3-DIMENSIONAL TRANSIENT ANALYSIS OF FAST-REACTORS WITH IMPROVED COARSE MESH METHOD

To estimate fast reactor core transient behavior in three-dimensional geometry, we have applied the improved coarse mesh method to time depe ndent problems. The improved quasistatic approximation is used with th e feedback of fuel temperature change and coolant temperature change. The shape function describing flux distribution is calculated using ef fective cross sections which are functions of mesh intervals and the a mplitude function. To allow large axial coarse meshes with control rod partially inserted, a bilinearly weighting method is used to calculat e homogenized cross sections. Using these methods KICOM code has been developed. The validity of this code has been checked by comparing wit h a two-dimensional transient code FX2 TH for the case when a control rod at core center is withdrawn. The KICOM code has been applied to ca lculate the power change of a prototype fast reactor and a large fast reactor for the case when a core center rod or an off-center rod is wi thdrawn. For the prototype reactor. the reactor power rapidly changes with time. but the power distribution (shape function) remains almost the same. However. for the large fast reactor, not only the reactor po wer but also the power distribution remarkably change with time. The m esh effect is also investigated by comparing the results with and with out the mesh correction.