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The MELCOR 2.1 computer code has been used to simulate the large-scale aerosol behavior experiment in the Containment System Test Facility (CSTF) vessel. The test LA2 of the Light Water Aerosol Containment Experiments (LACE) was designed to simulate the containment conditions and aerosol behaviors in the high-steam-concentration environment under a postulated severe accident. In this study, the Aerosol Condensation model’s (ACD) and Hygroscopic Model’s (HM) ability of the aerosol behavior prediction has been evaluated by the simulation of LA2 based on MELCOR 2.1 code. It has been found that the size distribution and depletion rate of dry aerosols are affected by the mass of water aerosol. The different masses of water aerosol in ACD and HM models are due to the different relative humidity calculated by these two models. The simulation models with different control volumes setup were evaluated to study the effect of water aerosol mass on aerosol depletion rate. The results show that higher water aerosol mass results are calculated and better airborne aerosol masses are predicted under the multi-control volume models. Based on this research①, it can be concluded that good results of aerosol deposition in the vessel are severely affected by the accuracy of water aerosol mass. The improvement of water aerosol mass results by the MELCOR code’s simulation should be thoroughly studied.

China Fusion Engineering Test Reactor (CFETR) is aimed to meet the technical gaps between International Thermonuclear Experimental Reactor (ITER) and future fusion DEMO reactors, tritium self-sufficiency must be guaranteed for the operation of fusion reactors. Helium Cooled Ceramic Breeder (HCCB) blanket is one of the promising candidate blanket schemes for CFETR. Tritium generated in Li4SiO4 pebble bed is carried out by purge gas, it is crucial to study the tritium behavior in the blanket from the tritium fuel cycle point of view. In this paper, the effective diffusivity of tritium in purge gas is conducted based on Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) coupled simulation considering the influence of the pebble bed structure. The flow characters analysis of purge gas is carried out by CFD simulation as well, which includes velocity field distribution, pressure field distribution and pressure drop. Based on the macro parameters conducted above, the tritium transport process in blanket including tritium permeation into coolant, tritium inventory in blanket and tritium carried out by purge gas is simulated based on a 2D model set up in COMSOL Multiphysics, the results can be the reference for the design of Tritium Extraction System (TES) and Coolant Purification System (CPS).

The steady impinging jets applied in turbomachine have been comprehensively studied but the pulsating jets still need to be further researched. The flow field and heat transfer characteristics of pulsating impinging jet impinging on a flat plate have been simulated using the improved very large eddy simulation established with SST k–ω model. Two time-mean Reynolds numbers (6,000 and 23,000) in the conditions of frequency = 10Hz and steady state at the constant jet–to–surface distance (6D) were considered. The velocity, vortices, and Nusselt number distributions on the plate surface were investigated to emphasize on the vortex structures in the flow and its relation to the heat transfer. The investigation has revealed the advantage of the improved very large eddy simulation for predicting the dynamical generating process of flow structures in pulsating jets. Calculated results showed pairs of vortices were organized and induced from the jet exit, and propagated along with the jet region periodically. The vortices grew with the entrainment towards the ambient fluid and resulted in accelerated interaction in the wall jet region. Meanwhile, the vortices had strong interaction with the core region and weakened velocity in the core region. Results showed that the time–mean local Nusselt number of pulsating jet was lower in the stagnation region at both investigated Re numbers but not reduced in the wall jet region.

Due to the low nuclear safety risk, low initial investment cost and short construction period, integrated small nuclear reactors have received wide attention from all over the world. As advanced new type of nuclear reactors, the technologies of integrated small nuclear reactors are in the process of exploration and development. Steam generators are used as the heat transfer system for energy exchange between the primary and secondary circuit in reactors, and their heat transfer analysis is very important for reactor design and development. Due to the simple structure, strong heat exchange capacity and timely load following, Once-Through Steam Generators (OTSGs) are the mainly used steam generators in the design of integrated small nuclear reactors. RELAP5/MOD4.0 is a commercial software developed by Innovative System Software, LLC for transient analysis of light water reactors (LWR). After years of development and improvement, RELAP5 has been a basic tool for analysis and calculation of various simulators of nuclear power plants. However, when RELAP5 models steam generators, only structural models related to straight pipes can be established, which is very inconvenient for the heat transfer research of Once-Through Steam Generators. In this paper, Once-Through Steam Generators with specified structural parameters are taken as the research object. The heat transfer calculation is performed on the simplified inclined tube models by RELAP5 code and the theoretical calculation of the spiral tube heat transfer models is also carried out. Comparing the steam outlet temperature on the primary and secondary sides, the heat exchange power, the average heat transfer coefficient and the tube length of different heat exchange zones under given primary and secondary side inlet fluid conditions, it is confirmed that the RELAP5 heat transfer calculation is verified for simplifying Once-Through Steam Generators with inclined tube models.

As key heat transfer system in small and medium size pressurized water reactors, once-through steam generators are important parts of energy exchange between primary and secondary circuits, and are very important for the design and operation of reactors. However, two-phase flow and heat transfer in once-through steam generators are very complicated. When a reactor experience power rising and descending transient, the heat removal of once-through steam generator, the flow rate, the inlet fluid temperature and outlet steam temperature will all change accordingly. Especially when a reactor is running at a low power, the flow rate of the secondary side of OTSG is extremely small and the single-phase region of the secondary side of OTSGs is also too small. The two-phase flow instability may occur, which has a serious impact on reactor operation and safety. So, a reasonable power-up and power-down transient scheme is required to ensure operational stability when starting up and shutting down a reactor. RELAP5/MOD4.0 is a commercial software developed by Innovative System Software, LCC for transient analysis of light water reactors (LWR). After years of development and improvement, RELAP5 has been a basic tool for analysis and calculation of various simulators of nuclear power plants. Scholars all over the world have carried out a large number of analysis of two-phase flow stability using RELAP5, and the results are reliable. This paper takes once through steam generators with given structural parameters as the research object, and uses RELAP5 as the calculation tool. The influencing factors of flow instability are discussed in this paper, and the operating parameters of the fluid on the primary and secondary sides are designed to satisfy the flow stability under different powers. And a set of power-up and power-down schemes for stable operation is proposed.

The subject of heat transfer behaviors of steam generator is of great interest for better heat transfer efficiency and safety in industry. On most previous study, the models of SG based on RELAP5 used heat structures as the power generation, which is different from heating by primary side. This paper investigates the distinctions of different heat transfer methods on the thermal-hydraulic parameters by RELAP5. Two different heat transfers are selected, case 1 is uniformly heated by heat structure and case 2 is heated by the primary side of SG. The results show that the onset position of critical heat flux for case 2 is ahead of that for case 1. Besides, the temperature of tube wall corresponding to position of critical heat flux increases abruptly in case 1, and the wall temperature in superheated region in case 1 increases faster than that in case 2 when the water becomes fully superheated vapor. Then instability distinctions for two heat transfer cases are studied in this investigation. It is detected the fluid flow heated by second method is more stable than the first one, and the system has a higher ultimate thermal power when density wave oscillation occurs for case 2.