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概要

In a hypothetical severe accident, the fuel assemblies and structure in the core region could melt. The corium from the core region may fall into the reactor lower plenum. Jet breakup phenomenon would happen while corium falling into water pool in the lower plenum. In a boiling water reactor lower plenum, the control rod guide tubes (CRGTs) will create flow channels for the falling jet. Therefore corium jet breakup behavior could be affected by these CRGTs. Thus, it is important to investigate CRGTs effect on the jet breakup behavior. In order to investigate CRGTs effect on the jet breakup behavior, a molten material (U-alloy) breakup experiment was conducted under isothermal condition. The experiment results indict that CRGTs restrain the jet breakup process. For the case with CRGTs pitch/diameter ratio of 1.37, the jet breakup fraction was approximately 20% of that for the case without CRGTs. The test was also conducted for the case with a coarser pitch/diameter ratio of 2.47, but the amount of jet breakup was only slightly reduced in this configuration. The experiments also indict that the departure droplets diameter was almost not affected by CRGTs. Furthermore, the particle image velocimetry (PIV) method was used to measure water velocity distribution around the jet. The water velocities surrounding the jet for the case without CRGTs were smaller than those in the case with CRGTs.

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