atw - International Journal for Nuclear Power | 10.2020

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atw Vol. 65 (2020) | Issue 10 ı October RESEARCH AND INNOVATION 510 Test Melt Power [kW] Remarks V1.5 V1.6 V1.7 V1.8 V1.9 V2.1 V2.2 V2.3 V5.1 V5.2 V5.3 300 kg Steel + 50 kg Oxide 300 kg Steel + 50 kg Oxide 300 kg Steel + 80 kg Oxide 350 kg Steel + 130 kg Oxide 350 kg Steel + 130 kg Oxide 300 kg Steel + 130 kg Oxide 300 kg Steel + 100 kg Oxide 300 kg Steel + 100 kg Oxide | Tab. 1. BETA Experiments [ALS95]. 300 kg Steel + 50 kg Oxide + 80 kg Zr 450 1000 1700 1900 400 - 200 120 - 150 400 - 1000 600 - 200 400 200 800 release and the heat transfer at the bottom are not influenced by the vertical walls [ALS86]. With a height of 175 cm of the cavern a maximum axial erosion of 105 cm and a maximum radial erosion of 35 cm are possible. In the BETA test series, the melt, which was generated by a thermite reaction, typically consists of 300 kg of a metal phase and 150 kg of an oxide phase [CRA13]. The metallic composition of iron, nickel and chromium is typical for a reactor case. Zirconium is added in some experiments in order to be able to assess its influence in a hypothetical severe accident. The oxide melt is made of aluminium oxide to which, depending on the purpose of the experiment, calcium oxide and/or silicon oxide is added. This allows the melt to be simulated in a very realistic way, both from the chemical and the physical point of view. [ALS86] The melt is then poured into the concrete crucible. The temperature of the melt at the beginning of interaction ranges from 2,000 K to 2,200 K. Using an induction coil enclosing the concrete, the melt is heated electrically with a net heating capacity of up to 2,500 kW. As a result, an extremely high heat flow from the melt to the concrete is possible. Due to the heating method, heating is only possible in the metallic melt, which dominates the MCCI process. The densities of the used oxide and metal melt correspond to a typical ratio for a very long MCCI phase in a reactor case after a few days. The different MCCI phases are reproduced in BETA by a series of experiments at various power and temperature levels. In addition, depending on the purpose of the experiment, the initial composition of the melt and the type of concrete are varied. The V1 series studied the MCCI of siliceous concrete and very high Siliceous concrete Siliceous concrete Siliceous concrete Siliceous concrete Siliceous concrete, CaO added Siliceous concrete Siliceous concrete, CaO added Siliceous concrete, CaO added Siliceous concrete, Zr added Siliceous concrete, Zr and Fission Product Mock-ups added heating power, while the V2 series used lower heating power. In the V3 series, limestone and LCS concrete was used. The V4.1 test studied the effect of a larger crucible and zirco nium addition. In V5 series, zirconium and fission product mockups were added to the melt, and in V6.1 there was a water pool behind the concrete sidewall. Further and more detailed information about the experiments considered here can be found in Table 1. The measurement of the progression of the melt, as well as the temperature of the melt, is possible via an instrumentation in the crucible with 110 equally distributed thermocouples. Additional temperature measuring lances, introduced from above into the melt, measure the temperatures of the metal melt and the oxide melt in dependence of time. Filter samples are used to analyse all gases evolved by the melt for their composition and containing aerosols. [ALS86] Modelling approach with COCOSYS V3.0 For the modelling of the BETA experiments, the concrete cavern must first be defined. Its geometrical data can be found in the test descriptions. For the respective structures, in addition to the type of structure, the heat conduction model and heat transfer model for the left and right areas of the zones must be selected. This part of the modelling has been taken from investigations already performed. [AGE18] The implementation of the MCCI interaction is done with some margin over some parameters. In addition to the failure time of the RPV, the required material data from the material database, the solidus and liquidus temperature of the melt and the geometry information of the cavity, the composition of the concrete must be specified. Changes in the composition of the concrete always involve a change in density, the enthalpy of decomposition and the decomposition temperature of the concrete. These values, especially the decomposition enthalpy, can often only be determined experimentally [PEE83]. Therefore, the decomposition enthalpy is determined using comparable concrete compositions [ALS92]. However, not all components of the concrete can be considered because some material data is missing in the material data bank (MDB). Accordingly, the simulated