Neutronic analysis of the unfolding process of the LOTUS reactor design

Abstract

The LOTUS reactor is a novel small thermal reactor concept designed for heat supply in winter. This pool-submerged reactor can unfold passively into four parts along its central axis solely under the buoyancy and gravity. When facing accidents like loss of coolant in the primary loop or in the pool, the unfolding process is expected to make the operational core sub-critical by reducing the volume of the fuel under the critical level and shut down the reactor immediately, which keeps the fuel assemblies in contact with water and delays the exposure in the air to ensure the safety. In this study, we implemented selected analyses using Serpent surrounding the unfolding process of the LOTUS reactor, including stepwise steady-state and transient calculations. This work showed that the fission power, keff value, flux, and reaction rate in the core decreased definitely as the reactor unfolded. The discrepancy between the results from the two modes of Serpent, however, indicated that in such dynamic behavior, considering the delayed neutrons and neutron moderation was crucial to the analysis, particularly to the water of the pool, which had a certain value of power deposited when the core power dropped during the unfolding process.