Speaker
Description
IAEA ATF-TS Benchmark FOR simulation of bundle tests
J. STUCKERT1, Z. HÓZER2, A. KHAPERSKAIA3
1Karlsruhe Institute of Technology, Karlsruhe, Germany
2HUN-REN Centre for Energy Research (HUN-REN EK), Budapest, Hungary
3Nuclear Energy Department, IAEA, Vienna International Centre, Vienna, Austria
Corresponding author: J. STUCKERT, juri.stuckert@kit.edu
INTRODUCTION: As part of the IAEA ATF-TS project, not only numerous single rod tests were carried out with ATF materials, but also two bundle tests with Cr coated claddings made of Zr alloys: the DEGREE-B3 bundle test at CRIEPI [1] and CODEX-ATF test at HUN-REN EK [2]. The advantage of bundle tests lies in the creation of prototypical adiabatic conditions and the possibility of studying the mutual influence of fuel rods. In addition, such integral tests are a good basis for verification and validation of computer codes. Therefore, it was decided to conduct a benchmark within the framework of this IAEA project using experimental data obtained both during tests and in post-test studies. In addition to the two bundle tests using chromium-coated zirconium claddings, it was proposed to also use the results of the QUENCH-19 bundle test with FeCrAl claddings previously conducted at FZK. Conducting benchmark for the QUENCH-19 test was initiated within the framework of the previous IAEA ACTOF project [3], but then only two research organizations managed to take part in this project. Now the range of organizations involved has been significantly expanded.
BENCHMARK ON THE QUENCH-19 BUNDLE TEST PERFORMED WITH FECRAL CLADDINGS
The QUENCH-19 bundle experiment with 24 B136Y cladding tubes and 4 Kanthal AF spacer grids as well as 7 KANTHAL APM corner rods and KANTHAL APM shroud was conducted at KIT on 29th August 2018 [4]. This was performed in cooperation with the Oakridge National Laboratory (ORNL). The test objective was the comparison of FeCrAl(Y) and ZIRLO claddings under similar electrical power and gas flow conditions. The experiment was performed in four stages. The electrical power supply was the same as in the reference test QUENCH-15 (ZIRLO) during the first two stages (pre-oxidation and transient). The third stage with constant electrical power was performed to extend the temperature increase period. The test was terminated at peak cladding temperature of about 1460 °C by water flooding similar to QUENCH-15. The total hydrogen production was 9.2 g (47.6 g for QUENCH-15).
Seven organizations provided results for exercises on the modelling of the QUENCH-19 bundle test (Table 1).
TABLE 1. ORGANIZATIONS AND CODES PARTICIPATED IN THE QUENCH-19 BENCHMARK
Participant CNEA
Argentina CTU
Czech Republic GRS
Germany IBRAE
Russia KIT/INR Germany NINE
Italy UPM/NFQ
Spain
Code DIONISIO MELCOR ATHLET-CD SOCRAT ASTEC MELCOR MELCOR
For almost all codes, the rod bundle was described by three concentric rings as shown in Fig. 1: an inner ring (ROD1) containing four central rods, a second ring containing eight intermediate rods (ROD2), and a third ring containing twelve peripheral rods (ROD3). When modeling with the MELCOR code, NINE and CTU applied a division into two groups of fuel rods: internal and external rods. Only one central rod was modelled with the DIONISIO code. The corner zirconium rods used for the bundle instrumentation were taken into account by their effect on reducing the flow area of the assembly. In addition, their outer surface area was taken into account when calculating the hydrogen release due to their oxidation. Also, when calculating the hydrogen release, the influence of the inner surface of the shroud was taken into account.
FIG. 1. Composition of the QUENCH-19 bundle
According to the benchmark conditions, each code had to calculate - based on specified boundary conditions and experimental data on the temporary change in electrical power supplied to the bundle - the temperature history at each of the seventeen elevations of the assembly. In addition, the most important parameter for comparing the efficiency of codes should have been the calculated value of hydrogen release.
QUENCH-19: Comparison of temperature predictions
Based on the readings of the thermocouples of the central and intermediate rods, an axial distribution of temperatures in the inner bundle part was obtained 300 s before the start of the reflood, namely at the time of 8800 s (in a later period, a number of thermocouples failed). Comparison of these experimental data with the results of calculations shows a good prediction of the position of the maximum temperature at the bundle elevation of 850 mm by most codes (Fig. 2). Below this level, the data from the four codes practically coincide with the experimental data. Above 850 mm, the data of the two codes coincide with the measured values. The other two codes give overpredicted temperature values.
Comparison of calculated temperatures with experimental ones at the bundle elevation of 950 mm throughout the experiment shows overestimated values for all codes - satisfactory for the first (Fig. 3) and second (Fig.4) groups of rods and significantly overestimated for the shroud (Fig. 5). The latter circumstance may be due to insufficient consideration of the steam-water mixture entering through leaks into the space between the shroud and the cooling jacket surrounding it [4].
FIG 2. Axial temperature profiles for QUENCH 19 FIG. 3. Temperature progress for internal rods of the QUENCH-19 bundle
FIG. 4. Temperature progress for external rods FIG. 5. Temperature progress for shroud
QUENCH-19: Comparison of hydrogen predictions
When metal M is oxidized in steam, hydrogen is released, the release rate of which is determined by the degree of oxidation:
xM + yH2O = MxOy + yH2 (1)
The enhanced oxidation resistance of FeCrAl alloys at high temperatures relies on the formation of a slowly growing and highly protective Al2O3 scale [5]. The formation of a protective alumina scale is determined by the competition between the oxidation rate governed by diffusion of O and Al through the oxide layer and the diffusion of aluminium in the substrate to the interface. Alumina performs its protective role at temperatures below approximately 1650 K. At higher temperatures, accelerated diffusion processes lead to increased Fe oxidation, leading to a catastrophic increase in the oxidation rate. Based on the results of oxidation experiments performed at MIT with the B136Y3 samples (FeCrAl alloy used for the QUENCH-19 claddings) [6], the following correlations for the parabolic rate constant of the sample mass gain have been proposed to use for all codes:
K_MIT [g^2/(〖cm〗^4 s)]={█(9.62×10^(-12), &T≤1473 K@A_B exp((-E_B)/RT), &1473