components are extrapolated to 100 %. The influence of the missing components can be neglected because of the very low mass fraction. After that, the melt is to be defined and since it is stratified, metal and oxide must be defined separately. Starting from the initial temperature, which is identical in both phases, the respective constituents need to be specified. In addition, in the metallic layer, the introduced heating power of the inductive heating method is dependent on the time, over which the decay heat is simulated. In the experiments, the planned inductive heating was never continuously introduced into the melt, since in the experiments dispersion of oxide and melt was observed. Due to the dispersion, the power could not be transmitted constantly. The individual metallic phases became too small and this influenced the effectiveness of the coil. Therefore, the energy in the melt measured during the experiment is entered into the melt. It was not always possible to keep the heating period of all experiments identical. For example, very high heat outputs could only be maintained for a short period of 500 s (V1.5), whereas, in experiments with low heat outputs (V2.1), heating could be maintained for more than 6,000 s. What follows is that a comparison of the experiments with each other is possible only in the phases continuous heating. Another important factor in the modelling of both layers is the effective heat transfer coefficient (HTC), which is given in the simulations via HEFF and has a great influence on the concrete erosion [SPE18]. The factor determines, in addition to the decomposition temperature, the total heat transfer of the melt to the concrete. The decomposition temperature determines the interface temperature Research and Innovation Simulation of Selected BETA Tests with the Severe Accident Analysis Code COCOSYS ı Maximilian Hoffmann and Marco K. Koch
atw Vol. 65 (2020) | Issue 10 ı October between melt and concrete. In this investigation a decomposition temperature of 1523 K is assumed, whereby ongoing investigations suggest a value of approx. 1800 K [FOI19]. The HTC must be defined for each layer upwards, to the side, and downwards. Also, between the two layers, a heat transfer must be determined. In metallic layers, HTC is always higher than in oxide layers. This can be explained by the higher conductivity of the metal compared to oxides. The first approach is that the coefficient determines the total heat, without individual effects, such as the crusting of the melt or the mixing of the melt by the released gas. A different approach would be to calculate these effects separately. The investigations here only relate to the experiments with silicate concrete, which are the experiments of the series V1.x, V2.x and V5.x. The values recommended by GRS for HEFF are shown in Figure 2. The first simulations are performed with these values for HEFF. In a second set of simulations, HEFF is adjusted separately for each experiment using a parameter variation and adjusted to the measured erosion. The parameters that best represent the measured erosion are shown in Figure 5 and will be evaluated and the end of this work. The results of both simulations are presented and compared in this work. Also, a possible relationship between the heat output and the effective heat coefficient is shown. | Fig. 2. Recommended values of the effective heat transfer, the decomposition enthalpy and the decomposition temperature for siliceous concrete. [SPE12] [ALS92] [ALS95] RESEARCH AND INNOVATION 511 Simulation results Before the results adapted to the erosion are shown, it is first of all advisable to present the simulated erosion of the initial simulations. This provides a better overview over the improvements. Figure 3 and Figure 4 show the axial erosions of the experiments and their simulations with silicate concrete, without additives in the melt. As already described above, the experiments can only be compared with each other during periods of simultaneous heating. Therefore, only the first 1,000 s of the experiments are shown in Figure 3 and Figure 4. The figures show the effective measured heating capacities, as the planned heating capacities could not always be achieved or maintained. Starting with V2.1 with an effective heating capacity of approx. 140 kW, the level is rising up to V1.8 with a heating capacity of 1,900 kW. | Fig. 3. Simulation results with recommended values for HEFF of the axial erosion of siliceous concrete without addition into the melt. | Fig. 4. Simulation results with adjusted values for HEFF of the axial erosion of siliceous concrete without addition into the melt. Research and Innovation Simulation of Selected BETA Tests with the Severe Accident Analysis Code COCOSYS ı Maximilian Hoffmann and Marco K. Koch
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