Preliminary interpretative analysis of the QUENCH-18 bundle test using SCDAPSim/mod3.5

A preliminary analysis of the bundle reflood experiment QUENCH-18 is performed with the SCDAPSim/Mod3.5/da code containing the PSI-developed model for oxidation in the presence of air. The simulation follows on from pre-test planning and prediction calculations using the same code and input model. The starting point for the post-test calculations differs from the pre-test only in respect of using the actual boundary conditions. Comparison with measured data enables several aspects of the experiment to be studied. Various treatments of steam and air oxidation kinetics investigate the effect of nitrogen on the oxidation and its continuing influence when air is no longer present. Concerning degradation, different assumptions on failure of the oxide crust indicate how the exposure of relocated metallic melt can enhance the oxidation excursion during reflood. Some modelling and knowledge limitations are identified, particularly regarding oxidation in steam-air mixtures, the roles of nitrogen and zirconium nitride as chemically active species. Several observed features of the facility operation remain unresolved. Simulations suggest that damage to the shroud affected the reflood progression. The bundle may also have been in a highly damaged state, with further impacts on reflooding. Interpretation is therefore provisional, pending more information on the bundle final state. However, the simulation results have significant implications for reactor calculations

Improvement of the high-temperature oxidation resistance of Zr alloy cladding by surface modification with aluminium-containing ternary carbide coatings

Alumina-forming MAX phase ternary carbides are being considered as protective coatings on zirconium alloys as occident tolerant fuel (ATF) cladding because of their resistivity against high-temperature steam oxidation during occident scenarios. This study attempted to synthesize three types of Al-containing MAX phase carbides (Ti2AlC, Cr2AlC and Zr2AlC) as coatings on Zircaloy-4 substrates via deposition of elemental nanoscale multilayer thin films using magnetron sputtering, and subsequent thermal annealing in argon. Formation of Ti2AlC and Cr2AlC MAX hases was confirmed after annealing at 800°C and 550°C, respectively, while growth of Zr(Al)C carbide rather than Zr2AlC AX phase was observed in the Zr-C-Al system. Oxidation of the three coated samples at 1000°C in steam for 1 hour evealed no protective effect of the Ti2AlC and Zr(Al)C coatings with significant spallation and cracking. The Cr2AlC oatings possess superior oxidation resistance and self-healing capability with a thin and dense α-Al2O3 layer growth on the surface, which shows good promise as a candidate for coated ATF claddings

Update of the QUENCH program

The main objective of the QUENCH program at KIT is the investigation of the hydrogen source term and materials interactions during LOCA and the early phase of severe accidents including reflood. Bundle experiments as well as separate-effects tests are conducted to provide data for the development of models and the validation of severe fuel damage code systems. The QUENCH bundle facility is a unique out-of-pile bundle facility with electrically heated fuel rod simulators and extensive instrumentation. So far, 19 experiments with various severe accident (SA) scenarios as well as a series of seven DBA LOCA experiments were conducted. The QUENCH-LOCA series was completed in 2016. One of the main results is the definition of the conditions for secondary hydriding around the burst position and its influence on the mechanical properties of the cladding rods. The last SA test QUENCH-19 was the worldwide first large-scale bundle test with ATF cladding. It was conducted with FeCrAl cladding in cooperation with ORNL, USA. First very promising results were presented by Stuckert. Separate-effects tests during 2018 were focused on the high-temperature behavior of various ATF cladding candidates. Many of these experiments were done in the framework of the EC ILTROVATORE and IAEA ACTOF programs. QUENCH bundle tests are part of the validation matrices of most SFD code systems which was also reflected during the session “Modelling and code validation”. Finally, the status of reporting and publishing as well as the numerous national and international cooperations were briefly described and acknowledged

Neutron Imaging Investigations of the Secondary Hydriding of Nuclear Fuel Cladding Alloys during Loss of Coolant Accidents

The hydrogen concentration and distribution at both sides of the burst opening of cladding tubes used in three QUENCH LOCA simulation bundle experiments were investigated by means of neutron radiography and tomography. The quantitative correlation between the total macroscopic neutron cross section and the atomic number density ratio between hydrogen and zirconium was determined by testing calibration specimens with known hydrogen concentrations. Hydrogen enrichments located at the end of the ballooning zone of the tested tubes were detected in the inner rods of the test bundles. Nearly all of the peripheral claddings exposed to lower temperatures do not show such enrichments. This implies that under the conditions investigated a threshold temperature exists below which no hydrogen enrichments can be formed. In order to understand the hydrogen distribution a model was developed describing the processes occurring during loss of coolant accidents after rod burst. The general shape of the hydrogen distributions with a peak each side of the ballooning region is well predicted by this model whereas the absolute concentrations are underestimated compared to the results of the neutron tomography investigations. The model was also used to discuss the influence of the alloy composition on the secondary hydrogenation. Whereas the relations for the maximal hydrogen concentrations agree well for one and the same alloy, the agreement for tests with different alloys is less satisfying, showing that material parameters such as oxidation kinetics, phase transition temperature for the zirconium oxide, and yield strength and ductility at high temperature have to be taken into account to reproduce the results of neutron imaging investigations correctl

Measurements of branching fraction ratios and CP-asymmetries in suppressed B^- -> D(-> K^+ pi^-)K^- and B^- -> D(-> K^+ pi^-)pi^- decays

We report the first reconstruction in hadron collisions of the suppressed decays B^- -> D(-> K^+ pi^-)K^- and B^- -> D(-> K^+ pi^-)pi^-, sensitive to the CKM phase gamma, using data from 7 fb^-1 of integrated luminosity collected by the CDF II detector at the Tevatron collider. We reconstruct a signal for the B^- -> D(-> K^+ pi^-)K^- suppressed mode with a significance of 3.2 standard deviations, and measure the ratios of the suppressed to favored branching fractions R(K) = [22.0 \pm 8.6(stat)\pm 2.6(syst)]\times 10^-3, R^+(K) = [42.6\pm 13.7(stat)\pm 2.8(syst)]\times 10^-3, R^-(K)= [3.8\pm 10.3(stat)\pm 2.7(syst]\times 10^-3, as well as the direct CP-violating asymmetry A(K) = -0.82\pm 0.44(stat)\pm 0.09(syst) of this mode. Corresponding quantities for B^- -> D(-> K^+ pi^-)pi^- decay are also reported.Comment: 8 pages, 1 figure, accepted by Phys.Rev.D Rapid Communications for Publicatio