Quenching of rod bundles with fuel rod simulators of different design

Reflood tests with 5 x 5 rod bundles consisting of electrically heated full length fuel rod simulators with Zircaloy claddings and a helium filled gap between cladding and internal Al2O3 pellets have been performed previously. For these tests the same facility has been used as for tests carried out previously with a 5 x 5 rod bundle consisting of rods with stainless steel claddings without gap between cladding and MgO insulation. The reflood parameters, e.g. bundle power, system pressures, flooding velocities were the same for both types of rod bundles. A comparison of selected data measured as well as evaluated from tests of both the bundles shows the following: Fuel rod simulators with Zircaloy claddings and a helium filled gap underneath the claddings quench significantly earlier than gapless fuel rod simulators with thick stainless steel claddings. Quenching is initiated from a higher level of the heat stored prior to quenching of rods with a gap