Development of multi-dimensional thermal hydraulic modeling using mixing factors for wire wrapped fuel pin bundles with inter-subassembly heat transfer in fast reactors

Nishimura, Motohiko; Kamide, Hideki ; Ohshima, Hiroyuki

Temperature distributions in fuel subassemblies of fast reactors interactively affect heat transfer from center to outer region of the core (inter-subassembly heat transfer) and cooling capability of an inter-wrapper flow, as well as maximum cladding temperature, The prediction of temperature distribution in the sub-assembly is, therefore one of the important issues for the reactor safety assessment. To treat the complex phenomena in the core, a multi-dimensional thermal hydraulic analysis is the most promising method. From the studies on the multi-dimensional thermal hydraulic modeling for the fuel sub-assemblies, the followings have been recommended through the analysis of sodium experiments using driver subassembly test rig PLANDTL-DHX and blanket subassembly test rig CCTL-CFR. (1)Staggered half pin mesh arrangements for the pin bundles. (2)MIT's distributed resistance correiation for the predictions of axial pressure drop. (3)MIT's Mixing factors for inter-subchannel mixing due to wire sweeps, turbulence and thermal plumes. Computations of steady states experiments in the test rigs using the above modeling showed quite good agreement to the experimental data. In the present study, the use of this modeling was extended to transient analyses, and its applicability was examined, Firstly, non-dimensional parameters used to determine the mixing factors were modified from the ones based on bundle-averaged values to the ones by local values. Sccondly, a new threshold function was derived and introduced to cut off the mixing factor of thermal plumes under inertia force dominant conditions. This function goes 1 to 0 when Richardson number decreasingly passes through value of 0.1. Since no thermal plumes were observed under the condition below Richardson numbcr equal 0.1, in the results of existing studies. A comparison or the multi-dimensional analysis to a subchannel analysis was also made to cxamine the consistency of formulations between the two analytical ...