Neutronic and thermal-hydraulic coupling analysis of the dynamic unfolding process of the LOTUS reactor

Common pool-type heating reactor designs have certain safety risks, particularly during natural disasters such as earthquakes. In such scenarios, if the pool water leaks, the reactor core is likely exposed to air, which can cause core meltdown. The proposed expandable LOTUS reactor offers a solution to this problem. Its core can unfold under the influence of gravity and buoyancy, passively shut down the reactor and remove the core heat when accidents occur. During the unfolding process, both geometric buckling and material buckling of the core change non-linearly, and this is a new problem for nuclear engineering that was not discussed yet. Therefore, the emergence of the LOTUS reactor entails an entirely new and intricate nuclear-thermal transient coupling process through numerical simulation. We used SERPENT and STAR-CCM + to conduct single-physical-field and nuclear-thermal coupling calculations. The former provides a fast, low-dimensional calculation that serves as references for the coupling calculation. The changes in power, effective multiplication factor and temperature indicate that the unfolding motion can force the reactor to shut down, and the remaining pool water effectively decreases the core temperature. For the coupling calculations, first, validation calculations confirmed the correctness and stability of the coupling strategy. Then, steady-state coupling calculations were conducted using Picard iteration method and converged after 207 iterations. There was a maximal power difference of up to 6 % between single-physical-field calculations and coupling calculations at the beginning of the unfolding motion. This difference gradually decreased over time and disappeared when the unfolding angle reached 0.6°. Compared to the single-physical calculations, the non-uniformly distributed temperature obtained from the coupling calculations was more consistent with the actual distribution. These differences highlight the necessity and importance of coupling calculations.