The small modular reactor (SMR) class core design concept of once-through SuperCritical light Water-cooled Reactor (SCWR) with fast neutron spectrum (Super FR) is being developed at Waseda University. The preceding studies revealed that SMR class Super FR core can be designed, whose performance is compatible to large Super FR core with respect to average power density and average coolant outlet temperature with small core pressure drop, which is favorable from the viewpoint of achieving natural circulation cooling after reactor shutdown. However, consequences of severe accident of such SMR class Super FR has never been considered. Two major issues need to be addressed. The first issue is the fundamental issue for a water-cooled reactor, which is the possibility of the whole core meltdown as the coolant boils off at low temperature under depressurized condition. Second issue is the fundamental issue for a fast spectrum core, which is to avoid re-criticality of molten core as the reactivity increases with core compaction. With the above described background, this study proposes In- Vessel Retention (IVR) of SMR class Super FR, which can avoid re-criticality even if the whole core relocates to the lower plenum of the Reactor Pressure Vessel (RPV). The debris coolability may be a subject for the future study. The core characteristics with a given set of design specifications and criteria are evaluated based on fully coupled neutronics and thermal-hydraulics core burnup calculations. The neutronics calculations are based on interpolation of macroscopic cross sections with neutron diffusion approximation (SRAC and COREBN codes with JENDL-4.0 nuclear data library). The thermal-hydraulics calculations are based on inhouse code that adopts single channel model with Watts' correlation for evaluating supercritical water heat transfer. The batch fuel shuffling pattern is considered to evaluate the equilibrium core characteristics. The debris criticality is evaluated based on Monte-Carlo based method (MVP) to consider the RPV lower plenum and debris configurations. The fuel debris are assumed to be homogeneous. Firstly, for a given discharge burnup, quantitative relationship between the core criticality and the debris criticality is evaluated. The results show that downsizing the core increases the debris criticality when relocated to the RPV lower plenum, because the Pu enrichment needs to be raised to attain the same discharge burnup with larger neutron leakage. The results also show that some measures are necessary to prevent re-criticality of the debris if the whole core is assumed to relocate to the lower plenum. Then, the new "debris dispersion IVR" concept is proposed. In this concept, heat resistant structure (tentatively ZrO2), which contains some neutron absorbing material (tentatively, B4C) is placed at the center of the lower plenum to disperse the debris and gain neutron leakage and absorption. Then, the trade-off relationships between the core discharge burnup and the size of the debris dispersion structure are evaluated with considerations of core design specifications such as average Pu enrichment, operation cycle length, and fuel shuffling patterns.