The CORA-15 bundle test was performed under transient conditions typical for many CORA-PWR tests with bundle containing two Ag-In-Cd neutron absorber rods. Unlike other CORA bundles, all heated and unheated fuel rod simulators (16 + 7 correspondingly) were filled with helium and pressurized to 6.0 MPa before transient. During the transient, all rods underwent ballooning and burst. Ballooning progressed for about 100 s. Bursts occurred within 150 s (between 3500 and 3650 s) in the temperature bandwidth between about 700 and 800 °C. The mostly probable burst elevation for the bundle was at 750 mm (which was the hottest elevation during the burst period).
During further heat-ups, temperature escalation due to the zirconium-steam reaction starts at elevations of 550 to 950 mm at temperature of about 1100 °C. In presence of PWR absorber rods, the sequence of failure starts with the release, relocation, and re-solidification of the (Ag, In, Cd) melt. The melt release occurred between 1290 and 1350 °C at elevations between 750 and 800 mm. Most of the melt reacted with the cladding and guide tube by liquefying the zirconium components, forming a metallic melt of the type (Ag, In, Zr). This melt was capable of dissolving Zircaloy as low as 1250 °C, i.e., clearly below the melting point of Zircaloy (1760 °C).
During the temperature escalation above 1800 °C, Zr melt formed in the gap between ZrO2 and UO2 was relocated partially inside the gap to lower bundle elevations and partially penetrated through the failed ZrO2 layer into the space between its neighbouring elements. The resulting melt of the fuel rod interaction, containing mainly U, Zr and O relocated downwards as a slug and solidified between 400 and 550 mm according to its solidus temperature as a large lump of porous structure. The maximum bundle blockage (almost 100%) was observed at the top of the Inconel grid spacer (at about 500 mm). The (Ag, In, Cd) absorber melt with the much lower solidus temperature solidified down at the lower elevation of about 150 mm.
Post-test investigations showed negligible oxidized cladding up to elevation of about 350 mm. The claddings were completely oxidized between elevations 480 and 1000 mm. The maximum hydrogen release rate of 210 mg/s was measured on the end of the transient (4800 s). The total hydrogen release was 145 ± 15 g
