge of the melt can be obtained with them. In general, extra sensors such Table 1 Pressure transducer locations KISTLER
Positiona (mm) KELLER
Positiona (mm)
K0 K1 K2 K3 K13 K4 K5
0 150 350 550 550 750 950
140 1450 1780
a
C1/C11 C2/C12 C3/C13
Measured from the bottom plate of the test section.
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as photocells, photoresistors and magnetic field detectors were introduced in the upper part of the test section above the water level in order to detect both the beginning and the duration of the melt release from the crucible without contacting the melt jet. However, these techniques were still at the development stage during this test series and they have failed to provide reliable data so far. Water level measurement is required for the estimation of the integral vapour fraction in the mixing zone. Two different kinds of level meters, an inductive (with a moving float) and TDR-type (time domain reflectometry), are used to measure the level swell during the premixing phase. The tests with the narrow test section utilise one of each of these types in a separate level meter vessel (location illustrated in Fig. 1c). In the tests with the wide test section, only TDR-type of level meters are used, because they are more robust and insensitive to thermo-mechanical loads that might be imposed during a test. It is important to note that the level meters in general do not function well in highly voided regions. However, the fast response time ( \ 1 kHz) of the TDR sensor and its ability to detect gas/water surface intersecting it give confidence in the measurements of the level swell even in the initial phase of mixing where high void fractions are expected. Qualitative check by visual observation of both types of level meters has been made in a transparent test section mock-up. Furthermore, a calibration of the level meters was done prior to each test with water (no void). The KROTOS data acquisition system consists of several transient data recorders. Pressure transducer and level swell probe signals are recorded using a transient recorder (Yokogawa 1600) with high data sampling rates of 50 kHz over a time period of about 5 s. Thermocouple data (8–12 channels) were also recorded at quite high sampling rate of 1 kHz (Yokogawa 1100) in most tests to capture the timing of the melt front progression accurately. A RACAL analog tape recorder and an HP 75 000 data acquisition system are used to record the long term behaviour of the system as well as to provide a data backup system. Data reduction and processing is done
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remotely using a PV-WAVE data analysis package in an HP 712 workstation which is connected to the KROTOS site via a TCP/IP network link.
3. KROTOS test procedure After having reached the desired melt temperature ( 3000 K), the crucible containing the corium melt (about 3 kg, density 7960 kg − 1 m3) is released from the furnace and falls by gravity through a 4 m long release tube of 95 mm inner diameter. Half-way down the tube, a rapid-acting slide valve separates the furnace from the test section below. This valve closes immediately after the crucible has passed. During its fall, the crucible cuts a copper wire generating the zero time signal for the data acquisition. Finally the crucible impacts onto a retainer ring at the end of the tube where a conical shaped metallic puncher breaks the bottom of the crucible thus allowing the melt to pour out (Fig. 1b). The exit of the melt can be delayed by some hundreds of milliseconds by a sacrificial disk of low melting point metal placed beneath the puncher (used only in one test). In this way the melt is slowed down to gravity release conditions. The melt jet diameter is defined by guiding the melt through a funnel of high temperature refractory material with an exit diameter of 30 mm. The melt arrival is detected with a thermocouple (TC 7) just at the exit of the funnel which, in turn, is located about 0.46 m above the water free surface. Another thermocouple (TC 6) is located about 10 mm above the water surface to allow for estimation of the velocity of the jet at the melt-water contact. The melt injection phase has been observed visually in numerous tin and wood metal simulant tests which were performed in a transparent test section mock-up. These tests demonstrated a coherent jet injection and verified the operation of the level meters. After the jet has penetrated sufficiently deep into the water column, either a thermocouple signal or a predetermined time delay can be used to activate the destruction of the membrane of the gas trigger device to generate a pressure pulse that propagates vertically upwards through the melt/water mixture in order to trigger a steam
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Table 2 Test conditions and results with corium melts KROTOS test No. Melt Composition Charged mass Temperature Brake disk Initial jet diameter Free fall in gas Coolant Water mass Height Initial temperature Subcooling Test section Initial pressure (He) Internal diameter Plexiglas liner Gas trigger Results Confirmed penetration Depth of the melt jet Maximum pressurisation Steam explosion Total debrisc Debris in test section
32
33
35
36
37
(mm) (m)
80.8 19.2 3030 3063 No 30 0.46
81.2 18.8 3170 3063 No 30 0.46
79 21 3102 3023 Yesa 30 0.46
79 21 3027 3025 No 30 0.46
79 21 3222 3018 No 30 0.44
(kg) (m) (K) (K)
7.1 1.08 351 22
7.7 1.08 298 75
7.7 1.08 363 10
7.7 1.08 294 79
(MPa) (mm)
0.1 95 Yes No
0.1 95 No No
0.1 95 No Yes
0.1 95 No Yes
0.1 200 No Yes
TC 5
TC 4
n.a.b
TC 5
TC 4
0.23 No 2608 1402
0.14 No 2802 1705
0.17 No 1424 331
0.13 No 2801 1142
0.07 No 2925 2925
UO2 (w/o) ZrO2 (w/o) (g) (K)
(MPa) (g) (g)
34.5 1.105 294 79
Tin/woods metal alloy (Tm413 K). Thermocouple wires destroyed by melt ejection from the test section. c Found in test section, level-meter vessel and pressure vessel. a
b
explosion. After the test, all the debris is collected, photographed and sieved. The debris is inspected before and after the sieving in order to detect any changes in debris form caused by the sieving process. Finally, the internal parts of the test facility are decontaminated before the preparations for the next test can be started.
4. KROTOS experimental results Five experiments with corium have been performed. The objective of this test series was to study premixing of corium melts with water at both low and high subcoolings and to determine if an energetic FCI could take place under such conditions.
The main test parameters and results are summarised in Table 2. The KROTOS test programme has been evolutionary in nature, the outcome of previous results contributing significantly to the planning of the future experiments. To reduce the number of test variables, the following parameters have been fixed in these tests: the initial system pressure (0.1 MPa), release nozzle diameter (30 mm), fall height ( 0.46 m) and the water depth ( 1.1 m). In the next sections a brief description of the tests and results is given. The discussion is subdivided into two classes as ‘saturated water conditions’ and ‘subcooled water conditions’ following the same convention as previously when the results of the alumina/water system were reported (Hohmann et al., 1995).
