Characterization of velocity and temperature fields in a 217 pin wire wrapped fuel bundle of sodium cooled fast reactor

Abstract

RANS based computational fluid dynamic (CFD) simulation of flow and temperature fields in a fast reactor fuel subassembly has been carried out. The sodium cooled prototype subassembly consists of 217 pins with helical wire spacers. An axial length of seven helical wire pitches has been considered for the study adopting a structured mesh having 36million points and 84 processors in parallel. The computational model has been validated against in-house and published experimental data for friction factor and Nusselt number. Also, the transverse flow in the central subchannel and swirl flow in the peripheral subchannel are compared against reported experimental data and those computed by subchannel models.The focus of the study is investigation of transverse and axial flows in different types of subchannels. Based on the 3-dimensional CFD study, correlations have been proposed for calculation of transverse flow, which forms an important input for development of subchannel analysis codes. Periodic variations have been observed in the subchannel axial flow rates. For the subchannels located in the central region, the peak to peak variation in the axial flow rate is ∼21% and it is found to be contributed by the changes in the flow area and hydraulic resistance due to frequent passage of helical wires through the subchannel. For the subchannels located in the periphery, this variation is as high as 50%. The transverse flow in the central subchannels follows a cosine profile, for all the faces. However, there is a phase lag of 120° among the three faces that bound the subchannel. In the peripheral subchannels, a strong unidirectional transverse flow prevails in the faces perpendicular to the hexcan wall. On the other hand, the transverse flow in the face parallel to the hexcan follows a square wave pattern. The CFD results indicate that the swirl velocity in the peripheral subchannel is non-uniform, contrary to that considered in the traditional subchannel models. The mean clad temperature is seen to exhibit a non-monotonic increase along the flow direction. This phenomenon is more dominant in the peripheral pins due to large gradient in subchannel temperature and the square wave profile of the transverse flow.