Investigation On Local Blockage In Flow Channel Of Multi-layer Annular-type Assembly

Flow blockage is hard to analyze among reactor accidents. The multi-layer annular-type assembly, which consists of annular-type fuel plates, has excellent heat transfer performance. However, due to the complex structure in the assembly, multi-layer coolant channels in the assembly are easily blocked. In such cases, local high temperature might damage the fuel element, leading to the leakage of radioactivity. Therefore, it is necessary to investigate flow and heat transfer characters of the assembly under flow blockage conditions. In this study, system analysis codes and computational fluid dynamics (CFD) methods are applied to simulate flow blockage in multi-layer annular-type assembly, in which CFD methods include traditional direct CFD modeling method and the newly developed porous-jump simplification method.According to flow blockage simulations on the flat-plate-type assembly of IAEA MTR, in which 60%,70%,80% and 90% flow blockages occur at the inlet of the blocked channel, the constant pressure drop boundary condition is proved to be the suitable flow condition for simulated domain with limited coolant channels. Comparisons between results of direct CFD investigations on four postulated blockage forms demonstrate that, for a given blockage ratio, blockage forms affect little on coolant channel flow rates. Then flow blockage investigations on the multi-layer annular-type assembly are conducted using above methods and blockage ratios. For both assemblies, in flow blockage scenarios, mass flow rate of the blocked channel decreases dramatically with blockage ratios, while no obvious changes occur for mass flow rates of non-blocked channels; considering both flow and heat transfer effects, besides the blocked channel, coolant channels which are adjacent to the blocked channel and fuel plates between these channels are mostly affected by the blockage. Compared with flat-plate-type assembly, heat could conduct annularly in annular-type plate. Therefore, heat transfer condition of the blocked channel could be mitigated better in flow blockage scenarios.Comparisons among system analysis codes, direct CFD modeling method and porous-jump simplification treatment indicate that the latter two could provide consistent results, and predict complex heat transfer phenomena; both relative geometric position of coolant channels and heat transfer in complicated structures could not be investigated by system analysis codes, this results in abnormal predictions patterns of both flow rate distribution and power distribution.