Investigations on the Phenomenology of Ex - Vessel Core Melt Behaviour (COMAS) W. Steinwarz 1), A. Alemberti 2), W. Häfner 3) Z. Alkan 4), M. Fischer 5) 1) Siempelkamp, Krefeld 2) Ansaldo, Genua 3) Battelle, Eschborn 4) Aachen University 5) Siemens Erlangen SUMMARY Within the COMAS (Corium on Material Surfaces) project representative experimental and analytical investigations on the ex - vessel spreading behaviour of prototypic corium were performed to provide a realistic technical basis for the development of core catchers for future nuclear reactor generations. In various large - scale experiments melts in the Mg - range with real corium compositions and temperatures around 2000 C were tested using different substratum materials (concrete, ceramics, cast iron). These tests were carried out at Siempelkamp's CARLA melting facility for radioactively contaminated metal scrap supported by laboratory experiments at Siemens / Erlangen. The COMAS results indicate that for a sufficiently high melt release rate a quick and homogeneous coverage of an ex - vessel spreading compartment as realized in the Franco - German EPR project can be expected. Regarding the analytical assessment it can be concluded that some spreading codes have reached a sufficient level to provide guidelines for the evaluation of spreading concepts. Open questions are mainly related to segregation and immobilization of the melt. A. INTRODUCTION Additional safety margins are required for the development of the next generation of nuclear power plants. These include the demands to ensure the integrity of the containment -even in the case of a core melt accident - in order to render emergency measures outside the reactor plant unnecessary. One of the optional concepts, designed for the Franco - German EPR (European Pressurized Water Reactor) project, is represented by the implementation of a dedicated large ex - vessel melt spreading compartment to stabilize the core melt (corium) after failure of the reactor pressure vessel [1]. To extend the existing knowledge on corium - material interactions and especially to deepen the understanding of the complex ex - vessel spreading process, a number of generic spreading experiments have been performed in Europe over the last decade. Among them, the COMAS experiments are unique due to realization of large masses, prototypic corium compositions, realistic temperature levels and a broad spectrum of substratum material characteristics.
- 2 - In a total of 9 one- and two - dimensional large - scale tests with superheated metallic melts, corium mixtures and pure oxidic (corium) melts, detailed phenomenological information, but also quantitative data should be provided and involved into further steps to validate relevant analytical codes [2], [3], [4]. The work was embedded as a cost - shared action into the European Community's 4th R & D Framework Programme under the contract no. F145 - CT95-003 with a duration from 1st December 1995 to 31st May 1999. B. WORK PROGRAMME The work programme was subdivided into five main work packages: Small - scale corium melting tests (task 1 / Siemens) were carried out in a laboratory induction furnace in order to gain experience on phenomena that may become crucial for the following large - scale experiments (i. e. resistance of the crucible in agressive melts) as well as to clarify boundary conditions and to determine material characteristics. The large - scale experiments (task 2 / Siempelkamp) were carried out in Siempelkamp's CARLA melting plant licensed for handling of radioactively contaminated materials. The main elements are a 3 Mg - MF - induction furnace (Fig. 1) and a redundant filter system according to nuclear standards. A spreading facility was developed and constructed for experiments with up to 3 Mg of prototypic corium consisting of a feed box equipped with a combined plug system and the spreading compartment using a 3 - channel arrangement with an overall length up to 8 m or a 2 D - spreading area representing a 1 : 6 scaling with respect to the EPR. To adapt the furnace to the high - temperature regime, a specific liner material on ZrO 2 - basis was developed similar to that foreseen for the EPR core catcher. For the measurement programme (task 3 / Battelle) a broad instrumentation equipment was installed consisting of various types of thermocouples, load cells, infrared cameras and video systems to monitor all parameters relevant for the overall process. To protect the standard thermocouples against the high - temperature melt attack and thus to prolongate their lifetime, a special covering based on a thick pulp layer was developed. By technical modifications of the high - qualified infrared cameras the measuring range could be extended, by a specially designed image analysis software complex evaluations and applications of the database could be realized. In order to determine the metallurgical structure and composition of all relevant materials the test specimens had to be analysed by microrange technique, like light microscopy, SEM and EDX (task 4 / Aachen University). For the pre- and post -calculations of the spreading tests numerical codes, such as CORFLOW and MELTSPREAD - 1 were applied (task 5 / Siemens, Ansaldo). By iterative modifications a stepwise improvement of the code modelling could be achieved initiating a final benchmark procedure as contribution to the code validation.
