Melting test of penetrating tube through BWR-RPV bottom wall

Highlights: • Melt behavior of a BWR in-core monitor guide tube made of steel was tested. • The test simulated severe accident condition with a mixture corium of UO₂ and ZrO₂. • Some metallic corium particles had fallen out from the bottom opening of the tube. • The tube was soon completely blocked by the solidification of steel melt. • There was almost no ingress of oxide melts into the tube. - Abstract: Melt behavior of a BWR in-core monitor (ICM) guide tube, which penetrates a RPV bottom wall, under a severe accident condition was tested using a corium with a mixture of UO₂, ZrO₂ and Zr. There are several BWR-ICMs; two of them are a short range monitor (SRM) and an intermediate range monitor (IRM), which are made of stainless steel, have the same tube dimensions and the bottom ends of the tubes open to the drywell (or, the pedestal region). In the test, the full scale SRM/IRM guide tube with the same dimension as one of Fukushima Daiichi Unit-1 but short length, was installed in the lower crucible which simulated the lower plenum and which bottom simulated the RPV bottom wall with about 200 mm thickness. The melt corium was generated in the upper crucible and it was discharged into the lower crucible. The test result showed that:(1)The corium discharged into the lower plenum attacked the guide tube first resulting in the tube melt from the top and then accumulated in the lower plenum. (2)Small amount of corium had fallen out from the bottom opening of the tube into the pedestal as particles, and such outflow stopped soon because a stainless steel melt had solidified in the tube and blocked the flow path. (3)The depth of the melt ingression into the tube was 60–65 mm from the inner surface of the RPV bottom wall, where the tube was completely blocked by the solidification of steel melt and no oxide melts had ingressed there.