Accidental water ingress in pebble-bed type, high temperaturereactor (HTR) cores can result in 1arge positive reactivity changesand in consequent reductions in shutdown margins, since such systemsgenera1ly tend to be undermoderated. The prediction of such effects,which are particu1arly 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, areduction in neutron streaming. Experiments have been carried out atthe PROTEUS facility of the Paul Scherrer Institute in Switzerland toquantify water ingress effects in LEU-HTR systems and to assess theperformance of certain deterministic codes in predicting them. Inorder to simulate water ingress, polyethylene rods were insertedbetween the pebbles of various deterministic configurations and apulsed-neutron-source technique was used to measure the variation ofK_cff with the stepwise removal of these polyethylene rods.Generation time, as well as reactivity measurements, were comparedwith ca1cu1ations, made with various types of streaming corrections,for three different subcritical states.
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