Results of the QUENCH-DEBRIS (QUENCH-17) test with strongly oxidized Zircaloy-4 and Hafnium claddings filled with segmented pellet simulators

In the framework of the SARNET-2 European program the QUENCH-DEBRIS test was conducted as the 17th severe accident test using the QUENCH facility at KIT to investigate the formation and coolability of a prototypic debris bed. The test scenario was defined by pre-test calculations using the MELCOR code. The test bundle with length of about 2 m contains the hafnium shroud tube and 12 heated peripheral rods with claddings made of hafnium. Hafnium was chosen because of its much higher melting temperature and the about one order of magnitude lower oxidation rate compared to zirconium. The claddings of the inner nine rods were made from Zry-4. They were filled with pre-fragmented zirconia pellets. The test started with an oxidation phase of 20 h at peek cladding temperature about 1800 K to reach complete oxidation of the Zry-4 claddings over a height of 500 mm. The temperatures expected from the pre-test simulation fitted well the experimentally observed evolution of the bundle conditions. The possibly surviving inner Zry-4 claddings were destroyed by the application of an axial mechanical force resulting in material relocation and formation of a heterogeneous debris bed consisting of segmented pellets and larger cladding tube fragments at the grid spacer #2 (350 mm bundle elevation) and in a smaller amount at grid spacer #3 (1050 mm elevation). The so destroyed bundle was quenched with 10 g/s water. The evaporation rate generally showed an increasing trend during the reflood, except at the time of water penetration through the debris bed at spacer #2 when stagnation was observed. During the oxidation phase about 100 g hydrogen was released. The post-test examinations comprised videoscope inspections, high energy X-ray-tomography and metallographic investigations. Most of the Zry-4 claddings experienced relatively limited damage, and there was generally only minor damage of the geometrical integrity of the internal 9 rod sub-assembly. All peripheral hafnium claddings survived the whole test. Oxidation degree of Hf claddings inside the high temperature region of the bundle was noticeably lower compared with the oxidation of Zry claddings. The porosity of debris bed was significant, no dense packing of debris particles was observed. Large empty volumes formed due to bending of rods