- 3 - C. MAIN RESULTS OBTAINED Reference corium melt For the COMAS test series two compositions, representative of EPR conditions, have been selected to reflect the main characteristics of the chosen ex - vessel mitigation concept (Fig. 2). Both fully oxidized corium compositions, called type R and Type R', were based on a ratio of oxidic to metallic melt of 60 : 40 weight percent. According to the present EPR design status, which includes temporary melt retention in the reactor pit and interactions with refractory and sacrificial materials, 10 wt.-% of SiO 2 was added to simulate the expected admixture of concrete to the melt leading from type R to type R' [5]. Laboratory investigation and calculations resulted in liquidus temperatures for both melts around 1900 C and a difference to the solid temperature of more than 600 K. What is remarkable with view to the spreading process are small density differences between the oxidic and metallic fractions. Experimental results In a total of 9 tests various mixed melts - as well as metallic and oxidic fractions separated from the reference melts were investigated since 1995. The most important new information came from the tests COMAS EU-1, COMAS EU-2b, COMAS EU-3a and COMAS EU-4 which will now be described in more detail: The test COMAS EU-1 showed the influence of the oxide content on the spreading length and gave further indications of the effect of crust formation on the immobilization of the melt. Approx. 1 Mg of corium R melt was spread with roughly 60 % oxidic fraction on the concrete channel and 70 % respectively 80 % on the cast iron and the ceramic channel. As the ceramic substratum saw the highest oxide content the melt front stopped here after the shortest distance of nearly 3 m. The initial melt temperature for the spreading process was monitored to 1750 C. This exhibits that good spreadability is given for such melts even with temperatures 200 K below liquidus. The maximum velocity was measured to 1.5 m/s. In the test COMAS EU-2b approx. 630 kg of pure oxidic melt separated from corium R' melt was spread over the 3-channel-course with an initial temperature of 2070'C (Fig. 3). The front temperature amounted to 1.2 m/s. The melt stopped at roughly 5 m, nearly equal for all substrata as expected. Fig. 4 shows the propagation and the shape of the melt front during the spreading process. Remarkable was the measured fast decrease of the melt surface temperature during the spreading process monitored by the IR camera system indicating the early formation of a crust.
- 4 - Cast iron was chosen as substratum material for the 2-dimensional spreading test COMAS EU-4 (Fig. 5). Approx. 2 Mg of mixed corium R melt was spread with an initial temperature of roughly 2,000 C. The maximum velocity reached nearly 2m/s and maintained an average value of 0.7 m/s before solidification. A spreading length of about 8.5 m along the direct line was reached (Fig. 6). The melt was homogeneously distributed over the spreading area. Using the IR camera technique it could be indicated that segregation of the oxidic and metallic fraction of the corium R mixture happened roughly at the half of the spreading period leading to a layering of the oxidic part on top due to its lower density. The samples taken from the oxidic layer consisted of nearly stoichiometric mixed oxides (> 97 %) (Fig.7). This experiment can be regarded as a demonstration test for the feasibility of the EPR spreading concept, especially in view of the large scale of approx. 1:6 (Fig. 8). Crust formation could be visualized by measuring the temperature difference between the melt surface and the melt interior, immediately after immobilization of the melt. In the case of the COMAS EU-3a test special dipping devices opened the crust. By means of the IR cameras temperature differences of up to 260 K were detected. The thermocouples, attached to the dipping devices, reached deep into the melt interior where they found an even higher overall difference of approx. 400 K. High ductility of the crust became evident in this test when the front area of the immobilized, initially-spread melt creased - due to pressure exerted by the following batches - but continued to confine the inner melt. Analytical efforts As full-scale, real material experiments are hardly feasible, the development of computational codes for the analytical simulation of a broader spectrum of spreading scenarios is absolutely essential. These codes have to be validated by means of representative experiments. Therefore, supporting the code development was one of the most important aspects of the COMAS project, leading to detailed benchmarking work. By doing this, various modelling approaches with different characteristics, such as Newtonian or Bingham flow behaviour, inclusion of crust formation and multi-component melts etc. were implemented into the codes and tested. All codes evidently had certain deficiencies when it comes to modelling the final immobilization phase of the complex spreading process. As one of the most effective post-calculations an analytical result of the 2Dexperiment COMAS EU-4 using the CORFLOW code should be shown
