Effect of rupture size in Ex-pin phenomena of severe accident in SFR

Abstract

In hypothetical core disruptive accident (HCDA) of sodium-cooled fast reactor (SFR), the metallic fuel melts and fuel pins are ruptured. After fuel pin failure, molten fuel ejected into the coolant channel due to accumulated pressure by fission gas, and this phenomenon is called as Ex-pin. Among the various parameters, rupture size of the cladding affects the original size of the molten jet, therefore, it directly affects the shape of the debris. Effects of the rupture size of the cladding on the Ex-pin phenomena were studied. The rupture shape in HCDA becomes various depending on the source of damage. The fuel cladding can be ruptured by ejected molten fuel adhered at the cladding after Ex-pin. In this experiment, various types of ejecting tubes were used to represent different rupture cases: Rupture size was varied from 3.125 mm to 12.7 mm in diameter. The other experimental conditions were fixed to simulate an unprotected loss of flow (ULOF). Average debris size decreased as debris diameter increases because molten metal jet was not fully fragmented until it smashed the channel wall. Debris bed formation becomes more compact and less porous as the average debris size decreases, which results in degradation of coolability.