In the framework of the QUENCH program at KIT, over the past 22 years, 21 bundle tests were performed under severe accident conditions with different cladding materials. Additionally, 7 QUENCH-LOCA bundle tests with fresh and pre-hydrogenated different cladding materials (Zry-4, M5®, opt. ZIRLO™) were performed according to a temperature/time-scenario typical for a LBLOCA in a German PWR.
Main purposes of severe accident tests were hydrogen source term, as well as investigation of phenomena on melt relocation, debris, and aerosol formation. Concerning the hydrogen source term, six parameters, enhancing hydrogen production during reflood, have been identified: 1) low reflood flow rates < 1 g/s/rod; 2) breakaway effect with weakness and spallation of protective oxide layer; 3) steam starvation; 4) nitride formation by air ingress with formation of very porous oxide layer during following reflood; 5) high temperatures with melt relocation outside claddings and intensive melt oxidation; 6) eutectic interactions between B4C, stainless steel and Zircaloy-4 leading to low melting point.
Post-test tensile experiments performed in the framework of the QUENCH-LOCA program evidenced fracture at hydrogen bands (formed during secondary hydriding) for claddings with local hydrogen concentrations >1500 wppm.