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4.1. KROTOS experiments in ‘saturated water conditions’ In the KROTOS 32 test, approximately 3 kg of corium was heated up to 3063 K in a tungsten crucible. After the test sequence initiation, the crucible was released and fell down onto the puncher. Since the test was performed without a brake disk, the melt was free to stream out through the punctured crucible bottom and funnel into the water without any time delay. A Plexiglas liner (2 mm thick) was used in the test section to reduce the risk of a spontaneous steam explosion upon the melt contacting the walls of the test section. The thermocouple data gives an estimate of 4.2 m s − 1 for the leading edge velocity using the thermocouples TC 6 and TC 7 as melt arrival indicators. This value is significantly lower than the theoretical value considering a gravity release of the melt from the furnace. Evidently, the puncher and funnel assembly slowed down the release rate. Thermocouples in the water allowed for the estimation of the melt velocity after penetration into the water. The estimated velocity of the melt jet between TC 5 and TC 6 was approximately 1.5 m s − 1. The thermocouple data demonstrates that the coherent jet penetrated at least down to TC 5. No energetic interactions occurred, and the pressurisation of the freeboard volume was mainly due to the steam generation by the melt (see Fig. 2). The different phases of the melt
Fig. 2. Test vessel pressure: subcooled vs. near saturated.
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injection can be distinguished in the pressure history. At about 200 ms after cutting the trigger wire, the crucible impacts the puncher. Immediately afterwards, the start of the melt pour increases the pressure rapidly due to the heating of the cover gas up to about 0.16 MPa (220–300 ms). The pressurisation rate changes when the melt jet penetrates the water column due to energy transfer to the water. Once the melt has penetrated and mixed with the water, the pressurisation rate increases again because of the enhanced heat transfer due to melt breakup. The pressure reached the maximum of 0.33 MPa at about 1.4 s and then it decreased quickly to the quasi steady-state value of about 0.15 MPa (at 60 s) due to condensation onto cool walls. The debris size distribution for KROTOS 32 is plotted in Fig. 5. The mass mean size of the debris is about 2.5 mm. The large particles of the debris consisted mainly of irregular shaped particles with some agglomerates. The smaller particles were more spherical in shape. An estimate of debris quenching rate can be obtained by assuming that the pressurisation of the freeboard volume was caused solely by saturated steam with negligible bulk water heating and that all the melt participated (i.e. entered into the water). This simple analysis gives an estimate of 0.879 10% MW kg-melt − 1 for the maximum melt quenching rate during the mixing phase. The KROTOS 35 test was essentially a repeat of the KROTOS 32 experiment except that the test section did not have a Plexiglas liner and a Woods metal-tin brake disk was installed to reduce the melt release rate. Additionally, a gas trigger device was mounted. The gas trigger was configured to trigger when TC 3 sensed the melt arrival. After the initial melt injection, a rapid pressurisation was observed. This initial rapid pressurisation of the cover gas lasted longer than in KROTOS 32 because of the longer duration of the melt release. The observed peak pressure of 0.27 MPa is lower than in KROTOS 32, because a significant amount of melt (about 1.1 kg) was blocked in the release funnel and, furthermore, vigorous steaming lead to expulsion of a significant fraction of unquenched melt (77% of the
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injected melt mass). Some of the melt swept out from the test section damaged the thermocouple and level-meter cables on the outside of the test section. Thus, the trigger device activated only after the set time delay (2.6 s) rather than with the TC 3 signal. No steam explosion took place. However, the time delay might have been too long for an appropriate triggering i.e. it is believed that a significant fraction of the melt was already quenched.
4.2. Experiments in ‘subcooled water conditions’ The KROTOS 33 test was essentially a repeat of the KROTOS 32 test except that the water subcooling was higher (75 K) and that the Plexiglas liner was removed from the test section. The same method as in KROTOS 32 was used to estimate the leading edge velocity. The TC 6 and TC 7 data indicated a somewhat higher velocity of 8 m s − 1 prior to melt penetration into the water. This is also confirmed by the shorter duration (220–270 ms) of the cover gas pressurisation (approximately 50.12 MPa) shown in Fig. 2. Once in the water, the jet decelerated rapidly from the average velocity of 4.4 m s − 1 between TC 6 and TC 5 to 0.9 m s − 1 between TC 5 and TC 4. Due to the higher subcooling, the pressure of the expansion volume is less than in KROTOS 32 with a maximum of about 0.24 MPa and a quasisteady-state level of about 0.125 MPa at 60 s. The debris size distribution for KROTOS 33 is plotted in Fig. 5. The morphology of the debris was the same as in KROTOS 32. One larger piece of material weighing 88.8 g was recovered from the level meter vessel. The particle sizes are somewhat smaller than in the near saturation conditions (KROTOS 32) with the mass mean size of about 2 mm (excluding the piece mentioned above). Accordingly, the estimate of the melt quenching rate (0.979 10% MW kg-melt − 1) is higher than in KROTOS 32. The KROTOS 36 test was performed with the same conditions as KROTOS 33 except that a trigger device was mounted. The trigger device was set to activate with the TC 3 signal as in the KROTOS 35 test. Moreover, the results from the KROTOS 35 test allowed for a better estimate of
Fig. 3. Test vessel pressure: corium vs. alumina.
the appropriate backup time delay for triggering. Melt injection was successful but again vigorous steaming at the melt-water contact lead to a partial expulsion of the melt from the test section. The gas trigger device was activated by the TC 3 signal at 1.6 s. However, no energetic propagating explosion was observed. Immediate rapid pressurisation and melt sweepout lead to the conclusion that the melt mass participating in the premixing process was limited due to the flow constraint imposed by the narrow test tube. The KROTOS 37 test was a repeat of the KROTOS 36 test with a new, larger diameter test section (200 versus 95 mm). The larger test section was utilised to reduce the superficial steam velocity above the water thus reducing early (fall stage) jet breakup, levitation and sweep-out of the melt as observed in previous tests. In KROTOS 37, the melt jet was successfully injected into the test section with an insignificant amount of sweep-out. The coherent melt jet penetrated at least down to the TC 4 level. The trigger activated with the TC 3 signal at 1.0 s, but no interaction was observed. However, due to the larger test section, the amplitude of the propagating trigger pulse was reduced from the previous tests because the trigger energy (200 J) was kept constant. The maximum pressurisation, shown in Fig. 3, was lower than the previous one (KROTOS 33) because of the greater mass of subcooled water (34.5 kg). A total fragmentation of the injected melt mass was observed. The resulting debris size distribution for KROTOS 37 is plotted in Fig. 5. The
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Table 3 Comparison of selected corium and alumina tests KROTOS test No.