- 5 - examplary. The good fitting of the analytical and experimental result is evident (Fig. 9). Overall prosect outcome In summarizing the main results of the overall COMAS project, the following main conclusions can be drawn: Spreading length was found to be virtually independent of the substratum, but did depend on mass flow rate, melt temperature and, for mixed melts, on metal/oxide ratio. Even for thin melt layers of only a few centimetres good melt distribution over the spreading area was observed. Spreading behaviour is dominated first by inertia and later by viscous forces. Good spreadability was observed even for oxidic melts with temperatures significantly below liquidus. Prior to immobilization, mixed melts separate into horizontal metallicloxidic layers even at small metalloxide density differences. Front immobilization is likely caused by crust formation. Measured heat losses to the upper and lower surfaces of the spread melt were in the same order of magnitude shortly after immobilization of the melt. For metal spread on cast iron the formation of an insulating gap was observed. Analytical spreading codes have reached a sufficient quality level to provide the main design requirements for corium retention systems based on the spreading concept. The questions still open relate to segregation and immobilization of the melt. D. INTERACTIONS WITH OTHER RESEARCH OR INDUSTRIAL ACTIVITIES The COMAS project was embedded within the in-vessel (INV)/ex-vessel (EXV) cluster of the 4 th R & D Framework Programme leading to a wide exchange of experiences between the various involved research teams on this overall subject. Via co-sponsoring the project was also part of the national German Reactor Safety Research Programme guided by GRS as programme manager. Last but not least, the COMAS results represented valuable input to the Franco- German EPR project with special view to the plant safety design [6]. E. CONCLUSIONS AND FUTURE PROSPECTS The EU-COMAS project could be successfully performed and provided a lot of new interesting scientific results, especially with view to the core melt mitigation concept. Furthermore, in connection with the development of the experimental facility for these research projects new techniques and optimized procedures for the melt generation process, temperature measurement and materials for hightemperature use were gained, which can find their application in other technical areas
- 6 - The COMAS project has helped to identify the key phenomena which control corium spreading and has provided an important basis for the validation of theoretical models which are used to simulate melt spreading in real reactor geometries. To improve understanding of complex spreading scenarios, further experimental research work should be concentrated on a few process-dominating phenomena such as segregation and crust formation. The results of this work should also help to finalize the validation activities for the relevant codes. In addition, large-scale demonstration tests - under representative conditions - should provide a significant contribution to final proof of a convincing corium mitigation concept. REFERENCES [1] Krugmann, Azarian Requirements and technical concept for the EPR Proc. Topical Session of the Annual Meeting on Nuclear Technology '99 Karlsruhe, May20, 1999, p. 5-20 [2] Steinwarz, Häfner, Alkan, Fischer Large-scale experiments using prototypic corium with view to mitigation of core melt accidents Proc. Topical Session of the Annual Meeting on Nuclear Technology '99 Karlsruhe, May20, 1999, p. 35-51 [3] Steinwarz, Dyllong, Koller COMAS spreading experiments with prototypic oxidic corium melts for optimization of core catcher designs Proc. ICONE-7, Tokio, April 19-23,1999, CD-ROM No. 7196 [4] Steinwarz, Alemberti, Häfner, Hurtado, Fischer Investigations on the phenomenology of ex-vessel core melt behaviour Proc. FISA-97, EUR 18258 EN, 167 (1997) [5] Helimann, Lansmann, Funke, Fischer Diverse internal reports by Siemens (1998) [6] Courtaud, Heusener, Peltier, Steinwarz Research and development related to the EPR Proc. KTG/SFEN Conference EPR, p. 107-130 Cologne, October 19-21, 1997
Figure 1: Charging of the induction furnace with oxidic corium material Figure 3: Spreading of the oxidic fraction of corium R' Figure 2: High-temperature compositions of the prototypic reference corium R and R' (after Fe 2 O 3 has been reduced to FeO by Fe) Figure 4: COMAS EU-2b: Melt from propagation
Figure 5: Spreading of corium R in the COMAS EU-4 test Figure 6: COMAS EU-4: Time dependent location of the melt front Figure 7: SEM/EDX analysis of an oxidic COMAS EU-4 specimen
Figure 8: Comparison of EPR spreading compartment and test COMAS EU-4, to scale Figure 9: Comparison of the final free surface location (left: calculated, right: experimental result) For the COMAS EU-4 test