Assessment of the Molten Pool Initial State During Degradation of the Reactor Core with Horizontal Fuel Assemblies Arrangement

The paper examines the first approximation in the assessment of the corium initial state in the lower plenum during the in-vessel phase late stage of severe accident of a promising small modular reactor of the ESS-SMART project [1]. The features of the presented analysis are related to both the initial supercritical state of the coolant and the horizontal layout of the core in the form of a system of parallel fuel assemblies with a seven-level coolant inlet system [2]. Existing industry computational system tools are limited in representing the original geometric configuration of the reactor flow path, and in particular the reactor core. Moreover, significant difficulties still arise taking into account the pressure decreasing from a supercritical state to a two-phase state, which seriously limits the performance of estimates using the analytical tool only. In this work, the main principles of sequential equivalent approximation are considered, which allow for a notable simplification of the analysis by isolating stages where only one phenomenon is dominant. For example, the evaluation of decompression parameters and impact loads is the first stage, while the immediate reactor core degradation and the formation of the core molten materials pool is the second one. The third stage is a detailed assessment of the reactor vessel degradation. In this work, attention is focused on the second stage. For this, a fast run model was prepared in the MELCOR 1.8.6 integral code with an equivalent approximation in mass fractions of the "components" in the reactor core flow part. In addition, the conceptual design of the lower plenum of the SCW-SMR reactor is similar to that of typical PWR reactors. The results of the design scenarios made it possible to form a general picture of the reactor core degradation progression at the early phase of a severe accident for the selected initial conditions, as well as to evaluate the characteristics of the accumulated melt in the lower plenum at the late phase of in-vessel severe accident stage. The obtained results are an input set for further evaluation of reactor vessel degradation.