30
33
37
38
Melt Composition Charged mass (g) Temperature (K) Brake disk Initial jet diameter (mm) Free fall in gas (m)
Al2O3 1516 2573 No 30 0.46
UO2 – ZrO2 3170 3063 No 30 0.46
UO2 – ZrO2 3222 3018 No 30 0.44
Al2O3 1533 2665 No 30 0.44
Coolant Water mass (kg) Height (m) Initial temperature (K) Subcooling (K)
7.5 1.08 293 80
7.7 1.08 298 75
34.5 1.105 294 79
34.5 1.105 294 79
Test section Initial pressure (MPa) Internal diameter (mm) Plexiglas liner Gas trigger
0.1 95 Yes No
0.1 95 No No
0.1 200 No Yes
0.1 200 No Yesa
Results Confirmed penetration depth of the melt jet Maximum dynamic pressureb (MPa) Steam explosion Total debris (g) Debris B250 mm (g) Debris B100 mm (g) Explosion efficiency (%)
TC 3 \100 Yes 1400 1210 1000 1.6
TC 4 — No 2802 136 37.4 —
TC 4 — No 2925 116 41.2 —
TC 3 67 Yes 1523 934 545 1.5
a b
Spontaneous explosion took place before triggering. In the test section.
particle sizes are somewhat smaller (1.4 mm) than observed under the same initial conditions but with the narrow test section (KROTOS 33). In the CCM-1 test, which was performed at Argonne National Laboratory (Spencer et al., 1994) with similar initial conditions (P0.1 MPa, DTsub 43 K), but with corium thermite melt, the mass mean particle size was 2.3 mm. The estimate of the maximum melt quenching rate (0.13910% MW kg-melt − 1) is significantly lower than in the tests with the narrow test section indicating more efficient bulk heating of the water which was not considered in the simple analysis and was not measured in the KROTOS 37 test during the mixing phase. By considering only the steam generation in the CCM-1 test, a melt quenching rate of about 0.09 MW kg-melt − 1
is obtained which compares well with the KROTOS 37 data. However, in the CCM-1 test a significant fraction of energy was transferred to the water, so that the total melt quenching rate was about 0.8 MW kg-melt − 1 which is comparable with the estimates in the KROTOS tests with the narrow test section (with less water) and to the results from FARO test analysis (including the bulk water heating). Therefore, it is likely that the bulk water heating in KROTOS 37 was at least as effective as in the CCM-1 test because of the higher initial subcooling and the smaller average particle mass mean diameter. It is interesting to contrast the corium results with the previous ones from the alumina test series where supercritical explosions were observed (Hohmann et al., 1995). Such a compari-
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son is shown in Table 3 where the initial conditions and some results of the alumina and corium tests are tabulated. In order to make consistent comparisons, it was imperative to repeat an alumina test with the larger diameter test section to see if lessening the constraint of the narrow test tube would indeed affect the outcome (normally an energetic interaction). Such a test (KROTOS 38) was performed with 1.5 kg of alumina (at 2665 K), see Table 3. A spontaneous energetic explosion took place before the trigger system was activated. Peak dynamic pressures up to 67 MPa were observed in the test section. The pressurisation of the freeboard volume for this test is shown together with the KROTOS 37 result in Fig. 3. A different type of premixing behaviour with corium and alumina is evident by comparing the initial steaming rates. Significantly greater steam generation with the corium melt is further illustrated by the level swell data, shown in Fig. 4. This preliminary data would therefore suggest that less breakup of the melt occurred and that the void fraction in the mixing region was smaller prior to triggering in the case of alumina. However, due to the steam explosion the debris is significantly finer than in the corium tests, see Fig. 5. Fig. 5 also indicates that about 65% of the interacting alumina melt mass fragmented in particles of less than 250 mm in diameter. In this investigation it was assumed that these particles participated in the explosion process. The efficiency of this explosion process is then the ratio of the kinetic energy to the thermal energy content of the melt which
Fig. 5. Particle size distribution in selected tests.
participated in the process. The kinetic energy of the mixture mass was determined from the dynamic pressurisation of the test section which allowed the estimation of the impulse ( pA dt) imparted to the water column. Using this approach and the experimental data from KROTOS 38, an explosion efficiency of 1.5% was calculated. These observations have important implications concerning tests with alumina simulants performed at various research centres. The differences between corium and alumina melts, in this respect, should be well understood. Currently, experimental investigations are pursued to study the differences between the prototypic corium melts and alumina simulant melts in parallel with general objectives of the KROTOS programme.
5. Conclusions
Fig. 4. Level swell in test section: corium vs. alumina.
Concerning KROTOS tests, it is important to note that the following conclusions are based on preliminary trends observed with only a few tests and need to be confirmed with further tests. The experimental results from the KROTOS corium programme so far indicate: Significant breakup of the melt into relatively fine debris ( 1.4–2.5 mm), No energetic interactions within the range of the following investigated parameters: low subcooling (1020 K), high subcooling (80 K) and external trigger (energy of 200 J),
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Due to lack of energetic interactions so far with corium melts, the data on effects of geometric constraints on explosions and the far field effects are limited to results from alumina tests, Simple analysis using steaming rates indicate melt quenching rates of 0.8 – 1.0 MW kg-melt − 1 in the narrow test section. The observed melt quenching rates in the larger scale FARO facility are comparable with the rates observed in the KROTOS tests. Significant differences between the behaviour of prototypic corium and simulant alumina melts have been observed and more data are needed to understand them.
