Development of an evaluation method for debris bed formation behavior focusing on the agglomeration mechanism observed in the DEFOR-A test using THERMOS/JBREAK–DPCOOL–MSPREAD

• A special model for agglomeration has been developed based on the new series of DEFOR-A tests (A23-27) conducted, in collaboration with KTH. • The debris agglomeration is found to be dominated not only by the droplet solidification but by the crust cracking and melt spreading. • The JBREAK-DPCOOL-MSPREAD analysis reproduces in order jet breakup, droplet formation and settling, and melt spread and agglomeration. • The coupled analysis of JBREAK–DPCOOL–MSPREAD can explain the mass fractions of agglomerated debris over a wide range of melt superheat levels. When the lower head of a reactor pressure vessel (RPV) is damaged during a severe accident in light water reactors (LWRs), after the jet breakup occurs in the water, the entrained pieces of molten debris (hereafter called droplets) is likely to form nonuniform, poorly coolable agglomerations on the floor. These debris agglomerates can impact the debris bed coolability. The authors are developing THERMOS, an analysis code composed of modules such as JBREAK, DPCOOL, and MSPREAD, to evaluate these behaviors. In the investigation of the new DEFOR-A test series conducted in collaboration with Kungliga Tekniska Högskolan (KTH), it has been identified that the formation of agglomerated debris is influenced not only by the solidification fraction of the droplets but also by crust cracking and melt spreading. To evaluate the formation of agglomerated debris at a wide range of superheat, the authors have developed the special model in JBREAK, one of the THERMOS modules, based on mechanism estimated from the investigation of DEFOR-A test series (A23-27). Additionally, the agglomeration process is affected by several complex phenomena, such as jet breakup, droplet sedimentation, deposition, and melt spreading behavior, so the authors developed an evaluation method that sequentially evaluates these behaviors using the THERMOS/JBREAK–DPCOOL–MSPREAD coupling. This evaluation method successfully simulated jet breakup, agglomeration, and debris bed formation observed in the DEFOR-A tests. The evaluation method has accurately explained the agglomerated debris mass fraction over a wide range of melt superheat levels by modeling droplet crust cracking, melt spreading, and agglomeration resulting from droplet–debris interactions.