[en] Two types of blockages have been described that may form in the reactor core when molten fuel is ejected into the coolant channels during a transient overpower accident. The first type of blockage forms when semi-molten fuel particles hit a wire wrap and freeze on the wire wrap, whereas the second type of blockage forms when solid or semi-solid particles get stuck in the coolant channels. The thermal-hydraulic analysis of these blockages indicates that under specific conditions the blockages can lead to melting of the wire wrap and cladding and to in-channel voiding. The second type of blockage, if formed as a plug, may re-melt before being quenched by subcooled or saturated sodium. (author)