Acknowledgements The authors greatly acknowledge the work and efforts of the whole KROTOS team. Performed in collaboration with the USNRC in the frame of Technical Exchange Agreement No. 4086-90-09 TG ISP USA. References Angelini, S., Yuen, W.W., Theofanous, T.G., 1995. Premixingrelated behaviour of steam explosions. Nucl. Eng. Des. 155, 115 – 157.
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Annunziato, A., Addabbo, C., 1994. COMETA (COre MElt Thermalhydraulic Analysis): a computer code for melt quenching analysis. Int. Conf. New Trends Nuclear System Thermohydraul., Vol. II, Pisa, Italy, May 1994, pp. 391– 398. Bu¨rger, M., Buck, M., Muller, K., Schatz, A., 1994. Stepwise verification of thermal detonation models: Examination by means of the KROTOS experiments. CSNI Specialist Meeting on Fuel Coolant Interactions, NUREG/CP-0127 January, 1994, pp. 218 – 232. Davis, F.J., Young, M.F., 1994. Integrated fuel-coolant interaction (IFCI) code. Technical Report NUREG/CR-6211, Sandia Nat. Laboratory, April 1994. Hohmann, H., Magallon, D., Schins, H., Yerkess, A., 1995. FCI experiments in the aluminium oxide/water system. Nucl. Eng. Des. 155, 391 – 403. Magallon, D., Hohmann, H., 1995. Experimental investigation of 150-kg-scale corium melt jet quenching in water. NUREG/CP-0142 3, pp. 1688 – 1711. Murphy, J., Corradini, M., 1994. An assessment of ex-vessel FCI energetics for ALWRs. Int. Conf. New Trends in Nuclear System Thermohydraulics, Vol. II, Pisa, Italy, May 1994, pp. 281 – 289. Spencer, B.W., Wang, K., Blomquist, C.A., McUmber, L.M., Schneider, J.P., 1994. Fragmentation and quench behavior of corium melt streams in water. Technical Rep. NUREG/CR-6133, Argonne Nat. Laboratory, February 1994. Tang, J., 1993. Modeling of the Complete Process of One-dimensional Vapor Explosions. PhD thesis, University of Wisconsin, Madison, 1993. Theofanous, T.G., Yuen, W.W., 1994. The prediction of dynamic loads from ex-vessel steam explosions. Int. Conf. New Trends in Nuclear System Thermohydraulics, Vol. II, Pisa, Italy, May 1994, pp. 257 – 270.
FCI experiments in the corium/water system I. Huhtiniemi *, H. Hohmann, D. Magallon European Commission, Joint Research Centre, Safety Technology Institute, I-21020 Ispra (Va), Italy
Abstract The KROTOS fuel coolant interaction (FCI) tests are aimed at providing benchmark data to examine the effect of fuel/coolant initial conditions and mixing on explosion energetics. Experiments, fundamental in nature, are performed in well-controlled geometries and are complementary to the FARO large scale tests. Recently, a test series was performed using 3 kg of prototypical corium (80 w/o UO2, 20 w/o ZrO2) which was poured into a water column of 5 1.25 m in height (95 and 200 mm in diameter) under 0.1 MPa ambient pressure. Four tests were performed in the test section of 95 mm in diameter (ID) with different subcooling levels (10 – 80 K) and with and without an external trigger. Additionally, one test has been performed with a test section of 200 mm in diameter (ID) and with an external trigger. No spontaneous or triggered energetic FCIs (steam explosions) were observed in these corium tests. This is in sharp contrast with the steam explosions observed in the previously reported alumina (Al2O3) test series which had the same initial conditions of ambient pressure and subcooling. The post-test analysis of the corium experiments indicated that strong vaporisation at the melt/water contact led to a partial expulsion of the melt from the test section into the pressure vessel. In order to avoid this and to obtain a good penetration and premixing of the corium melt, an additional test was performed with a larger diameter test section. In all the corium tests an efficient quenching process (0.8–1.0 MW kg-melt − 1) with total fuel fragmentation (mass mean diameter 1.4 – 2.5 mm) was observed. Results from alumina tests under the same initial conditions are also given to highlight the differences in behaviour between corium and alumina melts during the melt/water mixing. © 1997 Elsevier Science S.A.
1. Introduction In the event of a severe reactor accident a significant fraction of the core may melt and pour down into the lower plenum of the reactor pressure vessel. Unless outside cooling of the vessel wall is provided, the vessel integrity might be threatened and eventually core material could enter into the reactor cavity. In many countries the accident management strategies for such situa* Corresponding author. Fax: +39 33 2785412; e-mail: [email protected]
tions are based on having water in the cavity to quench the melt and ultimately to obtain a coolable configuration of debris on the basemat. However, such a strategy relies on the fact that no highly energetic fuel coolant interactions (FCI) will take place or that the cavity structures are sufficiently strong to cope with loads due to such an explosion. Active modelling work is currently underway in several research centres to investigate FCIs and, in particular, different stages of it, pre-mixing, triggering, propagation and expansion (in the case of an explosion), which determines the energetics
0029-5493/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 2 9 - 5 4 9 3 ( 9 7 ) 0 0 2 0 2 - 1
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and structural loading (Murphy and Corradini, 1994; Theofanous and Yuen, 1994). To support these modelling efforts, fundamental experimental investigations are being performed in the KROTOS facility at JRC-Ispra. These experiments are aimed at providing benchmark data to examine the effect of fuel-coolant initial conditions and mixing on explosion energetics. Detailed data is needed where mixing and explosion processes occur under controlled conditions. With such data one can also validate in small scale the fuel coolant mixing models such as IFCI [Sandia National Laboratory (Davis and Young, 1994)], TEXAS [University of Wisconsin (Tang, 1993)], PM-ALPHA [University of California (Angelini et al., 1995)] and COMETA [JRC Ispra (Annunziato and Addabbo, 1994)] and explosion models such as ESPROSE.m [Univ. of California (Theofanous and Yuen, 1994)], IDEMO [IKE-Stuttgart (Bu¨rger et al., 1994)] and TEXAS. These models are currently actively in use to analyse KROTOS test results with an aim to improve their predictive capabilities so that reliable extrapolation of results to the reactor scale could be achieved. The KROTOS facility is used for FCI studies in the molten corium/water system in conjunction with the largescale FARO facility (Magallon and Hohmann, 1995). The objectives of the current tests are to investigate in 1-D and 2-D geometries the premixing of molten fuel jets with nearly saturated and subcooled water and subsequently, the potential for an energetic interaction. During every experiment the dynamic pressurisation of the test section and pressure vessel, as well as the water and cover gas temperatures, and water level swell are measured. Post-test debris analyses provide additional qualitative information about explosion efficiency.
