Numerical Simulation of Heat Transfer Characteristic with Cell Type Spacer Grid for Annular Fuel Sub-channel

Abstract

Annular fuel is a new type of fuel used in pressurized water reactor. The fuel pellets are annular shape and loaded between internal and external cladding. Annular fuel consists of two coolant channels, namely the internal channel and the external channel. The pellets can be cooled by both sides simultaneously. The spacer grid is one of the core components of annular fuel assembly, which has an important influence on flow and heat transfer characteristics. In this paper, the numerical calculation model of 13×13 annular fuel fullspan grid was established for straightthrough cell grid and externallyexpanded cell grid based on computational fluid dynamics (CFD) method. The flow and heat transfer characteristics of the two grids were studied. Due to the symmetrical structure of annular fuel, 1/8 external subchannels were selected to evaluate the uniformity of velocity distribution by calculating the standard deviation (σ) of velocity distribution. Three evaluation factors including swirl factor, cross flow factor and turbulence intensity factor were defined, and the mixing characteristics of the two kinds of grids were analyzed based on central sub-channel. The axial heat transfer characteristics of the two grids were evaluated by calculating the average Nusselt number (Nu) distribution of the angular sub-channel, the side sub-channel and the central sub-channel. The circumferential heat transfer characteristics of fuel rods were evaluated by calculating the circumferential Nu. The resistance characteristics of the spacer grid were evaluated by comparing the axial pressure distribution of annular fuel and the resistance coefficient of the spacer grid. By comparing and calculating the coolant flow distribution ratio (φ) between the external channel and the internal channel, the adjustment effect of two kinds of grids on the flow distribution ratio was evaluated. It shows that externallyexpanded cell grid can form a constricted orifice with throttling function, compared with the straightthrough cell grid. It has a good regulating effect on the resistance characteristics and flow distribution characteristics of annular fuel, and can significantly improve the distribution uniformity of flow and heat transfer parameters of external sub-channels. By adopting the externally-expanded cell grid, the velocity distribution uniformity in the external sub-channel is increased by 54%, the maximum relative deviation of the average Nu of each subchannel is reduced from 65% to 24%, and the maximum relative deviation of the circumferential Nu of the fuel rods is reduced from 35% to 24%, which significantly improves the distribution uniformity of the flow and heat transfer parameters in the subchannels. The externally expanded grid has a more significant mixing effect on the fluid, and the active distance reaches 35Dh, which has an enhanced effect on the subchannel heat transfer. In addition, the resistance coefficient of the external expansion grid is 45 times that of the straightthrough grid, and the flow distribution ratio is only 53% of that of the straight-through grid, which has a good adjustment effect on the resistance characteristics and flow distribution characteristics of the annular fuel.

Keywords

• annular fuel
• cell grid
• flow and heat transfer characteristics
• cfd method