Accidental water ingress in pebble-bed type, high temperature reactor (HTR) cores can result in large positive reactivity changes and in consequent reductions in shutdown margins, since such systems generally tend to be undermoderated. The prediction of such effects, which are particularly important in the case of low-enriched uranium (LEU) fuel is a challenging task due to the complex interaction of, on the one hand, an increase in moderation and, on the other, a reduction in neutron streaming. Experiments have been carried out at the PROTEUS facility of the Paul Scherrer Institute in Switzerland to quantify water ingress effects in LEU-HTR systems and to assess the performance of certain deterministic codes in predicting them. In order to simulate water ingress, polyethylene rods were inserted between the pebbles of various deterministic configurations and a pulsed-neutron-source technique was used to measure the variation of keff with the stepwise removal of these polyethylene rods. Generation time, as well as reactivity measurements, were compared with calculations, made with various types of streaming corrections, for three different subcritical states