ous test series with molten alumina. At high temperatures (up to 3273 K), involved in the UO2 melting, the use of graphite heaters caused material problems due to unexpected chemical reactions. Therefore, the graphite heater elements had to be replaced by tungsten heater elements. The pre-tests also demonstrated that helium was better suited as furnace cover gas than argon. Furthermore, due to the high temperatures in these tests only tungsten could be used as the material for the melt crucible and the puncher (the device for perforating the crucible bottom), see Fig. 1b. Extensive work had to be done to refine the sophisticated fabrication techniques to machine the bottom membrane of the tungsten crucibles according to the required dimensions (0.2–0.3 mm thickness).
2. KROTOS facility
2.2. Pressure 6essel and test section
Fig. 1a illustrates the main components of the facility: the radiation furnace, release tube, pressure vessel and test section. A pre-test series with corium melts showed that some modifications of the facility were required with respect to the previ-
The lower part of the KROTOS facility consists of a pressure vessel and test section, both made of stainless steel. The pressure vessel is designed for 2.5 MPa at 493 K. It is a cylindrical vessel of 0.4 m inner diameter and 2.21 m in height (freeboard
2.1. Furnace The furnace consists of a cylindrical tungsten heater element which encloses the crucible containing the melt material. The crucible is held in place by means of a pneumatically operated release hook. Eight concentric tungsten, molybdenum and steel radiation shields are radially placed around the heater element. The top and bottom parts of the heated zone are insulated with thermal screens to reduce heat losses to the surroundings. The furnace is covered with a bell-shaped, water-cooled lid designed to withstand 0.25 MPa over-pressure (Ar, He) or vacuum. The 3-phase electric power supply has a maximum voltage of 30 V and a maximum power of 130 kW. Depending on the crucible design, melt masses in the range of about 1–10 kg can be used. Maximum achievable temperatures in the furnace are of the order of 3273 K. The melt temperature is controlled by an optical pyrometer measuring the wall temperature of the crucible.
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Fig. 1. KROTOS test facility.
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volume of 0.290 m3) with a flat bottom plate and flanged flat upper head plate. A number of feedtroughs exist in this vessel for auxiliary gas and water connections and mounting of instrumentation. Two test sections of different diameter, one of which is illustrated in Fig. 1a, are utilised in this test series. The test sections consist of strong stainless steel tubes either of inner diameter 95 mm and outer diameter 135 mm (designated as the narrow test section) or 200 and 240 mm (designated as the wide test section), respectively. Both of them can contain water at variable heights up to about 1.25 m. The bottom of both the test sections can be closed by either a flat plate or with a gas trigger device. In some of the first experiments with tin and alumina melts, spontaneous interactions occurred near the water surface (Hohmann et al., 1995), therefore additional means had to be introduced to improve the penetration of melt into the water and to minimise the risk of an early steam explosion. One of these measures consisted of providing the experimental tube with a 2 mm thick Plexiglas liner, thus avoiding direct contact between the melt and the wall of the test section. At the upper part of the narrow test section, 20 holes (diameter 50 mm) allow the vapour venting into the pressure vessel volume. A steel vessel of 205 mm inner diameter is mounted around the part of the test section with the holes. This vessel fills up with water to the test section water level and level meters are placed there to measure the water level swell during melt-coolant premixing, see Fig. 1c. The level meter vessel is not used with the wide test section. Instead, two level meters are mounted along the inside wall of this test section. In some tests a trigger device is attached at the bottom of the test section. The gas trigger device used in this test series is shown in Fig. 1d. The gas chamber volume of 15 cm3 can be charged to a pressure of up to 20 MPa (argon) and is closed by a 0.1–0.2 mm thick steel membrane. After melt penetration down into the lower region of the test section, the mechanical destruction of the membrane delivers a pressure pulse propagating vertically upwards through the mixture of melt, water and steam. The gas trigger device is activated either
by a thermocouple signal (normally TC 2 or TC 3, see below) or by a backup time delay circuit.
2.3. Instrumentation and data acquisition Pressures, temperatures and water level swell are the main experimental parameters measured in the KROTOS test section during melt-coolant interactions. Up to six piezo-electric pressure transducers (type KISTLER, designated as K0, etc.) with a pressure range up to 100 MPa are used in the test tube to monitor the pressure in the interaction zone and in the gas trigger device. The pressure increase in the cover gas atmosphere can be detected at six positions in the pressure vessel by means of piezoresistive pressure transducers (type KELLER, designated as C1, etc.). All pressure transducers have response frequencies of greater than 10 kHz and the signals are normally recorded on transient recorders with a sampling time of 20 s. The positions of the pressure transducers with respect to the bottom plate upper surface of the test section are given in Table 1. Several thermocouples (K-type) are used to measure the temperatures of the gas atmosphere and the water in the test section, and to detect the position of the melt jet leading edge during its penetration into the water. In order to improve the response times, thermocouples of 0.5 mm diameter are used. The thermocouples installed at the test section axis are positioned at the same elevations as the K-transducers and designated as TC 1, etc. The contact with the melt leads to immediate destruction of the thermocouples therefore no information of the trailing edge of the melt can be obtained with them. In general, extra sensors such Table 1 Pressure transducer locations KISTLER
Positiona (mm) KELLER
Positiona (mm)
K0 K1 K2 K3 K13 K4 K5
0 150 350 550 550 750 950
140 1450 1780
a
C1/C11 C2/C12 C3/C13
Measured from the bottom plate of the test section.
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as photocells, photoresistors and magnetic field detectors were introduced in the upper part of the test section above the water level in order to detect both the beginning and the duration of the melt release from the crucible without contacting the melt jet. However, these techniques were still at the development stage during this test series and they have failed to provide reliable data so far. Water level measurement is required for the estimation of the integral vapour fraction in the mixing zone. Two different kinds of level meters, an inductive (with a moving float) and TDR-type (time domain reflectometry), are used to measure the level swell during the premixing phase. The tests with the narrow test section utilise one of each of these types in a separate level meter vessel (location illustrated in Fig. 1c). In the tests with the wide test section, only TDR-type of level meters are used, because they are more robust and insensitive to thermo-mechanical loads that might be imposed during a test. It is important to note that the level meters in general do not function well in highly voided regions. However, the fast response time ( \ 1 kHz) of the TDR sensor and its ability to detect gas/water surface intersecting it give confidence in the measurements of the level swell even in the initial phase of mixing where high void fractions are expected. Qualitative check by visual observation of both types of level meters has been made in a transparent test section mock-up. Furthermore, a calibration of the level meters was done prior to each test with water (no void). The KROTOS data acquisition system consists of several transient data recorders. Pressure transducer and level swell probe signals are recorded using a transient recorder (Yokogawa 1600) with high data sampling rates of 50 kHz over a time period of about 5 s. Thermocouple data (8–12 channels) were also recorded at quite high sampling rate of 1 kHz (Yokogawa 1100) in most tests to capture the timing of the melt front progression accurately. A RACAL analog tape recorder and an HP 75 000 data acquisition system are used to record the long term behaviour of the system as well as to provide a data backup system. Data reduction and processing is done
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remotely using a PV-WAVE data analysis package in an HP 712 workstation which is connected to the KROTOS site via a TCP/IP network link.
3. KROTOS test procedure After having reached the desired melt temperature ( 3000 K), the crucible containing the corium melt (about 3 kg, density 7960 kg − 1 m3) is released from the furnace and falls by gravity through a 4 m long release tube of 95 mm inner diameter. Half-way down the tube, a rapid-acting slide valve separates the furnace from the test section below. This valve closes immediately after the crucible has passed. During its fall, the crucible cuts a copper wire generating the zero time signal for the data acquisition. Finally the crucible impacts onto a retainer ring at the end of the tube where a conical shaped metallic puncher breaks the bottom of the crucible thus allowing the melt to pour out (Fig. 1b). The exit of the melt can be delayed by some hundreds of milliseconds by a sacrificial disk of low melting point metal placed beneath the puncher (used only in one test). In this way the melt is slowed down to gravity release conditions. The melt jet diameter is defined by guiding the melt through a funnel of high temperature refractory material with an exit diameter of 30 mm. The melt arrival is detected with a thermocouple (TC 7) just at the exit of the funnel which, in turn, is located about 0.46 m above the water free surface. Another thermocouple (TC 6) is located about 10 mm above the water surface to allow for estimation of the velocity of the jet at the melt-water contact. The melt injection phase has been observed visually in numerous tin and wood metal simulant tests which were performed in a transparent test section mock-up. These tests demonstrated a coherent jet injection and verified the operation of the level meters. After the jet has penetrated sufficiently deep into the water column, either a thermocouple signal or a predetermined time delay can be used to activate the destruction of the membrane of the gas trigger device to generate a pressure pulse that propagates vertically upwards through the melt/water mixture in order to trigger a steam
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Table 2 Test conditions and results with corium melts KROTOS test No. Melt Composition Charged mass Temperature Brake disk Initial jet diameter Free fall in gas Coolant Water mass Height Initial temperature Subcooling Test section Initial pressure (He) Internal diameter Plexiglas liner Gas trigger Results Confirmed penetration Depth of the melt jet Maximum pressurisation Steam explosion Total debrisc Debris in test section
32
33
35
36
37
(mm) (m)
80.8 19.2 3030 3063 No 30 0.46
81.2 18.8 3170 3063 No 30 0.46
79 21 3102 3023 Yesa 30 0.46
79 21 3027 3025 No 30 0.46
79 21 3222 3018 No 30 0.44
(kg) (m) (K) (K)
7.1 1.08 351 22
7.7 1.08 298 75
7.7 1.08 363 10
7.7 1.08 294 79
(MPa) (mm)
0.1 95 Yes No
0.1 95 No No
0.1 95 No Yes
0.1 95 No Yes
0.1 200 No Yes
TC 5
TC 4
n.a.b
TC 5
TC 4
0.23 No 2608 1402
0.14 No 2802 1705
0.17 No 1424 331
0.13 No 2801 1142
0.07 No 2925 2925
UO2 (w/o) ZrO2 (w/o) (g) (K)
(MPa) (g) (g)
34.5 1.105 294 79
Tin/woods metal alloy (Tm413 K). Thermocouple wires destroyed by melt ejection from the test section. c Found in test section, level-meter vessel and pressure vessel. a
b
explosion. After the test, all the debris is collected, photographed and sieved. The debris is inspected before and after the sieving in order to detect any changes in debris form caused by the sieving process. Finally, the internal parts of the test facility are decontaminated before the preparations for the next test can be started.
4. KROTOS experimental results Five experiments with corium have been performed. The objective of this test series was to study premixing of corium melts with water at both low and high subcoolings and to determine if an energetic FCI could take place under such conditions.
The main test parameters and results are summarised in Table 2. The KROTOS test programme has been evolutionary in nature, the outcome of previous results contributing significantly to the planning of the future experiments. To reduce the number of test variables, the following parameters have been fixed in these tests: the initial system pressure (0.1 MPa), release nozzle diameter (30 mm), fall height ( 0.46 m) and the water depth ( 1.1 m). In the next sections a brief description of the tests and results is given. The discussion is subdivided into two classes as ‘saturated water conditions’ and ‘subcooled water conditions’ following the same convention as previously when the results of the alumina/water system were reported (Hohmann et al., 1995).
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4.1. KROTOS experiments in ‘saturated water conditions’ In the KROTOS 32 test, approximately 3 kg of corium was heated up to 3063 K in a tungsten crucible. After the test sequence initiation, the crucible was released and fell down onto the puncher. Since the test was performed without a brake disk, the melt was free to stream out through the punctured crucible bottom and funnel into the water without any time delay. A Plexiglas liner (2 mm thick) was used in the test section to reduce the risk of a spontaneous steam explosion upon the melt contacting the walls of the test section. The thermocouple data gives an estimate of 4.2 m s − 1 for the leading edge velocity using the thermocouples TC 6 and TC 7 as melt arrival indicators. This value is significantly lower than the theoretical value considering a gravity release of the melt from the furnace. Evidently, the puncher and funnel assembly slowed down the release rate. Thermocouples in the water allowed for the estimation of the melt velocity after penetration into the water. The estimated velocity of the melt jet between TC 5 and TC 6 was approximately 1.5 m s − 1. The thermocouple data demonstrates that the coherent jet penetrated at least down to TC 5. No energetic interactions occurred, and the pressurisation of the freeboard volume was mainly due to the steam generation by the melt (see Fig. 2). The different phases of the melt
Fig. 2. Test vessel pressure: subcooled vs. near saturated.
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injection can be distinguished in the pressure history. At about 200 ms after cutting the trigger wire, the crucible impacts the puncher. Immediately afterwards, the start of the melt pour increases the pressure rapidly due to the heating of the cover gas up to about 0.16 MPa (220–300 ms). The pressurisation rate changes when the melt jet penetrates the water column due to energy transfer to the water. Once the melt has penetrated and mixed with the water, the pressurisation rate increases again because of the enhanced heat transfer due to melt breakup. The pressure reached the maximum of 0.33 MPa at about 1.4 s and then it decreased quickly to the quasi steady-state value of about 0.15 MPa (at 60 s) due to condensation onto cool walls. The debris size distribution for KROTOS 32 is plotted in Fig. 5. The mass mean size of the debris is about 2.5 mm. The large particles of the debris consisted mainly of irregular shaped particles with some agglomerates. The smaller particles were more spherical in shape. An estimate of debris quenching rate can be obtained by assuming that the pressurisation of the freeboard volume was caused solely by saturated steam with negligible bulk water heating and that all the melt participated (i.e. entered into the water). This simple analysis gives an estimate of 0.879 10% MW kg-melt − 1 for the maximum melt quenching rate during the mixing phase. The KROTOS 35 test was essentially a repeat of the KROTOS 32 experiment except that the test section did not have a Plexiglas liner and a Woods metal-tin brake disk was installed to reduce the melt release rate. Additionally, a gas trigger device was mounted. The gas trigger was configured to trigger when TC 3 sensed the melt arrival. After the initial melt injection, a rapid pressurisation was observed. This initial rapid pressurisation of the cover gas lasted longer than in KROTOS 32 because of the longer duration of the melt release. The observed peak pressure of 0.27 MPa is lower than in KROTOS 32, because a significant amount of melt (about 1.1 kg) was blocked in the release funnel and, furthermore, vigorous steaming lead to expulsion of a significant fraction of unquenched melt (77% of the
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injected melt mass). Some of the melt swept out from the test section damaged the thermocouple and level-meter cables on the outside of the test section. Thus, the trigger device activated only after the set time delay (2.6 s) rather than with the TC 3 signal. No steam explosion took place. However, the time delay might have been too long for an appropriate triggering i.e. it is believed that a significant fraction of the melt was already quenched.
4.2. Experiments in ‘subcooled water conditions’ The KROTOS 33 test was essentially a repeat of the KROTOS 32 test except that the water subcooling was higher (75 K) and that the Plexiglas liner was removed from the test section. The same method as in KROTOS 32 was used to estimate the leading edge velocity. The TC 6 and TC 7 data indicated a somewhat higher velocity of 8 m s − 1 prior to melt penetration into the water. This is also confirmed by the shorter duration (220–270 ms) of the cover gas pressurisation (approximately 50.12 MPa) shown in Fig. 2. Once in the water, the jet decelerated rapidly from the average velocity of 4.4 m s − 1 between TC 6 and TC 5 to 0.9 m s − 1 between TC 5 and TC 4. Due to the higher subcooling, the pressure of the expansion volume is less than in KROTOS 32 with a maximum of about 0.24 MPa and a quasisteady-state level of about 0.125 MPa at 60 s. The debris size distribution for KROTOS 33 is plotted in Fig. 5. The morphology of the debris was the same as in KROTOS 32. One larger piece of material weighing 88.8 g was recovered from the level meter vessel. The particle sizes are somewhat smaller than in the near saturation conditions (KROTOS 32) with the mass mean size of about 2 mm (excluding the piece mentioned above). Accordingly, the estimate of the melt quenching rate (0.979 10% MW kg-melt − 1) is higher than in KROTOS 32. The KROTOS 36 test was performed with the same conditions as KROTOS 33 except that a trigger device was mounted. The trigger device was set to activate with the TC 3 signal as in the KROTOS 35 test. Moreover, the results from the KROTOS 35 test allowed for a better estimate of
Fig. 3. Test vessel pressure: corium vs. alumina.
the appropriate backup time delay for triggering. Melt injection was successful but again vigorous steaming at the melt-water contact lead to a partial expulsion of the melt from the test section. The gas trigger device was activated by the TC 3 signal at 1.6 s. However, no energetic propagating explosion was observed. Immediate rapid pressurisation and melt sweepout lead to the conclusion that the melt mass participating in the premixing process was limited due to the flow constraint imposed by the narrow test tube. The KROTOS 37 test was a repeat of the KROTOS 36 test with a new, larger diameter test section (200 versus 95 mm). The larger test section was utilised to reduce the superficial steam velocity above the water thus reducing early (fall stage) jet breakup, levitation and sweep-out of the melt as observed in previous tests. In KROTOS 37, the melt jet was successfully injected into the test section with an insignificant amount of sweep-out. The coherent melt jet penetrated at least down to the TC 4 level. The trigger activated with the TC 3 signal at 1.0 s, but no interaction was observed. However, due to the larger test section, the amplitude of the propagating trigger pulse was reduced from the previous tests because the trigger energy (200 J) was kept constant. The maximum pressurisation, shown in Fig. 3, was lower than the previous one (KROTOS 33) because of the greater mass of subcooled water (34.5 kg). A total fragmentation of the injected melt mass was observed. The resulting debris size distribution for KROTOS 37 is plotted in Fig. 5. The
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Table 3 Comparison of selected corium and alumina tests KROTOS test No.
30
33
37
38
Melt Composition Charged mass (g) Temperature (K) Brake disk Initial jet diameter (mm) Free fall in gas (m)
Al2O3 1516 2573 No 30 0.46
UO2 – ZrO2 3170 3063 No 30 0.46
UO2 – ZrO2 3222 3018 No 30 0.44
Al2O3 1533 2665 No 30 0.44
Coolant Water mass (kg) Height (m) Initial temperature (K) Subcooling (K)
7.5 1.08 293 80
7.7 1.08 298 75
34.5 1.105 294 79
34.5 1.105 294 79
Test section Initial pressure (MPa) Internal diameter (mm) Plexiglas liner Gas trigger
0.1 95 Yes No
0.1 95 No No
0.1 200 No Yes
0.1 200 No Yesa
Results Confirmed penetration depth of the melt jet Maximum dynamic pressureb (MPa) Steam explosion Total debris (g) Debris B250 mm (g) Debris B100 mm (g) Explosion efficiency (%)
TC 3 \100 Yes 1400 1210 1000 1.6
TC 4 — No 2802 136 37.4 —
TC 4 — No 2925 116 41.2 —
TC 3 67 Yes 1523 934 545 1.5
a b
Spontaneous explosion took place before triggering. In the test section.
particle sizes are somewhat smaller (1.4 mm) than observed under the same initial conditions but with the narrow test section (KROTOS 33). In the CCM-1 test, which was performed at Argonne National Laboratory (Spencer et al., 1994) with similar initial conditions (P0.1 MPa, DTsub 43 K), but with corium thermite melt, the mass mean particle size was 2.3 mm. The estimate of the maximum melt quenching rate (0.13910% MW kg-melt − 1) is significantly lower than in the tests with the narrow test section indicating more efficient bulk heating of the water which was not considered in the simple analysis and was not measured in the KROTOS 37 test during the mixing phase. By considering only the steam generation in the CCM-1 test, a melt quenching rate of about 0.09 MW kg-melt − 1
is obtained which compares well with the KROTOS 37 data. However, in the CCM-1 test a significant fraction of energy was transferred to the water, so that the total melt quenching rate was about 0.8 MW kg-melt − 1 which is comparable with the estimates in the KROTOS tests with the narrow test section (with less water) and to the results from FARO test analysis (including the bulk water heating). Therefore, it is likely that the bulk water heating in KROTOS 37 was at least as effective as in the CCM-1 test because of the higher initial subcooling and the smaller average particle mass mean diameter. It is interesting to contrast the corium results with the previous ones from the alumina test series where supercritical explosions were observed (Hohmann et al., 1995). Such a compari-
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son is shown in Table 3 where the initial conditions and some results of the alumina and corium tests are tabulated. In order to make consistent comparisons, it was imperative to repeat an alumina test with the larger diameter test section to see if lessening the constraint of the narrow test tube would indeed affect the outcome (normally an energetic interaction). Such a test (KROTOS 38) was performed with 1.5 kg of alumina (at 2665 K), see Table 3. A spontaneous energetic explosion took place before the trigger system was activated. Peak dynamic pressures up to 67 MPa were observed in the test section. The pressurisation of the freeboard volume for this test is shown together with the KROTOS 37 result in Fig. 3. A different type of premixing behaviour with corium and alumina is evident by comparing the initial steaming rates. Significantly greater steam generation with the corium melt is further illustrated by the level swell data, shown in Fig. 4. This preliminary data would therefore suggest that less breakup of the melt occurred and that the void fraction in the mixing region was smaller prior to triggering in the case of alumina. However, due to the steam explosion the debris is significantly finer than in the corium tests, see Fig. 5. Fig. 5 also indicates that about 65% of the interacting alumina melt mass fragmented in particles of less than 250 mm in diameter. In this investigation it was assumed that these particles participated in the explosion process. The efficiency of this explosion process is then the ratio of the kinetic energy to the thermal energy content of the melt which
Fig. 5. Particle size distribution in selected tests.
participated in the process. The kinetic energy of the mixture mass was determined from the dynamic pressurisation of the test section which allowed the estimation of the impulse ( pA dt) imparted to the water column. Using this approach and the experimental data from KROTOS 38, an explosion efficiency of 1.5% was calculated. These observations have important implications concerning tests with alumina simulants performed at various research centres. The differences between corium and alumina melts, in this respect, should be well understood. Currently, experimental investigations are pursued to study the differences between the prototypic corium melts and alumina simulant melts in parallel with general objectives of the KROTOS programme.
5. Conclusions
Fig. 4. Level swell in test section: corium vs. alumina.
Concerning KROTOS tests, it is important to note that the following conclusions are based on preliminary trends observed with only a few tests and need to be confirmed with further tests. The experimental results from the KROTOS corium programme so far indicate: Significant breakup of the melt into relatively fine debris ( 1.4–2.5 mm), No energetic interactions within the range of the following investigated parameters: low subcooling (1020 K), high subcooling (80 K) and external trigger (energy of 200 J),
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Due to lack of energetic interactions so far with corium melts, the data on effects of geometric constraints on explosions and the far field effects are limited to results from alumina tests, Simple analysis using steaming rates indicate melt quenching rates of 0.8 – 1.0 MW kg-melt − 1 in the narrow test section. The observed melt quenching rates in the larger scale FARO facility are comparable with the rates observed in the KROTOS tests. Significant differences between the behaviour of prototypic corium and simulant alumina melts have been observed and more data are needed to understand them.
Acknowledgements The authors greatly acknowledge the work and efforts of the whole KROTOS team. Performed in collaboration with the USNRC in the frame of Technical Exchange Agreement No. 4086-90-09 TG ISP USA. References Angelini, S., Yuen, W.W., Theofanous, T.G., 1995. Premixingrelated behaviour of steam explosions. Nucl. Eng. Des. 155, 115 – 157.
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